A study on the Methodology for Integrated Safety Assessment for Accidental Analysis on LILW managed in Temporary Storage Facility (TSF) at NPP

In Korea, the low and intermediate level wastes (LILW) generated from the operation of NPPs have been gradually increased since 1978 because of the absence of the repository for the disposal of LILW. And then, the LILW has being stored in the temporary storage facility (TSF) at each reactor sites. Fortunately, in 2005, the Gyeongju city was selected by the resident's vote in four competitive provinces for the LILW disposal facility. The repository for the disposal of LILW will be operated in end of 2009. In opposition to many of researches on the disposal of LILW, however, the risk assessment on the TSF has scarcely been conducted. Furthermore, the details in regards of the safety analysis on this facility have not been considered in the preliminary and final safety analysis report because this report focused on the nuclear reactor system rather than this facility. As a consequence of these situations, the number of the researches on the arbitrary accidents occurring in the TSF has not been enough. The objective of this study is to establish arbitrary accident scenarios and to evaluate the exposure dose in terms of the effective dose and the thyroid equivalent dose due to the arbitrary accidents originating in the TSF for LILW management. For the establishment of arbitrary accident scenarios in the TSF for LILW, the initiating event was derived by master logic diagram (MLD) method based on the fault tree analysis (FTA), and then arbitrary accident scenarios were developed by the event tree analysis (ETA) through the derived initiating events. The main initiating events led to the arbitrary operational accident, which is the dropping of a drum and fire, were derived from MLD method. The exposure effects resulted from release of radioactive materials related to arbitrary accidents in the TSF for LILW can be divided with the internal exposure caused by breathing and external exposure by radioactive plume. The 13 radionuclides for internal and external exposure dose assessment were considered: 3H, 14C, 55Fe, 58Co, 60Co, 59Ni, 63Ni, 90Sr, 94Nb, 99Tc, 129I, 137Cs, and 144Ce. According to the U.S. NRC Regulatory Guide 1.145, the atmospheric dispersion factors (?/Q) for workers and public were derived by meteorological data measured in Kori NPP: wind speed, wind direction, and atmospheric stability. For the evaluation of internal exposure, the breathing rate (m3/sec) described in U.S. NRC Regulatory Guide 1.8 was considered. In conclusion: Based on the MLD method, the dropping of drums and fire were used to evaluate the exposure dose by arbitrary accidents originating in the TSF for LILW management. For the exposure dose assessment, a variety of parameters were considered: the amount of radionuclides released (Bq), the atmospheric dispersion factor for workers and public (sec/m3), the breathing rate for internal exposure (m3/sec), and the dose conversion factor for internal and external exposure (mSv/Bq, mSv/hr per Bq/m3). Furthermore, an exposure dose assessment was conducted in terms of the effective dose and the thyroid equivalent dose for workers and public, considering the release rates of the radionuclides stemming from the dropping of drums and fire. The number of damaged drums was taken as 2 for the dropping of drums and 40 for fire relative to each waste stream. Seven waste streams were considered: the general DAW (200 L), the shielded DAW (200 L), the concentrated waste solidified by cement (200 L), the concentrated waste stabilized by paraffin (200 L), the general spent resin (200 L), the spent resin solidified by cement (200 L), and the general spent filter (200 L). The effective doses resulted from the dropping of drums were in the range of 4.95 E-15 to 2.19 E- 7 mSv for workers and 9.21 E-17 to 1.34 E-9 mSv for public. Also the thyroid equivalent doses originating from the dropping of drums were in the range of 3.59 E-14 to 1.09 E-6 mSv for workers and 6.53 E-16 to 6.61 E-9 mSv for public. On the other hand, the effective doses resulting from fire were in the range of 6.18 E-17 to 1.01 E- 4 mSv for workers and 3.77 E-19 to 6.15 E-7 mSv for public. Furthermore, the thyroid equivalent doses originating from fire were in the range of 4.49 E-16 to 5.45 E-7 mSv for workers and 2.72 E-18 to 3.31 E-9 mSv for public. Consequently, the exposure doses resulting from two main accidents derived in this study did not exceed the regulatory dose limits with respect to arbitrary accidents originating in the TSF for LILW management. (authors)