Assessment of Radiological Dispersal Devices in Densely Populated Areas: Simulation and Emergency Response Planning

Abstract

The increasing threat of terrorism involving Radiological Dispersal Devices (RDDs) necessitates comprehensive evaluation and preparedness strategies, especially in densely populated public areas. This study aims to assess the potential consequences of RDD detonation, focusing on the effective doses received by individuals and the ground deposition of radioactive materials in a hypothetical urban environment. Utilizing the HotSpot code, simulations were performed to model the dispersion patterns of ¹³⁷Cs and ²⁴¹Am under varying meteorological conditions, mirroring the complexities of real-world scenarios as outlined in recent literature. The results demonstrate that ¹³⁷Cs dispersal produces a wider contamination footprint, with effective doses exceeding the public exposure limit of 1 mSv at distances up to 1 km, necessitating broad protective actions. In contrast, ²⁴¹Am generates higher localized contamination, with deposition levels surpassing cleanup thresholds near the release point, creating long-term remediation challenges. Dose estimates for first responders highlight the importance of adhering to operational dose limits, with scenarios approaching 100 mSv under urgent rescue conditions. Overall, the findings underscore the need for rapid dose assessment, early shelter-in-place orders, and targeted decontamination to reduce population exposure. These insights provide actionable guidance for emergency planners and first responders, enhancing preparedness protocols for RDD incidents in major urban centers.

1. Introduction

The 11 September 2001 terrorist attacks fundamentally shifted global security perspectives and heightened concerns regarding unconventional weapons, particularly radiological dispersal devices (RDDs). In the aftermath, intelligence agencies documented increased interest among various terrorist organizations in acquiring and deploying radiological materials for malicious purposes, with RDDs, commonly referred to as “dirty bombs”, emerging as a significant threat vector [1,2].

Unlike nuclear weapons, RDDs do not produce nuclear explosions or cause immediate mass casualties through blast effects. Instead, these devices combine conventional explosives with radioactive materials to disperse contamination across potentially extensive areas, creating long-term health risks, economic disruption, and psychological impact on affected populations. The primary hazards associated with RDDs include radiation exposure through inhalation, ingestion, and external contamination, along with the substantial costs of decontamination and area denial.

Historical incidents demonstrate the serious consequences of radiological material misuse and mishandling. The 1995 Chechen rebel incident in Moscow, where militants buried a radiological device in a public park and threatened detonation, illustrated the psychological warfare potential of such threats [3,4,5]. More significantly, the 1987 Goiânia accident in Brazil provided a stark example of radiological contamination consequences when an abandoned medical ¹³⁷Cs source (1375 Ci) was breached by scavengers, resulting in four fatalities and widespread environmental contamination affecting hundreds of individuals [6]. Similarly, a tragic incident in Morocco involved an abandoned ¹⁹²Ir industrial source (16.3 Ci) that was unknowingly brought into a family residence, remaining there for several weeks. This exposure resulted in eight deaths, including four children, with diagnosis delayed by 80 days after initial exposure. These cases, along with additional documented events summarized in

Table 1. Documented incidents involving RDDs [4].

Date	Country	Incident Description
1995	Russia	Chechen militants planted a device containing 137Cs in a Moscow park, but it failed to detonate.
June 2002	United States	Jose Padilla was detained for allegedly plotting to use a “dirty bomb”.
December 2002	Nigeria/Germany	A radioactive well-logging source with 137Cs was stolen in Nigeria and later discovered in Germany in September 2003.
May 2003	Tbilisi, Georgia	Authorities stopped an attempt to traffic radioactive materials across borders into Turkey or Iran.
June 2003	Bangkok, Thailand	Police apprehended a suspect trying to sell 137Cs for over $200,000.
2011	New York, USA	A man attempted to construct an RDD using material from smoke detectors. He was arrested, convicted, and sentenced to 15 years.
December 2013	Mexico	Thieves hijacked a truck transporting 60Co from a hospital in Tijuana to a waste storage facility. The material was recovered after the abandoned vehicle was located near Temascalapa, 35 km northeast of Mexico City.
2016	Iraq	ISIS claimed responsibility for an RDD attack in Mosul, reportedly using a car bomb containing radioactive material.

, demonstrate the real-world implications of radiological material misuse [4].

The radioactive materials potentially used in RDDs are commonly found in legitimate applications across medical, industrial, and research sectors [4,5,6,7,8,9,10]. The severity of contamination and health impacts from an RDD incident depends on multiple variables: the radioisotope type and quantity, detonation location, and prevailing meteorological conditions during dispersal. Security risk assessment of radioactive sources considers four primary factors: prevalence of use, radioactivity level, portability, and dispersibility potential. Sources exhibiting high levels across these categories pose the greatest security concerns. Cesium chloride exemplifies a high-risk material, containing significant ¹³⁷Cs activity in powder form that facilitates dispersal. When housed in portable containers without adequate security measures, such sources become attractive targets for malicious acquisition [11]. Notably, ¹³⁷Cs exists in forms readily adaptable for dispersal, and solid forms can be converted to dispersible powders through conventional mechanical or chemical processes [3,11].

Research has identified eight radioisotopes of primary security concern based on their characteristics and widespread availability. These high-risk isotopes include reactor-produced ²⁴¹Am, ²⁵²Cf, ¹³⁷Cs, ⁶⁰Co, ¹⁹²Ir, ²³⁸Pu, and ⁹⁰Sr, along with naturally occurring ²²⁶Ra. Most of these radioisotopes possess half-lives spanning years to decades, meaning they retain significant radioactivity throughout substantial portions of human lifespans, contributing to their long-term health threat potential [3,6,9,10]. The health hazards posed by these isotopes vary according to their radiation emission characteristics.

Alpha-emitting isotopes (²⁴¹Am, ²⁵²Cf, ²²⁶Ra, and ²³⁸Pu) primarily present internal hazards through inhalation or ingestion pathways, as alpha particles lack sufficient energy to penetrate human skin. However, some of these nuclides also emit accompanying gamma radiation. ²⁴¹Am, for example, emits a 59.54 keV photon that can contribute to measurable external doses for MBq-level sources, although internal exposure remains the dominant hazard. Similarly, ²²⁶Ra’s external radiation risk arises less from its own 186 keV gamma emission and more from its short-lived progeny, particularly ²¹⁴Bi, which accounts for the majority of the external gamma dose. Conversely, gamma-emitting isotopes (¹³⁷Cs, ⁶⁰Co, and ¹⁹²Ir) pose both external and internal health risks due to gamma radiation’s ability to penetrate tissue. ⁹⁰Sr, which emits high-energy beta particles, presents external hazards in unshielded scenarios but poses its greatest threat when ingested, as it accumulates in bone tissue [3,6,9,10,11].

Recognizing these vulnerabilities, the IAEA has intensified efforts to improve radioactive source security, with accelerated initiatives following the September 11 attacks to prevent these materials from becoming instruments of radiological terrorism [12,13,14,15,16,17,18,19]. Despite these enhanced security measures, incidents involving nuclear and radioactive materials continue to occur. According to the IAEA Incident and Trafficking Database (ITDB), 168 incidents were reported by 31 states in 2023, representing a 22-incident increase from 2022 [16,20]. The ITDB categorizes these incidents into three groups: Group I encompasses incidents connected or likely connected to trafficking or malicious use; Group II includes incidents of undetermined intent; and Group III covers incidents unlikely to be associated with trafficking or malicious activities. Analysis of ITDB data from 1993 to 2023 reveals important trends regarding radiological security threats, as the temporal distribution shows concerning fluctuations, with notable spikes in incidents occurring in 2006 and around 2020, though the latter may be influenced by COVID-19 pandemic-related reporting disruptions (Figure 1). Material analysis indicates that radioactive materials account for 59% of all reported incidents, significantly outnumbering nuclear material incidents (14%) and other materials (27%) (Figure 2A). Particularly relevant to RDD threat assessment is the vulnerability during transportation, with 65% of theft-related incidents occurring during authorized transport of materials, a percentage that has increased to 70% in the last decade (2014–2023) (Figure 2B). These persistent incidents and evolving threat patterns underscore the ongoing challenges in securing radioactive materials and highlight the continued relevance of comprehensive radiological threat assessment and emergency response planning.

While no RDD events have been reported in Morocco, the current global security threats and documented incidents of radioactive material theft create realistic scenarios that warrant serious consideration for any nation. Morocco’s proactive approach to radiological emergency preparedness demonstrates recognition of these global threat patterns. The Moroccan Agency for Nuclear and Radiological Safety and Security (AMSSNuR) has established comprehensive response capabilities through participation in national emergency simulation exercises, international cooperation frameworks, and development of the National Intervention Plan for Nuclear or Radiological Emergency Situations (PNI-SUNR) [21,22]. Despite these extensive preparedness measures and the absence of reported RDD events in Morocco, the persistent global incidents and evolving threat patterns underscore the ongoing challenges in securing radioactive materials and highlight the continued relevance of comprehensive radiological threat assessment and emergency response planning.

Morocco, covering approximately 710,850 km² in northwestern Africa and strategically positioned with the Atlantic Ocean to the west and the Mediterranean Sea to the north, faces unique nuclear security challenges as it prepares to host several major international events in the near future. These high-profile gatherings present attractive targets and necessitate comprehensive radiological threat assessments, particularly given the broader context of regional security concerns and heightened vigilance linked to recent international developments. Effective emergency response training fundamentally depends on realistic scenario development. The goal of such training is to prepare responders to effectively mitigate the consequences of hazardous material releases through combinations of tabletop exercises, field exercises, and classroom instruction [22,23,24,25]. Therefore, developing realistic RDD scenarios serves the critical objective of generating drill and exercise data for emergency response training programs.

Extensive literature exists on hypothetical urban RDD scenarios; therefore, this study adopts a pragmatic methodology by incorporating actual documented data in the IAEA ITDB, recognizing that unrealistic scenarios compromise training effectiveness and emergency response performance. The research examines credible theft scenarios involving ¹³⁷Cs from a calibration irradiator or ²⁴¹Am from industrial nuclear density gauges. These isotopes were selected for comparative analysis to quantify the range and magnitude of potential radiological consequences, as they represent the most accessible targets for malicious acquisition.

The assessment centers on a detailed scenario wherein terrorists covertly deploy an RDD within a vehicle near a fan zone, simulating a vehicle-borne improvised explosive device during an international sporting event broadcast from the capital. The scenario models 200,000 spectators congregated in an open area environment, representing a high-density population target typical of major international gatherings. Upon detonation, the conventional explosive component disperses the sealed radioactive source throughout the immediate vicinity, generating immediate blast casualties and panic while simultaneously contaminating the surrounding environment with aerosolized radioactive particles.

The comprehensive radiological consequence assessment evaluates Total Effective Dose Equivalent (TEDE) distributions, cumulative external and internal radiation exposures, and ground deposition contamination patterns. The analysis employs the HotSpot general plume dispersion code to model atmospheric transport and deposition under various meteorological stability classifications to provide robust assessment capabilities across the full spectrum of realistic environmental conditions that emergency responders might encounter during an actual incident. It also aims to ascertain the suitable stay time for emergency workers, ensuring that response personnel can operate effectively while maintaining radiation exposure below recommended occupational limits.

2. Materials and Methods

2.1. Study Area Details

The hypothetical scenario involving the detonation of an RDD was situated in the capital city of Morocco, positioned on the Atlantic coast. This city spans an area of approximately 118 km². The city has an estimated population of 580,000 residents, resulting in a population density of 4380 inhabitants per km² and an annual population growth rate of 1.1%. During large public events, the capital city accommodates multiple fan zones distributed throughout the urban area. For the purposes of this study, the largest fan zone, accommodating more than 200,000 spectators, was selected as the focal point for the radiological threat assessment (Figure 3). The radiological impact of dispersal events is contingent upon numerous environmental factors, including release height, atmospheric conditions, source material quantity, precipitation patterns, aerosol particle size distribution, and characteristics of the atmospheric inversion layer. Meteorological characterization was conducted using wind rose analysis, which revealed that dominant winds originated from the northwest and north sectors, with speeds predominantly ranging from 2 to 6 m/s (Figure 4). These wind conditions correspond to neutral to slightly unstable atmospheric stability (Pasquill classes C–D), which facilitates effective atmospheric dispersion of airborne contaminants. The coastal location engenders favorable meteorological conditions for pollutant dispersion, significantly influencing the spatial extent of contamination spread and radiation exposure patterns following a potential RDD incident. This atmospheric behavior constitutes a critical input parameter for accurate dispersion modeling and radiological consequence assessment in the urban environment.

2.2. Modelling the RDD (Scenario Narrative)

The study examines a hypothetical RDD incident designed to assess radiological consequences and emergency response requirements during a major international sporting event. The scenario assumes deployment of a vehicle-borne improvised explosive device containing radioactive material near the largest fan zone, with 200,000 spectators present in an open area environment typical of major international gatherings. The conventional explosive component was modeled as equivalent to 522 kg TNT. Two radioisotopes were selected for comparative analysis based on their accessibility and radiological characteristics: ¹³⁷Cs and ²⁴¹Am (

Table 2. Radionuclide characteristics [3,10].

Isotope	T1/2 Phys	T1/2 eff	Specific Activity (MBq/kg)	Decay Mode	Energy (MeV) α	Energy (MeV) β	Energy (MeV) γ
137Cs	30.1 y	109 d	3256.0	β, IT, γ	n/a	0.19; 0.06	0.660
241Am	432.7 y	2.5 y	129.5	α, γ	5.5	0.052	0.033

IT—isomeric transition.

). These isotopes represent credible source terms identified by Argonne National Laboratory as posing elevated radiological terrorism risks due to their widespread availability in industrial and medical applications [4]. ¹³⁷Cs sources are commonly found in calibration irradiators in the form of cesium chloride powder or salt, while ²⁴¹Am is typically present in density gauges as pressed ceramic powder (AmO₂) [1,10]. The modeled source activities were 3000 Ci (1.11 × 10¹⁴ Bq) for ¹³⁷Cs and 20 Ci (7.4 × 10¹¹ Bq) for ²⁴¹Am, representing realistic values found in industrial applications. ¹³⁷Cs presents primarily an external gamma radiation hazard, while ²⁴¹Am poses predominantly an internal alpha radiation threat following inhalation or ingestion. Results evaluations were established following IAEA recommendations to ensure consistency with international radiological emergency response standards. Results are evaluated against three ICRP dose threshold levels that define protective action requirements for different population groups. The 1 mSv threshold represents the public effective dose limit recommended by ICRP for general population protection during emergencies. The 20 mSv threshold reflects the ICRP-recommended dose limit for radiation workers, calculated as an average over defined five-year periods with a total limit of 100 mSv over five years, while maintaining the additional provision that effective doses should not exceed 50 mSv in any single year. The 100 mSv threshold establishes the emergency response limit for first responders undertaking urgent rescue actions, with ICRP recommendations emphasizing that all reasonable efforts should be made to keep responder doses below this limit during lifesaving operations [6].

2.3. HotSpot Health Physics Codes

Radiological consequence assessment was conducted using HotSpot (Version 3.1.2), a Health Physics software designed for evaluating short-term radionuclide releases. The software calculates four primary dose pathways: (i) inhalation dose, (ii) submersion dose, (iii) groundshine, and (iv) Total Effective Dose (TED). Based on the Gaussian Plume Model, HotSpot is widely used for emergency assessments and safety planning applications [1,27]. The general explosion module within HotSpot modeled atmospheric dispersion following radioactive material detonation. The software calculates time-dependent cloud height (H) as a function of explosive quantity (w, in pounds) and time since detonation (t, in seconds), using different formulations for various atmospheric stability conditions. For unstable conditions (Pasquill classes A, B, C), cloud height calculations differ from those used for stable and neutral conditions (classes D, E, F, G). Four meteorological scenarios were developed based on the local wind rose data to represent the range of atmospheric conditions likely to occur in the study area (

Table 3. HotSpot meteorology scenarios derived from the wind rose.

Scenario ID	Use Case	10-m Wind Speed (m/s)	Wind Direction (from, °)	Stability Class	Likelihood	Notes
Scenario 1	Baseline / typical	5	330 NNW	D (Neutral)	Highest (most probable)	Best single representative hour.
Scenario 2	Sunny, well-mixed day	7	330 NNW	C (Slightly unstable)	Medium	Center of 6–8 m/s band.
Scenario 3	Night, slightly stable	2	330 NNW	E (Slightly stable)	Low–medium	Common conservative night case.
Scenario 4	Rare stagnation (upper-bound)	1	330 NNW	F (Moderately stable)	Very low	Use only for bounding; uncommon on the coast.

).

The software calculates the time-dependent cloud height (H) as a function of explosive quantity (w, in pounds) and time since detonation (t, in seconds). Key equations include the time to maximum cloud rise (Equation (1)), cloud height for unstable conditions (Classes A, B, C) (Equation (2)), and cloud height for stable/neutral conditions (Classes D, E, F, G) (Equation (3)). Several modeling assumptions are common across all simulations: the radionuclide mass is assumed to be completely dispersed into the atmosphere; in the evaluation of the Total Effective Dose (TED), ground shine and resuspension are included; and Dose Conversion Coefficients are derived from the U.S. Federal Guidance Report 13 (FGR-13), with the lung model for internal contamination based on the International Commission on Radiological Protection (ICRP) Publication 66. Results are reported as TED received by affected individuals in the short period following detonation, using HotSpot default settings for exposure time and breathing rate. Consequently, the reported TED values may not fully represent doses received by emergency personnel or rescue workers, who are likely to arrive after the blast and may be equipped with protective gear [27,28].

$$t_i=21.6 w^{0.33}$$

(1)

Cloud height for unstable conditions (Class A, B, C):

$$H_{A,B,C}\left(w\right)=27.4 w^{0.48}$$

(2)

Cloud height for stable/neutral conditions (Class D, E, F, G):

$$H_{D,E,F,G}\left(w\right)=23.3 w^{0.44}$$

(3)

3. Results

3.1. Simulation Results

The HotSpot simulations generated comprehensive dose assessments for both ¹³⁷Cs and ²⁴¹Am across four meteorological scenarios, with results presented as Total Effective Dose Equivalent (TEDE) and pathway-specific contributions. Figure 5 illustrates the TEDE distribution as a function of distance from the detonation point, while Figure 6, Figure 7, Figure 8 and Figure 9 provide detailed breakdowns of dose contributions from inhalation, submersion, ground shine, and resuspension pathways. Further details can be found in the tables in the Appendix A section.

The TEDE results reveal striking differences between the two radioisotopes that cannot be explained by source activity alone. Despite ²⁴¹Am having 150 times lower activity, it consistently produces TEDE values 50–100 times higher than ¹³⁷Cs across all scenarios and distances. At 0.01 km from the detonation point, ²⁴¹Am TEDE ranges from 5.2 mSv (Scenario 2) to 10 mSv (Scenario 3), while ¹³⁷Cs TEDE ranges from 0.049 mSv (Scenario 2) to 0.097 mSv (Scenario 3).

The meteorological scenarios demonstrate distinct atmospheric dispersion behaviors that significantly influence dose distributions. Scenario 3 (nighttime, 2 m/s, Class E slightly stable) produces the highest doses for both isotopes due to reduced atmospheric mixing and limited vertical dispersion. Scenario 2 (sunny day, 7 m/s, Class C slightly unstable) generates the lowest doses through enhanced atmospheric turbulence and dilution. Scenarios 1 and 4 produce intermediate values, with the baseline conditions (Scenario 1) showing moderate dispersion characteristics typical of neutral stability. Dose attenuation with distance follows steep gradients for both isotopes, with TEDE values decreasing by 2–3 orders of magnitude at 1 km distance compared to 0.01 km values.

The inhalation pathway presented in Figure 6 accounts for the overwhelming majority of ²⁴¹Am dose contributions, representing 95–98% of the total dose across all scenarios. At 0.01 km, ²⁴¹Am inhalation doses range from 5.1 mSv to 10 mSv, closely matching the TEDE values. This dominance reflects the high biological effectiveness of alpha radiation when deposited in lung tissue, where the radiation weighting factor of 20 amplifies the absorbed dose compared to gamma radiation. The biological basis for this enhanced effectiveness lies in the high linear energy transfer characteristics of alpha particles, which, at approximately 100 keV/μm compared to 0.3 keV/μm for gamma radiation, create dense ionization tracks that cause concentrated cellular damage. When ²⁴¹Am particles are inhaled, they deposit preferentially in the pulmonary region with a biological half-life of 2.5 years, ensuring prolonged internal irradiation of sensitive lung tissues.

External exposure pathways demonstrate distinct radiological signatures between the isotopes. For ¹³⁷Cs, characterized by its 0.660 MeV gamma emission, submersion (Figure 7) doses at 0.01 km reach up to 9.7 × 10⁻⁴ mSv under scenario 3, with values on the order of 10⁻⁴ mSv persisting at 0.1 km and gradually decreasing with distance. These contributions confirm that external exposure from ¹³⁷Cs remains measurable across multiple scenarios, highlighting the need for shielding and external radiation protection measures. In contrast, ²⁴¹Am submersion doses are negligible in all scenarios, with peak values near 1.7 × 10⁻⁷ mSv at 0.01 km. This means that at very close distances, ¹³⁷Cs produces external submersion doses nearly 5700 times higher than ²⁴¹Am. The difference reflects the limited range of alpha particles in air (≈4 cm) and the low-energy gamma emissions (59.54 keV) associated with ²⁴¹Am, which together prevent it from contributing meaningfully to external dose. Consequently, external exposure is a significant pathway for ¹³⁷Cs but remains effectively irrelevant for ²⁴¹Am, where inhalation dominates the dose burden.

Ground shine dose contributions, summarized in Figure 8, highlight the dominant role of ¹³⁷Cs compared to ²⁴¹Am. For ¹³⁷Cs, near-field doses reach up to 2.16 × 10⁻² mSv at 0.01 km and remain measurable at 10⁻³–10⁻⁴ mSv even at 1 km, confirming the operational relevance of this pathway for external exposure control. In contrast, ²⁴¹Am ground shine is negligible across most scenarios, rarely exceeding 10⁻⁶ mSv, except scenario 2, where stable meteorological conditions yield localized deposition values of 2.88 × 10⁻⁶ mSv at 0.01 km. Even under this conservative case, ²⁴¹Am ground shine is three to four orders of magnitude lower than the corresponding ¹³⁷Cs values. These results demonstrate that while ¹³⁷Cs demands time-distance-shielding countermeasures for external exposure, protective actions for ²⁴¹Am must prioritize inhalation control, surface contamination management, and respiratory protection for first responders.

Resuspension presented in Figure 9 doses reveals significant long-term exposure potential following initial deposition. ²⁴¹Am produces resuspension doses of 5.7 × 10⁻² to 9.2 × 10⁻² mSv at 0.01 km, representing a substantial secondary exposure pathway that could persist for years following the initial incident. The high specific activity and long half-life (432.7 years) of ²⁴¹Am ensure that resuspended particles remain radiologically significant for extended periods. ¹³⁷Cs resuspension doses range from 3.4 × 10⁻⁴ to 6.7 × 10⁻⁴ mSv at 0.01 km, though the 30.1-year half-life provides a more manageable timeframe for natural decay.

Finally, it is important to note that the reported TED and deposition values represent conservative point estimates derived from HotSpot defaults. The results are sensitive to several key inputs, particularly atmospheric stability class, wind speed, release height, and particle size distribution. For example, stable nighttime conditions expanded the affected areas by approximately fourfold compared to unstable daytime conditions, while lower wind speeds increased near-field doses. These drivers of variability illustrate that uncertainty in meteorological conditions and source parameters can significantly alter consequence outcomes, and results should therefore be interpreted as indicative planning values rather than precise predictions.

3.2. Emergency Response Planning and Best Practices

The simulation results translate into concrete operational guidance for emergency response during the critical first operational period. Immediate actions must prioritize rapid confirmation and lifesaving operations without compromising responder safety. Upon arrival, emergency responders should confirm radiation presence using the “two-two-two” protocol—two readings, two locations, two instruments—before declaring elevated radiation levels. Shelter-in-place messaging should be issued immediately using pre-approved communications while radiation monitoring capabilities are established, with initial messages ideally transmitted within 10 min of confirming an explosion with visible smoke and debris. Critically, lifesaving rescue operations must commence without waiting for comprehensive radiation surveys, as emergency protocols explicitly direct that lifesaving operations should not be delayed due to radiation presence [8].

Zone delineation requires measurable criteria to establish defensible and updatable operational boundaries. The Hot Zone should be defined using >0.1 mSv h⁻¹ exposure rates or surface contamination exceeding 60,000 dpm cm⁻² for gamma radiation at 1.5 cm or >6000 dpm cm⁻² for alpha radiation at 0.5 cm. Shelter-in-place zone boundaries must be established for public protection and adjusted as field data become available. Emergency management agencies should visualize and update Hot Zone and shelter-in-place boundaries in RadResponder systems to maintain shared, real-time operational awareness across all responding organizations [8].

Protection strategies must be tailored to the specific isotope characteristics identified in the modeling results. For ²⁴¹Am incidents, where alpha radiation and internal hazards dominate with inhalation representing 95–98% of total dose, early emphasis must focus on respiratory protection and contamination control measures. Emergency responders require appropriate respiratory protection for field operations, as alpha particles present exceptional internal hazards when inhaled or ingested, while standard radiation detection instruments respond poorly to alpha emissions. The limited range of alpha particles in air (approximately 4 cm) and their inability to penetrate paper or moisture confirm that external radiation fields are not the primary hazard driver for ²⁴¹Am incidents.

For ¹³⁷Cs incidents, where gamma radiation creates measurable external exposure representing 20–40% of total dose, emergency response must emphasize time-distance-shielding principles supported by continuous exposure-rate mapping. Standard sodium iodide gamma survey instruments are well-suited for detecting and tracking external dose fields from ¹³⁷Cs characteristic 0.662 MeV gamma emissions, enabling effective field monitoring and work planning activities. Rapid measurement and mapping protocols must replace default response footprints with field-validated data collection. Response teams should execute 1-km orthogonal transect surveys and implement 10-Point Monitoring Plans to locate plume centerlines and constrain deposition patterns. All radiological measurements should be uploaded to RadResponder systems to maintain common operational pictures and support real-time boundary updates and decision-making processes [8].

Population monitoring and decontamination operations require scalable implementation capabilities. Community Reception Centers must be established to provide screening, decontamination, registration, and medical referral services for affected populations, with center planning, site selection, and operational timelines integrated into incident action plans. Simple nasal swab procedures and pragmatic screening criteria should be employed to identify potentially significant inhalation cases requiring medical follow-up, particularly critical for alpha-emitting isotope incidents. Protective action scales must align with meteorological conditions and the dose thresholds established in this study. Based on the simulation results, ²⁴¹Am public protection measures should extend approximately 0.2–0.4 km from the detonation point to maintain individual exposures below the 1 mSv public dose limit. Under optimal dispersion conditions (Scenario 2: sunny, well-mixed, 7 m/s winds), this protective zone contracts toward the 0.2 km range, while under stable atmospheric conditions (Scenario 3: nighttime, 2 m/s winds), the affected area expands toward 0.4 km, representing approximately a four-fold increase in protected area. For ¹³⁷Cs incidents, the modeling demonstrates that acute radiological evacuation is not required based on dose projections alone, as maximum doses remain below 0.1 mSv even at 0.01 km from the detonation point across all meteorological scenarios.

Integration of these findings into operational doctrine requires alignment with established emergency response frameworks. The “Radiological Dispersal Device (RDD) Response Guidance: Planning for the First 100 Minutes” handbook [8] provides a mission-and-tactic sequence that can be directly embedded into local standard operating procedures and incident action plan templates (Recognize → Inform → Initiate → Measure & Map → Evacuate & Monitor). This framework ensures that radiological dispersal device response capabilities remain consistent with established emergency management practices while addressing the unique radiological characteristics and meteorological sensitivities identified through this comprehensive modeling analysis.

3.3. Public Perception and Psychological Response

Beyond the quantitative estimates of dose and deposition, public perception of radiation plays a decisive role in the effectiveness of emergency response during an RDD incident. Decades of research have demonstrated that fear of radiation, often referred to as radiophobia, has historically exceeded scientifically supported risk levels and has hindered the beneficial use of radioactive materials. Brooks et al. (2023) emphasize that this fear is rooted not only in past nuclear accidents but also in cultural symbolism and misinformation, and has been amplified by conservative applications of the linear-no-threshold (LNT) model [29].

Our HotSpot simulations show that modeled doses from ¹³⁷Cs and ²⁴¹Am RDD scenarios are largely within the stochastic risk domain and far below thresholds for deterministic health effects. Without proper context, however, reporting exposures in millisieverts can inadvertently reinforce public anxiety. It is therefore essential to frame dose estimates against familiar benchmarks (e.g., annual background radiation, medical diagnostic exposures) to provide perspective and avoid misinterpretation.

Effective emergency management must integrate risk communication strategies alongside technical countermeasures. Evidence from Chernobyl and Fukushima demonstrates that fear-driven evacuations and stigma caused more long-term harm than the radiological exposures themselves. Communicating clearly that modeled doses, while requiring protective actions, are not immediately life-threatening can reduce panic, support compliance with sheltering or evacuation orders, and maintain public trust. Aligning protective measures with established decision frameworks (ICRP, IAEA, DHS/NUSTL guidance) while addressing public concerns directly will ensure that RDD response is both scientifically sound and socially effective.

4. Conclusions

This comprehensive assessment of radiological dispersal device consequences in densely populated urban environments reveals critical insights for emergency response planning and public safety preparedness. The HotSpot simulations demonstrate that radiological impacts are determined more by isotope characteristics than by source activity alone, with ²⁴¹Am producing doses 50–100 times higher than ¹³⁷Cs despite having 150 times lower activity. The dominance of inhalation pathways for alpha-emitting isotopes, representing 95–98% of the total ²⁴¹Am dose, contrasts sharply with the mixed internal and external exposure patterns observed for gamma-emitting ¹³⁷Cs. The meteorological sensitivity analysis establishes that atmospheric conditions critically influence protective action requirements, with stable nighttime conditions expanding affected areas by approximately fourfold compared to optimal dispersion scenarios. These findings necessitate isotope-specific emergency response strategies: ²⁴¹Am incidents require immediate respiratory protection and compact evacuation zones (0.2–0.4 km), while ¹³⁷Cs incidents demand external radiation monitoring and time-distance-shielding protocols without acute evacuation requirements.

The study’s realistic source terms, derived from documented IAEA incident data, provide credible foundations for emergency response training and preparedness planning. The integration of these findings into operational frameworks, particularly the structured “Recognize → Inform → Initiate → Measure & Map → Evacuate & Monitor” sequence, enables evidence-based response protocols that balance public safety with operational efficiency.

Finally, this study highlights that meteorological and source-term assumptions introduce inherent uncertainty into RDD consequence modeling. While HotSpot provides rapid and conservative estimates, future research should incorporate explicit uncertainty quantification (e.g., Monte Carlo sensitivity analysis) to generate confidence intervals around TED estimates. This will strengthen the utility of such simulations for decision-making under uncertainty. Future research should therefore expand meteorological scenario coverage to include precipitation effects, examine mixed-isotope devices reflecting realistic compositions, and integrate ingestion-pathway calculations into consequence modeling to provide an even more complete basis for emergency planning.