Journal of Nuclear Fuel Cycle and Waste Technology

For several decades, many countries operating nuclear power plants have been studying the various disposal alternatives to dispose of the spent nuclear fuel or high-level radioactive waste safely. In this paper, as a direct disposal of spent nuclear fuels for deep geological disposal concept, the rod consolidation from spent fuel assembly for the disposal efficiency was considered and analyzed. To do this, a concept of spent fuel rod consolidation was described and the related concepts of disposal canister and disposal system were reviewed. With these concepts, several thermal analyses were carried out to determine whether the most important requirement of the temperature limit for a buffer material was satisfiedin designing an engineered barrier of a deep geological disposal system. Based on the results of thermal analyses, the deposition hole distance, disposal tunnel spacing and heat release area of a disposal canister were reviewed. And the unit disposal areas for each case were calculated and the disposal efficiencies were evaluated. This evaluation showed that the rod consolidation of spent nuclear fuel had no advantages in terms of disposal efficiency. In addition, the cooling time of spent nuclear fuels from nuclear power plant were reviewed. It showed that the disposal efficiency for the consolidated spent fuel rods could be improved in the case that cooling time was 70 years or more. But, the integrity of fuels and other conditions due to the longer term storage before disposal should be analyzed.

Keywords

Spent nuclear fuels,Deep geological disposal(DGD),Disposal efficiency,Rod consolidation,Disposal tunnel,Deposition Hole,Cooling time

Radionuclide migration through the underground multibarrier repository system of high-level radioactive waste (HLW) and transportation to the ecosystem should be thoroughly understood to evaluate the proof-of-safety of the repository system [1]. Groundwater flow is a major route of radionuclide migration, and the migration quantity depends on the total amount of solubilized radionuclides [2]. It is difficult to experimentally assess nuclide solubility under vast variations in spatiotemporal conditions. Geochemical modeling based on a thermodynamic database (TDB) has been employed to evaluate the solubility under diverse repository conditions [3]. The complex formation of radionuclides with groundwater components determines the solubility of radionuclide compounds and directly affects sorption and diffusion through engineered and natural barriers [4, 5]. The oxidation state of a radionuclide is an important factor affecting its chemical reactivity. Ideally, all possible chemical processes including aqueous chemical reactions, solid-phase dissolution, and redox reactions should be considered. The reliability of geochemical modeling depends on the accuracy of the thermodynamic data and the comprehensiveness of the TDB. An effective way to validate a TDB, that is, whether it can describe domestic conditions appropriately, is to directly compare the modeling results with the experimentally measured values. In our previous study, Am solubility was measured in synthesized groundwater (Syn-DB3) by simulating groundwater conditions at the DB3 site in the KAERI underground research tunnel (KURT) [6]. Due to the shortage of Am in stock to prepare solid phases, the experiment was conducted under oversaturation conditions, where aqueous Am (241Am, t1/2 = 432.7 years) [7] was added to Syn-DB3, and the Am concentration in the supernatant was monitored using LSC. In this study, three main issues were identified: 1) The precipitated solids in the samples were not visible and thus could not be characterized. Solid-phase characterization before and after the solubility measurements is important for evaluating the experimental results. 2) The pH values of the samples were not checked at the end of the experiments despite the probable pH changes over a period of long reaction time. 3) The supernatant contained colloidal particles, which would have resulted in overestimation of the dissolved Am concentrations. More rigorous phase separation should be employed to exclude colloidal contributions to the dissolved Am concentrations. In the thermodynamic studies of aqueous species, Nd(III) and Eu(III) are widely used as nonradioactive analogs of Am(III). The chemical behavior of trivalent actinide (An) and lanthanide (Ln) ions is highly correlated with their ionic radii [8, 9]. Am3+ (1.10 Å), Cm3+ (1.09 Å), Nd3+ (1.11 Å), and Eu3+ (1.07 Å) ions have similar ionic radii [8, 10] and differences between their activities in aqueous solutions are often less than the experimental errors [2, 11]. The NEA-TDB project has used these analogies to evaluate the thermodynamics of aqueous Am(III) species using experimental data for Cm(III), Nd(III) and Eu(III) [2, 12]. Notably, the solubility properties of the isostructural solids of Am(III) and Ln(III) can differ considerably, unlike in aqueous chemistry. Even in these cases, the NEA-TDB project discussed the solubility constants of Am(III) solids in comparison with those of isostructural Ln(III) solids, as Ln are free from alpha-radiation damage and theirs crystallinities can be relatively well characterized [2, 12]. The present study aimed to measure solubility of Ln(III) compounds of Ln2(CO3)3·xH2O(cr) (Ln = Sm, Eu) in the Syn-DB3 in under-saturation conditions. Eu was employed as the nonradioactive analog of Am. Sm was also examined because it is a trivalent Ln element included in the safety assessments of the HLW repository system conducted by the Institute for Korea Spent Nuclear Fuel (iKSNF) [13]. The solid compounds were characterized by x-ray diffraction (XRD) before and after the solubility study. Ultrafiltration was used to separate the dissolved metal ions from the solid phase. The pH and composition of the samples were analyzed at the end of the experiment. The aqueous Eu(III) species were investigated using time-resolved laserinduced fluorescence spectroscopy (TRLFS). For comparison, the solubility of Am in Syn-DB3 was also measured under oversaturated conditions in continuation of the previous study [6]. The measured solubility values were compared with geochemical modeling results based on ThermoChimie (ver. 11a) TDB [14].