Improvement of cement solidified radioactive waste acceptance criteria reflecting the disposal condition of silo-type repository

Abstract

Owing to the generation of radioactive waste in countries operating nuclear-related facilities, the disposal of radioactive waste is an inevitable problem that should be solved. To dispose of the radioactive waste, the radioactive waste repositories were constructed and in operation in several countries. For the sufficient isolation of radioactive waste from the biosphere, the stability of the radioactive is necessary. Therefore, in the radioactive waste repositories, the waste acceptance criteria (WAC) were established which are different according to the radioactive waste repository. In the Gyeongju low- and intermediate-level waste (LILW) silo-type repository, the WAC for cement solidified radioactive waste is established which is similar to the WAC stated in the ‘NRC, Technical Position of Waste Form, Rev 1’. The type of the repository in U.S. is generally the trench-type type repository which has a approximately few tens of disposal depth and allows the disposal of low-level radioactive waste. In the case of the Gyeongju LILW silo-type repository, the burial depth is larger than 100 m and the intermediate-level waste can be disposed of in the repository in maximum. Thus, the disposal condition difference between the trench-type and silo-type is significant, and the importance of assessing the adequacy of the current WAC of the Gyeongju LILW repository arose. Among the WAC of the Gyeongju LILW silo-type repository, the main criteria are compressive strength and leachability index. To check the adequacy of the current compressive strength criterion and its test method, the structural stability of the waste packages which is a concrete disposal container containing the solidified waste was analyzed under the expected static and dynamic loadings. Then, the effect of test conditions, loading rate, and height-to-diameter ratio, on the compressive strength of cement solidified radioactive waste was evaluated. The effective coring methods for the assessment of the homogeneity of the cement solidified radioactive waste were developed during the compressive strength measurement of the waste. Finally, the leachability index difference of Cs, Sr, and Co according to the type of leachant and initial concentration was analyzed by performing several ANS 16.1 test methods. By performing the structural stability analysis on the 16-pack container disposal container through the finite element method (FEM), COMSOL Multiphysics, the structural stability of the disposal containers on the top and bottom positions was checked. From the analysis, the structural instability of the top container was observed by the weight of backfilling materials. For the validation of the modeling results, the fracture experiment for a 1:6 scaled concrete disposal container was performed together with the COMSOL modeling. As a result, similar results obtained from the previous modeling were revealed, and the well-matched results between the experiment and modeling were checked. In addition, the seismic data reaching the silo was derived from deconvolution analysis reflecting the geological characteristics of the repository site. By using the obtained seismic data as input, the seismic analysis of stacked 16-pack disposal containers was performed with ABAQUS. The effect of test conditions, loading rate, and height-to-diameter ratio on the compressive strength of the cement solidified radioactive waste was evaluated. The cement solidified specimens were prepared by varying the cement content. The loading rates of ASTM C39 and KS F2405, and the height- to-diameter ratios following two and 200 L drums were compared. The effect of test conditions on the compressive strength of cement solidified specimens was statistically analyzed in parametric and non- parametric methods. From the results, the effect of test conditions on the compressive strength of cement solidified specimens was significant when the cement contents were below 50 w.t%. The effective coring method for cement solidified specimens was developed by performing the homogeneity analysis made of MATLAB code. By comparing the detection rate of inhomogeneous 200L cement solidified waste according to the coring methods, the most effective coring method for cement solidified waste during the compressive strength measurement was revealed. Finally, the effects of the initial concentration of Cs, Sr, and Co inside the cement solidified waste and the type of leachant on the leachability indexes of Cs, Sr, and Co were estimated. A total of four concentrations of Cs, Sr, and Co were prepared by applying the Portland type I cement as binding material. The crystal structures of Cs, Sr, and Co inside the cement were analyzed by X-ray diffraction analysis and energy dispersion spectroscopy. Two types of leachant were prepared, deionized water and concrete saturated groundwater which are the leachant suggested in ANS 16.1 and the actual leachant of a silo-type repository, respectively. The concentrations of Cs, Sr, and Co in the Portland type I cement were measured. As a result, the significant effect of initial concentration and type of leachant was found for Co, and the unexpected effect of Cs, Sr, and Co concentrations in the Portland type I cement was revealed. As a result of this dissertation, the adequacy of the current WAC of the Gyeongju LILW silo- type repository was assessed, and the development direction of the WAC was suggested.