Content of Short-Lived Radionuclides (¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce) in Fish

Abstract

This review is part of a series of studies on short-lived radionuclide accumulation in aquatic organisms following nuclear weapons testing, routine facility discharges, and accidental releases. It examines the pathways of uptake, accumulation, and internal redistribution of ¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce in fish representing different ecological groups. The analysis combines published literature data with our original findings obtained from studies conducted in the cooling pond of the Chornobyl Nuclear Power Plant and the Kaniv Reservoir during the post-accident period. It has been established that radionuclide accumulation is governed by their physicochemical properties, environmental speciation, and the trophic characteristics of fish. ¹²⁵Sb demonstrates high bioavailability and accumulates in internal organs, gills, roe, and muscle depending on its chemical form in the aquatic environment. ¹³¹I is characterized by high solubility, rapid incorporation into biological processes, and transient retention in tissues. ¹⁴¹Ce and ¹⁴⁴Ce exhibit low mobility, strong association with particulate matter, and preferential accumulation in the gastrointestinal tract, external, and mineralized tissues. At the same time, the presence of ¹⁴⁴Ce in the muscle tissue of carnivores and piscivores suggests possible trophic transfer and does not exclude potential manifestations of limited biomagnification of this radionuclide under conditions of elevated environmental contamination. It has been determined that the ratio of ¹²⁵Sb to ¹⁴⁴Ce can be used to identify contamination sources: their co-occurrence is interpreted as evidence of fuel particle input, explaining their predominant localization in the gastrointestinal tract and, to a lesser extent, in external tissues. Conversely, their separate detection reflects differences in mobility and bioavailability. It has been shown that the principal pathways for the uptake of the investigated radionuclides by fish are particle ingestion and absorption from the dissolved phase; thus, trophic dilution predominates over biomagnification, although trophic transfer of ¹⁴⁴Ce cannot be excluded.

1. Introduction

This review examines the accumulation of three chemical elements in aquatic ecosystems: antimony (Sb), a metalloid (heavy metal); iodine (I), an essential trace element; and cerium (Ce), a representative of the rare earth elements (REEs). These elements differ in their biological functions and environmental behaviour, and each has short-lived radioactive isotopes (¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce) that enter aquatic environments under conditions of anthropogenic radioactive contamination. Despite these differences, these elements are unified by the presence of short-lived radioactive isotopes capable of entering aquatic environments under conditions of anthropogenic impact. Such inputs may occur as a result of nuclear waste disposal, routine discharges associated with the nuclear fuel cycle, nuclear weapons production and testing, accidental releases from terrestrial nuclear facilities, as well as maritime accidents or losses involving nuclear materials [1].

Among the radionuclides considered, ¹²⁵Sb has a half-life (T_½) of 2.8 years; ¹³¹I, 8.0 days; ¹⁴¹Ce, 32.5 days; and ¹⁴⁴Ce, 285 days [2,3]. Differences in biological significance, half-life, and chemical properties largely determine the environmental persistence, migration patterns, and overall ecological relevance of these radionuclides. In radioecological classifications, anthropogenic radionuclides are commonly divided into conservative and non-conservative (particle-reactive) categories. Conservative radionuclides, which include radioisotopes of iodine, are highly soluble in seawater and may remain in the dissolved phase for extended periods. In contrast, particle-reactive radionuclides, among them cerium isotopes, exhibit a pronounced affinity for suspended matter and natural particle surfaces, which promotes their relatively rapid removal from the water column through sedimentation and downward particle flux processes [4].

The environmental pathways of the radionuclides under consideration reflect their physicochemical properties and modes of release. ¹²⁵Sb, a fission product, may enter aquatic systems both in dissolved form and in association with suspended particles and bottom sediments, demonstrating a general tendency toward particulate binding while retaining a fraction in bioavailable form. ¹³¹I is released into the environment primarily during accidental discharges from nuclear installations and atmospheric nuclear weapons testing; owing to its short half-life, its presence in aquatic ecosystems is largely transient, although it may be rapidly incorporated into biological processes during the initial post-deposition period. Similarly, cerium isotopes (¹⁴¹Ce and ¹⁴⁴Ce), generated during nuclear fission and dispersed during weapons testing or nuclear accidents, are transported predominantly via atmospheric pathways and deposited through precipitation. After deposition, these radionuclides tend to associate with particulate matter and may accumulate in bottom sediments, reflecting their limited solubility and particle-reactive behaviour [5]. Their detection in global fallout confirms their participation in large-scale atmospheric and hydrological transport processes and their entry into aquatic systems via precipitation and surface runoff [6].

1.1. Radionuclide Release from Nuclear Weapons Testing

Nuclear weapons testing represented one of the earliest and most significant global sources of artificial radionuclides to aquatic environments. According to data from the Stockholm International Peace Research Institute, a total of 2053 nuclear tests were conducted worldwide between 1945 and 2006. Of these, 85% were carried out by the United States and the Soviet Union between 1945 and 1992; 14.5% (300 tests) by the United Kingdom, France, and China; and less than 1% by India, Pakistan, and North Korea. Tests were conducted in the atmosphere, underground, and underwater. Over several decades, aquatic basins, both marine and freshwater, functioned as long-term repositories for radionuclides originating from global fallout. A substantial fraction of radioactive particles released into the atmosphere was ultimately deposited onto the surface of oceans, seas, lakes, and rivers. Although isotopes such as ¹³⁷Cs and ⁹⁰Sr are traditionally regarded as principal components of fallout, iodine isotopes (including ¹³¹I and the long-lived ¹²⁹I) were also consistently detected in aquatic environments [6].

Temporal patterns of fallout deposition further illustrate the role of radionuclide half-life. During the period of maximum global fallout intensity, short-lived radionuclides such as ¹³¹I reached peak deposition rates in 1962, whereas longer-lived nuclides such as ¹⁴⁴Ce exhibited peak deposition in 1963 [7]. These patterns reflect both the chronology of atmospheric testing and the differing environmental persistence of individual isotopes.

Concrete examples from specific test series demonstrate how these global processes translate into regional contamination of aquatic ecosystems. The United States test series, including the thermonuclear test “Castle Bravo” (1 March–14 May 1954), conducted in the Bikini–Enewetak region of the Marshall Islands, resulted in substantial emissions of radionuclides detected both on land and in adjacent coastal waters. A portion of the fallout was adsorbed onto suspended particles or dissolved in seawater, subsequently accumulating in bottom sediments and aquatic organisms [8]. Seawater samples collected on 20 June 1954 at a distance of 450 km from Bikini Atoll contained, among other radionuclides, ¹⁴¹Ce and ¹⁴⁴Ce [9]. Soil samples obtained in 1957 on Kabelle Island at seabird nesting sites revealed the presence of radioactive isotopes of Sb and Ce: ¹⁴⁴Ce constituted 41.7% of the identified radionuclide inventory, ¹²⁵Sb 3.74%, and ¹³⁷Cs 0.81% [10]. Although radioactive Ce was not detected in living organisms, stable Sb was present in very small quantities in non-planktonic algae, fish, tunicates, and seawater [10]. According to [8], biological effects of radioisotopes in the marine environment were expected to be most pronounced in the Bikini–Enewetak region. On a broader scale, following global fallout, the proportion of ¹⁴¹Ce in plankton of the World Ocean was approximately 2%, whereas ¹⁴⁴Ce accounted for less than 1%.

Continental atmospheric nuclear tests conducted by the United States in Nevada and New Mexico (94 tests between 1945 and 1962) led to widespread fallout across the United States. Artificial radionuclides, including ¹³¹I, ¹³⁷Cs, ⁹⁰Sr, and other fission products, were deposited in soils and aquatic systems, affecting both surface waters and groundwater [11]. These events demonstrate that even inland test sites contributed to contamination of distant aquatic environments through atmospheric transport and hydrological redistribution.

In addition to military programs, the United States conducted nuclear tests aimed at exploring peaceful applications of nuclear energy. The experiment codenamed “Cabriolet” (Nevada Test Site, 1968) investigated the feasibility of creating artificial canals, harbors, or reservoirs. Subsequent laboratory studies examined radionuclide accumulation in freshwater and marine organisms exposed to materials released by this explosion [12]. The highest concentrations of ¹²⁴Sb (T_½ = 60.2 days [2]) were recorded in freshwater mollusks. In contrast, concentrations of ¹³¹I in biota did not exceed those measured in water during the experiment. The highest ¹³¹I levels were observed in internal organs of crayfish, whereas in fish and mollusk tissues, the concentrations were lower than those in the surrounding water. In crabs, ¹³¹I was detected in both muscle tissue and visceral organs; in marine fish, it accumulated predominantly in non-muscular tissues at levels comparable to those in water. ¹⁴⁴Ce, largely present in a dispersed particulate fraction, was likely not fully bioavailable for uptake by aquatic organisms.

The Soviet Union conducted parallel activities at the Novaya Zemlya test site, where numerous atmospheric, near-surface, and underwater nuclear tests were carried out. Subsequent investigations established that underwater nuclear weapons tests conducted in the 1950s in Chernaya Bay, Novaya Zemlya, resulted in the contamination of sediments, water, polychaetes, the mollusk Macoma calcarea (Gmelin, 1791), and macroalgae (Laminaria sp. and Fucus sp.) with radionuclides including ¹³⁷Cs, ⁶⁰Co, and transuranic elements ^239,240Pu. The influence of discharges from Chernaya Bay extended over considerable distances (up to 100 km) to the northwest along the coast of Novaya Zemlya. Radionuclides formed as a result of radioactive fallout were detected as well as in the bottom sediments of the Barents Sea and the Kara Sea, as well as in the water column, indicating their transport into Arctic marine ecosystems [13,14]. On the western coast of Norway, total beta activity of 1–10 Bq/m³ associated with Novaya Zemlya tests was recorded in 1962 [7], illustrating the transboundary character of radioactive contamination.

Between 1966 and 1974, France conducted 41 atmospheric and 137 underground nuclear tests in French Polynesia at the Mururoa and Fangataufa atolls in the southeastern Tuamotu Archipelago [15]. These tests resulted in substantial radioactive contamination due to significant atmospheric releases of fission products, followed by their deposition into marine and lagoon ecosystems [6]. Reconstruction studies indicate that fallout included a broad spectrum of radionuclides, among them radioactive isotopes of ¹³¹I, ¹²⁵Sb, ¹⁴¹Ce, and ¹⁴⁴Ce, as well as ¹³⁷Cs, ⁹⁰Sr, and transuranic elements. These radionuclides were deposited onto the ocean surface, dissolved or remained suspended in the water column, accumulated in bottom sediments and biota, and became the principal source of radioactive contamination in the regional marine environment [15]. As emphasized in [6], the principal ecological consequences of radionuclide inputs into marine ecosystems are associated with their accumulation in biota, which ultimately determines the scale and persistence of biological effects.

Thus, consideration of nuclear weapons testing provides a historical framework for understanding the large-scale introduction of radioactive isotopes of Sb, I, and Ce into aquatic ecosystems and establishes the basis for subsequent analysis of their environmental behavior, migration pathways, and biological accumulation.

1.2. Radionuclide Release from Normal Operation of Nuclear Facilities

Following the period of intensive atmospheric nuclear weapons testing, which constituted the first large-scale global source of artificial radionuclides to aquatic systems, a second, structurally different but chronically significant source emerged: routine discharges associated with the normal operation of civilian nuclear facilities and spent fuel reprocessing plants. In contrast to accidental releases, these inputs are characterized not by short-term pulses but by sustained, regulated emissions extending over decades. Their cumulative impact has been most clearly documented in European marine environments, particularly in the North Sea and adjacent basins.

One of the earliest and most influential sources of anthropogenic radionuclides in the marine environment was the Windscale Works plant operated by the United Kingdom Atomic Energy Authority (UKAEA) at Sellafield (Cumberland). Beginning in 1952, radioactive liquid effluents were discharged via pipeline into the Irish Sea. Investigations conducted in 1959–1960 confirmed the presence of ¹⁴⁴Ce among other radionuclides in these effluents [16]. This finding demonstrated that anthropogenic ¹⁴⁴Ce had become a persistent component of the coastal marine environment already in the early phase of nuclear industry development.

Although ¹⁴⁴Ce was confirmed in early releases, it has not emerged as a dominant indicator of routine discharges. As of 1 January 2000, newly authorized atmospheric release limits for Sellafield were established at 70 GBq·yr⁻¹ for ¹²⁹I, 55 GBq·yr⁻¹ for ¹³¹I, and 5 GBq·yr⁻¹ for ¹²⁵Sb, while permitted liquid discharge limits were set at 2 TBq·yr⁻¹ for ¹²⁹I and 8 TBq·yr⁻¹ for ¹⁴⁴Ce [17]. These regulatory values reflect the comparatively lower radiological importance assigned to cerium under routine operating conditions.

The Windscale coastal monitoring programme, initiated in 1961, provided further insight into the spatial behavior of ¹⁴⁴Ce. The radionuclide was detected in beach sand, mud, and the marine alga Porphyra umbilicalis ((Linnaeus) Kützing, 1843) at varying distances from the discharge point, yet it was not detected in fish muscle tissue, seawater, or bottom sediments [18]. This distribution underscores its localized occurrence and strong association with particulate matter rather than with dissolved transport pathways.

Bioindicator investigations near the UKAEA discharge pipeline at Windscale demonstrated that in plaice (Pleuronectes platessa L.), the principal isotope detected in muscle tissue was ¹³⁷Cs, whereas ¹⁴⁴Ce was found mainly in intestinal contents [19], indicating limited systemic incorporation of ¹⁴⁴Ce into fish tissues.

As monitoring programmes expanded, it became evident that long-lived ¹²⁹I (T_½ = 1.6 × 10⁷ years [2]) represents one of the most environmentally significant products of routine discharges. The occurrence of anthropogenic ¹²⁹I in the waters of the North Sea, the Irish Sea, the Baltic Sea, and the English Channel is primarily associated with regular liquid and gaseous releases from the Sellafield (United Kingdom) and La Hague (France) reprocessing plants [20,21].

By 2005, cumulative marine discharges amounted to approximately 1380 kg from Sellafield and 3330 kg from La Hague [20].

In contrast to particle-reactive radionuclides, ¹²⁹I exhibits conservative behavior in seawater, resulting in a markedly broader spatial footprint. Discharges from the La Hague facility dominate its distribution in North Sea surface waters. Brown seaweed (Fucus vesiculosus (Linnaeus, 1753)) collected along the Swedish coast in 1982 contained ¹²⁹I at concentrations ranging from 0.82 to 5.89 × 10⁹ atoms·g⁻¹ [22,23]. Elevated levels were also recorded in the Baltic Sea, which exchanges water with the North Sea through the Danish Straits; surface water concentrations reached up to 16 × 10⁻¹³ g·L⁻¹ [23]. These findings illustrate the ability of ¹²⁹I to undergo regional-scale marine transport over prolonged periods.

A similar pattern of incomplete characterization applies to ¹²⁵Sb. Although its chemical speciation in effluents from Sellafield Ltd. remains insufficiently described in the open literature, available data indicate that ¹²⁵Sb discharged from the La Hague reprocessing plant is more soluble than ⁶⁰Co, ¹⁵⁴Eu, and ¹⁵⁵Eu [24]. This greater solubility enhances its potential mobility compared with strongly particle-bound activation products.

A comparable, though quantitatively less intense, transport pathway is associated with the Marcoule reprocessing plant, from which fourteen artificial radionuclides are conveyed via the Rhône River to the Mediterranean Sea. The mean total activity is approximately 70 GBq·day⁻¹, an order of magnitude lower than La Hague discharges. Suspended particles constitute the principal carrier of radioactivity, except for ¹²⁵Sb and likely isotopes of Ru, which are transported predominantly in dissolved form; notably, ¹²⁵Sb was detected exclusively in the “organic” fraction (>75%) [25]. This observation again highlights the intermediate geochemical behavior of antimony between fully conservative and strongly particle-reactive elements.

The dispersion of anthropogenic ¹²⁹I extends beyond European seas. Studies of Chinese marginal seas have documented the presence of anthropogenic ¹²⁹I in coastal marine sediments, reflecting long-range transport via atmospheric circulation and marine currents, including contributions from European reprocessing releases [26]. Thus, routine reprocessing discharges can contribute to hemispheric-scale redistribution of long-lived iodine isotopes.

Similar processes occur in other regions, though generally at smaller scales. The Korea Atomic Energy Research Institute (KAERI) in Daejeon operates a Radioisotope Production Facility (RIPF) processing approximately 1000 Ci of ¹³¹I annually. Although ¹³¹I was not detected in surface waters within the RIPF site, it was recorded in wastewater downstream of treatment facilities. Over one year, ¹³¹I concentrations in aquatic plants near the discharge outlet ranged from 27.9 to 627.6 Bq·kg⁻¹, while fish contained 11.35 Bq·kg⁻¹ in common carp and 3.93 Bq·kg⁻¹ in crucian carp. The authors suggested that environmental ¹³¹I may also originate from medical facilities utilizing radioisotopes [27].

In Japan, studies were conducted on the environmental pollution resulting from the operation of the commercial nuclear fuel reprocessing plant in Rokkasho (Aomori Prefecture) under both laboratory and field conditions. Laboratory exposure of olive flounder (Paralichthys olivaceus (Temminck & Shlegel, 1846)) demonstrated minimal incorporation of ¹²⁵I into viscera, with the highest accumulation in gills after seven days [28], consistent with branchial uptake of dissolved iodine species. Field investigations of marbled flounder (Pseudopleuronectes yokohamae (Günther, 1877) and olive flounder (P. olivaceus) showed that during the fishing season, the ¹²⁹I/¹²⁷I ratio in fish tissues did not differ from seawater (p > 0.1), whereas after the fishing season, the ratio in fish was significantly higher (p < 0.05). Although stronger effects might be expected in benthivorous species if sediments were the principal source, this trend was not confirmed [29].

The plant’s influence was also assessed in Lake Obuchi, where concentrations of stable ¹²⁷I and ¹²⁹I in coastal sediments were associated with both terrestrial inputs (riverine particles and terrestrial plant material) and marine sources (particles settling from plankton) [30]. Maximum ¹²⁹I concentrations (wet weight) were recorded as 1.5 × 10⁻⁷ Bq·g⁻¹ in Zostera marina (Linnaeus, 1753), 3.9 × 10⁻⁸ Bq·g⁻¹ in Hypomesus nipponensis (McAllister, 1963), 9.7 × 10⁻⁸ Bq·g⁻¹ in Chaenogobius annularis (Gill, 1859), and 6.4 × 10⁻⁸ Bq·g⁻¹ in Crassostrea nippona (Seki, 1934) [31], demonstrating incorporation into coastal trophic networks.

In North America, civilian nuclear power plants are subject to stringent environmental monitoring programs conducted under the supervision of regulatory authorities. Long-term environmental monitoring programs include systematic measurements of isotopes most commonly associated with reactor operations, notably tritium (³H) and gamma-emitting radionuclides [32], as well as measurements in water, air, and soil surrounding facilities regulated by the U.S. Nuclear Regulatory Commission (NRC) and the Canadian Nuclear Safety Commission (CNSC). Published scientific data on less frequently measured radionuclides, such as ¹²⁵Sb or ¹⁴⁴Ce, in waters near nuclear power plants are scarce in the available literature. This likely reflects the very low concentrations of these isotopes in routine reactor discharges.

Monitoring programs also include systematic measurements of iodine isotopes in surrounding waters, notably ¹²⁹I, reflecting both accidental and routine releases from reactors and industrial sources. For example, in waters off the coast of California and the Pacific Northwest of the United States, a series of ¹²⁹I concentration measurements in seawater has been conducted as part of ongoing monitoring following the Fukushima Daiichi Nuclear Power Plant accident and background sources [33].

Taken together, these observations demonstrate that routine nuclear facility operation produces a geographically differentiated yet persistent anthropogenic influence on aquatic systems. The scale and mechanism of dispersion of ¹²⁹I, ¹²⁵Sb, and ¹⁴⁴Ce are determined primarily by their physicochemical properties, iodine behaving conservatively, cerium exhibiting strong particle reactivity, and antimony occupying an intermediate position, resulting in distinct patterns of transport and ecological incorporation.

1.3. Radionuclide Release from Accidents at Nuclear Facilities

In addition to global fallout from nuclear weapons testing and radionuclide inputs associated with the routine operation of nuclear facilities, severe reactor accidents have constituted episodic yet highly significant sources of radioactive contamination of aquatic ecosystems. Unlike chronic and regulated releases, accidental emissions occur abruptly and involve large amounts of radionuclides in diverse physicochemical forms. Their subsequent environmental behavior depends not only on the total activity released, but also on volatility, chemical speciation, and the degree of association with aerosol particles or fuel fragments. These factors determine atmospheric transport, deposition pathways, entry into surface waters, partitioning between dissolved and particulate matter, and long-term redistribution within river basins and marine systems.

One of the most serious nuclear accidents prior to Chornobyl was the Kyshtym disaster on 29 September 1957 at the Mayak Production Association in the Soviet Union (INES level 6 of 7). The total release activity was approximately 740 PBq (20 MCi), with about 91% attributable to short-lived isotopes, notably ¹⁴⁴Ce and ⁹⁵Zr, which were the primary components of the radioactive fallout. Precise data regarding ¹³¹I, ¹²⁵Sb, or ¹⁴¹Ce in open sources are limited due to the secrecy regime prevailing at that time; however, ¹⁴⁴Ce was part of the main release, whereas less volatile nuclides, including ¹²⁵Sb, potentially contributed to localized contamination. Following atmospheric dispersion, radioactive dust settled onto terrestrial surfaces within the affected catchment area. Subsequent remobilization by precipitation and surface runoff facilitated the transfer of radionuclides into the Techa River system. There, they accumulated both in the water column and, more persistently, in bottom sediments, forming long-term contamination reservoirs [34]. This sequence of atmospheric deposition, hydrological transport, and sediment accumulation illustrates how accident-derived fallout may exert prolonged effects on freshwater ecosystems even when the initial release is predominantly terrestrial.

In the same year, another serious accident occurred at Reactor No. 1 of the Windscale Works complex (now Sellafield) in Cumbria, United Kingdom (INES level 5 of 7) on 10 October 1957 [35,36]. In contrast to Kyshtym, where refractory elements such as ¹⁴⁴Ce dominated the fallout, the Windscale fire was characterized by the predominance of volatile radionuclides. According to reconstructions, ¹³¹I was the dominant radionuclide released, whereas ¹³⁷Cs and ²¹⁰Po principally determined the long-term radiological impact after deposition [35,37]. The total activity of ¹³¹I in the release is estimated in the hundreds of terabecquerels (approximately 600–740 TBq) [36]. In addition to iodine, ¹⁴⁴Ce was present in the release but in significantly smaller amounts than ¹³¹I [38].

As a result of atmospheric transport and deposition, ¹³¹I contaminated a large area extending beyond the United Kingdom into parts of Western and Northern Europe, as confirmed by measurements of ¹³¹I activity in the radioactive plume and associated precipitation [39]. Analysis of the long-lived isotope ¹²⁹I in bottom sediments provides evidence of fallout deposition and the subsequent input of radionuclide material into aquatic systems, thereby demonstrating transfer to freshwater ecosystems [40].

Concentrations of ¹³¹I in reservoirs and streams near Windscale during the first ten days after the accident ranged from 3.7 to 37 Bq·L⁻¹ [35]. Contamination of marine algae near Hunterston by fission products, notably ¹⁰⁶Ru and ¹⁴⁴Ce, has been attributed primarily to the Windscale accident [41]. Thus, compared to the pattern of radionuclide dominance in sediments observed after Kyshtym, the Windscale event demonstrated the rapid transfer and short-term predominance of volatile iodine in aquatic environments.

The accident at the Three Mile Island Nuclear Generating Station on 28 March 1979 remains the most significant event in the history of commercial nuclear power in the United States. The principal radioactive component of the releases resulting from the accident was ¹³³Xe. According to the official technical investigation prepared for the U.S. Nuclear Regulatory Commission, the total release of ¹³¹I into the atmosphere was estimated at approximately 13–17 Ci (≈0.5–0.6 TBq) [42]. Shortly after the accident on March 28, more than 100 drinking water sources were identified within a 20-mile radius of the station. A sampling program was developed for these water sources, including locations along the Susquehanna River where local communities obtained their drinking water. No significant concentrations of radioactive materials were detected in these water samples. Analysis of water from the Delaware River likewise indicated that no airborne radioactive substances were detected within this watershed [43]. Overall, the data do not demonstrate any statistically significant or long-term effects on aquatic ecosystems associated with the release of ¹³¹I.

The accident at the Chornobyl NPP on 26 April 1986 (INES level 7 of 7) represented the largest uncontrolled nuclear accident in the history of civilian nuclear power and resulted in an unprecedented release of radionuclides into the environment. The reactor explosion and subsequent core fire discharged a substantial amount of fission products into the atmosphere, including volatile isotopes (¹³¹I) and less volatile isotopes (¹⁴¹Ce, ¹⁴⁴Ce, ¹²⁵Sb).

It is estimated that the total activity of released ¹³¹I was about 1760 PBq, which determined its dominant role in the early radioecological impact. The activity of ¹⁴¹Ce is estimated at approximately 84 PBq, and that of ¹⁴⁴Ce at approximately 116 PBq; these isotopes belonged to the so-called “refractory elements,” which tended to deposit together with fuel particles and settle onto soil and water surfaces. The estimated activity of ¹²⁵Sb in the total release was considerably lower than that of major volatile radionuclides such as ¹³¹I and ¹³⁷Cs [11].

Contamination of terrestrial and aquatic ecosystems within the 30 km zone around the Chornobyl NPP was especially intense, including the lower reaches of the Pripyat River and the upper part of the Kyiv Reservoir. The accident caused a sharp deterioration of the radioecological situation in the Dnipro catchment. In the first weeks after the accident, total water radioactivity was determined mainly by ¹³¹I: during the first month, its contribution was 80–90%, and in June, it was around 30%. Beginning in the second half of June, owing to the decay of ¹³¹I, total water radioactivity was defined by short-lived radionuclides, ⁸⁹Sr, ¹⁴⁰Ba, ¹⁰³Ru, ¹⁴¹Ce, ¹⁴⁴Ce, ⁹⁵Zr, as well as, to a lesser extent, ¹³⁴Cs and ¹³⁷Cs. Over time, their contributions decreased, and long-lived radionuclides, ⁹⁰Sr, ¹³⁷Cs, ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Am, began to play a leading role [44].

The behaviour of individual radionuclides in aquatic systems reflects their intrinsic physicochemical properties. ¹³¹I is characterized by high volatility and environmental mobility. Following atmospheric transport, it was deposited primarily with precipitation, entering rivers, lakes, and reservoirs through direct fallout onto water surfaces and via runoff from contaminated catchments. Owing to its high solubility, ¹³¹I exhibited substantial mobility within the water column and was temporarily incorporated into aquatic food webs during the early post-deposition phase. Monitoring data from several European countries indicated a rapid increase in ¹³¹I concentrations in surface waters within weeks after the Chornobyl accident [45].

Cerium isotopes (¹⁴¹Ce and ¹⁴⁴Ce), in contrast, were predominantly associated with aerosol particles and fuel fragments. They settled with the solid phase of fallout and entered aquatic systems via surface runoff and erosion processes. ¹⁴¹Ce and ¹⁴⁴Ce were also detected in distant marine waters, including the Mediterranean Sea, after the passage of radioactive fallout. Calmet et al. (1991) [46] demonstrated the presence of these isotopes in seawater and biota (marine plants), confirming their broad geographic distribution and incorporation into aquatic trophic chains.

¹²⁵Sb was part of the global fallout and was transported mainly in aerosol form. After deposition, it entered aquatic systems through surface runoff and partly in dissolved form; however, to a large extent, it remained associated with suspended particles and bottom sediments [47].

On a broader scale, inputs of ¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce into aquatic ecosystems following the Chornobyl accident occurred through atmospheric transport, wet and dry deposition, and subsequent hydrological runoff. The high mobility of ¹³¹I led to its short-lived but substantial presence in the water column, whereas Ce isotopes and ¹²⁵Sb predominantly accumulated in bottom sediments, forming secondary sources of redistribution. Studies of aquatic biota confirmed the incorporation of Chornobyl-derived radionuclides into various components of marine and freshwater ecosystems [46,48,49,50,51].

The accident at Fukushima Dai-ichi NPP on 11 March 2011 represented another large-scale source of radionuclide input into aquatic environments, combining significant atmospheric releases with direct discharges of radioactively contaminated water into the ocean (INES level 6 of 7). According to source term reconstructions, the principal radionuclide was ¹³¹I. Total atmospheric releases of ¹³¹I are estimated in the range of 100–500 PBq [52,53], with most estimates near 200 PBq [54,55]. The highest rate of release (10¹⁶ Bq·hr⁻¹) was recorded on 15 March 2011 [55].

Iodine occurred in both gaseous and aerosol forms. Based on approximately 210 paired measurements carried out up to 12 April, the average gas/total ¹³¹I ratio in Europe was 77.2% ± 13.6%, indicating the predominance of the gaseous form during atmospheric transport. About 1 PBq of ¹³¹I passed over Europe (<1% of the total release). Less volatile isotopes, including ¹⁴¹Ce, ¹⁴⁴Ce, and ¹²⁵Sb, were identified in samples of aerosols, soils, and bottom sediments, although their quantities were several orders of magnitude lower than those of ¹³¹I and ¹³⁷Cs [53].

Radionuclides entered the ocean both by atmospheric deposition and through direct discharge of contaminated emergency water. It is estimated that approximately 10–40 PBq of ¹³¹I and 3–6 PBq of ¹³⁷Cs were discharged directly into the Pacific Ocean [56,57]. In seawater near the reactor, ³H, ¹⁴C, ¹³⁴Cs, ¹³⁷Cs, ⁶⁰Co, ¹²⁵Sb, ⁹⁰Sr, ¹²⁹I, ⁹⁹Tc, ¹⁰⁶Ru, and ²³⁸Pu were detected [58].

Post-accident observations demonstrated the transfer of radionuclides into marine biota. After the accident, concentration ratios (CR_water) for ¹⁴⁴Ce ranged from 23 to 100 for pelagic and benthic fish, octopuses, crustaceans, and macroalgae [59]. Brown algae effectively concentrated ¹³¹I (CR ≈ 10⁴), although, due to its short half-life, its activity declined rapidly [56]. ¹³¹I, ¹³⁴Cs, and ¹³⁷Cs were detected in Japanese sardine (Sardinops sagax (Jenyns, 1842)) and Japanese anchovy (Engraulis japonicus (Temminck & Schlegel, 1846). Concentrations of ¹³¹I were higher in internal organs than in muscle; maximum values for anchovy (24 March 2011) were 309.1 ± 2.1 Bq·kg⁻¹ (¹³¹I), 61.0 ± 0.5 Bq·kg⁻¹ (¹³⁴Cs), and 59.6 ± 0.4 Bq·kg⁻¹ (¹³⁷Cs) [60].

1.4. Accumulation of Stable Isotopes of Antimony, Iodine, and Cerium in Aquatic Ecosystems

In contrast to radionuclide inputs associated with nuclear activities and accidents, stable isotopes of Sb, I, and Ce are continuously present in aquatic ecosystems as natural trace constituents and as products of diffuse anthropogenic emissions. Their environmental behavior is governed primarily by geochemical controls, speciation, and biological uptake mechanisms. Data on the distribution of these elements in water bodies and aquatic organisms provide a basis for understanding accumulation patterns, organ-specific partitioning, and trophic transfer.

Ce is generally characterized by low background concentrations in aquatic systems and comparatively weak enrichment relative to many other trace metals. According to [61], Sb concentrations in freshwater and marine algae range from 0.1 to 0.2 µg·g⁻¹ dry weight, placing it among the least abundant trace metals in algal biomass. This relatively limited enrichment at the primary producer level influences its subsequent transfer through trophic chains.

Studies of fish demonstrate a pronounced organ-specific distribution of Sb. In Salmo trutta (Linnaeus, 1758), the highest Sb content was detected in the kidneys, with the following sequence of distribution among organs: kidneys > gills > intestine > gonads > liver > muscles [62]. Other investigations of the same species reported elevated Sb levels in the gills and liver [63], indicating that accumulation patterns may vary depending on environmental conditions, exposure pathways, and physiological status.

Comparative analyses of different species further illustrate the role of ecological behavior in Sb uptake. In Carassius auratus (Linnaeus, 1758), Sb concentrations in the gills were higher than in Ctenopharyngodon idella (Valenciennes, 1844) [64]. This difference has been attributed to the benthic feeding habits of crucian carp, which increase contact with contaminated bottom sediments and suspended particles. In contrast, muscles, swim bladder, and skin exhibited lower accumulation coefficients [64], reflecting limited redistribution of Sb into less metabolically active tissues.

Field investigations across aquatic ecosystems consistently demonstrate the absence of Sb biomagnification within trophic chains [65,66]. The highest concentrations were recorded in benthic invertebrates (up to 316 µg·g⁻¹), whereas levels in fish were substantially lower [66]. Such patterns indicate trophic dilution rather than progressive biomagnification along food webs.

Laboratory experiments with Danio rerio (Hamilton, 1822) confirm these observations, showing that the liver serves as the primary site of Sb accumulation, followed by the gills and muscles, while the lowest concentrations were measured in the brain [67,68]. Together, field and experimental data confirm the organ-specific and exposure-dependent nature of Sb distribution in aquatic organisms.

Iodine exhibits a markedly different biogeochemical behavior. Investigations of lakes in North America and Europe indicate that the dominant fraction of iodine in both aqueous and solid phases consists of organically bound forms. Comparable patterns have been documented in atmospheric aerosols and precipitation [69,70,71], underscoring the central role of organic matter in iodine cycling. This strong association with organic compounds influences its mobility, transformation processes, and availability to aquatic biota.

In contrast, Ce belongs to the rare earth elements (REEs) and is among the most abundant representatives of this group in the Earth’s crust. Its environmental distribution reflects both lithogenic inputs and geochemical fractionation processes characteristic of REEs. Numerous studies demonstrate that REEs, including Ce, accumulate predominantly in bottom sediments and in organisms occupying lower trophic levels, whereas concentrations in fish muscle tissue generally remain low. In most examined ecosystems, trophic dilution rather than biomagnification has been observed [72,73,74,75].

Within the REE group, Ce may display relatively high or moderate proportions depending on prevailing redox and geochemical conditions. Elevated concentrations are most frequently reported in benthic fish species [76,77], consistent with the tendency of Ce to associate with particulate matter and sediments. This sediment affinity enhances exposure of bottom-dwelling organisms and shapes the vertical distribution of Ce within aquatic food webs.

Laboratory investigations focusing on Ce dioxide nanoparticles (n-CeO₂) further elucidate uptake pathways and tissue distribution. Experiments with Oncorhynchus mykiss (Walbaum, 1792) demonstrated predominant accumulation in the gills, while liver concentrations were comparatively lower [78,79]. Oral ingestion was identified as the principal uptake pathway for n-CeO₂; nevertheless, penetration through the gill epithelium or skin is also possible [80]. These findings highlight the importance of both dietary and direct aqueous exposure routes in determining cerium bioavailability.

Taken together, available evidence indicates that stable Sb, I, and Ce display distinct but internally consistent patterns of distribution in aquatic ecosystems. Sb and Ce tend to associate with particulate matter and bottom sediments, leading to higher concentrations in benthic organisms and specific target organs, whereas iodine is largely controlled by organic complexation processes. In all cases considered, trophic dilution predominates over biomagnification, and accumulation is strongly influenced by ecological niche, exposure pathway, and physicochemical speciation.

Comparative analysis, therefore, reveals several common features for Sb, I, and Ce in both radioactive and stable forms: predominance of organ-specific accumulation, preferential association with metabolically active tissues or particulate matters, minimal presence in muscle tissue, and the absence of clearly expressed biomagnification in aquatic trophic networks.

At the same time, a review of the literature highlights the limited availability of systematized data on ¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce in aquatic ecosystems, particularly regarding organ distribution patterns and long-term ecological dynamics. Compared with extensively studied radionuclides such as cesium and strontium, the behavior of these isotopes within aquatic food webs remains insufficiently characterized. Existing evidence is often fragmentary and restricted to case-specific investigations, underscoring the need for comprehensive, integrative, and long-term research addressing their migration, bioavailability, and trophic transfer in diverse aquatic environments. Simultaneous or separate detection of ¹²⁵Sb and ¹⁴⁴Ce in fish tissues is also considered an indicator of the source of contamination, making it possible to distinguish between the intake into the organism of fuel particles containing both of these isotopes in large quantities and the accumulation of ¹²⁵Sb alone from the condensation component of fallout.

2. Methodology

2.1. Literature Sources and Search Strategy

The literature search strategy followed that described in our previous works [81,82]. The databases Scopus, PubMed, and Google Scholar were searched using Boolean operators, with key terms including bioaccumulation, fish, radionuclides, antimony, iodine, cerium, Chernobyl, Chornobyl, nuclear testing, global fallout, and nuclear facilities. No restriction on publication year was applied; the review includes studies measuring short-lived radionuclides in fish and other aquatic biota from the mid-1950s to 2025, with sampling campaigns conducted within a few half-lives of the respective source events (e.g., Chornobyl: 1986 to the early 1990s; Fukushima: 2011 to the mid-2010s; weapons tests: 1950s to 1960s). Publications from national scientific journals were additionally accessed through the library of the Institute for Nuclear Research of the National Academy of Sciences of Ukraine in Kyiv.

2.2. Observational Data Collection and Analytical Procedures

Illustrative observational data provided in Tables 1–4 and Figures 3–8 were collected as part of routine environmental monitoring of the ChNPP Cooling Pond and the Kaniv Reservoir on the Dnipro River, in the area between the village of Staiky and the town of Rzhyshchiv (Figure 1 and Figure 2).

Samples were obtained from water, aquatic plants, mollusks, and fish. Sampling methods, sample preparation, and radionuclide analyses followed procedures described previously [83]. The specific activities of ¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce were determined using non-ashed tissue samples, as ashing has been shown to cause substantial loss of short-lived radionuclides, leading to false-negative results. Activity concentrations in fish organs and tissues are expressed on a fresh weight basis; those in aquatic plants are expressed on a dry weight basis.

3. Discussion

Case Study: Radionuclide Content in Aquatic Vegetation and Fish of the ChNPP Cooling Pond and Kaniv Reservoir

Unlike ⁵⁴Mn, ⁶⁰Co, ⁶⁵Zn, and ⁹⁵Zr [81,82], the radionuclides ¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce were not detected in either the Chornobyl Nuclear Power Plant cooling pond or the Kaniv Reservoir prior to the Chornobyl accident.

Antimony ¹²⁵Sb: According to [84], approximately 4 MCi of ¹²⁵Sb (1.48 × 10¹⁷ Bq) was present in the reactor of Unit 4 of the Chornobyl Nuclear Power Plant at the time of the accident. It is noted in [85] that nearly all ¹²⁵Sb released into the environment was associated with fuel particles. Compared to other accident-derived radionuclides, the contribution of this isotope to the contamination of various components of aquatic ecosystems was relatively minor. The highest specific activity levels of ¹²⁵Sb were recorded in aquatic vegetation of the cooling pond, particularly in Cladophora spp. Figure 3 presents the content of this radionuclide in Cladophora spp. from the cooling pond in comparison with the content of ¹⁴⁴Ce in the same species.

The maximum value over the entire observation period was recorded in Cladophora spp. in August 1987 and amounted to 36,820 ± 3820 Bq/kg dry weight. A similar pattern was observed in the Kaniv Reservoir, where the highest specific activity levels of ¹²⁵Sb were also associated with algae (Figure 4).

Despite the fact that ¹²⁵Sb has the longest half-life among the radionuclides studied, it was detected in samples only up to and including 1990. During the period from 1986 to 1990, approximately 1800 fish samples were collected in the cooling pond of the Chornobyl NPP. This radionuclide was detected in only 88 samples, 42 of which were found in planktivorous Hypophthalmichthys molitrix (Valenciennes, 1844) [86].

Table 1. Content of ¹²⁵Sb in fish tissues and organs (Bq/kg, fresh weight) from the Cooling Pond of the ChNPP, with sampling periods in the early years following the Chornobyl accident.

Sample Collection Period	Tissue or Organ	125Sb Min	125Sb Max	Fish Ecological Groups
August 1987–March 1990	Head	124	1020	Omnivores, Planktivores, Carnivores, Piscivores
October 1987–March 1990	Gills	27	112	Carnivores, Piscivores
March 1988–April 1989	Muscles	312	6768	Omnivores, Planktivores
February 1987–March 1990	Internal Organs	121	7191	Omnivores, Planktivores, Carnivores, Piscivores
September 1988–March 1990	Liver	35	257	Omnivores, Planktivores
March 1987–February 1990	Roe	159	1917	Omnivores, Planktivores, Piscivores
March 1988–March 1990	Fat	14	65	Omnivores, Planktivores
February 1987–October 1987	Swimming Bladder	335	403	Planktivores, Carnivores
April 1987–April 1989	Fins	100	1458	Omnivores, Planktivores, Carnivores
February 1987–April 1989	Scales	200	1851	Planktivores
March 1988	Skin	478	1074	Planktivores, Piscivores
April 1987	Mucus	1576		Carnivores
March 1988	Whole Fish	294	445	Omnivores, Planktivores

presents data on the content of radioactive antimony in fish from the Chornobyl NPP cooling pond.

The highest concentrations were recorded in the internal organs of fish belonging to Omnivores and Planktivores (H. molitrix, Abramis brama (Linnaeus, 1758) Cyprinus carpio (Linnaeus, 1758)). The main proportion of accumulation occurred in the internal organs, including the stomach and intestines, which is particularly characteristic of fish at lower trophic levels. It should be noted that the highest content of ¹²⁵Sb in muscle tissue was recorded only in March 1989 in Planktivores H. molitrix (Figure 5).

In Carnivores Ictalurus punctatus (Rafinesque, 1818), ¹²⁵Sb was detected much less frequently: once in internal organs (1989), once in the head (1988), once in the gills in 1987 and 1988, and twice in the fins (1987 and 1989). In Piscivores Sander lucioperca (Linnaeus, 1758) and Aspius aspius (Linnaeus, 1758), this radionuclide was recorded in internal organs only once, in S. lucioperca in 1989. At the same time, in this species, it was also detected in the gills in 1989 and 1990, and, notably, in the roe in 1990. In A. aspius, ¹²⁵Sb was recorded only once, in the head in 1988.

In the Kaniv Reservoir, ¹²⁵Sb was not detected in the first samples collected on 3 May 1986. In general, its accumulation pattern in this reservoir differed from that in the cooling pond: a very low frequency of occurrence was observed in the internal organs of fish across all ecological groups. However, in 1986 this radionuclide was detected in Piscivores Esox lucius (Linnaeus, 1758) in a whole-body sample, and in 1988 it was recorded once in the kidneys and scales (

Table 2. Content of radionuclides (¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce) in fish tissues and organs (Bq/kg, fresh weight) from the Kaniv Reservoir, with sampling periods in the early years following the Chornobyl accident.

Sample Collection Period	Tissue or Organ	125Sb Min	125Sb Max	Fish Ecological Groups
November 1987	Roe	41		Omnivores
May 1988	Kidneys	20		Piscivores
October 1986	Internal Organs	31	64	Omnivores
June 1987	Fins	17		Omnivores
May 1988	Scales	150		Piscivores
October 1986–March 1987	Whole Fish	8	21	Omnivores, Piscivores
		131I min	131I max
May 1986	Internal Organs	1736		Piscivores
May 1986–June 1986	Whole Fish	148	2245	Omnivores, Carnivores, Piscivores
		141Ce min	141Ce max
May 1986	Internal Organs	167		Piscivores
May 1986–June 1986	Whole Fish	312	1307	Omnivores
		144Ce min	144Ce max
May 1986–October 1986	Internal Organs	20	119	Omnivores, Carnivores, Piscivores
June 1987	Head	29		Carnivores
May 1986–October 1988	Whole Fish	12	435	Omnivores, Carnivores, Piscivores

).

Iodine (¹³¹I). Due to the fact that the first samples of fish and algae in the Chornobyl NPP cooling pond were collected only in August 1986, and the main sampling began in November 1986, ¹³¹I was not detected in these samples.

In contrast, in the Kaniv Reservoir, where the first samples were collected on 3 May 1986, ¹³¹I was detected both in algae and in fish. The maximum content of this radionuclide was recorded in a higher aquatic plant, sedge, and amounted to 727,390 ± 54,370 Bq/kg dry weight (Figure 6). In Cladophora spp. collected on 15 May 1986, the content of ¹³¹I was 11,880 ± 1110 Bq/kg.

In fish, ¹³¹I was detected only in whole-body samples, and it was recorded in representatives of all studied ecological groups: A. aspius and E. lucius—Piscivores; Perca fluviatilis (Linnaeus, 1758)—Carnivores; Alburnus alburnus (Linnaeus, 1758)—Planktivores; Abramis brama, Blicca bjoerkna (Linnaeus, 1758), Carassius carassius (Linnaeus, 1758), Tinca tinca (Linnaeus, 1758), and Rutilus rutilus (Linnaeus, 1758)—Omnivores. In addition, this radionuclide was several times detected in internal organs (stomach and intestinal contents) of A. brama and A. aspius in May and June 1986 (Figure 6).

The specific activity of ¹³¹I in Piscivores (E. lucius) reached 2200 Bq/kg and was significantly higher compared to fish of lower trophic levels (890 Bq/kg in B. bjoerkna) [87].

Cerium (¹⁴¹Ce): The highest concentrations of ¹⁴¹Ce among all studied radionuclides were recorded in aquatic vegetation of the Kaniv Reservoir in May 1986 at the sampling sites (Figure 2). Specifically, on 3 and 5 May, in Phragmites australis australis ((Cav.) Trin. ex Steud., 1841) of the previous year’s vegetation, the content of this radionuclide amounted to 1,191,487 ± 84,698 and 3,758,636 ± 271,189 Bq/kg dry weight, respectively. The fact that such high concentrations were observed in plants of the previous year’s vegetation indicates surface contamination caused by the deposition of fuel particles. In the same species of plants, but of the current (1986) vegetation, the content of ¹⁴¹Ce in September was 2490 ± 25 Bq/kg dry weight. This contamination was likely also associated with fuel particles that could have been present in bottom sediments or migrated within the water column.

In Cladophora spp., the maximum content of ¹⁴¹Ce was recorded on 15 May, at 45,038 ± 3322 Bq/kg. By mid-November, the concentration of this radionuclide decreased to 403 ± 144 Bq/kg, which is likely related to its short half-life. The lower levels in Cladophora spp. compared to P. australis may indicate the predominance of atmospheric transport of the radionuclide from the Chornobyl NPP rather than waterborne pathways. In addition, partial “washing off” of surface contamination due to water flow cannot be excluded. In Potamogeton perfoliatus (Linnaeus, 1753), collected in September, October, and November 1986, the content of ¹⁴¹Ce was 83 ± 21, 25 ± 6, and 23 ± 5 Bq/kg dry weight, respectively.

In the Chornobyl NPP cooling pond, ¹⁴¹Ce was not detected in aquatic plants in 1986, which is likely due to the fact that plant sampling began only in October–November. However, in P. perfaliatus, this radionuclide was recorded once in February 1987 at a level of 2235 Bq/kg dry weight. After this, ¹⁴¹Ce was not detected in the aquatic vegetation of the cooling pond.

In fish of the Kaniv Reservoir, ¹⁴¹Ce was recorded in May and June 1986 (Table 2). All recorded cases of its detection were associated with covering tissues or internal organs. The highest concentrations were observed in May in fish belonging to different ecological groups, E. lucius, B. bjoerkna, and Gymnocephalus cernua (Linnaeus, 1753), in whole-body samples.

In the cooling pond of the Chornobyl NPP, ¹⁴¹Ce was most frequently detected in whole-body samples and internal organs of fish belonging to various ecological groups, including C. carpio, I. punctatus, Alburnus alburnus (Linnaeus, 1753), and H. molitrix. The dynamics of ¹⁴⁴Ce in C. carpio across different organs and tissues are shown together with ¹⁴¹Ce in Figure 7.

In the head and internal organs of this species, ¹⁴¹Ce was detected only in 1986. After March 1987, ¹⁴¹Ce was no longer recorded in fish from the cooling pond. At the same time, in H. molitrix, this radionuclide was detected in roe in March 1987 (

Table 3. Content of ¹⁴¹Ce in fish tissues and organs (Bq/kg, fresh weight) from the Cooling Pond of the ChNPP, with sampling periods in the early years following the Chornobyl accident.

Sample Collection Period	Tissue or Organ	141Ce Min	141Ce Max	Fish Ecological Groups
November 1986	Head	287		Omnivores
November 1986–April 1987	Internal Organs	154	4364	Omnivores, Planktivores
March 1987	Roe	920		Planktivores
November 1986–December 1986	Whole Fish	182	820	Omnivores, Planktivores

).

Cerium (¹⁴⁴Ce): Similar to ¹⁴¹Ce, the maximum content of ¹⁴⁴Ce in the Kaniv Reservoir was recorded in P. australis of the previous year’s vegetation in May 1986; however, the concentrations were lower, 865,287 ± 64,432 and 2,923,240 ± 209,660 Bq/kg for the same samples. In plants of the current year’s vegetation, the content in September was 26,589 ± 2072 Bq/kg. The last detection in this species occurred in the summer of 1987.

In Cladophora spp. of the Kaniv Reservoir, the content of ¹⁴⁴Ce was noticeably lower, which may indicate the predominance of atmospheric transport of the radionuclide. Its detection in this species continued until the end of October 1988 (Figure 4).

In P. perfaliatus, the content of ¹⁴⁴Ce was the lowest among the studied plant species; the maximum value was recorded in November 1986 and amounted to 518 ± 91 Bq/kg, after which this radionuclide was no longer detected in this species.

In fish of the Kaniv Reservoir, the highest content of ¹⁴⁴Ce was recorded on 2 May 1986, in a whole-body sample of A. brama, 435 ± 113 Bq/kg. Subsequently, a significant decrease in concentrations was observed. The last detections were recorded in October 1988 in E. lucius and P. fluviatilis (whole-body samples), 35 ± 6 and 37 ± 11 Bq/kg, respectively. In internal organs, this radionuclide was recorded three times: in October 1986 in B. bjoerkna (119 ± 27 Bq/kg wet weight), in June 1987 in A. aspius (20 ± 4 Bq/kg wet weight), and in T. tinca (45 ± 7 Bq/kg wet weight). A single case of detection in the head of a fish was recorded in June 1987 in P. fluviatilis (Table 2).

In the Chornobyl NPP cooling pond, concentrations of ¹⁴⁴Ce in aquatic plants of different species exceeded 1,000,000 Bq/kg during the period 1987–1989. In P. perfoliatus, the content in November 1986 was 111,995 ± 8008 Bq/kg, and in August 1987 it increased to 2,897,525 ± 205,891 Bq/kg. By the end of 1988, specific activity decreased and generally did not exceed 1,000,000 Bq/kg, except in one case: in March 1988, a value of 16,157,037 ± 1,142,822 Bq/kg was recorded in this species, representing the highest level of this radionuclide in the biota of the cooling pond over the entire post-accident study period. By 1990, the content of this radionuclide decreased and ranged between 7031 and 50 Bq/kg. The last detection of ¹⁴⁴Ce in P. perfoliatus was recorded in November 1994 at a level of 2450 ± 1020 Bq/kg.

In Cladophora spp., ¹⁴⁴Ce was not detected in 1986; it was first identified in March 1987 (Figure 3). By 1990, a gradual decrease in its concentrations was observed.

Out of approximately 1800 fish samples collected in the water area of the Chornobyl NPP cooling pond, ¹⁴⁴Ce was detected in about 800 cases (

Table 4. Content of ¹⁴⁴Ce in fish tissues and organs (Bq/kg, fresh weight) from the Cooling Pond of the ChNPP, with sampling periods in the early years following the Chornobyl accident.

Sample Collection Period	Tissue or Organ	144Ce Min	144Ce Max	Fish Ecological Groups
November 1986–March 1990	Head	41	27,355	Omnivores, Planktivores, Carnivores
November 1986–March 1990	Internal Organs	107	7,504,518	Omnivores, Planktivores, Phytivores, Carnivores, Piscivores
February 1987–March 1990	Gills	94	10,348	Omnivores, Planktivores, Carnivores, Piscivores
March 1987–February 1990	Roe	128	202,662	Omnivores, Planktivores
August 1987–February 1990	Fat	25	1726	Omnivores, Planktivores, Carnivores
March 1987–March 1990	Intestines with Content	160	105,802	Omnivores, Planktivores, Phytivores, Carnivores, Piscivores
August 1987–March 1990	Skin	135	5770	Omnivores, Planktivores, Carnivores, Piscivores
August 1987–March 1990	Bones	92	12,254	Omnivores, Planktivores, Carnivores, Piscivores
October 1987–August 1993	Muscles	272	72,039	Omnivores, Planktivores, Carnivores, Piscivores
September 1988	Liver	150		Omnivores
November 1986–March 1990	Fins	76	89,367	Omnivores, Planktivores, Phytivores, Carnivores, Piscivores
August 1987–March 1990	Kidneys	294	1534	Omnivores, Planktivores, Carnivores, Piscivores
April 1987	Mucus	11,196	196,875	Carnivores
November 1986–March 1990	Scales	76	11,051	Omnivores, Planktivores, Piscivores
November 1986–March 1990	Whole Fish	267	35,445	Omnivores, Planktivores, Piscivores

).

The highest levels of this radionuclide were observed in fish belonging to the Planktivores and Omnivores [88].

In fish, the content of ¹⁴⁴Ce was lower than in aquatic plants and generally did not exceed 370,000 Bq/kg. An exception was recorded in October 1987, when the concentration in the internal organs of H. molitrix reached 7,504,518 ± 536,566 Bq/kg. This radionuclide was detected in the internal organs of fish belonging to various ecological groups: Omnivores (B. bjoerkna, C. carpio, A. brama), Herbivores (Scardinius erythrophthalmus (Linnaeus, 1753)), Carnivores (I. punctatus), Planktivores (H. molitrix), and Piscivores (A. aspius) (Table 4).

Unlike ¹⁴¹Ce, ¹⁴⁴Ce was detected both in covering tissues and in various internal organs of fish. The highest concentrations were observed in mucus and stomach contents, which indicates the dominant role of the particulate fraction. In the liver, this radionuclide was recorded only once, in September 1988, in C. carpio; in the kidneys of different fish species, it was detected up to 1990.

In fish roe of different species, ¹⁴⁴Ce was detected during the period from 1987 to 1990, which may indicate its involvement in biological and reproductive processes.

¹⁴⁴Ce was widely present in the muscle tissue of fish from different ecological groups until 1989; however, from 1990 onward, it was detected only in a limited number of species, namely S. glanis, I. punctatus, S. lucioperca, and H. molitrix. The last detection in muscle tissue dates to March 1993 (I. punctatus, 272 ± 19 Bq/kg), indicating prolonged retention of the radionuclide in organisms despite an overall decline in environmental levels.

This temporal pattern, in which ¹⁴⁴Ce is no longer detected over time in the muscle tissue of fish from lower trophic groups but persists for a longer period in predatory species, reflects not only a general decrease in its environmental abundance but also an uneven distribution among organisms with different feeding strategies. When considered together with earlier detections of this radionuclide in the muscle tissue of fish occupying various trophic positions, these observations suggest that under conditions of elevated contamination, ¹⁴⁴Ce may persist for longer periods in higher levels of the food chain.

Under such conditions, the accumulation of ¹⁴⁴Ce along the food chain cannot be excluded, i.e., biomagnification, which is atypical for a chemical element that is not biologically essential and is traditionally considered to be only weakly involved in metabolic processes.

In our previous work on ⁵⁴Mn, ⁶⁰Co, ⁶⁵Zn, ⁹⁵Zr, ⁹⁵Nb, ¹⁰³Ru, and ¹⁰⁶Ru, as well as in the published literature, muscle tissue has consistently shown the lowest radionuclide concentrations among fish tissues. The prolonged detection of ¹⁴⁴Ce in muscle tissue until 1993, as well as the persistence of ¹²⁵Sb in the muscle of planktivores, therefore does not fully conform to this general pattern. It is noteworthy that the exceptionally high initial activities of ¹⁴⁴Ce recorded in the cooling pond in the early post-accident period may have extended its effective environmental availability beyond what would be expected based solely on its physical half-life, thereby providing sufficient exposure time for accumulation processes in muscle tissue to become operative.

A similar delayed accumulation in muscle tissue was previously reported for ¹⁰⁶Ru in fish from the same water body [82]. The temporal dynamics of ¹⁴⁴Ce in the muscle tissue of I. punctatus from the Chornobyl Nuclear Power Plant Cooling Pond (Figure 8) further support this pattern, although the underlying mechanism responsible for such behaviour in particle-associated radionuclides with low biological mobility remains to be elucidated. At the same time, the pronounced decrease in ¹⁴⁴Ce content by 1993 is associated with the relatively short half-life of this radionuclide.

The co-occurrence or separate detection of ¹²⁵Sb and ¹⁴⁴Ce in fish tissues provides a direct means of distinguishing the two principal contamination pathways following the Chornobyl accident: input via fuel particles and input via dissolved or aerosol-derived forms. The presented data clearly demonstrate fundamental differences in the behavior and accumulation of ¹⁴⁴Ce and ¹²⁵Sb in fish, reflecting both their physicochemical properties and their modes of occurrence in the environment.

Cases where high concentrations of both ¹⁴⁴Ce and ¹²⁵Sb are observed simultaneously, particularly in 1987 and in internal organs or the gastrointestinal tract, strongly suggest the intake of both radionuclides as components of fuel particles. For example, in C. carpio in early 1987, extremely high concentrations of ¹⁴⁴Ce (exceeding 300,000 Bq/kg) accompanied by ¹²⁵Sb levels of several thousand Bq/kg represent a typical signature of the fuel component. A similar pattern is observed in the intestine and its contents in 1988–1989, where elevated levels of both radionuclides persist, likely due to the retention of particles in the digestive tract or their continuous ingestion with food.

At the same time, a significant number of cases are characterized by the presence of only ¹⁴⁴Ce without the accompanying ¹²⁵Sb. Such patterns prevail in most tissues, including bones, scales, fins, gills, and even muscles. This reflects the low mobility of ¹⁴⁴Ce: it is either not absorbed from the intestine and is subsequently excreted, or it becomes fixed in tissues associated with mineral metabolism. For example, in numerous samples of C. carpio, S. lucioperca, and I. punctatus collected in 1987–1988, this radionuclide is detected in bones or scales in the complete absence of antimony, indicating its inert behavior and tendency to precipitate. At the same time, the detection of ¹⁴⁴Ce in the muscles of Piscivorous fish, in particular A. aspius and S. lucioperca, in 1987, 1988, and 1990, may indicate biomagnification of this radionuclide, which is consistent with general concepts of trophic transfer of low-mobility elements. In this context, it should be noted that radionuclides released into the environment after the Chornobyl accident as components of fuel particles (for example, ¹⁰⁶Ru), even with relatively short half-lives, can gradually accumulate in fish muscles provided that they are present in sufficient quantities in the water body [82].

The most illustrative cases for understanding the differences between these radionuclides are those in which only ¹²⁵Sb is detected in tissues without ¹⁴⁴Ce. Although less frequent, these cases show a clear pattern: they are more common in later years (1988–1990) and in tissues directly exposed to water or characterized by high physiological activity. This is consistent with the data presented in Figure 5 for H. molitrix, where ¹²⁵Sb persists in the liver, roe, and muscles even under conditions of substantially reduced ¹⁴⁴Ce levels. For instance, only ¹²⁵Sb was detected in the gills of S. lucioperca in 1989–1990, in the head of A. aspius in 1988, and similarly in the liver of H. molitrix in 1989–1990, as well as in the roe of A. brama and S. lucioperca in 1990. In these cases, the absence of ¹⁴⁴Ce is explained by its low solubility and limited ability to penetrate tissues, whereas ¹²⁵Sb in dissolved form readily enters through the gills or intestine and is transported to internal organs.

Thus, the presence of ¹²⁵Sb alone in internal organs does not indicate greater “biological significance” compared to ¹⁴⁴Ce, but rather reflects its substantially higher bioavailability and mobility. ¹²⁵Sb behaves as a typical dissolved radionuclide capable of active incorporation into physiological processes. This is particularly evident in the later post-accident period, when ¹⁴⁴Ce is largely fixed in bottom sediments or has decayed, whereas ¹²⁵Sb continues to circulate in the aquatic environment.

4. Summary

This study presents a comprehensive analysis of the accumulation patterns of short-lived radionuclides (¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce) in fish, based on the integration of the literature data and results obtained in the Chornobyl Nuclear Power Plant cooling pond and the Kaniv Reservoir. The findings indicate that radionuclide behavior, bioavailability, and tissue distribution are primarily governed by their physicochemical properties, environmental forms, and ecological characteristics of fish [4,6].

Antimony (¹²⁵Sb) shows intermediate behavior between conservative and particle-reactive radionuclides. In the present study, its accumulation depends on environmental form. When associated with fuel particles, it co-occurs with ¹⁴⁴Ce and accumulates in the gastrointestinal tract. When present independently, it reflects dissolved or aerosol-derived forms and shows higher bioavailability, accumulating in internal organs and muscles. No biomagnification was observed, in agreement with published data [4].

Iodine (¹³¹I) is highly soluble and conservative. Literature indicates rapid incorporation into biological processes [28,31,33]. In this study, it was detected mainly in early post-accident samples. Its short half-life leads to transient presence [53]. Its accumulation reflects rapid uptake and turnover rather than biomagnification [28].

Cerium isotopes (¹⁴¹Ce and ¹⁴⁴Ce) exhibit a pronounced tendency to associate with particulate matter in aquatic environments. According to the literature, ¹⁴⁴Ce is predominantly bound to suspended particles and bottom sediments [4,5], which leads to enhanced accumulation in benthic organisms and external tissues, whereas concentrations in muscle tissue are typically low, and biomagnification is generally considered absent or highly unlikely.

Within the present study, these general patterns are broadly confirmed; however, several important features were identified. In particular, very high activities of ¹⁴⁴Ce were recorded, especially in omnivorous and planktivorous fish species, with maximum values observed in the gastrointestinal tract, intestinal contents, mucus, and external tissues (see Table 4). This indicates that ingestion of contaminated particles represents the primary pathway of uptake.

At the same time, our results demonstrate the presence of ¹⁴⁴Ce in muscle tissue, kidneys, and roe over several years following the accident, indicating a gradual internal redistribution of the radionuclide within organisms. Importantly, the detection of ¹⁴⁴Ce in the muscle tissue of piscivorous fish may suggest possible trophic transfer and limited biomagnification.

Notably, ¹⁴⁴Ce was detected in muscle tissue until 1993, later than in most other organs and tissues, which is consistent with the pattern previously reported for ¹⁰⁶Ru in fish from the same water body [82]. This may indicate prolonged redistribution processes and slow elimination of the radionuclide from muscle tissue, potentially creating conditions for its persistence in higher trophic levels.

The analysis of the relationship between ¹²⁵Sb and ¹⁴⁴Ce indicates fundamentally different pathways of their input and accumulation in fish and allows differentiation of the main forms of radioactive contamination following the Chornobyl accident. It was established that simultaneously high activities of both radionuclides, particularly characteristic of the early post-accident period, are associated with the input of fuel particles and reflect their common origin and limited mobility. In contrast, the predominance of ¹⁴⁴Ce alone in most tissues indicates its strong association with the particulate matter, low solubility, and limited bioavailability, resulting in its accumulation mainly in external and mineralized structures. At the same time, cases where only ¹²⁵Sb is detected, especially in later post-accident years and in physiologically active organs, reflect its occurrence in dissolved form, higher mobility, and greater capacity for uptake and internal transport. Thus, differences in the accumulation of these radionuclides reflect the combined influence of two principal contamination pathways, particulate and dissolved, and determine their contrasting bioavailability and tissue distribution in fish.

5. Conclusions

• The accumulation patterns of ¹²⁵Sb, ¹³¹I, ¹⁴¹Ce, and ¹⁴⁴Ce in fish are primarily controlled by their physicochemical properties and environmental forms (dissolved vs. particulate), rather than by their radioactive nature.
• ¹²⁵S exhibits higher mobility and bioavailability, accumulating in internal organs (liver, kidneys, gills) and, under certain conditions, in muscle tissue, reflecting uptake from dissolved forms.
• ¹³¹I shows high mobility and rapid biological incorporation, with a short-term presence in fish tissues, mainly in internal organs and whole-body samples.
• ¹⁴¹Ce and ¹⁴⁴Ce are predominantly associated with particulate matter and accumulate mainly in the gastrointestinal tract, external tissues, and mineralized structures, with limited but detectable redistribution into internal organs and muscles.
• In general, accumulation patterns of radionuclides are consistent with those of their stable analogues; no fundamental qualitative differences were identified.
• Biomagnification is not a dominant process for the studied elements; trophic dilution prevails, although limited trophic transfer for ¹⁴⁴Ce may occur under specific conditions.
• The co-occurrence of radionuclides (especially ¹⁴⁴Ce and ¹²⁵Sb) serves as an indicator of fuel particle contamination, while their separate occurrence reflects different environmental pathways and bioavailability.