Performance evaluation of a resonance ionization mass spectrometer developed for the FFDL system of fast reactors

高速炉のFFDLシステム用に開発されたレーザー共鳴イオン化質量分析計の性能評価

岩田 圭弘   ; 伊藤 主税   ; 原野 英樹*; 井口 哲夫*

Iwata, Yoshihiro; Ito, Chikara; Harano, Hideki*; Iguchi, Tetsuo*

高速炉の破損燃料位置検出(FFDL)用に開発されたレーザー共鳴イオン化質量分析計(RIMS)の性能評価を行った。RIMSでは、波長216.7nm又は249.6nmのパルスレーザー光を用いてクリプトン又はキセノンを共鳴イオン化し、飛行時間型質量分析計で同位体比を測定する。高速増殖原型炉「もんじゅ」の設計を想定した数値計算において、破損燃料集合体を99%の信頼度で識別する観点から、RIMSの要求性能を系統誤差1%以下、統計誤差を濃度7pptの核種に対して3%以下と設定した。開発したRIMSはクリプトン,キセノンともに測定時間約40分で上記の要求性能を満たしており、迅速性かつ信頼性が要求されるFFDLに適用可能であることが実証された。

To prevent a fuel failure event from becoming a serious radiation accident, sodium-cooled fast reactors are equipped with a system for failed fuel detection and location (FFDL). The FFDL instrument employed in the prototype fast breeder reactor Monju is based on the gas tagging method, in which precise and accurate measurements of krypton and xenon isotope ratios (Kr/Kr, Kr/Kr and Xe/Xe) must be performed in a short time. Burnup measurements also contribute to accurate determination of Kr/Kr. We have developed a highly sensitive resonance ionization mass spectrometer for the isotopic analyses, which uses resonance ionization of Kr and Xe atoms by a pulsed laser at wavelengths of 216.7 nm and 249.6 nm, respectively. In evaluating the performance of our spectrometer, we find that systematic errors caused by isotope shifts can be reduced to negligible levels, and that statistical errors of 3% at a nuclide concentration of 7 ppt can be achieved with a single measurement time of about 40 minutes for each Kr and Xe isotope ratio. This means that, within one hour, about 200 fuel assemblies can be individually identified with a probability of 99%, verifying the applicability of our spectrometer to the FFDL system of fast reactors.