Examination of Atmospheric Dispersion Model s Performance - Comparison with the Monitoring Data under the Normal Operation of the Tokai Reprocessing Plant - M. Takeyasu 1, M. Nakano 1, N. Miyagawa 1, M. Takeishi 1 1 Tokai Works, Japan Nuclear Cycle Development Institute, 4-33, Muramatsu, Tokai-Mura, Naka-Gun, Ibaraki, 319-1194, Japan E-mail: take@tokai.jnc.go.jp Abstract. The computer system named SIERRA-II (Simulation Code (II) for Emergency Dose by Released Radioactive Substances) is that for nuclear operators to calculate a maximum environmental dose when nuclear accident happens. This system is based on the computer codes named EXPRESS (Exact Preparedness Supporting System) which solves numerically the advection and diffusion processes of particles which represented released radioactivity. One of the main modification points of SIERRA-II from EXPRESS is that the vertical diffusion coefficient is derived from Pasquill-Gifford chart in accordance with downwind distance. In this report, the examination of SIERRA-II was performed by using the monitoring data of ambient -ray dose rate around the Tokai Reprocessing Plant under its normal operation. The observed fluctuation of dose rate was simulated well. From the results of statistical measures of the performance examination, the calculation by SIERRA-II was somewhat under-estimation, and agreements within a factor of 2 and 5 were 30% and 51%, respectively. 1. Introduction After the JCO criticality accident happened in Japan in September 30 in 1999, the detail information of the accident of nuclear facility became more important than before which nuclear operators have to report to the central and local governments for deciding an effective emergency response plan[1]. As one of the information, there are predicted environmental doses including a maximum dose. For predicting a maximum environmental dose, it is important to obtain the spatially detailed distribution of dose around the facility site boundary, because a maximum dose point due to -cloud normally appears less than a few kilometers of the facility. In and around the site boundary, ambient -ray dose rate is measured at monitoring-stations(st) or monitoring-posts(p)[2]. Because of the limitation of the number of ST and P, monitoring data should be interpolated and accumulated for obtaining a maximum dose. Furthermore, a prediction of environmental dose may
be needed, although the effluent monitoring data is not yet obtained before the release from the dtack. In Tokai Works of Japan Nuclear Cycle Development Institute (JNC Tokai Works), The computer system named SMAP(Simulation and Mapping System for Emergency Environmental Effects) was developed in 1997 for calculating environmental dose in an emergency[3]. By SMAP, we can calculate air concentration of released radioactivity, based on a puff model. This system has been used for calculating the atmosphere dispersion of radioactive noble gas released when JCO criticality accident happened in 1999, and was useful for explaining the temporal variation of ambient -ray dose rate observed at ST and P of JNC Tokai Works. Besides SMAP, the new calculation system named SIERRA-II (Simulation code (II) for Emergency dose by Released Radioactive substances) was developed which solves numerically the advection and diffusion processes of particles which represented radioactivity released to the atmosphere[4,5]. By this system, we can calculate dispersion of radioactivity on the complicated topography more in detail than SMAP. We have established SIERRA-II for the facilities of JNC. In this report, the outline of SIERRA-II for JNC Tokai works was described, then the examination of SIERRA-II was performed by using the monitoring data of ambient -ray dose rate around the Tokai Reprocessing Plant(TRP) under its normal operation. 2. Outline of SIERRA-II 2.1. Functions In SIERRA-II, the maximum environmental dose is calculated when an accident happens on the facility in JNC Tokai Works. For this function, SIERRA-II acquires on-line meteorological and effluent monitoring data, then calculates and outputs a real-time environmental dose distribution with simple and rapid computer operation. Main functions of SIERRA-II are as follows: (1) Objective facilities are the nuclear fuel cycle facilities inside JNC Tokai Works. For the main facilities such as TRP, their stack data such as its hight were set beforehand. (2) The objective area is that of 40km in horizontal direction and 400m in vertical direction around JNC Tokai Works, the Oarai Engineering Center (JNC OEC) located about 20 km south of the Tokai works is included. The grid resolution is 1.25km in horizontal direction and 20m in vertical direction. For the area of 4km in horizontal direction around Tokai Works where a maximum dose point may appear, the grid resolution in horizontal direction is 50m by the
nesting function[6], in order to calculating a spatially detailed dose distribution. (3) The objective period is from a past arbitrary time to the time of 48 h from the present. Time resolution is 10 min in the past and 1 h in the future. (4) Input data are on-line local meteorological observation data based on the guideline of Japan Nuclear Safety Commission[7] and on-line effluent monitor data, both of which are processed every 10 min by the telemetering systems in JNC Tokai Works and JNC OEC. For predicting dose, it can be used a local meteorological data of every 1 h which is predicted by the atmospheric dynamic model of Japan Weather Association[8]. (5) Output data are vector plots of wind field, contour maps and external and internal accumulated doses. The objectives of contour map are air concentration, external and internal dose rates and so on. (6) There are three calculation modes, that is, real-time calculation every 10 min, re-calculation every 10 min for a past arbitrary period by using on-line observed data, and prediction calculation every 1 h for up to 48 h from the present by using the predicted data. (7) We operate SIERRA-II on a Windows PC. Introduction of GUI(graphical user interface) makes it simple and easy that operations of the system such as the calculation condition setting, the calculation execution and the output condition setting. The data such as topographic data is set beforehand as a database. 2.2. Models for wind field, atmospheric dispersion and dose calculation The models of SIERRA-II for wind field, atmospheric dispersion and dose calculation were based on the computer codes, EXPRESS (Exact Preparedness Supporting System)[9]. In EXPRESS, it is calculated that 3-dimensional mass-consistent wind field and the atmospheric dispersion of radioactivity based on a random-walk method. In SIERRA-II, the same calculation procedure is performed with on-line local meteorological observation data, local meteorological prediction data, on-line effluent monitor data and topographic data, as shown in Fig.1. The main modification points of SIERRA-II from EXPRESS are the followings: (1) In EXPRESS, the horizontal and vertical diffusion coefficients of particles are derived from Pasquill-Gifford chart[10]. The longer the downwind distance is, the larger the vertical diffusion coefficient is. The vertical diffusion coefficient saturates at about 2 km downwind in the neutral and stable meteorological conditions. In EXPRESS, the saturated values are employed in spite of downwind distance, because this employment makes it possible that three thermally stratified vertical layers can be considered. But the input data in SIERRA-II is only for one thermally stratified vertical layer above the ground. And the saturated values employed
On-line local meteorological observation data Local meteorological prediction data Calculation of three-dimensional local meteorological field with the nested grid system Topographic data Three-dimensional local meteorological data in the nested grid system On-line effluent monitor data Calculation of atmospheric dispersion of radionuclides in the nested grid system Air concentration, radiological doses FIG.1. Calculation procedure of SIERRA-II. in EXPRESS are too large for SIERRA-II, because the main objective domain of SIERRA-II is around the site boundary which is several hundred meters of a facility. In SIERRA-II, the coefficient was derived from Pasquill-Gifford chart in accordance with downwind distance. A particle at the position of (x t, y t, z t ) at the time t moves to the position of (x t+ t, y t+ t, z t+ t ) when the time t passes, according to the following equations: [ 0.5,0.5] [ 0.5,0.5] 1/ 2 xt + t = xt + u t + (24K t) (1) 1/ 2 yt + t = yt + v t + (24K t) (2) 1/ 2 Zt+ t = Zt + w t ± ( 2K t) (3) where (u, v, w) wind velocity at the particle position (m/s); K diffusion coefficient of the particle (m 2 /s); [-0.5, 0.5] uniform random number from -0.5 to 0.5. The value of K is obtained from the following equation: 2 1 dσ K = (4) 2 dt
where standard deviation of the plume distribution. The value of is derived from Pasquill-Gifford chart. (2) To calculate with high resolution in space around the facility and to shorten calculation time, the nesting function was employed. By this function, the site boundary lies in the small-scale domain with the finer grid which is nested in the larger domain with larger grid. 3. Performance examination of SIERRA-II 3.1. Measurement of ambient -ray dose rate around the TRP Around the TRP, there are two monitoring stations(st-1 and ST-2) located at the distance of about 0.45 km, and eight monitoring posts(from P-1 to P-8) located at the distance of 0.3 to 1 km of the stack of the TRP (about 100m high above the sea level), shown in Fig.2. At ST and P, ambient -ray dose rate are continuously measured by 2 2 energy compensated NaI(Tl) scintillation detector (BG level is from 30 to 40 ngy/h). In 2003, the TRP was operated from September 17 to December 3. Figure 3 shows an example of temporal increase of dose rate, which was observed in October 2 in 2003. Figure 3 also shows the discharge rate of 85 Kr and wind direction. In Fig.3, 85 Kr was discharged at the rate of more than 1 P-1 P-2 ST-1 P-3 P-5 stack P-7 ST-2 P-4 P-8 P-6 FIG.2. Locations of monitoring stations and monitoring posts inside the Tokai works (ST : monitoring station, P : monitoring post).
80 70 W NW NNW NW 5.E+04 4.E+04 Dose rate (ngy/h) 60 50 40 30 P-8 P-4 3.E+04 2.E+04 Discharge rate of 85 Kr(GBq/h) 1.E+04 20 10 0.E+00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 hour 2003/10/2 FIG.3. An example of temporal increase of dose rate. 10 4 GBq/h from 15:30 to 16:30 when wind blew from northwest to north-northwest, and the increases were observed at P-8 and P-4 which were located on the downwind direction of the stack. 3.2. Comparison between calculation and observation The dose rate calculated by SIERRA-II was compared with the observed one. An example of these results is shown in Fig.4, where the observed dose rate is an additional dose rate due to the discharge of 85 Kr. In Fig.4, relatively large increase of dose rate was observed at ST-1 from 10:00 to 11:00 in November 5 in 2003. The observed fluctuation of dose rate was simulated well by SIERRA-II with a factor of 2. In Fig.4, the calculation result was also shown when the saturated value of the vertical diffusion coefficient which was derived from the Pasquill-Gifford chart was employed in spite of downwind distance, as the same manner as EXPRESS. The result was smaller Dose rate (ngy/h) 8 7 6 5 4 3 2 1 Observation Calculation(a) Calculation(b) 0 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 2003/11/5 FIG.4. Comparison between calculated and observed dose rates; calculation(a) is the result of SIERRA-II, calculation(b) is that at the case of using the vertical diffusion coefficient derived by the same manner as EXPRESS. hour
100 R=5 R=2 R=1/2 R=1/5 Calculated dose rate (ngy/h) 10 1 0.1 0.1 1 10 100 Observed dose rate (ngy/h) FIG.5. Scatter plot of calculated and observed dose rate. than that of SIERRA-II, because the atmospheric dispersion was larger than that in SIERRA-II due to the difference of the vertical diffusion coefficient. Figure 5 shows the scatter plot of calculated and observed dose rates. In Fig.5, the point indicated the calculated dose rate when the increase of dose rate was observed meaningfully, or the observed dose rate when the increase of dose rate more than 0.1 ngy/h was calculated. The observed to calculated ratios are shown by lines indicated letter R. The points on each axis indicate the calculated or the observed values less than 0.1 ngy/h. We examined the performance of SIERRA-II by using statistical measures. As statistical measures, we employed mean fraction bias, FA2 and FA5[11]. The value of mean fraction bias ranges from -2.0 (extreme over-estimation) and +2.0 (extreme under-estimation). FA2 and FA5 mean the factor of agreement of 2 and 5, respectively. Mean fraction bias is defined by the following equation: Mean fraction C o C C bias = (5) 0.5( C o + C C ) where C mean value of observed dose rate (ngy/h); o C mean value of calculated dose rate (ngy/h). c
Table 1 shows the results of statistical measures of the performance examination. From Table 1, the calculation by SIERRA-II was somewhat under-estimation, and the agreements within factor of 2 and 5 occurred in 30% and 51% of all observation data, respectively. Table I. Results of the performance examination of SIERRA-II. Mean fraction bias FA2 FA5 0.15 0.30 0.51 4. Conclusion The computer system named SIERRA-II was developed for nuclear operators to calculate a maximum environmental dose when nuclear accident happens. The examination of SIERRA-II was performed by using the monitoring data of ambient -ray dose rate around the Tokai Reprocessing Plant under its normal operation. The observed fluctuation of dose rate was simulated well. From the results of statistical measures of the performance examination, the calculation by SIERRA-II was somewhat under-estimation, and the agreements within factor of 2 and 5 occurred in 30% and 51% of all observation data, respectively. 5. Acknowledgment We thank Dr. Takao Iida of Nagoya University for his very useful comments, and Mr. Asao Yamamoto and Mr. Yoichi Yatake of Hitachi Engineering Co. Ltd. for developing SIERRA-II. REFERENCES 1. Japan nuclear safety commission, Nuclear emergency preparedness in the vicinity of nuclear facilities, (in Japanese), partly revised on November 2002, (1980). 2. Japan nuclear safety commission, Guideline of environmental radiation monitoring, (in Japanese), partly revised on March 2001, (1989). 3. Takeyasu, M., Shimizu, T., Suto, T., Katagiri, H., Evaluation of Time Variation of Radionuclides Released in JCO Criticality Accident, 2000 Annual Meeting of the At. Energy Soc. Jpn., (in Japanese), A16, Matsuyama, Japan, March, (2000). 4. Takeyasu, M., Takeishi, M., Real-time Simulation of Environmental Dose in the Normal Operation of Tokai Reprocessing Plant by Dose Evaluation Computer Code (SIERRA-II), AOCRP-1, OP 6C-3, Seoul, Korea, Oct., (2002). 5. Miyauchi, K., et al., Development of atmospheric dispersion prediction system for emergency
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