Radiation shielding structures used in nuclear power plants are designed to withstand both mechanical loads and harsh environmental conditions, including elevated temperature exposure, thermal and humidity cycling related to fuel cycles, and long-term exposure to ionizing radiation. The normal service temperature range is up to 65 °C, and can be as high as 95 °C at localized hot zones [1
]. The elevated temperature and high relative humidity (RH) in the concrete core of usually massive shielding structures may promote the development of a detrimental alkali-silica reaction (ASR), provided that both reactive minerals in the aggregate and sufficient alkali in concrete pore solution are present. The risk of ASR-induced damage in the concrete of nuclear power plants is not acceptable. Moreover, the long-term ionizing radiation from the reactor or other sources may further intensify the reaction in the nearby concrete elements [3
]. Although the current knowledge of radiation-induced damage to concrete is still fragmented, it was suggested that quartz may exhibit increased chemical instability following neutron irradiation [4
]. Quartz would eventually dissolve faster in the caustic concrete pore solution and therefore enhance the formation of alkali-silica gel provided that the other necessary conditions are met. ASR-related damage to concrete was detected in a nuclear power plant containment wall and an ageing management program was proposed [5
]. The proper selection of concrete aggregates to be used in radiation shielding structures is of primary importance for this application.
Current aggregate selection procedures refer to ASR mitigation strategies defined in ASTM C1778 [6
] and RILEM AAR-0 [7
]. Nuclear power plant (NPP) safety-related structures are directly classified as “high risk” structures that require the highest level of ASR prevention. However, the materials selection is limited due to numerous requirements associated with radiation-shielding concrete [8
]. For massive structures, the limitation of aggregate grain size is not applicable. The use of highly efficient supplementary cementitious materials in concrete, like siliceous fly ash, is questionable when considering potential activation of elements, which heavily influence the decommissioning time after the termination of the NPP service period.
Lee et al. [10
] described petrographic examination methods that can be used to evaluate aggregates for use in radiation-shielding concrete. The authors stated that the aggregates used in this type of concrete should be relatively clean, free of deleterious materials, and chemically inert. The common types of potentially deleterious materials in aggregates include, among others, siliceous components of aggregates that are known to be associated with potentially harmful alkali-silica reactivity, so the aggregates should be examined to detect the presence of harmful minerals. Under the influence of ionizing radiation and elevated temperature in concrete shields, conditions for ASR promotion may occur [11
]. Detailed mineralogical characterization and laboratory tests of special aggregates are thus necessary to prevent expansive alkali-silica reactions in concrete structures in NPP.
According to ACI 221R-96 [12
], heavy fine, and coarse aggregates range in specific gravity from about 3500 to about 7500 kg/m3
and produce concrete ranging in unit weight from about 2800 to 5600 kg/m3
. The aim of this work was the evaluation of high-density aggregates selected for radiation shielding concrete with respect to deleterious alkali-silica reactions. The scope of the investigation was limited to high-density aggregates that could be used for the construction of the first Polish NPP and that are economically reasonable. The cost of the aggregate, the expense involved in its transport, the aggregates properties, and availability of deposits were considered.
Petrographic analysis on thin sections revealed the presence of SiO2
crystals in all tested high-density aggregates. In hematite aggregate H1 and the barite aggregate B2, microcrystalline quartz was observed. In barite aggregate B2, cristobalite was found. Opposite results were reported by Topçu [38
], who analyzed a barite aggregate from an unknown source using thin sections, but no mineral other than barite was found. Barite crystals showed dimensions that ranged between 25 and 1200 µm. Castro and Wigum [39
] showed that image analysis petrography can be successfully used as a supplementary technique to overcome some of the limitations of the petrographic method RILEM AAR-1.
Several researchers [40
] reported the results of mechanical tests on high-density concrete containing hematite aggregates from different sources. Gencel et al. [41
] showed that mechanical properties, especially the compressive strength of concrete with hematite, did not differ from those of plain concrete. The influence of barite and magnetite aggregate on the permeability of radiation shielding concrete was reported by Kubissa et al. [29
]. ACI limits the normal operating temperature in NPP concrete structures to 65 °C in general and 90 °C locally, so the exposure of concrete to high temperature is unavoidable, which may create favorable conditions for ASR. Pillai et al. [40
] reported that, among all tested concretes, the concrete containing hematite aggregate showed better compressive strength than the reference concrete with granite aggregate after sustained thermal ageing for 56 days (84 MPa and 76 MPa, respectively). However, according to Jones and Clark [42
], although the Young’s modulus of concrete can be significantly reduced by ASR, the apparent deterioration of concrete compressive strength due to ASR is dependent on the test method used. The cube test is particularly insensitive to ASR, with strengths after significant expansion often being greater than those at 28 days. So, the large expansion of the hematite H1 mortar and the threat of the ASR occurrence should be considered during the high-density concrete design process.
The mortar bar expansion test results showed that one barite aggregate was susceptible to ASR. The obtained results are in contrast to those of Pomaro et al. [43
]. They stated that high specific weight aggregates like barites have almost null reactivity with alkalis in cement. It seems that the potential for ASR in barite aggregates is related to the content of secondary minerals, and particularly to the size and content of SiO2
(microcrystalline quartz). This is consistent with the review presented by Rajabipour et al. [44
]. Aggregate reactivity depends not only on the type of silica mineral it contains, but also on the size and distribution of these minerals within the aggregate structure. However, Hagelia and Fernandes [17
] suggested that the size of the quartz grains might be less important, and that the dissolution of feldspars and micas contribute to the reactivity. It was shown that products of the reaction were associated with coarse-grained quartz (up to 1500 μm in size and essentially free of deformation), whereas granitic mylonites containing microcrystalline quartz and low amounts of mica were found to be innocuous. However, according to Tiecher et al. [45
], under favorable conditions of temperature, humidity, and access of alkali over a long time period, each type of quartz tends to provoke the alkali-silica reaction.
All the presented conclusions regarding reactivity or innocuousness are based only on laboratory tests. The data from real structures and from field exposure are unknown or cannot be published.
The results from the ASTM C1293 test confirm observations made by Islam et al. [46
]. They showed that the mortar bars with hematite/magnetite submerged in 1 M NaOH expanded mostly during the early state of testing up to 28 days, and the expansion rate decreased with an increase in test duration. The concrete prism from barite (B1 and B3) and magnetite (M1) aggregates demonstrated the greatest elongation during the first days of testing up to 90 days. The sharp slope of the curve during the first 90 days may suggest a large potential for ASR, but after this time, the beams did not show such a rapid increase in length. Carles-Gibergues and Cyr [35
] presented interpretation of expansion curves of concrete subjected to accelerated ASR tests. Besides the commonly known expansion curve of a concrete prism affected by ASR, which is characterized by three successive phases (an initial phase where swelling begins, a phase of significant expansion at a nearly constant rate, and a decrease in the expansion rate to reach a final plateau), the author showed another form of expansion curve. The initial phase was not clearly separated from the expansion phase. Nevertheless, all the curves clearly presented the same initial period—the highest expansion of prisms.
This research was limited to materials that could be used for the construction of the first Polish NPP and that are economically reasonable. There are no high-alkali cements available on the Polish market, so the tests were performed according to ASTM standards. According to ASTM C1260, the alkali content of the cement has a negligible or minor effect on the expansion in this test because the specimens are stored in a 1 M NaOH solution. Contrary to the RILEM guidelines for mortar bar, where the Na2
content has to be 1%, the maximum level of Na2
in Polish cement was equal to 0.88%, so met the ASTM C1293 requirements. In Jóźwiak-Niedźwiedzka et al. [27
], the effect of alkali content in cement on the expansions due to ASR was studied. Two ordinary Portland cements (Na2
= 0.8% and 0.6%), which are commonly available in the market, and one special cement were tested. One cement was specially made for the purpose of nuclear shielding concrete CEM I NA-SR-LH with low-alkali content, increased sulphate resistance, and low heat of hydration (Na2
= 0.4%). For aggregates susceptible to ASR, the content of alkali in cement had no effect on the final test result, whereas for ASR-resistant aggregates, although the results of the expansion were lower than 0.1%, they differed depending on the amount of Na2
in the cement. Finally, depending on the environmental conditions in which the concrete structure is expected to work, the choice of the appropriate cement should be considered.