先進高速炉の検討 -長半減期FPの消滅特性解析-

Study on advanced fast reactor; Analysis of burning Characteristics of long-lived FPs

山岡 光明*; 川島 正俊*

Yamaoka, Mitsuaki*; Kawashima, Masatoshi*

使用済燃料中に含まれる核分裂生成物(FP)には、TRU核種と同様にきわめて長半減期のものが存在している。中性子束の高い高速炉では、これらFPを消滅できるポテンシャルがある。FPの断面積は低エネルギーで増大する傾向があるので、消滅効率を上げるために、FPを含むピンの他に水素化ジルコニウム(ZrH1.7)等の中性子減速材ピンを装荷したターゲット集合体による消滅が考えられる。平成6年度は、Tc-99等のFPおよび減速材を装荷したFP消滅ターゲット集合体を炉心内部領域に配置することを前提とした検討を行った。その結果、減速材により消滅率は増大するものの、炉心燃料の出力スパイクが増大することがわかった。これを避けるには炉心周囲でのFP装荷が示唆された。本年度は、Tc-99と減速材を装荷したターゲット集合体を炉心周囲に配置することを前提として、高速炉において効率的に消滅を行うための解析を行った。解析は主として、非均質性の強いターゲット集合体におけるTc-99の共鳴吸収効果を厳密に扱うために有効と考えられる連続エネルギーモンテカルロコードを用いて実施した。

Some of fission products (FPs) in spent fuels have very long half lives as transuranic nuclides. Fast reactors have a potential to transmute these FPs into short-lived ones because of high neutron flux. As neutron capture cross sections of FPs increase in low energy region, one of the effective means to transmute them is to irradiate them using target assemblies that contain pins loaded with neutron moderating material such as ZrH as well as FP loading Pins. In the work carried out in 1994, a study was performed on transmutation of long-lived FPs, assuming that FP target assemblies are loaded in the core region of fast reactors. As a result, it was found that neutron moderation tends to improve transmutation rate whereas it causes to significant power spike in the adjacent core fuel. So as to suppress the power spike, FPs' loading at the outer periphery of the core was suggested. In the present study, an analysis was carried out to transmute FPs in fast reactors, assuming that target assemblies containing ZrH pins and Tc pins are loaded at the outer periphery of the core. The analysis was mainly performed using the continuous energy Monte Carlo code that is effective for rigorous trestment of rcsonance absorption of Tc99 in the target assembly with large heterogeneity. The results are shown below. (1)To increase absorption rate of Tc99, the neutron spectrum where absorption in the resonance region is more than in the thermal region is advantageous. This is because the spectrum helps to suppress absorption by structural material. Therefore, the appropriate loading mass of moderator depends upon the moderating power. (2)The target assembly selected from the survey indudes 19 ZrH pins with large diameter and 36 Tc pins with small diameter. The transmutation rate of Tc99 is about 4-5%/year and the mass is about 15-20kg/year. (3)The effect of loading target assembly upon main core characteristics were analyzed. It was found that the ...