Year
2015
Abstract
In response to the accident at Fukushima Daiichi Nuclear Power Station, passive gamma spectrometry is being researched and developed as one of the candidates of a mass measurement method for the special nuclear materials in molten core material. Among Fission Products (FPs) accompanied in molten core materials, some of them are very low-volatile and emit high-energy gamma rays, which enable us to derive the mass of those FPs by passive gamma spectrometry. Using the mass ratio of the FPs and nuclear materials, this technique provides the mass estimation of nuclear materials. This technique is relatively simple and was applied to the analysis of nuclear materials in the clean-up process of damaged Three Mile Island unit-2 (TMI- 2) reactor. We supposed a small diameter canister and a large diameter canister (hypothetical TMI-2 fuel type canister), and the characteristics of leakage gamma-ray from these canisters were compared. There was a lead shield with a collimator of 1cm surrounding the canister. It was assumed that the debris was cooled for 10 years after the accident, and then we measured gamma rays passed through the collimator with the F8 pulse height tally in MCNP with Gaussian Energy Broadening (GEB) option, which allowed us to obtain the detector response. We simulated the detector responses of NaI, LaBr3 and HPGe, respectively, and checked if the predominant gamma ray peaks were observable with those detectors. As the result, the gamma ray peaks of Eu-154 in high-energy regions (E>1MeV) were clearly seen, even with the low-resolution NaI detector. Peaks of Eu-154 and Cs-137 in the middle energy region (0.5MeV