Studies into the Effect of Insoluble Fission Products on the Generation of Ag(II) for the Dissolution of MOx Fuel

With new light water reactors and advanced reactors such as Generation IV fast reactors having the ability to more efficiently utilise recycled uranium and plutonium as mixed oxide (MOx) fuels, future options to close the fuel cycle would require advanced reprocessing capability that could deal with spent MOx fuels as well as uranic fuels. As such, investigation into the dissolution of MOx fuel is required because this is known to be more difficult to dissolve in nitric acid for reprocessing. One known method for the efficient dissolution of MOx fuel is through the use of an electrogenerated Ag²⁺ mediator as an advanced oxidation process (AOP) reagent that may promote the oxidative dissolution of the spent fuel.<br/>One particular aspect of the dissolution of irradiated MOx fuel that is being increasingly recognised as being in need of further investigation is the effect of the presence of insoluble fission products (IFPs) on the efficacy of dissolution. These occur mostly in the form of metallic particles, referred to as ε-particles, consisting largely of the noble metals Mo, Pd, Rh, Ru, and Tc. As they are fission products arising from the in-reactor irradiation of fuel, they have varying compositions dependant on fission yield, oxygen potential, temperature gradient and fuel burnup. Being predominantly noble metals, they are also known to be resistant to dissolution by concentrated nitric acid. Too, such noble metal particles are known to catalyse water oxidation by AOP reagents such as Ag²⁺; thus, they may act to promote a parasitic water oxidation by Ag(II), potentially impacting severely on the capacity of Ag(II) to participate in the target oxidative dissolution of spent fuel. Due to the release of the component metals into solution being proportional to their oxidation potential, the effect of their presence on the dissolution of MOx needs to be further understood.<br/>Thus, here we present the results of experiments exploring the efficacy of electrogenerated Ag(II) for fuel dissolution in the absence and presence of IFPs. In the first instance, exploration of the fundamental electrochemistry of the Ag(I)/Ag(II) system under conditions relevant to fuel dissolution plant – so-called headend – has shown the electrogeneration of Ag(II) from Ag(I) to be an EC’ process i.e. an electrochemical step E, followed by a chemical step, C’, that regenerates the reactant for the electrochemical step. In the case of the silver system, these correspond respectively to the electrogeneration of Ag(II) and its loss through water oxidation – the latter of which may be catalysed by the presence of the IFPs. Through simple voltammetric measurements and the use of extant analytical models of the voltammetric behaviour of EC’ systems, kinetic rate parameters for both steps have been determined both on commonly used platinum electrodes and on boron-doped diamond (BDD). A newly available electrode material, BDD exhibits lower rates than platinum for oxygen evolution from the electrooxidation of water; as such oxygen evolution may compete with Ag(I) oxidation, use of BDD may be advantageous to the electrical efficiency of Ag(II) electrogeneration.<br/>Baseline electrochemical characterization experiments were conducted as a function of headend-relevant nitric acid concentrations, both in the absence and presence of IFP particles, so allowing for quantitative assessment of the effect of acidity and the presence of ε-particles on the net rate of Ag(II) generation. These studies into investigating the effect of IFPs on the rate of Ag(II) generation has successfully demonstrated that the rate parameter for the C’ step increases with increasing amounts of Pd, Ru, and Rh as IFP simulants – and that this IFP-enhanced rate of Ag(II) reduction is proportional to the surface area of IFP particles present in solution.