ランプ型過出力時の燃料破損限界 - 既存炉内試験のサーベイとFCMI緩和メカニズムの検討 -

Fuel pin failure threshold under the slow TOP condition; Survey on the existing In-pile tests and investigation of the FCMI mitigation mechanism

深野 義隆  ; 佐藤 一憲 

not registered; not registered

CABRI-2及び、CABRI-FAST試験、EBR-II TOPI-1E試験、以前のTREAT試験等、既存の炉内ランプ型過出力試験(以下スローTOPと呼ぶ)のデータを広範にサーベイし、燃料破損限界に係わるこれらの試験データを統一的に説明し得る解釈を得た。これにより基本的な燃料ピン破損メカニズムが把握でき、低中スミア密度燃料ではFCMIが緩和され高い破損限界が得られることを確認した。このような低中スミア密度燃料の高い破損限界は、(1)燃料内の気相空間による燃料熱膨張及びスウェリングの吸収、(2)自由空間への早期ガス放出によるスウェリングの抑制、及び(3)燃料溶融時の溶融領域圧力の抑制の3つの主要な効果によるものであると考えられる。これらの効果を過渡時燃料挙動解析コードPAPAS-2Sのモデルに反映するとともに、既存スローTOP試験の解析に適用した。その結果、試験結果との整合性が確認され、前述の考え方の妥当性が示唆された。

Existing data of in-pile ramp-type transient-overpower tests (slow TOPs hereafter), such as those of the CABRI-2 and CABRI-FAST tests, the EBR-II TOPI-1E test and the former TREAT tests, were extensively surveyed and this led to a global interpretation which provided a consistency among the tests. Through this study, a basic fuel pin failure mechanism was comprehended and it was confirmed that fuel pins with low to intermediate smear density have a very high failure threshold with significant mitigation effects against fuel-cladding mechanical interactions. Such high failure threshold of low to intermediate smear density fuel is considered to be attributed to the following three effects: (1)absorption of fuel thermal expansion and fuel swelling by void space (porosity or cracks) within the fuel, (2)mitigation of fuel swelling by an early gas escape into the free volume, and (3)mitigation of molten cavity pressurization upon fuel melting. These effects were refrected to the analytical model of the transient fuel behavior code PAPAS-2S. Application of this improved PAPAS-2S model to representative slow TOP tests provided results consistent with the test data, suggesting that the above-mentioned consideration is valid.