Performance confirmation of Monju failed fuel detection and location system

もんじゅタギング法破損燃料検出装置の性能確認

諸橋 裕子 ; 鈴木 敏

Morohashi, Yuko; Suzuki, Satoshi

高速増殖原型炉もんじゅのタギング法破損燃料検出装置は、あらかじめ集合体ごとに特有なタグガス(Kr, Xe)を燃料ピンに封入しておき、燃料破損時に1次アルゴンガス系へ放出されたタグガスを回収、分析し、破損燃料を同定する。設計上希ガス濃縮率200倍以上で同定できるとされており、過去の試験では、希ガス濃度1ppmの試料ガスにて確認している。今回、燃料破損時に想定されるレベルの低濃度ガスにて希ガス濃縮率を確認するとともに、希ガス濃度依存性についても確認した。

Monju has failed fuel detection systems which consist of the delayed neutron (DN) monitoring system, the cover gas (CG) monitoring system, and the failed fuel detection & location (FFDL) system based on the tagging gas analysis. The DN method and the CG method are used to watch the integrity of fuel assemblies continuously. When a fuel failure of pin-hole level is detected by the CG monitoring, the FFDL system starts operation to identify the location of the failed fuel assembly in the reactor core. In case of a larger fuel failure, the DN monitoring system works and the reactor shuts down automatically. The FFDL system collects the tagging gas which migrates into the reactor cover gas from a failed pin. The tagging gas is made of stable isotopes of Kr and Xe. 270 types of isotopic composition are available using 15 types for Kr and 9 types for Xe. Thus, the isotopic composition of the tagging gas can be made specific to each assembly. The assembly containing a failed fuel pin in the reactor core can be identified by analyzing the isotopic composition. The FFDL system is comprised of two tagging gas concentration devices. The device collects and concentrates the tagging gas by adjusting temperature of activated carbon from 110 K to 420 K. The concentration rate is designed to be higher than 200. In the past examination performed, it was confirmed that the concentration rate meets the requirement with a noble gas concentration of 1 ppm. However, the actual noble gas concentration emitted from a failed fuel is assumed to be much lower. In the present study, the performance of FFDL system was demonstrated by measuring low concentration gas of the actual fuel failure level. The sample gas of concentrations ranging 0.1 ppb to 10 ppb was used. As a result, the concentration rate was confirmed to be more than tens of thousands, which sufficiently satisfies the design demand. Moreover, it was confirmed that the concentration dependence of noble gas was small.