Gamma-radiolysis of NaCl and NaHCO3 aqueous solutions relevant to high-level radioactive waste disposal

Introduction: The concept of geological disposal of radioactive wastes has been investigated in several countries. In Japan, high-level radioactive wastes arising form reprocessing of nuclear fuel are planned to be disposed in a deep underground repository after being solidified with borosilicate glass in canister and encapsulated by metal overpack and situated in buffer material (i.e. bentonite). These materials are expected to act as engineered barrier system to isolate radionuclides from human beings. However, corrosion and failure of overpack will occur after certain period of time and then radiolysis of groundwater is induced around the vitrified waste. Oxidative radiolytic products diffused into bentonite would alter the oxidation states of radionuclides and eventually affect their migration. Although several groups have already studied radiolysis of several groundwater types, they have not given a systematic understanding on radiolytic processes in groundwater. In this presentation we show our preliminary study on radiolysis of NaCl and NaHCO3, which are one of the most important solutes of saline and fresh groundwater. Experiment and Analysis: Aqueous solutions of NaCl and NaHCO3 were prepared by using distilled water. Glass vials were filled with solutions and capped with rubber septums. These vials were subjected to 60Co γ-irradiation (dose rate: 1kGy/h). After irradiation, H2O2 was quantified by the Ghormley method and dissolved H2 and O2 were measured with UG-510 atomic pressure injection mass spectrometer (Renesas Eastern Japan Semicon. Inc., Japan). FACSIMILE (MCPA Software, U.K.) and CHEMSIMUL (Risφ National Lab., Denmark) softwares were used to simulate concentrations of radiolytic products as a function of time. Results and Discussion: Measured concentration of H2O2, H2 and O2 in irradiated air-saturated pure water reached steady value after several hours gamma-irradiation and steady state concentration of H2O2 as well as the time profile to reach the steady state were successfully simulated by typical sets of G-values and chemical reactions of gamma-radiolysis for pure water. In contrast, measured concentration of radiolytic products in air-saturated NaCl and NaHCO3 aqueous solutions showed continuous accumulation of molecular products during irradiation up to about 100h, and initial rise of H2O2 concentration became slower as NaCl and NaHCO3 concentration increased. Slight modification of the reaction data set was necessary to reproduce the continuous increase of H2O2 concentration by the simulation of gamma- radiolysis of NaCl solution: Generation of Cl, which is usually omitted in neutral solutions due to equilibrium Cl- +.OH = ClOH-., was added to the data set. Furthermore, G-values of NaCl solutions have been modified to reproduce lower initial increase of H2O2 concentration. On the other hand, no modification of chemical reaction data sets was necessary to reproduce accumulation of molecular products by gamma-radiolysis of NaHCO3 solutions. In these aqueous solutions primary radicals produced by gamma-radiolysis are converted to solute radicals. They are less reactive towards molecular products and thus might have resulted in accumulation of molecular products. Acknowledgment: This presentation is a part of results from the research project 'Radiation effects on engineered barrier systems in high level radioactive waste disposal' endorsed by Ministry of Economy, Trade and Industry, Japan. (authors)