ボイド反応度に対するガドリニアポイズンの効果

Gadolinia-poison effect on coolant void reactivity

小綿 泰樹

Kowata, Yasuki

燃料集合体内にガドリニア入燃料棒を数本組み込んだGd入燃料を、DCA炉心の中央部に少数体装荷した炉心を対象にして、圧力管型重水炉の冷却材ボイド反応度に及ぼすガドリニアの効果を実験及び解析によって考察した。ガドリニア濃度は0.0,0.1,0.5及び1.0wt%に変化させた。ボイド反応度の実験値と計算値の比較によって、解析に使用した核計算コードの計算精度は0.2となった。解析では、格子群定数計算に衝突確率法を用いた格子計算コードWIMS-D、炉心計算に少数群拡散計算コードCITATIONをそれぞれ使用した。ガドリニアの添加はボイド反応度を正側へ移行させるが0.5wt%以下の濃度で飽和させる。ボイド反応度の正側への変化量は、Gd入燃料の炉心への装荷割合が高いほど、またGD入燃料棒をクラスタの外側へ配置するほど大きい。しかしガドリニアを含まない標準燃料の核分裂性核種をウランからプルトニウムに変更すれば、この正側への変化量は緩和される。ガドリニアの添加がボイド反応度を正側へ移行させる主な理由は、ボイド発生によるGdの中性子吸収の増加が標準燃料領域で中性子インポータンスの増加を助長させて核分裂効果を高めるためである。

The effect of the gadolinia-poison on coolant void reactivity in a pressure-tube-type heavy water reactor has been clarified by critical experiments and analyses. Experiments were carried out in the 25 cm lattice pitch DCA (Deuterium Critical Assembly) core in which few gadolinia-poisoned fuels were loaded in the central region. Four or three gadolinia-poisoned UO fuel rods were used in the middle layer of 28 rod or 36 rod fuel cluster as the gadolinia-poisoned fuel respectivity. The concentration of gadolinia in gadolinia-poisoned fuel pellets was changed from 0.0 to 0.1, 0.5 and 1.0 wt% in order. Void reactivity calculated by the lattice code WIMS-D using the collision probability method and the diffusion code CITATION were compared with experimental ones. Calculated values for cores with gadolinia-poisoned fuels agree with experimental ones within 0.2$. The void reactivity tends to shift toward positive side with adding gadolinia-poison, but it soon saturates around the 0.5wt% gadolinia concentration. These tendencies are remarkable as an increase in loading ratio of gadolinia-poisoned fuels in the core or as a location to the outer layer of gadolinia-poisoned fuel rods in the cluster. However, this tendency is relieved when fuel material of reference fuel without gadolinia-poisoned fuel rods changed from UO to PuO-UO mixed oxide. The reason why the coolant void reactivity shifts to the positive side with the addition of gadolinia to the fuel is that an increase in the neutron absorption of gadolinium due to voiding make an increase in the neutron importance at the reference fuel region to promote, so that it tone up the fission yield component of the void reactivity.