Leaching Behavior of Cs-137 from Cement-Based Matrices for Immobilization of Irradiated Uranium–Graphite Fuel

During the operation of the first core of the IGR pulsed graphite reactor (1961–1966), 2.6 tonnes of irradiated highly enriched uranium (HEU)–graphite fuel were accumulated. For its immobilization, cementation with preliminary down-blending using depleted uranium was proposed, with the 235U concentration in the final matrix limited to less than 50 ppm. Among the radionuclides accumulated in irradiated uranium–graphite fuel, 137Cs is of particular concern because it is a long-lived uranium fission product with high mobility in cementitious systems. In this study, four cement compositions with water-to-binder ratios of 0.38–0.42 and Portland-cement-to-fly-ash ratios of 1:2.5 and 1:1.77 were investigated for the immobilization of irradiated uranium–graphite fuel. The 137Cs activity in the cement samples was determined using the substitution method with a point calibration gamma source and the MCNP5 code to calculate correction factors accounting for the measurement geometry and gamma-ray self-absorption. For all compositions, the initial 137Cs leaching rates exceeded 1 × 10−3 g/(cm2·day) during the first 1–3 days of testing; however, from day 7 onward, the leaching rates decreased below the regulatory limit and continued to decline during the subsequent test period. Based on these results, as well as on the leachability of calcium and silicon dioxide, two candidate cement–matrix compositions were selected. The results provide an experimental basis for further scale-up studies aimed at the immobilization of irradiated HEU graphite fuel from the IGR reactor.