The extensive studies of used fuel dissolution inside a failed nuclear waste container have been reviewed. The primary controlling factor is the redox condition set at the fuel surface by water radiolysis, its evolution with time as radiation fields decay, and how it is influenced by the presence of oxidant scavengers, especially H2, produced by corrosion of the steel liner in the container.

If the container fails early, and oxidizing conditions are established, dissolution will be a corrosion reaction driven by radiolytically-produced H2O2. As radiation fields decay and conditions become less oxidizing, the corrosion rate will decrease, and its evolution with time, will be influenced by the formation of insoluble UVI corrosion product deposits, stabilized by calcium and silicate in the groundwater. These deposits could partially block fuel corrosion, but also lead to locally acidified sites at which the corrosion rate is increased. These sites would be pores in the deposit and/or flaws in the fuel surface. If the groundwater contains sufficient bicarbonate, deposition would be inhibited and the fuel corrosion possibly accelerated by the formation of bicarbonate-uranyl ion complexes.

As conditions become less oxidizing these issues become less important. The generation of acidity within deposits is unlikely, and since corrosion would be limited by the available concentration of oxidants, the influence of bicarbonate on the corrosion rate disappears. For sufficiently low dose rates, a threshold for the transition from corrosion to chemical dissolution has been identified and validated by dissolution rate measurements on used and alpha-doped fuels. Beyond this threshold dissolution will be solubility-controlled and extremely slow. The transition from radiolytic corrosion to chemical dissolution can be rapidly induced by oxidant scavengers, especially H2. Small concentrations of H2 suppress the redox condition to the threshold even for used fuels with high gamma/beta radiation fields. For sufficiently high concentrations, the establishment of the reversible H2/H+ reaction on noble metal epsilon particles can lead to galvanic protection of the fuel against corrosion. It is possible that this effect could lead to protection of the fuel against corrosion almost from the time of container failure. Under these conditions fuel dissolution and radionuclide release would be extremely small.

Subject
Corrosion rate, Containment, Dissolution, Solubility, Corrosion reactions, Acidity, Groundwater, Corrosion products, Oxide surfaces, Oxidants, Deposit corrosion, Risk reduction, Oxidation
Keywords:
Nuclear waste, uranium dioxide, nuclear fuel corrosion, radiolytic corrosion
© 2008 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2008
Association for Materials Protection and Performance (AMPP)
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