Small Gas Leakage Rate Measuring and Monitoring System

A sealing function of transport and/or storage casks of radioactive materials such as spent fuels is essential to prevent radioactive materials from being released excessively into the environment The sealing function of a transport cask is inspected at shipment, and the release rate of radioactive materials into environment is evaluated based on the correlation specified in ANSI N 14.5 1987 (ANSI 1987}. The inspection and evaluation methods on the leakage rate of gas from a cask is being standardized by the International Organi7lltion for Standardization Reprocessing the spent fuels and l&lge of plutoniwn are the national policy of nuclear energy in Japm. Ho~. the interim storage of the spent fuels may be an alternative oJXjon for continuously smooth operation oflight Wclter reactors because of world-wide public opinion against the utilization of plutoniwn, and for a dry storage system such as a storage cask, monitoring the sealing func. tion is essential from safety point of view. Release of radioactive materials from a transport cask is eval uat.ed from a diameter and length of the leak pith obtained from gas leakage rate measured in a pre-shipment test Gas pressure decrease or increase method is usually applied for the test In this test, a pressure change rate in a~ between double O-rin~ is measured by a pressw-e transducer. If the pre-shipment test is carried out under choked flow conditions specified in ANSI N 14.5 1987, the leak JlUh diameter evaluated for a choked flow is one order less than that evaluated for a laminar flow. However, the evaluation method of gas leakage discussed at the ISO has deleted the conce}X of choked flow. Aritomi et al. ( 1993,1994} also indicated that a consideration for a choked flow is not necessary for the gas leakS!,\"e rate of 104 to 10· 2 cmJ/s through the leak pith whose length is 3 mm to 10 mm, COrresJX>nding to an 0-ring thickness. In addition, since the weight of cask is mostly about 100 tons and its thermal ~ity is so large, it takes a significantly long time to reach the thermal equilibriwn. And the temperature of the cask is rising during the test Moreover, the change in an ambient ternperatw\"e, which varies in a day or in a year, affects the temperature of the cask. For these reasons, the le3kab'e rate is often overestimated even for a high leaktight cask