Small Gas Leakage Rate Measuring and Monitoring System for Spent Fuel Transport/Storage Cask.

Abstract

A containment function of transport and/or storage casks of radioactive materials is essential to prevent the materials from being released excessively into the environment. It is not practical for containment tests to measure directly the radioactivity release so that gas volumetric leakage rates are usually assessed and gas pressure decrease or increase method is usually applied. As gas flow model for evaluation, the ISO standards has deleted the concept of choked flow which is adopted by ANSI N14.5. Provided that the choked flow is not adopted to the leakage rate evaluation, the criteria of the test should be severer, and a new leakage rate measuring system with high accuracy and reasonable measuring time is required. Transport casks are often inspected in a temporary cask-storage facility where simultaneous measurement of many casks is required. In a storage cask system, multiple casks are monitored on their containment function during a storage period, and the method for simultaneous monitoring at many points for long term is required. In this study, two kinds of small gas leakage rate measuring systems are developed. One is to measure gas leakage rates directly and is called “flow measuring system”, which can measure gas leakage rate of 10−4 to 10−2 cm3/s with high accuracy and short measuring time. The other is to measure the pressure decreasing rate and is called “pressure decreasing rate measuring system”, which can monitor the pressure change at many points simultaneously.