弥生炉の標準照射場を用いたHAFM、タグガスの校正照射

Irradiation test of HAFM and tag gas samples at the standard neutron field of "YAYOI"

井口 哲夫*

not registered

高速炉の新型中性子ドシメトリー法であるHe蓄積型中性子フルエンスモニタ(HAFM)測定システムの総合的な精度評価およびタグガスの放射化分析による高速炉の燃料破損検出法への適用性評価を目的として、東京大学工学部附属原子力工学研究施設の高速中性子源炉「弥生」の標準照射場を用い、これらの試料の校正照射を行った。HAFM試料では、平成7年度から継続して、炉心中央(Gy孔)に93%濃縮B1mg入りVカプセル、また高速中性子柱実験孔(FC孔)に、同仕様の天然B10mg入りカプセル等が照射された。また、平成8年度新たに、ブランケット実験孔へ濃縮B40mg及びB100mg入りカプセルが装荷された。これらの照射場の中性子束及び中性子スペクトルは、Fe,Co,Ni,Au,235U,237Np等を用いた放射化法でモニターされ、平成7年度の測定結果の追試・確認とともに、今年度は特に試料近傍のより詳細な中性子束分布の評価がなされた。平成9年3月末現在、すべてのHAFM試料につき、予定のHe生成量目標値(10の13乗Heatoms/試料)を越える照射が達成された。一方、SUS製カプセルに封入された6種類のKr/Xe混合タグガス試料について、Gy孔で積算炉出力4.57.5kWh(中性子フルエンスにして、10の16乗n/cm2乗相当)の3回の標準照射を行い、照射直後における各試料のガンマ線スペクトル測定を行った。タグガス組成に応じて、125Xeや79Krなどの生成放射性核種からのガンマ線光電ピークが異なるパターンで明確に同定でき、高速炉破損燃料検出に適用する際の放射化計算の精度評価に有用なベンチマーク実験データが得られた。

To check the accuracy of helium accumulation neutron fluence monitors(HAFM) as new technique for fast reactor neutron dosimetry and the applicability of tag gas activation analysis to fast reactor failed fuel detection, their samples were irradiated at the standard neutron field of the fast neutron source reactor 'YAYOI' (Nuclear Engineering Research Laboratory, University of Tokyo). Since october in 1996, the HAFM samples such as 93% enriched boron(B) powders of 1mg and natural B powders of 10 mg contained in vanadium(V) capsule were intermittently irraditated at the reactor core center (Glory hole:Gy) and/or under the leakage neutron field from the reactor core(Fast column :FC). In addition, new V capsules filled with enriched B of 40mg and Be of 100mg, respectively, were put into an experimental hole through the blanket surrounding the core. These neutron fields were monitored by the activation foils consisting of Fe, Co, Ni, Au, U, Np etc., mainly to confirm the results obtained from 1995's preliminary works. In particular, neutron flux distributions in the vicinity of irradiated samples were measured in more detail. At the end of March in 1997, the irradiated neutron fluences have reached the goal necessary to produce the detectable number of He atoms more than 10 in each HAFM sample. Six kinds of tag gas samples, which are the mixed gases of isotopically adjusted Xe and Kr contained in SUS capsules, were separately irradiated three times at Gy under the neutron fluence of 10n/cm in average. After irradiation, -ray spectra were measured for each sample. Depending on the composition of tag gas mixtures, the different patterns of -ray peak spectra from Kr, Xe, etc. produced through tag gas activation were able to be clearly identified. These experimental data will be very useful for the benchmark test of tag gas activation calculation applied to the fast reactor failed fuel detection.