Evaluation of uncertainties in FEMAXI-6 calculations for predicting MOX fuel behavior in FLWR design

FLWR燃料設計のためのFEMAXI-6によるMOX燃料ふるまい予測の不確実性の評価

山路 哲史; 鈴木 元衛; 大久保 努

Yamaji, Akifumi; Suzuki, Motoe; Okubo, Tsutomu

軽水炉技術に立脚して早期のプルトニウムのマルチサイクルを実現するFLWRの燃料設計及びその健全性を評価するために、汎用性の高いFEMAXI-6コードを用いることが有用と考えられる。本研究では、FEMAXI-6による軽水炉MOX燃料ふるまい解析の不確実性を検証した。検証計算にはHalden炉で照射されたMOX燃料(IFA-514)の照射データ(TFDB)を用いた。検証の結果、測定された燃料棒内圧を再現するにはモデル中のFPガス放出閾値を8から10倍程度にする必要があり、FPガス放出計算に大きな予測誤差があることが明らかになった。燃料中心温度の予測結果に対してFPガス放出以外に、ペレットの焼きしまり,スエリング,リロケーションのモデルの感度が高かった。これらのモデル中のパラメータを燃料物性の変動と考えられる範囲で変化させた結果、測定された燃料中心温度に対してプラスマイナス50K程度の範囲で予測できた。

The concept of Innovative Water Reactor for Flexible Fuel Cycle (FLWR) has been proposed and being studied at JAEA to achieve effective and flexible utilization of the uranium and plutonium resources based on the well-developed LWR technology. FLWR is a BWR type concept and it is planned to be introduced by two stages. In the first stage, the MOX fuels are irradiated in a similar condition to that of the current BWR but with a harder neutron spectrum. The core average discharge burnup is about 45 GWd/tHM. In the second stage, the neutron spectrum is further hardened to achieve a multiple recycling of plutonium with higher burnups. In order to design and evaluate the integrities of FLWR fuel rods, the uncertainties in FEMAXI-6 calculations and models for predicting LWR MOX fuel behavior need to be evaluated. As an introduction to the evaluation process, the Test-Fuel-Data-Base (TFDB) obtained from the Halden reactor experiments (IFA-514) were used for the evaluations. The maximum discharge burnup was about 40 GWd/tMOX. Based on the present investigation, the following models were found to be particularly important. Namely the FGR, pellet densification, swelling, and relocation models. These models of FEMAXI-6 have been developed and the parameters have been optimized based on the past UO irradiation test data. For predicting MOX fuel behavior, the FGR model has a relatively large uncertainty and causes a large uncertainty in the FGR calculations. On the other hand, the uncertainties in the other models are within the range expected by the property variations of typical UO fuels. Hence, the densification, swelling, and the relocation models of FEMAXI-6 can be applied to MOX fuel analyses provided the corresponding MOX property variations are taken into account in the input parameters of these models.