Leaching behavior of radionuclides from samples prepared from spent fuel rod comparable to core debris in the 1F NPS

Abstract

To investigate the leaching behavior of radioactive nuclides in leaching samples comparable to core debris (partially molten ZrO 2 /UO 2 between fuel rods) in the Fukushima Daiichi Nuclear Power Station (1 F NPS), the concentration of radionuclides in the leaching solution was measured. The leaching behaviors of actinides (U, Pu, and Np) and Cs from the samples were similar to those from spent fuel. As predicted from the current thermodynamic database, the leaching of U and Pu depends on the pH in the cooling water of the core debris. However, if Mo and Tc are surrounded by zircaloy in the core debris, their leaching amount may increase by one order of magnitude than those from spent fuel.

Ten years after the NPP accident at Fukushima : review on fuel debris behavior in contact with water

“…Highly oxidizing conditions resulting from external gamma irradiation are observed to significantly increase the concentration of plutonium (Pu (+V)) and uranium (U(+VI)) compared with a benchmark experiment (without external irradiation). The uranium and plutonium concentrations are, respectively, 1 × 10 −5 mol.L −1 and 1 × 10 −6 mol.L −1 under these strong oxidizing conditions (E>+600 mV/SHE) whereas respective concentrations under hydrogen saturated reducing conditions are in the range between 1 × 10 −8 mol.L −1 to 1 × 10 −11 mol.L −1 Rather similar behaviors have been observed recently on MOX spent fuel debris leached in deionized water [83]. For less oxidizing conditions, Pu is found mainly at oxidation state +IV.…”

“…The microstructure of the fuel debris and the coexistence of phases with different electrochemical properties can also have notable effects through galvanic couplings. The oxidation behavior of metallic fission products like Mo-97 and Tc-99 depends on the nature of the phases surrounding these metallic particles [83]. Contacting electrochemically Zircaloy promotes their oxidation and their release from the fuel debris contrary to what is observed for spent fuel.…”

“…The location of radionuclides in the different solid phases and their distribution in relationship to the microstructure remains relatively unknown, only the total inventories are evaluated thanks to the complete dissolution of the samples. In addition to these approaches with surrogates, recent studies on irradiated fuel (MOX 47 GWd/t HM ) have sought to melt crushed samples with their cladding in order to simulate fuel debris [83]. This approach allows more easily a direct comparison of the leaching behavior of the fuel debris with an unmelted spent fuel used as a benchmark.…”

“…Contacting electrochemically Zircaloy promotes their oxidation and their release from the fuel debris contrary to what is observed for spent fuel. According to the authors [83] for spent fuel, the above described epsilon particles are surrounded by UO 2 grains, which are oxidized preferentially. An electron transfer takes place on the fuel surface from UO 2 to the metallic phases, which remain reduced.…”