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「常陽」使用済燃料の崩壊熱の測定評価(2)

Measurement and analysis of JOYO MK-II spent MOX fuel decay heat(2)

前田 茂貴   ; 野瀬 正一; 長崎 英明*; 青山 卓史 

Maeda, Shigetaka; Nose, Shoichi; not registered; Aoyama, Takafumi

使用済燃料の崩壊熱を精度良く把握することは、原子炉冷却系、使用済燃料の取扱・貯蔵設備等の設計において重要であり、「常陽」では実際の使用済MOX燃料の崩壊熱データを蓄積し、それを基に解析法の精度向上を進めている。今回、使用済のMK-II燃料(集合体平均燃焼度約6.6万MWd/t)について、冷却期間40$$sim$$150日における崩壊熱を詳細に測定し、実測値1446$$pm$$24$$sim$$663$$pm$$20Wを得た。崩壊熱計算では"ORIGEN2"計算に加え、炉心管理コードシステム"MAGI"を用いた中性子スペクトルを反映した燃焼計算を基に崩壊熱を算出し、C/E=0.94$$sim$$0.90を得た。測定対象集合体は、全照射期間を通じて「常陽」の炉心第1列に装荷されており、計算条件が単純なため、解析上の誤差要因が少ない。今回みられた計算値と測定値の系統的な差はFP崩壊熱の計算誤差(1$$sim$$2%)や測定誤差(1$$sigma$$=1.7$$sim$$3.0%)より大きく、その主要因はアクチニド崩壊熱の計算誤差によるものと考えられ、これらの核種生成に係る断面積や燃焼組成の不確かさ等の影響を検討する必要があることが明らかとなった。

Decay heat of the spent MOX fuel is important not only from the viewpoint of reactor safety concerning a decay heat removal at the reactor shut down, but also for the thermal design of the spent fuel storage and handling facility. In order to obtain the experimental data and to improve the accuracy of calculation, the decay heat of spent fuel subassemblies of the JOYO Mk-II core was measured. The burn-up was 66 GWd/t and the cooling time was between 40 and 150 days. Measured decay heat of the spent fuel subassemblies was approximately 1446$$pm$$ 24$$sim$$663$$pm$$20W. The decay heat was calculated by "ORIGEN2" code and then it was compared with the measured value. In the "ORIGEN2" calculation, the JENDL-3.2 cross section library and the JNDC-V2 decay data library were used and fuel power calculated by the core management code system "MAGI" was used as a input. The ratios between calculated and experimental values, C/Es, were approximately between 0.94 and 0.90. The discrepancy between calculation and measurement was considered to be larger than the experimental error (1$$sigma$$ = 1.7$$sim$$3.0%) or the uncertainty of calculated FP decay heat (1$$sim$$2%). It appears due to the uncertainty of actinides decay heat and that indicates cross sections of actinides and initial composition of actinides are important to evaluate decay heat accurately.

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