A Study on Electrochemical Decontamination of Irradiated Zr-Nb Alloys in LiCl-KCl Eutectic Molten Salts

Abstract

The number of operational nuclear power plants over the world is 445 as of June 2016 and almost age of reactors is more than 30 years. In Korea, Kori unit #1 and Wolsung unit #1 are doing the long term operation after their design life (30years), and it is essential to replace obsolete equipment of Nuclear Power Plants (NPPs) to acquire the license renewal. For example, Wolsung unit #1 replaced Pressure Tubes (PTs). They are core components in a Pressurized Heavy Water Reactor (PHWR). Retired PTs were stored in radioactive waste storage facilities built newly in Wolsung unit #1. If the long term operation of Kori unit #1 finishes in June 2017, it will be decommissioned as related laws after sending a final decommissioning plan to an authority and receiving an approval of the decommissioning. KORAD had built the #1 underground repository cavern for low and intermediate level wastes (LILWs) and began operating the repository in 2015. But according to KHNPs research, the LILWs repository will be saturated in 2015 due to operational wastes and decommissioning wastes. Therefore, study on decommissioning should be continued the effectively operate the repository and to reduce disposal cost for LILWs. Zr, one of metals used in in NPPs, is so expensive metal, since a thermal neutron absorption cross section of zirconium is low and mechanical properties of zirconium are excellent, Zr based alloys are used variously in NPPs. If niobium is added to zirconium alloy, corrosion resistance of zirconium alloys is better. Zr-Nb alloys are used for structural components, claddings, pressure tubes in PWRs or PHWRs. And KAERI developed HANA claddings by adding niobium to zircaloy-4 and by modulating niobium. And have conducted verification tests in commercial reactors and research reactors. If HANA cladding is commercialized, it is expected that radioactive wastes of Zr-Nb will increase rapidly. ORIGEN-2 computational simulation results for PHWRs pressure tubes(Zr-2.5Nb) after 30 years of operation time and 10 years of cooling time showed that specific radioactivity of Nb94, Co60, Fe55, Ni63 is higher than other nuclides in the order named. In particular, Nb94 is critical for irradiated Zr-Nb alloys, because a half-life of Nb94 is 20,300 years and is longer than others. Irradiated PHWR pressure tubes by simulating ORIGEN-2 are intermediate level wastes and are need to dispose of in the cave repository as related laws. According to chapter 8.2 in the Safety Analysis Report (SAR) of #1 LILW repository, total radioactivity of Nb94 is 8.59×1010Bq in this repository. Based on this value, it is possible to dispose of about 10kg of irradiated pressure tubes for Nb94. Considering that mass of replaced Pressure Tubes for Wolsung #1 unit is 23 ton, waste to be possible to dispose of is almost nothing. And ILWs are impossible to be disposed of in the #1 LILWs repository, because KORADs acceptance criteria is equal to a low level waste standard, thus intermediate radioactive level of this waste should be lowered to LLWs or Very LLWs (VLLWs). If Nb94 is removed from irradiated pressure tubes, it will be possible to dispose of almost them. Therefore this study aims for assessment of electorefining steps to need to be exemption level wastes and decontamination effects by using electrorefining of Zr-Nb alloys. Electrorefining of zircaloy-4 and zirlo of main materials in nuclear fuel cladding was previously studied. Their results showed that purity of zirconium at the cathode increased and concentration of impurities at the cathode decreased. Like this, it is expected to reduce Nb94 from radioactive wastes by using electrorefining. Molten salts used in electrorefining are mainly fluorides or chlorides. In molten salts of fluorides, a high purity zirconium is acquired and the redox behavior of Zr is one-step reaction, but operating temperature is higher than chlorides due to melting point and there is a corrosion problem. In contrast, operating temperature of chloride is low and corrosion problem occurs less than the fluoride, but disproportion reaction like that, Zr(IV) + Zr ↔ 2Zr(II), occurs because zirconium is very unstable in chloride and ZrCl is electrodeposited at the cathode. This dissertation decided to use chloride molten salts due to operating temperature. Considering half-life and specific radioactivity of each nuclide, Nb, Co, Ni as impurities were selected to separate from PTs. After dissolving ZrCl4, NbCl5, CoCl2, NiCl2 in LiCl-KCl, cyclic voltammetry(CV) of each element is conducted at 500℃ to check the behaviors of redox for each element. Oxidation and reduction peaks were compared to previous studies and redox behaviors were checked. CV results of Zr have usually 3 reduction peaks. For low concentration (0.2wt.%, 0.5wt.%) of ZrCl4 in the molten salt, there is no reduction peak(ZrCl + e- → Zr) at -1.5V(vs. Ag/AgCl). Because reduction velocity from Zr(VI) to ZrCl is slower than that from ZrCl to Zr metal. Therefore, it is expected that ZrCl is electrodeposited at a cathode in LiCl-KCl will get solved for low concentration of ZrCl4. A reduction peak at -1.0V (vs. Ag/AgCl) is Zr(IV)↔Zr(II) and a reduction peak at -1.2V (vs. Ag/AgCl) is Zr(IV)↔ZrCl. Mainly Oxidation peak was at -0.9 ~ -0.7V (vs. Ag/AgCl). Disproportionate reaction of the niobium also occurs in LiCl-KCl, and redox behaviors of niobium are more complicated than Zr. Niobium chlorides are made up subchlorides due to nonstoichiometric reaction in LiCl-KCl. For niobium subchlorides(NbCly), the value of y is from 2.33 to 3.13. A dominant subchloride is Nb3Cl8. In contrast, cobalt and nickel ions are simply reduced to metals and are equal to previous studies. Since Zr is the most oxidizing among Nb, Co, Ni as results of CV, a applied potential at a anode electrode was decided -0.85V(vs. Ag/AgCl) to dissolve only Zr. No shielding facility is in our laboratory, so irradiated material should not be used. Because Zr-2.5Nb equal to PTs material for nuclear grade couldnt be bought, HANA cladding scraps were used as surrogate materials at the anode. The lab-scale potentiostatic electrorefining experiments of unirradiated Zr-Nb alloys are conducted in LiCl-KCl-5wt% ZrCl4. ICP-MS result of Zr-Nb alloys used for experiments was not included Co, Ni, so these impurities are additionally inserted to an anode basket. First, the experiment was conducted at -0.85V(vs. Ag/AgCl) as the applied potential, but there was no electrodeposition at a cathode and black powders were sunken in molten salts. Thus after oxidizing anode for 15 hours, a working electrode was changed from the anode to the cathode and the applied potential was changed to reduction potential of Zr(-1.0V~-1.6V(vs. Ag/AgCl)). At reduction potential of Zr, electrodeposition was formed at cathode and was analyzed by ICP-MS and XRD to check compositions. Results of XRD showed that the electrodeposition was mainly ZrCl, because ZrCl4 concentration (5wt.%) in molten salts was higher than those of CV condition(0.1~2.0wt.%), and partailly ZrO2 was checked at the electrodeposition. It is expected that Zr was oxidized during XRD analysis, because oxygen concentration in the glovebox was lower than 0.1ppm. In the all of the experiment results, the purity of zirconium increased from 91~93% to 99% when anode composition before electrorefining was compare with the electrodepostion at cathde after electrorefining. A very small amount of Nb was measured, it is expect that impurities of anode for a counter electrode was chemically dissolved during electrorefining experiment and co-electrodeposit with molten salt at cathode. In all the experiment, Co was not discovered and Ni was only discovered within background concentration in LiCl-KCl. Therefore it was thought that Co and Ni would not be dissolved during electrorefining experiment. Decontamination factors were evaluated by using results of electrorefining. And the number of electrorefining steps needed to be exemption level wastes is three. Because recovered zirconium from irradiated wastes is included Zr93, one of the radioactive nuclides, it can be not recycled for general industry, but can be reused for storage drums of radioactive wastes or shielding materials in nuclear industry. Considering only one replacement of PTs for domestic four PHWRs, it is assessed that about 500 waste-drums will be reduced and economic effect is about 19 billion won. If nuclides (Nb94, Co60, Ni63) separated by electrorefining in molten salts or anodes are conducted nuclear transmutation to nuclides which have low radioactivity and short half-life through fast reactors or accelerators or dispose of in High Level wastes disposal repository which will be built, it is expected to solve disposal problem of radioactive wastes. Existing surface decontamination or melting decontamination cant remove selectively activated products as this study. Therefore it is certainly necessary that volume reduction of radioactive wastes and limited recycling of valuable radioactive metals by developing electrochemical decontamination for electrorefining continuously to improve public acceptance for nuclear power as well as to reduce disposal cost and to effectively operate the repository.