Verification of interlid pressure monitoring using lead pipe for a spent fuel transport & storage cask

In Japan, a dry metal cask is used for interim storage of spent fuels. For the containment of radioactive material in dry metal cask, the inside of a cask is filled with helium gas having negative pressure and the interlid space is pressurized by helium gas. The metal cask is required to be designed so as to enable operators to monitor the containment function. Therefore pressure monitoring sensors are employed to continuously monitor interlid pressure. Usually, in the interim storage facility, for the increase in efficiency of storage capacity, dry metal casks are kept standing. Therefore the interlid pressure monitoring sensors will be put on top of the cask. We call these the “direct system” The pressure monitoring sensors are required to be correct periodically. For the maintenance of pressure monitoring sensors, it is necessary to work not only at high place but also under the environment of high temperature and radiation. To work safely, we considered using a lead pipe (stainless steel tube, 6 mm in diameter and 6 m in length) so that the work might be possible done on the floor. We made the mock-up device to conduct various kinds of experiments in order to verify whether this system, we call the “lead pipe system”, could be able to monitor gas pressure accurately under various conditions. The contents of experiments are as follows. ? Verification that there is no meaningful difference between interlid pressure indicated values in the direct system and the lead pipe system in spite of environmental temperature differences. ? Verification of there is no meaningful detection delay with the lead pipe system at the time of the small leak from a cask top-side. ? Verification of an alarm value being set up above the pressure change by environmental temperature, and being able to secure management time. Finally we confirmed the “lead pipe system” is useful and contributes to worker's dose reduction and work environment improvement.