冷却材ボイド反応度の中性子吸収材効果

Neutron absorber effect of coolant void readivity in pressure-tube-type heavy water reactor

小綿 泰樹*

not registered

軽水冷却圧力管型重水炉の減速材重水に用いられる2種の中性子吸収材が及ぼす冷却材ボイド反応度に対する炉物理効果について検討した。中性子吸収材は減速材に溶解したポイズンと局所挿入した制御棒であり,いずれも中性子吸収元素はほう素である。重水臨界実験装置を用いて,臨界水位差法によってボイド率を変えたときのボイド反応度を測定した。WIMS‐Dコードによる計算値と測定値を比較し,計算結果を摂動解析によって吸収材の効果を評価した。ボイド反応度の正側への移行の理由を記述した

Reactor physics effect of two kinds of neutron absorbers used in the moderator of light-water-cooled pressure-tube-type heavy water reactors on coolant void reactivity is clarified by experiments and by theoretical analysis. These neutron absorbers are boron-10 burnable poison uniformly dissolved into the moderator and BC control rods locally inserted into it. Void reactivity arised by changing void fraction from 0% to 100% is measured by critical level difference method using a deuterium critical assembly (DCA). Measurements are compared with calculations by design code WIMS-D4 for Japanese co㎜ercial reactor. Experimental and calculated results presented in the range of 0-10$ agreed each other within 1$. If an absorber of either poison or control rod is contained in the moderator, the neutron absorber has such an effect to move void reactivity to positive side. Amount of change of void reactivity to positive side is nearly proportional to poison concentration. Dependence of void reactivity on the number of control rods inserted is smaller than that of poison case and at the same time, it exhibits saturation tendency with regard to increment of control rod worth. The reason for a behavior of void reactivity due to the neutron absorber is investigated by the perturbation analysis. Neutrons are easily thermalized by light water in the pressure tube at lower void fraction and are diffused in part into the heavy water moderator. The absorption in the heavy water of the thermal neutrons is relatively increased by the existence of an absorber compared with that at higher void fraction.