2023 年 22 巻 1 号 p. 1-11
Thermodynamic calculation of volatile radionuclides such as Cs, I and Te in a cladding failure accident of a sodium-cooled fast reactor has been conducted for the evaluation of an in-vessel source term. Important chemical reactions and chemical forms such as Cs, Cs2, Te, CsI, Cs2I2, Cs2Te, CsNa, NaI, Na2I2, Cs2Te, Na2Te, AgTe and Cs2MoO4 were suggested by the results of interaction among fuel, fission products, a cladding tube and Na coolant at high reduction rates inside the fast reactor vessel. As the cladding failure accident proceeds, it was confirmed that 15% and 60% of Cs was in gas phase at 1,155 K and 5.7 MPa and at 1,670 K and 10.5 MPa, respectively. 58% and 66% of Cs was in the gas phase at 1,155 K and 1,670 K at 0.1 MPa owing to cladding failure. Under Na mixing conditions, almost 100% of Cs became gas. Furthermore, we confirmed the same behavior as in light water reactors, that is, the release of Cs is suppressed at high pressures, but we observed that Cs rarely oxidizes and that the amount of Cs released increases with Na mixing in the fast reactor.
