Failure thresholds of high burnup BWR fuel rods under RIA conditions

高燃焼度BWR燃料の反応度事故時破損しきい値

中村 武彦* ; 更田 豊志; 杉山 智之 ; 笹島 栄夫 

Nakamura, Takehiko*; Fuketa, Toyoshi; Sugiyama, Tomoyuki; Sasajima, Hideo

反応度事故を模擬した条件での高燃焼度BWR燃料の変形挙動を測定し、破損限界を検討した。NSRRで行った実験では、パルス照射のごく初期に0.4%程度の小さな周方向歪みが生じた時点で被覆管は脆性的に破損した。この変形の歪み速度は数10%/s程度であった。これらの結果をペレットの熱膨張の計算値と比較した結果、この被覆管変形はペレットの熱膨張によって生じており、ペレットに蓄積されたFPガスの影響は小さいことが示された。また、被覆管温度が破損しきい値に及ぼす影響を個別効果試験によって調べた。高燃焼度BWR燃料の破損しきい値に及ぼすパルス幅の影響について、歪み速度,変形の程度、及び被覆管温度の観点から議論した。

Transient deformation of high burnup BWR fuel rods was measured and failure limit was examined under simulated RIA conditions. Brittle cladding failure occurred at a small strain of about 0.4% during an early phase of the pulse irradiation tests at the NSRR. Strain rates were in an order of tens %/s at the time of the failure. Comparison of the results with thermal expansion of pellets suggested that the deformation was caused by thermal expansion of the pellets. In other words, the influence of fission gases in the pellets was small in the early phase of the deformation. Separate effect tests were conducted to examine influence of the cladding temperature on the failure behavior of cladding. Influence of the pulse width on the failure threshold was discussed in terms of the strain rate, magnitude of the deformation and temperature of the cladding for high burnup BWR fuel rods under RIA conditions.