RANNS code analysis on the local mechanical conditions of cladding of high burnup fuel rods under PCMI in RIA-simulated experiments in NSRR

NSRRのRIA模擬試験における、高燃焼度燃料被覆管のPCMIにおける局所的な力学的状態に関するRANNSコードによる解析

鈴木 元衛; 斎藤 裕明*; 更田 豊志

Suzuki, Motoe; Saito, Hiroaki*; Fuketa, Toyoshi

RANNSコードは反応度事故条件における1本の燃料棒ふるまいを解析する。このコードは二種の力学モデルを持つ。軸方向セグメント全長に対する一次元モデルと、新開発の、ペレット1個長に対する局所変形モデルである。NSRR実験OI-10及びOI-11の解析を行い、被覆管の変形をこの二モデルとPIEデータとの間で比較した。FEMAXI-6で解析したパルス照射前の燃料状態に基づき、RANNSによりパルス実験における変形を計算した。二次元モデルにより、被覆管の塑性歪みの局所的な不均一やそれに伴う応力の差を計算し、PIEデータと比較し、応力の差の影響を検討した。

The RANNS code analyzes behaviors of a single fuel rod in reactivity-initiated accident (RIA) conditions. The code has two types of mechanical model; one-dimensional deformation model for each axial segment length of rod, and newly-developed two-dimensional local deformation model for one pellet length. Analyses were performed on the two RIA-simulated experiments in the NSRR, OI-10 and OI-11 with high burnup PWR rods, and results of cladding deformation were compared between calculations by the two models and PIE data. RANNS calculated the deformation profiles of claddings during the power transient of the experiments on the basis of the pre-pulse conditions of rods predicted by FEMAXI-6 code. In the calculations by the two-dimensional model, the plastic strain increase at the cladding ridges was compared with those in between the ridges and with the PIE data, and effect of stress variance induced by local non-uniformity of strain on the crack growth was discussed.