The amount of radioactive materials released from nuclear power plant must be evaluated before construction stage for the shielding design and radioactive systems. These methodologies for the radioactive source term evaluation are developed from the mid of 1970s to the mid of 1980s. Since 1984, any new methodologies are not provided. Especially, although the radionuclide concentration at reactor coolant system is predominant factor at source term evaluation, the base-data measured in USA during the 1980s are not updated. At present, a few codes such as PWR-GALE and DAMSAM used for evaluation of source term have some limitations for application to the next generation plants of Korea.
The purpose of this study is to provide the method and evaluation tool for radionuclide concentrations at reactor primary and secondary coolant systems respectively, and to compare the results with those of the well-known and recognized tools. In this study, an evaluation method for radionuclide concentrations at fuel pellet region and primary and secondary coolant systems is suggested and a corresponding code for source term evaluation is developed.
The code named as SYCOS (System for Calculation Of Source term) is able to predict the radionuclide concentration of fission product at primary and secondary coolant region with various reactor design parameters based on the simple calculations.
SYCOS uses the simplified equation with the assumption of the steady state condition both for the fission product radioactivity at fuel pellet region and the fission product concentrations at reactor coolant systems. At fuel pellet region, ORIGEN 2 code was used for the calculation of the effective fission yield.
The reference system is pressurized water reactor with U-tube steam generator. Also Chemical and Volume Control System and Boron Recovery System are considered as primary coolant purification system and steam generator blowdwon and condensate demineralizer as secondary coolant purification system.
By applying SYCOS code to YGN unit 3 and 4 plants, the results are compared with the actual data measured from the object plants and calculation results of PWR-GALE and FSAR (Final Safety Analysis Report) of YGN 3&4.
In this study, specifically, the expected fuel defect rate and the concentration distribution of the fission product was focussed to be analyzed in detail.
The comparison has shown that the well-known and recognized tools relatively overestimate radionuclide concentrations at the reactor coolant systems, and the resultant concentration distribution of the fission product from SYCOS is similar to the that of actual data measured from the object plants and calculation results of PWR-GALE and FSAR (Final Safety Analysis Report) of YGN 3, 4. For the fission product, the fuel defect rate has a range of 1.57E-7-9.00E-5.
In conclusion, SYCOS can be used to evaluate the fission product concentrations at fuel pellet region and primary and secondary coolant systems. Despite of the simple method, SYCOS showed reasonablely good results as compared to the well-known and recognized tools such as PWR-GALE and DAMSAM.
For evaluation of the source term with various fuel defect rate, the SYCOS is turned out to be useful and applicable, for next generation reactor, to estimate the radionuclide concentrations in the reactor coolant systems. Especially, for fission product, SYCOS can predict the fuel defect rate with sufficient actual data and operational history.
발전소에서 누출되는 방사성물질의 양인 방사선원항은 발전소 폐기물 처리계통 및 차폐 설계를 위하여 건설이전 예측 가능하여야 한다. 현재 사용되고 있는 선원항 평가 도구들은 국내 원전에 적용하기에는 몇 가지 문제점을 가지고 있다. 특히, 선원항 평가 시 주요한 요소인 냉각재 내 핵종농도는 차세대 원전등에서 크게 달라지 것으로 예상된다.
본 연구에서는 냉각재 내 핵종농도 예측을 위한 방법과 평가 도구(SYCOS)를 개발하였으며, 이를 영광 3, 4호기에 적용하였다. 평형상태를 가정하여, 계산을 단순화 하였으며, 핵연료 영역에서의 방사능과 냉각재 영역에서의 냉각재 내 핵종 농도를 구하였다. 핵연료 영역에서는 ORIGEN 2 코드를 이용하여 유효 핵분열 생성율을 구하였으며, 냉각재 영역에서는 대상원전의 냉각재 정화계통을 반영하여 핵종농도를 계산하였다. 이러한 결과를 영광 3, 4호기의 실측값, PWR-GALE, FSAR의 자료들과 핵종 농도 분포 및 핵연료 결함율 관점에서 비교하였다.
단순화된 수식이지만, 대부분의 핵분열 생성 핵종에 대하여 의미 있는 결과를 얻을 수 있었다.
결과적으로, 본 연구를 통하여 제시된 SYCOS는 핵연료 결함율 및 기타 발전소 설계 조건에 따라 냉각재 내 핵종농도를 제시할 수 있으며, 이를 통하여 KNGR등 운전 자료 구축이전의 발전소에 대하여 적용가능 하다고 판단된다.
