Numerical analysis on the cladding failure behavior of high burnup PWR fuels in NSRR experiments

NSRR実験における高燃焼度PWR燃料の被覆管の破損ふるまいに関する数値解析

鈴木 元衛

Suzuki, Motoe

高燃焼度PWR燃料を用いたNSRR実験VA-1及びVA-2(室温実験), VA-3及びVA-4(高温実験)について、事故時燃料ふるまい解析コードRANNSを用いて解析を行った。ベース照射からパルス照射直前までの燃料の状態変化を燃料解析コードFEMAXI-6によって計算し、初期条件としてRANNSに与えた。RANNS解析では、パルス出力を与えてペレット及び被覆管の温度と応力歪み,PCMIなどを計算し、実際の実験における破損時刻の状態や非破損実験VA-4における内圧変化を推測し、破損した被覆管の断面形状と結びつけて破損を左右する要因を考察した。また、VA-3については同一の積分エンタルピーでパルス半値幅を長くした条件でのシミュレーションを行い、破損が生じない可能性を示した。

Numerical analyses were performed by using the RANNS code for the NSRR experiments VA-1, VA-2 (room temperature tests), VA-3 and VA-4 (high temperature tests) using high burnup PWR fuels. The fuel changes during base-irradiation were calculated a by the fuel performance code FEMAXI-6 and the results were fed to RANNS as initial conditions prior to the pulse irradiation tests. The RANNS analyses were conducted for the temperature, stress/strain and PCMI of pellet and cladding, and the fuel conditions at failure instant or internal pressure change in the non-failed VA-4 test were evaluated. These results were compared with the actual cladding rupture morphology and factors dominating cladding failure were discussed. Also, a simulation calculation was carried out with a widened pulse power which has the same integrated enthalpy as that of the original NSRR pulse power in the VA-3 test. This suggests no possibility of cladding failure.