大型炉心におけるSASSへの要求条件の整備

Summary of requirements on SASS for a large FBR Core

糸岡 聡*; 澤田 周作*; 橋口 禎郎*; 藤又 和博*; 堂元 昇*

not registered; Sawada, Shusaku*; not registered; not registered; not registered

受動的安全機能の採用によってFBRにおける安全系の信頼度を向上するために、自己作動型炉停止機構(SASS)を具備する平成11年度サイクル機構設計の大型均質炉心を対象として、流量減少型スクラム失敗事象(ULOF)における冷却材最高温度に対する不確実因子の感度を安全解析コード(SAS4A)を用いて解析し、SASSへの要求条件を整理した。その結果、SASSの切り離し温度への要求は、誤落下防止の観点から638以上、さらに、ULOF時における冷却材沸騰防止(冷却材最高温度960以下)の観点から666以下となった。また、不確実因子の不確定巾の検討結果と感度解析結果を基に感度係数を定量化したところ、最大感度係数を有する不確実因子はSASS隣接集合体出口温度であり、その感度係数は-20.5/(1=6.7)であった。以上の結果を基に、仮に、SASSの切り離し温度をノミナル値として666に設計した場合でも、SASS切り離しの非同時性を適切に考慮するとともに、本炉心における現実的な冷却材沸騰温度(1018)を炉心損傷の判断基準として適用するならば、SASSはULOFに対して最大感度を持つSASS隣接集合体出口温度換算で約3.4の安全裕度を有するといえる。

In order to improve the reliability of safety system of FBR employing passive safety functions, requirements on a self-actuated shutdown system (SASS) have been summarized, which employs Curie-point magnets in its safety rods, for a large homogeneous reactor core (1500MWe), which was designed by JNC in JFY1999 as one of candidates in the Feasibility Studies on Commercialized FBR System. The requirements were based on the sensitivity analyses, conducted by using the safety analysis code SAS4A, of uncertainty factors that affect the maximum coolant temperature in unprotected loss-of-flow events. The study has given the following requirements: (1)The detachment temperature of the SASS magnet is to be set above 638C (911K) to avoid the possibility of unintended rod drop under normal operations, and to be set below 666C (939K) to prevent coolant boiling under ULOF conditions conservatively assuming a boiling temperature of 960C in the plant. (2)The parameter that has the largest sensitivity on the maximum coolant temperature under ULOF conditions is the exit coolant temperatures of the adjacent fuel assemblies to the SASS assembly, with a sensitivity coefficient of 20.5C/ (1= 6.7C for the exit coolant temperature). It has been concluded based on the parametric analyses that a SASS design even with a nominal detachment temperature of 666C, the SASS has a safety margin of about 3.4 for the exit coolant temperature of the most sensitive adjacent fuel assembly in preventing coolant boiling under ULOF conditions, if the incoherency of SASS detachments are appropriately taken into account together with a more realistic criterion on the coolant boiling temperature (1018C) under the ULOF conditions of this plant.