Prediction accuracy improvement for neutronic characteristics of a fast reactor core by extended bias factor methods

拡張バイアス因子法による高速炉核特性予測精度の向上

久語 輝彦 ; 森 貴正; 横山 賢治   ; 沼田 一幸*; 石川 眞

Kugo, Teruhiko; Mori, Takamasa; Yokoyama, Kenji; Numata, Kazuyuki*; Ishikawa, Makoto

拡張バイアス因子法、すなわちLC法とPE法の2手法を種々の核特性実験値を活用して、典型的な高速増殖炉の臨界性とナトリウムボイド反応度の予測精度評価に適用した。拡張バイアス因子法は従来バイアス因子法より効果的である。活用する実験の数が少ない場合は、PE法の持つ実験の組合せ方に自由度が多いという性質のために、PE法がLC法よりも効果的に働く。活用する実験値の数が十分多い場合は、その利点はほとんど影響しない。設計値に含まれる断面積起因誤差は、種々の核特性に関する約200個の実験値を活用することにより、そのほとんどが消滅した。臨界性については、設計値に含まれる全体の不確かさのうち、断面積起因誤差が大部分を占めているため、全体の不確かさも大きく低減した。ナトリウムボイド反応度については、設計値の不確かさのうち、断面積起因誤差が手法起因誤差より小さいため、全体の不確かさの低減は大きくない。

The extended bias factor methods, the LC and the PE methods, are applied to the prediction accuracy improvement for criticality and sodium void reactivity of a traditional two-region homogeneous fast reactor core with utilizing various experimental results. The extended bias factor methods are more effective than the conventional bias factor method. The PE method is more effective than the LC method when a small number of experimental results are used because of the advantage of the former method, its higher degree of freedom in combining experimental results. The advantage hardly affects on the prediction accuracy improvement when a sufficiently large number of experimental results are used. The variances due to cross sections originally included in the design calculation values for the criticality and sodium void reactivity are almost eliminated by the extended bias factor methods with use of about 200 experimental results regarding various neutronic characteristics. The uncertainty of the criticality is considerably reduced, because the uncertainty due to cross sections largely occupies in the original total uncertainty. The uncertainty reduction in the sodium void reactivity is not so much, because the uncertainty due to cross sections is smaller than that due to calculation methods.