W. XUAN, L. LIAO, D. SUN
Shanghai Nuclear Engineering Research & Design Institute Shanghai, China
Email: wangxuan@snerdi.com.cn Abstract
In China, due to the requirement of "Criteria for emergency planning and preparedness for nuclear power plants: Part1, The dividing of emergency planning zone." (GB/T 17680.1-2008), for PWR nuclear power plant, its external plume EPZ should be within 7km-10km, and its internal plume EPZ should be within 3km~5km. However, the scope of the standard for the emergency planning area is currently limited to conventional nuclear power plants, and for the current SMR, its emergency planning size is not included.
The paper presents emergency planning zone (EPZ) sizing method for Small Modular Reactor (SMR), as well as NNSA requirement, calculation process, the probability of accident truncation and the choice of meteorological data, and gives suggestions on EPZ determination for CAP200 SMR.
The paper also gives a case study, and Shidaowan nuclear power plant is chosen as the study site. According to the CAP200 source term and meteorological data of the site, use MACCS2 computer program to calculate the severe accidents consequence. Conclusion show that project dose exceeding probability is less than 30% at the distance of 500m, which directs that for CAP200 SMR, its plume emergence is limited to the on-site area, and off-site emergency response can be simplified.
1. INTRODUCTION
Nuclear power plant off-site emergency plan is the last step of the nuclear safety principle, which is very important to protect the safety of the public and protect the environment. The emergency planning zone is an important technical basis and main issue for making emergency plan. According to requirement of China standard
"Criteria for emergency planning and preparedness for nuclear power plants-Part1: The dividing of emergency planning zone." (GB/T 17680.1-2008) [1], after taking into account for the reactor power, internal plume emergency planning zone should be within 3km~5km, and external plume emergency planning zone should be within 7km~10km. However, this standard is limited to large pressurized water reactor nuclear power plant, but for the current small reactor, it does not give instructions.
Compared to large reactors, SMR can achieve higher safety, shorter construction cycle time, better economy and application flexibility than the traditional pressurized water reactors, and SMR can also be applied to different requirements and conditions, it is a new nuclear energy system. SMR can meet the needs of small and medium-sized grid power supply, urban heating, industrial process heating and desalination and other special areas [2].
In the paper, case study of CAP200 is used to illustrate the calculation process of SMR plume emergency planning zone, and give recommendations for its size.
2. GENERAL METHODS AND CRITERIA FOR THE DETERMINATION OF PLUME EMERGENCY PLANNING ZONE
2.1. Concept of emergency planning zone
The emergency planning zone refers to the area that has been set up around the nuclear power plant in advance to formulate contingency plans and make emergency preparedness to protect the public in a timely and effective manner in the event of an accident at the nuclear power plant. The emergency planning area generally includes the plume emergency planning zone and the ingestion emergency planning zone. China's plume emergency planning zone is divided into internal and external areas, in the internal area, evacuation and other emergency protection measures should be prepared. Since the ingestion and drinking water control is not a "short term emergency" protection strategy, and the determination method of ingestion emergency planning zone is
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2.2. Method of determining emergency planning zone
Whether the large PWR or SMR, the method to determine the emergency planning zone is usually divided into three steps: firstly, choose suitable accident type and source term; secondly, estimate individual project dose and averted dose during the early plume exposure; thirdly, compare the estimated dose level and the contamination level with the generic optimization intervention level, then determine the size of EPZ.
In addition to the requirements of "Criteria for emergency planning and preparedness for nuclear power plants-Part1: The dividing of emergency planning zone." (GB/T 17680.1-2008), the (NUREG-0396) report [3]
issued by the National Nuclear Regulatory Commission (NRC) and the white paper for the establishment of theSmall Modular Reactor Emergency Response Plan zone issued by NEI [4] are also considered. NUREG-0396 noted that at the recommended emergency planning zoneboundary, the probability of dose exceed the corresponding intervention level should be less than 30% during the entire core melt accident sequence. The NEI White Paper states that the cutoff probability of a severe accident can be 1E-07 / year in the case of a SMR emergency planning zone determination.
"Principles for the Safety Review of SMR(Trial) in China" [5] states: Under the condition of not taking off-site interventions, the public should be provided with higher off-site intervention levels than large PWR nuclear power plants. For an important sequence of the beyond design basis accident, the effective dose for individuals (adults) at the site boundary should be less than 10 mSv throughout the entireaccident.
3. CAP200 PLUME EPZ DETERMINATION 3.1. Site overview
In the paper, Shandong Shidaowan nuclear power plant site is chosen as ancase study site, to illustrate the calculation process of CAP200 SMR plume EPZ. Shidaowan nuclear power plant site is located in Weihai City, Shandong Province. The site is about 120km away from Yantai City in the northwest, about 68km away from Weihai City in the northwest, about 23km away from Rongcheng City in the northwest and about 105km away from Shandong Haiyang Nuclear Power Plant in the southwest. The site is near the Yellow Sea, most of the natural ground elevation is between 0m~30m.
The site's dominant wind direction is SSW, calm wind frequency is 3.9% and the average wind speed is 3.63m/s.
3.2. Choose of accident type
When calculating the CAP200 plume EPZ, its input conditionsinclude the total core inventory, release share, accident probability, meteorological condition, distance segment, etc. The core inventory of CAP 200 is calculated using the "total amount of radioactivity at the end of the compact reactor cycle", which has the similarsevere accident prevention and mitigation strategy as the CAP1000 reactor type and a similar containment leak rate. Its severe accident sequence is also consistent. The severe accident source term of CAP200 reactor contains six release classes. The following table gives a description of the six accidents types and their release frequency.
TABLE 1. CAP200 SEVERE ACCIDENT RELEASE CATEGORIES
Release type Name Description Frequency
（/reactor.yr）
IC Complete
containment
The containment remains intact, and conventional leaks cause the release of radionuclides into the environment.
1.78E-07
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Release type Name Description Frequency
（/reactor.yr）
BP Containment bypass Containment failure occurs before the core damage, fission products from the reactor coolant system through the secondary circuit or other connecting system into the
environment.
6.53E-09
CI Containment
isolation
Containment failure occurs before the core damage, because the failure of closure of the containment or the valve, leading to fission product release.
3.19E-10
CFE Early failure of containment
The release of fission products into the failing containment is caused by a dynamic severe accident after the core has melted (before core collapse).
9.05E-09
CFI Medium term
containment failure
The release of fission products to the failure containment was caused by a dynamic severe accident after the core was melted (after the collapse of the core, 24 h before).
2.08E-10
CFL Late containment failure
The release of fission products to the failure containment was caused by a severe accident after 24 hours.
5.96E-11
It can be seen from the above table that only the release frequency of the IC accident sequence in the six release categories exceeds 1E-07 persite year. According to the NEI white paper, the cutoff probability of severe accident for the SMR is 1E-07 persite year, thus, for CAP200, only the IC accidents is considered.
3.3. Methods and parameters
When calculating CAP200 plume EPZ, MACCS2 computer program is used. The MACCS2 program was developed by the US Sandia National Laboratories for the US Nuclear Regulatory Commission (NRC), its purpose is to calculate the off-site consequences of severe accidents.
MACCS2 program is an important component of the three-stage PSA study in nuclear power plants.
MACCS2 computer model consists of three basic modules which are ATMOS, EARLY and CHRONC. The ATMOS module mainly calculates the diffusion and transport of plume. The EARLY module mainly calculates the early dose, acute health effects and gives early emergency response actions. The CHRONC module mainly calculates long-term dose, chronic health effects, medium and long-term emergency response actions, as well as the economic costs. This paper mainly uses ATMOS and EARLY two modules. The main calculation parameters are as follows:
1) Weather sequence
Based on the MACCS2 program's data entry requirements for atmospheric diffusion and transport, meteorological data is based on yearly data from the site tower observations, including hourly data such as wind direction, wind speed, stability, and precipitation.
The Monte-Carlo sampling method is used to classifythe meteorological data. The weather classification is divided into 32 classes, and four representative weather series are extracted from each category. Therefore, the total number of representative weather series are 128.
2) Atmospheric diffusion parameters and mixing layer height
Atmospheric diffusion parameters of the site are in Table 2, mixed layer height is in Table 3.
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Stability Horizontal diffusion parameter
（σy= axb）
Vertical diffusion parameter
（σz= cxd）
a b c d
A 0.300 0.933 0.143 0.972
B 0.247 0.931 0.176 0.871
C 0.218 0.919 0.197 0.784
D 0.169 0.906 0.209 0.725
E 0.115 0.909 0.140 0.723
F 0.093 0.896 0.119 0.678
TABLE 3. MIXED LAYER HEIGHT
Stability A B C D
Mixing layer. height
（m） 900 900 350 200 3.4. Results
The figure below shows the results of IC accident.
FIG. 1. Project dose exceeding probability.
It is calculated that the conditional probability of effective dose exceeding 10mSv is 1.26% at 300m, 0.54%
at 500m, and only 0.01% at 800m. The plant boundary of CAP200is 500m. Thus, according to the calculation results, the probability of the effective dose higher than 10mSv at 500m from the center of CAP200 reactor is far less than 30%. This indicates that for CAP200 SMR, its plume EPZis limited to the site area and the off-site emergency response can be simplified accordingly.
4. CONCLUSION
This paper chooses Shidaowan nuclear power plant as a case study site, and CAP200 plume EPZ is calculated. The core inventory of CAP 200 is calculated using the "total amount of radioactivity at the end of the
0.00%
1.00%
2.00%
3.00%
4.00%
5.00%
6.00%
7.00%
0 0.2 0.4 0.6 0.8 1
Beyond Probability
Distance(km) 0.01Sv
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compact reactor cycle", and for CAP200 reactor type, only IC severe accident should be considered during the determination of plume EPZ.
The plant boundary of CAP200is 500m, according to the calculation results, the probability of the effective dose higher than 10mSv at 500m from the center of CAP200 reactor is far less than 30%. This indicates that for CAP200 SMR, its plume EPZis limited to the site area and the off-site emergency response can be simplified accordingly.
Because the meteorological conditions at different sites have certain influence on the calculation of accident consequences, results of this paper only reflect the specific characteristics of the coastal site, and other sites still need to be analyzed according to the site characteristics.
REFERENCES
[1] National Standard of the People’s Republic of China, Criteria for emergency planning and preparedness for nuclear power plants, Part 1: The dividing of emergency planning zone, GB/T 17680.1-2008, China (2008).
[2] DONG, F., WANG, W., LUO F., WU, Q., Study on SMR EPZ determination. Research and Discussion, 1 (2014) 7.
[3] US NUCLEAR REGULATORY COMMISSION, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants, NUREG-0396, Washington, DC, USA (1978).
[4] Establishment of the Small Modular Reactor Emergency Response Planning.
[5] Principles for the Safety Review of SMR (Trial) in China.
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