Pulse radiolysis study on the reactivity of NO3˙ radical toward uranous(iv), hydrazinium nitrate and hydroxyl ammonium nitrate at room temperature and at 45 °C.

Concentrated nitric acid solutions subjected to radiation produce radicals of extreme importance in the reprocessing of spent nuclear fuel. Knowledge of the different rate constants of the reactions involved in this chemistry is needed to improve the efficiency of the process and to define safe operating practices. Pulse radiolysis measurements are performed to find the rate constant of the reaction between NO3˙ radicals and U(iv) in highly concentrated nitrate solution. The optimal stabilization conditions toward thermal oxidation are defined for the considered solutions at room temperature and at 45 °C by adding anti-nitrous agents such as hydrazinium nitrate (HN) and hydroxyl ammonium nitrate (HAN). The decay of the NO3˙ radical is monitored and its reaction rates with HN, HAN and U(iv) are found to be 1.3 × 105, 1.5 × 107 and 1.6 × 106 M-1 s-1 at room temperature. The latter value is more than 10 times lower than the one currently used in numerical codes for simulation of the long-term radiolytic degradation associated with the reprocessing and storage of spent nuclear waste. At 45 °C, conditions similar to the reprocessing of spent fuel, the values of the rate constants of NO3˙ radical toward HN, HAN and U(iv) increase and are found to be 2.6 × 105, 2.9 × 107 and 9.3 × 106 M-1 s-1.

Radiolysis of water in nanoporous gold.

Nanoporous gold was used as a high specific surface material to describe the water radiolysis in the vicinity of metal surfaces. Porous gold monoliths were prepared by electrolytic dissolution of silver in Au-Ag alloys and characterized by small angle neutron scattering. The hydroxyl radical production under gamma irradiation was measured by benzoate scavenging in water confined in 50 nm porous gold or in silica glasses of similar pore size, and in bulk water. Whereas a silica interface induces minimal modifications to the HO˙ radical production with respect to bulk water, HO˙ production near gold is enhanced more than seven times on the short time scale, and almost completely suppressed by reaction with the metal on the long time scale.