確率論的燃料設計手法を用いた設計合理化検討

Study on the rationalization of fuel design by adopting the probabilistic fuel design method

松山 慎一郎*; 坪井 靖

not registered; not registered

現行の高速炉燃料設計においては各種の不確かさを保守側に積み重ねて設計裕度を設定している。しかし、将来的には高速炉炉心の高性能化(高線出力化、高燃焼度化)が求められ、燃料設計における設計裕度の切り詰め等の設計合理化が必要となることが考えられる。確率論的燃料設計手法は、設計裕度の定量的把握が可能となることから過度の保守性を排除するには有効な手段と考えられる。現行の決定論的設計手法を確率論的手法に変えるためには、設計基準の確立、評価手法の開発、評価の為のデータベースが必要と考えられる。これまで、高速炉燃料設計における燃料最高度評価、被覆管応力評価、CDFの評価に確率論を適用するために、軽水炉における統計的手法の現状を調査し、BORNFREE-DIRAD、BORNFREE-STRESSを開発してきた。本研究では物性データに関する検討を行った。被覆管応力評価に関係する、FPガス放出率、内面腐食等の物性について、常陽等のデータに基づき新しい物性式と不確かさを設定した。新しい物性式は線出力等の関数としており、よりさまざまな設計パラメータの燃料挙動に与える感度を評価可能とした。ガス放出のノミナル値は従来式より小さくなり、不確かさは従来式より大きくなった。被覆管応力及びCDFの確率論的評価結果より2ベースでは従来用いてきたものより下がることを示した。これにより燃焼度の増大等を可能であることを示した。この評価結果に基づき、確率論で必要とされるデータを摘出した。

The design margin for uncertainties is evaluated by accumulating the uncertainties conservatively in the present LMFBR fuel design. However rationalization, such as reduction of design margin for uncertainties, is required to increase the linear heat rate and burn-up in the future. Probabilistic design method is considered to be effective to cut the excess design margin, because the design margin can be evaluated quantitatively by this method. In order to apply the probabilistic design method instead of present deterministic method, it is considered that the design criteria, design method and database should be provided. In the previous studies, the LWR statistic design method was investigated and BORNFREE-DIRAD and BORNFREE-STRESS were developed to apply the probabilistic fuel design method to evaluation of CDF, the cladding stress and the maximum fuel temperature. In this study, the database of material properties was investigated. The FP gas release ratio and FCCI (Fuel cladding chemical Interaction) related to evaluation of cladding stress were investigated based on irradiated data in JOYO and other foreign LMFBR. New correlation of FP gas release ratio and FCCI were proposed and their uncertainties were evaluated. These correlation were made as the function of various parameters such as linear heat rate and burn-up and the sensitivity of these parameter to the fuel performance were able to be evaluated by using these correlation. It was evaluated that the nominal FP gas release ratio by new correlation was smaller than that of old one, and uncertainty of new correlation was larger than that of old one. The 2 -base values of cladding stress and CDF decreased comparing to these values evaluated by old correlation. It was shown that using new correlation could increase the burn-up. The necessity database for probabilistic method was clarified by above results.