Preliminary Study of the Emergency Planning Zone Evaluation for the Nuclear Power Plant in Taiwan by Using MACCS2 Code

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1 Preliminary Study of the Emergency Planning Zone Evaluation for the Nuclear Power Plant in Taiwan by Using MACCS2 Code Chung-Kung Lo, Ing-Jane Chen, Yu-Hao Huang and Yuan-Ching Chou Institute of Nuclear Energy Research, Lungtan, Taiwan INTRODUCTION The purpose of this preliminary study is to perform the evaluation of the emergency planning zone of nuclear power plants in Taiwan by using MACCS2 (MELCOR Accident Consequence Code System version 2) code. MACCS code series were developed by SNL (Sandia National Laboratory) for replacing the CRAC (Calculations of Reactor Accident Consequences) code series. The function of these codes is to estimate the radiological doses, health effects and economic consequences that could result from postulated accidental release of radioactive materials to the atmosphere. This preliminary study has not only provided an opportunity to establish our capability to use MACCS2, but also helped us know better the deference between MACCS and CRAC code series. The Reactor Safety Study presented the first comprehensive assessment of the consequences and risks to society from PRA evaluated nuclear power plant accidents in As part of the Reactor Safety Study, the CRAC code was developed to calculate the consequences from accidental release of radioactive material to the atmosphere. Since the Reactor Safety Study, consequence modeling has received widespread attention and application throughout the world and a significant number of consequence models have been developed. CRAC2, released in 1982, incorporated major improvements over CRAC in the areas of weather sequence sampling and emergency response modeling. Because CRAC2 was not portable across computer systems and did not offer sufficient flexibility for the evaluation of alternative parameter values for its models, the goal of the MACCS development effort was to produce a portable code with a modular architecture and flexible database. In order to implement a number of other changes that enhance the code s usefulness for all types of reactor and nonreactor facilities, the MACCS2 development effort was initiated at SNL in The purpose of this effort was to develop a generally applicable analysis tool for use in assessing potential accidents at a broad range of reactor and nonreactor nuclear facilities. The version we used to perform the emergency planning zone evaluation is MACCS2 V Since NUREG-0396 report introduced the concept of the emergency planning zone as a basis for the planning of response actions in the event of a severe power reactor accident in 1978, this concept has been accepted all over the world. According to government regulation in Taiwan, the emergency planning zone of nuclear power plant must be designated before operation. The related emergency response planning in the emergency planning zone must be planned to guarantee all necessary resources available under the postulated accidents of nuclear power plant. Thus the pre-planned necessary actions will be helpful to protect inhabitants from the damage during the possible accident. Up to now, there are three nuclear power plants operating in Taiwan, and the fourth one is to be constructed. We evaluated the emergency planning zone of the three operating nuclear power plants by using CRAC2 code in 1992 and designated it with a conservative value, 5.0km. According to government regulation, we have to finish the evaluation of the fourth one s emergency planning zone by 2004, because the plant will be operated at that time if everything goes right by schedule. This preliminary study focuses on performing the evaluation of the emergency planning zone of all the four nuclear power plants by using MACCS2 code. We have established our capability to use MACCS2 code, rechecked the emergency planning zone of the three operating nuclear power plants and confirmed that 5.0km is still a conservative value for all the four nuclear power plants in Taiwan. MODEL DESCRIPTION MACCS2 is used to estimate the radiological doses, health effects, and economic consequences that could be resulted from postulated accidental release of radioactive materials to the atmosphere. The specification of the release characteristics, designated as source term, can consist of up to four Gaussian plumes. The radioactive materials released are modeled as being dispersed in the atmosphere while being transported by the prevailing wind. During transport, whether or not there is precipitation, particulate material can be modeled as being deposited on the ground. If contamination levels exceed a user-specified criterion, mitigative actions can be triggered to limit radiation exposures. MACCS2 is divided into three primary modules: ATMOS, EARLY, and CHRONC. ATMOS calculates the dispersion and deposition of material released to the atmosphere as a function of downwind distance. It utilizes a Gaussian plume model with Pasquill-Gifford dispersion parameters. The phenomena that ATMOS treats are (1) building wake effects, (2) buoyant plume rise, (3) plume dispersion during transport, (4) wet and dry deposition, and (5) radioactive decay and ingrowth. The method of weather sampling is a modified version of the weather bin sampling method used in CRAC2, which sorts weather sequences into categories and assigns 1

2 a probability to each category according to the initial conditions (wind speed and stability class) and the occurrence of rain (intensity and distance). The results of the ATMOS calculations are stored for use by EARLY and CHRONC. EARLY performs all of the calculations pertaining to the emergency phase. The exposure pathways considered during this period are cloudshine, groundshine, and resuspension inhalation. Two kinds of doses are calculated: (1) acute doses used for calculating early fatalities and injuries and (2) lifetime dose commitment used for calculation cancers resulting from the early exposure. In general, the dose equation for an early exposure pathway in MACCS2 in a given spatial element is the product of the following quantities: radionuclide concentration, dose conversion factor, duration of exposure, and shielding factor. Mitigative actions that can be specified for the emergency phase include evacuation, sheltering, and dose-dependent relocation. CHRONC performs all of the calculations pertaining to the intermediate and long-term phases. The exposure pathways considered during this period are groundshine and resuspension inhalation. Food and water ingestion is only considered during long-term phase. CHRONC also calculates the economic costs of the longterm protective actions. The region surrounding the facility is divided into polar-coordinate grid. All of the calculations of MACCS2 are stored on the basis of this spatial grid system centered on the location of the release. The angular divisions used to define the spatial grid are fixed in the code and correspond to the 16 directions of the compass, each being 22.5 degrees wide. Figure 1 provides an example of a MACCS2 spatial grid and the numbering system associated with the 16 compass directions. NNW N NNE NW NE 15 3 WNW 14 4 EN E W 13 5 E 12 6 WSW 11 7 ESE SW SE SSW S Figure 1. A MACCS2 Polar Coordinate Grid with 3 Radial Divisions. The Numbers on the Grid Refer to 16 Compass Directions. This preliminary study only considered the effects during emergency phase. Mitigative actions that can be specified for this phase include evacuation, sheltering, and dose-dependent relocation. For the most conservative concern, no mitigative action was specified in the EARLY module. DATA SOURCE To perform the emergency planning zone evaluation of nuclear power plants, we need the specific data such as source terms, meteorological data, and population distribution data. For comparing the results with the CRAC2 model, the specific data of the three operating nuclear power plants was identical to that used in CRAC2 analysis. We transformed those data files of CRAC2 to fit the MACCS2 data file format. The specific data of the fourth nuclear power plant was also established accordingly. The source term data including inventory, sensible heat content, timing, duration, the fraction of inventory released with each segment etc. was based on a preliminary design result of the facility. Table 1, table 2, and table 3 list the source term data used in the emergency planning zone evaluation. The hourly meteorological data including wind direction, velocity, stability, and rainfall etc. was collected during July 1997 to June 1998 at the planned site. A preprocessor named METRAN was developed for transforming the huge data contained 8760 hourly records to fit MACCS2 meteorological data file format. Figure 2 shows the flowchart of METRAN. The population distribution data including normal and peak value was reinvestigated in April 1999 and recorded on 16 compass sectors every 0.5km. SSE 2

3 Table 1. The Important Parameter Related to Each Release Categories Prob. TL DR TLL FPR RH Accident (1/yr) (sec) (sec) (sec) (watts) (m) Case 0 2.1E-07 9,720 36,000 6, E Case l 3.0E-06 72,000 3,600 69, E Case 2 <1.0E-12 68,400 3,600 65, E Case 3 4.8E ,000 36, , E Case 4 <1.0E-12 72,000 3,600 69, E Case 5 <1.0E-12 68,400 3,600 65, E Case 6 <1.0E-12 68,400 36,000 65, E Case 7 1.2E ,000 69, E Case 8 6.3E-08 7,200 36,000 4, E Case 9 5.1E-08 84,960 36,000 43, E Case E-08 2,880 29, E Case E-09 95,400 36,000 9, E Case E-09 46,800 36,000 7, Case E ,200 36,000 7, Case E ,320 36,000 7, TL = Time between reactor shutdown and release to atmosphere DR = Duration of release TLL = Warning time between notification of public and release FPR = Sensible heat rate RH = Release height Table 2. The Inventory of Each Radionuclide Present in the Facility No. Name Group Inventory (Bq) No. Name Group Inventory (Bq) 1 Co E Te-131m E+17 2 Co E Te E+18 3 Kr E I E+18 4 Kr-85m E I E+18 5 Kr E I E+18 6 Kr E I E+18 7 Rb E I E+18 8 Sr E Xe E+18 9 Sr E Xe E Sr E Cs E Sr E Cs E Y E Cs E Y E Ba E Y E Ba E Y E La E Zr E La E Zr E La E Nb E Ce E Mo E Ce E Tc-99m E Ce E Ru E Pr E Ru E Nd E Ru E Np E Rh E Pu E Sb E Pu E Sb E Pu E Te E Pu E Te-127m E Am E Te E Cm E Te-129m E Cm E+16 3

4 Table 3. The Release Fractions of Radioactive Element Groups Inventory for Each Release Category Group number Accident Xe-Kr I-Br Cs-Rb Te-Sb Sr Co-Mo La-Y Ce-Pu Ba Case E E E Case E E E E E E E E-06 Case E E Case E E Case 4 l 1.6E E Case E E Case E E Case E E Case E E Case E E E E E E E E-03 Case E E E E E E E E-05 Case E E E E E E E-06 Case E E E E E E E E-05 Case E E E E E E E E-04 Declare Dimension Array Read Hourly Meteorological Records Files Wind Speed WSTMP (DAY, HR) Wind Direction WDTMP (DAY, HR) Temp. Difference TEMP (DAY, HR) Rainfall RAINTP (DAY, HR) Degree to Compass Conversion Stability Class Judgement mm to inch unit conversion Wind Speed WS (DAY, HR) Wind Direction WD (DAY, HR) Stability STAB (DAY, HR) Rainfall RAIN (DAY, HR) Output File (MACCS2 Meteorological Input File) Case E E E E E E E E-04 Figure 2. The Flowchart of METRAN (meteorological transformation) Preprocessor Dose conversion factors, which express the relationship between environmental concentrations and resultant human doses or dose rate, are provided to MACCS2 through a data file read by the code. A preprocessor in MACCS2 code package named DOSFAC2 was developed to produce a MACCS2 input file of DCFs for the 60 radionuclides considered important for NPP analyses. DOSFAC2 obtains DCFs for cloudshine and groundshine from a DOE (1988) database, referred to later by its report number, DOE/EH DCFs for exposure resulting from the inhalation or ingestion of radionuclides are generated from a 1987 DCF database provided by Keith Eckerman of Oak Ridge National Laboratory. METHOD To perform consequence calculations, MACCS2 requires all above mentioned data sets. Figure 3 depicts the progress of a MACCS2 consequence calculation for one source term, one weather sequence, and one exposed population distribution. Severe accidents can lead to source terms of quite different magnitudes, and the weather conditions at the time of the release can greatly alter consequence magnitudes. Because consequences vary with source term magnitude, weather, and population density, in order to develop statistical distributions of consequence measures that depict the range and probability of consequence for the reactor being examined, consequence assessments must examine all possible combinations of representative sets of source terms, weather sequences, and exposed populations. MACCS2 estimates the distributions that display the variation of consequences with weather and population density for each representative source term. A postprocess interface was developed to construct an integral depiction by weighted summation of these source-term dependent distributions, with each distribution weighted by the estimated absolute probability of occurrence of its underlying source term. 4

5 ATM OS Dose Factor EARLY & CHRONC Plume Rise Source Terms Weather Data Dispersion and Transport Population Site Data Dosimetry and Mitigative Action Health Effects Outputs Costs ATMOS Data EARLY Data Plume Rise CHRONC Data Figure 3. Progression of a MACCS2 Consequence Calculation MACCS2 is itself only a consequence code system. To perform the emergency planning zone evaluation, we have to compare the consequence with some safety criteria to make our decision. Four proposed guidelines are: 1. The risk to an individual or to the population in the vicinity of a nuclear power plant site of prompt fatalities that might result from reactor accidents should not exceed 0.1% of the sum of prompt fatality risks resulting from other accidents to which members are generally exposed. 2. The risk to an individual or to the population in the area near a nuclear power plant site of cancer fatalities that might result from reactor accidents should not exceed 0.1% of the sum of cancer fatality risks resulting from all other causes. 3. The anticipate whole body dose and thyroid dose beyond the emergency planning zone will not exceed the PAG value in the design base accidents and most of core-melt accidents. 4. There are no prompt fatalities beyond the emergency planning zone even if the most severe accident occur. According to the guidelines above, we collected the prompt fatality and cancer fatality data resulting from other accidents and causes, and then decided the safety criteria beyond the emergency planning zone are: 1. The individual risk is less than 6.50E-07 per year. 2. The societal risk is less than 2.17E-06 per year. 3. The probability of whole body dose exceeding 0.1Sv and thyroid dose exceeding 1.0Sv (PAG value) is less than 3.0E-05 per year. 4. The probability of whole body dose exceeding 2.0Sv (prompt fatality dose) is less than 3.0E-06 per year. By comparing the consequences of individual risk, societal risk, whole body dose, and thyroid dose vs. distance to related safety criteria above, we may then propose a reasonably conservative suggestion of the emergency planning zone. RESULT AND CONCLUSION Using the MACCS2, we estimated the radiological doses and health effects that could result from each postulated accidental release categories of all four nuclear power plants. The probability that a consequence magnitude will be equaled or exceed against consequence magnitude called the complementary cumulative distribution function (CCDF). Using post-process interface, we constructed the consequence results to integral depiction by weighted summation of release categories. Then, we plotted the distribution of individual risk, societal risk, whole body dose, thyroid dose, and the CCDF of thyroid dose 1.0Sv, whole body dose 0.1Sv, and whole body dose 2.0Sv versus distance from the facilities. Figure 4 shows the estimated results of the fourth nuclear power plant. 5

6 1.0E-8 Individual Risk 1.0E-9 1.0E-10 Societal Risk 1.0E-8 1.0E E-12 Distance (km) 1.0E-9 Distance (km) 1.0E-3 1.0E-6 1.0E-4 Thyroid Dose ( Sv ) Whole Body Dose ( Sv ) Thyroid Dose 1.00 Sv Whole Body Dose 0.10 Sv Whole Body Dose 2.00 Sv Dose ( Sv ) 1.0E-5 Probability (1/yr) 1.0E-8 1.0E-6 Distance ( km ) 1.0E-9 Distance ( km ) Figure 4. The Individual Risk, Societal Risk, Dose Distribution, and CCDF Results of the Fourth NPP Although the models, parameters, and DCFs used in CRAC2 and MACCS2 are not identical; there are no significant difference between the consequences they estimated for the three operating nuclear power plants. Comparing the consequence with the safety criteria, we confirmed that 5.0km is still a conservative value of the emergency planning zone for all the four nuclear power plants in Taiwan. We can also conclude that being the advanced design reactor, the fourth nuclear power plant is more safe than the others are. Because the source term data of the fourth nuclear power plant was based on a preliminary design result, this study of its emergency planning zone was thus not a formal emergency planning zone evaluation and not accepted by the regulatory department. The emergency planning zone must be re-estimated by using the official source term data before the plant operating. REFERENCE 1. H.E. Collins, B.K. Grimes and F. Galpin, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants. NRC/EPA Task Force Report, NUREG-0396, EPA 520/ , Washington, USA (1978). 2. L.T. Ritchie, J.D. Johnson and R.M. Blond, Calculations of Reactor Accident Consequences Version 2 CRAC2 Computer Code User s Guide. NUREG/CR-2326, Sandia National Laboratories, Albuquerque, USA (1983). 3. H.N. Jow, J.L. Sprung, J.A. Rollstin, L.T. Ritchie, and D.I. Chanin, MELCOR Accident Consequence Code System (MACCS). NUREG/CR-4691, Sandia National Laboratories, Albuquerque, USA (1990). 4. D.I. Chanin, and Mary L. Young, Code Manual for MACCS2: Volume 1, User s Guide. SAND , Sandia National Laboratories, Albuquerque, USA (1997). 5. Chung-Kung Lo, The Final Report of the Evaluation of the Planning Zone for the Fourth Nuclear Power Plant Project. Institute of Nuclear Energy Research, Taiwan (1999). 6. Hsueh- Li Yin, Chien-Liang Shih, and Hai-Yung Huang, Assessment of Nuclear Power Plant 6

7 Emergency Planning Zone. RS22-J10-04, institute of Nuclear Energy Research, Taiwan (1993). 7