Investigation of irradiation behavior of SiC-coated fuel particle at extended burnup

燃焼延長時におけるSiC被覆燃料粒子の照射挙動の検討

沢 和弘; 飛田 勉*

Sawa, Kazuhiro; Tobita, Tsutomu*

照射健全性を補償するためにHTTRの最高燃焼度は3.6%FIMAに制限されている。燃焼度を延長した場合の燃料挙動を検討するために、照射試験を行った。被覆燃料粒子のバッファ層とSiC層は5%FIMAを超えても健全性を保てるよう厚く設計した。これらの燃料コンパクトは、ORNLのHFIRと原研のJMTRのキャプセルで独立に照射した。放出率の測定値と計算値の比較から、両方の照射試験中に追加の破損が生じたことがわかった。内圧破損モデルでは、照射末期においても健全粒子のSiC層には引張応力は作用せず、破損は生じないと評価された。考えられる破損機構として、製造時のSiC層破損粒子の貫通破損又は加速照射による過度の内圧上昇が考えられるが、さらに検討が必要である。

The maximum burnup of the first-loading fuel of the HTTR is limited to 3.6%FIMA to certify its integrity during the operation. In order to investigate fuel behavior under extended burnup condition, irradiation tests were performed. The thickness of buffer and SiC layers of the irradiated fuel particles were increased to keep their integrity up to over 5%FIMA. The fuel compacts were irradiated in independent capsules at the HFIR of ORNL, and at the JMTR of JAERI, respectively. The comparison of measured and calculated (R/B)s showed that additional failures occurred in both irradiation tests. A pressure vessel failure model analysis showed that no tensile stresses acted on the SiC layers even at the end of irradiation and no pressure vessel failure occurred in the intact particles. The presumed failure mechanisms are additional through-coatings failure of as-fabricated SiC-failed particles or an excessive increase of internal pressure by the accelerated irradiation. The further study is needed to clarify the failure mechanism.