국내 PWR 원전 C-14 배출량 계산결과 비교평가 연구

Abstract

The radionuclide carbon-14(14C) has been produced continuously in the upper atmosphere by cosmic ray interaction with nitrogen at an estimated rate of 1.5 x 1015 Bq/yr. The total global inventory of 14C is estimated to be 1.28 x 107 TBq. 14C is integrated into atmosphere carbon dioxide and mixed throughout the atmosphere and hydrosphere. Because carbon dioxide(CO2) is absorbed by plants through photosynthesis, 14C eventually becomes incorporated into all living organisms, including humans. Although the production rate of 14C is quite small compared to naturally producing rate, today the major 14C producer is the nuclear power industry, comprising about 440 nuclear power reactors worldwide and a few fuel reprocessing plants. The total gaseous 14C release from all nuclear power plants is estimated 1.1 x 1013 Bq/yr(UNSCEAR 2000). 14C in the nuclear fuel cycle is produced by neutron interaction with 13C, 14N, 15N, 16O and 17O, which may be in the nuclear fuels, moderator and primary coolant system of nuclear reactors. According to the RETS(Radiological Effluents Technical Specification) there is no specific requirement to report 14C in ether gaseous or liquid effluents in the U.S. Since 14C is a pure beta emitter, it is not identified in a mixture of radionuclides, and in general it has not been given significant consideration since the nuclear power plant release of 14C has minimal impact on the world inventory of 14C. In addition, its impact on local vegetation prior to its dispersion has been given little attention. Improvements in nuclear power plant effluent management practices have resulted in a decrease in the concentration of gaseous radionuclides released to the environment. As a result, 14C may become a principal nuclide for the gaseous effluent pathway. Revision 2 of Regulatory Guide 1.21 issued in june 2009 introduced the 'Risk-informed' principals of the Reactor Oversight Process and included a discussion of 14C as a potential principal radionuclide for effluent monitoring. It stated that radioactive effluents from commercial nuclear power plants have decreased to a point that 14C is likely to be a principal nuclide in gaseous effluents. It requested that licensee evaluate whether 14C is a principal radionuclide for gaseous release from their facility. In Korea, MOST(Ministry of Science and Technology) Notice was announced in January 2001, requested nuclear power operator evaluate the amount of 14C released to the environment and its impact on general public. The public dose was limited to 0.15 mSv a year by the Notice. The regulatory body asked KHNP(Korea Hydro and Nuclear Power Corporation, a licensee for nuclear power plant operation) to monitor and evaluate 14C release from Hanbit units 5, 6 as a Operation License condition in June 2001. KHNP conducted a study for 14C measurement and its impacts on public dose from January 2003 to December 2008, and based on this study it established a implementing program for monitoring 14C in PWR plants. Considering for the preparation such as procurement of sampling devices, measurement procedure and training personnel, theoretical estimation (calculation, Indirect method) was carried in from January 2012 to March 2013. Since then the quantity of 14C releases from the PWR plants and public doses were evaluated by actual measurements(Direct method) data. This thesis presents the results of measurements carried out on some Korean PWR plants. Comparison study among the expected release described in FSAR(Final Safety Analysis Report), estimation using EPRI methodology and actual measurements was conducted. It is confirmed that actual measurements(Direct method) data showed far below the expected release in FSAR(about 10.4 % ∼ 32.4% ) and 17.7% ∼ 103.5% compared to estimations. Off-site dose calculation results using actual 14C release data for Hanbit units 5 & 6 in 2013 was 2.757 x 10-3 mSv, which was about 1.84% of design limit(0.15mSv). But the public dose contribution by gaseous 14C release was extremely high, 94.4%, compared to total dose of 2.921 x 10-3mSv. For Hanbit site, operating 6 PWRs, total public dose by gaseous effluents including 14C was 5.824 x 10-3mSv. Because the dose by 14C was 5.139 x 10-3mSv, it is assumed that the dose contribution was around 88.24%. It is confirmed that regardless plant's design characteristics, the expected annual 14C releases in gaseous pathway described in some Korean PWR FSAR are the same. It is assumed that the value of 7.3 Ci/yr which described in the FSAR was taken from revision 2 of NUREG-0017 in 1985.