Improvement of the RIMS Detection Sensitivity for Application to Failed Fuel Detection and Location System of the Fast Reactor

In the fast reactors, rapid and accurate detection of fuel failures as well as subsequent identification of failed fuel location are essential to achieve their safety operation and high plant availability. The gas tagging method, currently employed in the prototype fast breeder reactor Monju, is one of the efficient ways for the failed fuel detection and location (FFDL) technique, the principle of which is the isotope analysis of the argon (Ar) cover gas that includes, in case of fuel failure, a partial amount of leaked krypton (Kr) and xenon (Xe) originally loaded into each fuel pin. We propose a new type of FFDL technique using laser resonance ionization mass spectrometry (RIMS) for the isotope analysis of the cover gas in view of selective ionization of a specific element to obtain high S/N ratio. Nevertheless, the actual experimental data shows the existence of Ar and Ar₂ non-resonant ionization by the photoelectron generated in the vacuum chamber to hinder precise measurement of Kr and Xe. We could successfully decrease the effect of these ions by one to two orders of magnitude by applying both a set of a neutralization apparatus and a Brewster window, and an electrode with a slit-type hole in the ion acceleration region, resulting in reliability improvement of RIMS in the FFDL system.