The Radiolytic Steady-State and Factors Controlling H2 Production

UoM administered thesis: Phd

  • Authors:
  • Thomas Donoclift

Abstract

Sellafield is home to the UK's largest repository of nuclear waste, including reprocessed uranium and plutonium, as well as a backlog of unprocessed used fuel and waste kept in outdated storage facilities; commonly referred to as "legacy waste". For this reason, Sellafield has often been called the most hazardous place in Western Europe and as such, is currently undergoing a multi-billion pound decommissioning and clean-up operation. Each on-site facility has unique challenges associated with it, many of them presenting situations where the radiation chemistry aspects of the material degradation are not well understood. The key factors that can affect water radiolysis processes in the Sellafield challenges are a high pH environment, the presence of magnesium hydroxide, the presence of iron oxide, and the presence of organic materials. This work examines the effect each of these factors has on H2 and H2O2 production in water radiolysis as well as developing a computational model to offer some understanding to the kinetic behaviour of water radiolysis under such conditions. The computational model was able to replicate experimental measurements of radiolytic H2 and H2O2 production in both aerated and deaerated water at neutral pH, and provide a further understanding of the role of dissolved oxygen in water radiolysis. Measurements of H2O2 from solutions containing NaOH have shown that an increase in pH generally results in a higher steady state of H2O2, while measurements of H2 show a similar increase with a maximum production rate at pH ~11. The model was also able to closely replicate these experimental measurements with some over prediction, which highlights a gap in our understanding of high pH radiolysis and also brings into question the validity of the estimated rate constant for the reaction: O- + O2- -> 2OH- + O2 k= 6.0×10^8 M^-1 s^-1which was originally determined from kinetic model calculations designed to describe the decay of ozonide (O3ˉ) during pulse-radiolysis studies of high pH solutions conducted byK. Sehested et al in 1982.The radiolysis of magnesium hydroxide slurry also resulted in an increased yield of hydrogen gas but had little effect on the yield of hydrogen peroxide. The hydrogen yield was 0.52 molecules per 100eV while a NaOH solution of equivalent pH gave a yield of 0.27, however interference from carbonate may be the cause of the increased yield. A surface effect was also estimated to contribute 0.05 molecules per 100 eV to the hydrogen gas yield.Hydrogen gas and hydrogen peroxide was measured from the radiolysis of aqueous methanol. This was modelled with a near agreement, but modifications to the model were necessary; highlighting areas of the model that need improvement, as well as providing a reaction scheme from which a more comprehensive model for aqueous methanol radiolysis could be developed.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • Simon Pimblott (Supervisor)
Award date1 Aug 2017