Research and Application of an Airborne Radioactive Source Term Analysis Program for Nuclear Facility Buildings

Chang Yedi¹, Zhang Liying¹, Fan Yuxuan¹, Tian Yingnan¹, Wang Xiaoxia¹, Gao Guiling¹
¹China Nuclear Power Engineering Co., Ltd., Reactor Engineering Department, Haidian District, Beijing 100044, China

Abstract

In nuclear facility buildings, leaked radioactive nuclides exist in the form of gases or aerosols, forming airborne radioactive source terms that cause internal exposure to workers and represent one of the primary sources of occupational radiation dose. To analyze the distribution of airborne source terms and personnel exposure within nuclear facility buildings, we independently developed the "Airborne Radioactive Source Term Analysis Program for Nuclear Facility Buildings." This paper introduces the main calculation models employed by the program, including generation models, removal models, and airborne source term calculation models. Using three compartments (containment, annular space, and environment) as examples, we compared the numerical solutions from the program with analytical solutions to verify the accuracy of the numerical algorithm. The program can be used to evaluate and analyze source term distribution characteristics within buildings, providing a research basis for analyzing worker exposure doses. Finally, based on various influencing factors in the calculation program, measures to reduce airborne radioactive levels in nuclear facility buildings are discussed.

Keywords: Airborne radioactivity; Source term calculation model; Radiation safety

1. Research Background

Airborne radioactive source terms in nuclear facility buildings serve as a critical basis for calculating radiation doses to workers. During normal operation of nuclear facilities, radioactive material leakage from equipment (such as drips, seeps, and spills) and evaporation from open pools containing radioactive liquids cause airborne radioactive substances to permeate nuclear island buildings. The portion retained within these buildings leads to internal exposure through inhalation when workers perform operations. In accident scenarios such as a Loss-of-Coolant Accident (LOCA), radioactive nuclides from the reactor core are released in large quantities into the containment and may subsequently be released to other buildings and the environment through multiple pathways. For radiation protection and safety considerations, it is essential to analyze and optimize airborne radioactive source term levels in nuclear island buildings.

Traditional analysis methods often employed a "large room" model, which assumes uniform distribution of airborne radioactivity within a building. These methods primarily considered generation terms such as leakage and pool evaporation, and removal terms such as decay and ventilation, but the treatment of these terms was not comprehensive. Additionally, constrained by equation solving methods, these approaches could not account for migration processes involving more than three compartments. In reality, airborne radioactive concentrations vary significantly between different compartments within a building, and existing calculation methods cannot meet the demands for refined analysis. Therefore, it is necessary to develop versatile airborne radioactive source term analysis software based on refined models that can be applied throughout the lifecycle of various nuclear facilities, incorporating generation, migration, and deposition pathways of airborne radioactive source terms and integrating operational experience data from existing nuclear facilities.

2. Software Model

Based on research into the generation, migration, and deposition pathways of airborne radioactive source terms in nuclear facilities, we developed versatile analysis software using refined models applicable to all lifecycle stages of various nuclear facilities. The software comprehensively considers generation and removal models for airborne radioactive materials in each compartment while also accounting for migration processes between different compartments, as illustrated in [FIGURE:1].

[FIGURE:1]

Fig.1 Main generation and removal terms considered in the multi-compartment model

2.1 Calculation Program Assumptions

To ensure broad applicability of the calculation program and its models, the following assumptions were adopted for the airborne radioactive source term analysis program:

1. Radioactive nuclides are assumed to be uniformly distributed within a compartment.
2. Once released into a compartment, radioactive nuclides are assumed to diffuse sufficiently to achieve uniform distribution.
3. During specified generation or release phases, the generation rate of each generation model remains constant.
4. During specified removal phases, the removal rate of each removal model remains constant.

2.2 Generation Models

Based on operational and potential accident conditions in nuclear facilities, the program comprehensively considers generation terms for airborne radioactivity within compartments, including: instantaneous release models for radioactive liquid leakage, uniform release models for radioactive liquid leakage, pool evaporation generation models, resuspension models, instantaneous release models for radioactive gases and aerosols, and uniform release models for radioactive gases and aerosols. Migration generation terms between compartments include ventilation or leakage generation models and ventilation filtration generation models.

2.3 Removal Models

Removal models are categorized into intra-compartment removal terms and inter-compartment migration removal terms. Intra-compartment removal terms include decay removal models, deposition removal models, filtration removal models, and spray removal models (the latter is not used for reprocessing plant airborne radioactive source term calculations and is applied only to other nuclear facility airborne radioactive source term calculations). Inter-compartment migration removal terms include ventilation or leakage removal and ventilation filtration removal.

2.4 Airborne Source Term Calculation Equation

Based on the generation and removal models, the time-dependent relationship for radioactive nuclide i in compartment k during time stage j is expressed by Equation (1).

The equation employs the Gear numerical method for solution, and the airborne radioactive source term analysis program for buildings was developed accordingly.

3.1 Model Validation Analysis

To verify the accuracy of the analysis models and algorithms, three compartments that permit analytical solutions were selected as examples. The three compartments represent containment, annular space, and environment, with their connections shown in [FIGURE:2].

[FIGURE:2]

Fig.2 Schematic diagram of the three-compartment connection

The program was used to calculate airborne radioactive source term concentrations in the three compartments from 0 to 2 hours. The results demonstrate good agreement between the numerical solutions from the program and the analytical solutions, with deviations below 0.05%, thereby verifying the accuracy of the program models and algorithms.

[TABLE:1]

Table 1 Comparison of airborne radioactive source terms in three compartments from 0 to 2 hours

3.2 Complex Building Model Application

A five-building model was designed with flow between all buildings, as shown in [FIGURE:3]. The airborne radioactivity in Building 1 originates from pool evaporation, while radioactivity in the other buildings comes from ventilation leakage between buildings.

[FIGURE:3]

Fig.3 Schematic diagram of the five-building model connection

The airborne radioactive source term analysis program was used to calculate the airborne radioactivity concentrations in each building at different times. [FIGURE:4] and [FIGURE:5] show the temporal variation of airborne source terms in Building 1 and Building 5, respectively. Since Building 1 receives radioactive material from pool evaporation, its airborne radioactivity concentration is highest at time zero. As gas flows in from other buildings, the decay rate of airborne radioactivity in Building 1 decreases and approaches equilibrium, with this behavior being most pronounced for iodine and alkali metal nuclides.

Observing the variation patterns of airborne radioactivity in other buildings using Building 5 as an example, the concentrations of iodine, alkali metals, and inert gases first increase and then decrease, reaching peak values at 3-4 hours. These calculation results can provide recommendations for determining when workers should enter buildings and what protective measures should be implemented.

[FIGURE:4]

Fig.4 Airborne radioactivity concentration of inert gases, iodine, and alkali metals in Building 1

[FIGURE:5]

Fig.5 Airborne radioactivity concentration of inert gases, iodine, and alkali metals in Building 5

Conclusion

This study independently developed an airborne radioactive source term analysis program for nuclear facility buildings, detailing the main calculation models involved, including generation models, removal models, and airborne source term calculation models. The program can be used to calculate airborne source term distributions within nuclear facility buildings. Example cases were selected to compare numerical solutions with analytical solutions, verifying the accuracy of the program's numerical algorithms. The program can be applied to evaluate and analyze source term distribution characteristics within buildings, providing a reference basis for analyzing radiation doses to personnel in nuclear facility buildings. Additionally, it provides analytical support for normal operation, decontamination and maintenance, accident prevention, accident management, and environmental protection of nuclear facilities. Combined with actual radiation monitoring values, the program ensures that workplace dose levels meet design requirements, enables timely identification of radiation safety issues and potential hazards, and supports the ALARA (As Low As Reasonably Achievable) principle by providing data for worker dose calculations to ensure personnel safety.

References

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[2] Zhang Puzhong, Li Pengfei, Feng Jia, et al. Airborne radioactive concentration calculation and monitoring threshold value analysis of HPR1000 nuclear island building[J]. Radiation Protection, September 2023, Vol. 43 No. 5: 510-514.

[3] Fan Yuxuan. Research on Post-Accident Radioactive Source Term Calculation for Pressurized Water Reactor Nuclear Power Plants[M]. Beijing: North China Electric Power University, 2020.