The Safe Storage of Plutonium Oxide

The safe and secure storage of plutonium (Pu) materials is a matter of international concern with ~250 tonnes of separated Pu stockpiled worldwide. Over half of this, resulting from ~50-years civil nuclear fuel reprocessing, is in long term storage in the UK whilst the Government considers options for its final treatment and disposition. This Pu is stored as calcined PuO2 powder in nested, sealed steel storage cans.

Under certain circumstances, gas generation may occur with consequent storage package pressurisation. In practice, this is rarely seen and empirically derived criteria are used to maintain safe storage conditions. Nonetheless, this is a potential scenario that must be avoided in practice - thus the fundamental mechanisms that could lead to pressurisation must be understood. 5 main routes have been suggested:

(i) Helium accumulation from alpha decay; (ii) Decomposition of polymeric packing material; (iii) Steam produced by H2O desorption from hygroscopic PuO2 due to self-heating; (iv) Radiolysis of adsorbed water; and, (v) Generation of H2 by chemical reaction of PuO2 with H2O.

The last 4 mechanisms are being studied as part of the work of the EPSRC TRANSCEND (TRANSformative SCience & Engineering for Nuclear Decommissiong) consortium (www.transcendconsortium.org). This project, a collaboration between the University of Lancaster, the National Nuclear Laboratory (NNL) and Sellafield Ltd, seeks to understand the role that the first of these mechanisms, helium from alpha decay, might play in pressurisation.

Innovative sample preparation and thermal / gravimetric analysis methods for the study of this will first be developed at the university and then deployed on real samples at NNL. Data & knowledge generated will be transferred to Sellafield Ltd via the NNL and used in better underpinning of the Pu storage safety cases.

This studentship is offered as part of the "Growing skills for Reliable Economic Energy from Nuclear" (GREEN) Centre for Doctoral Training (https://www.nuclear-energy-cdt.manchester.ac.uk/), a collaboration between the Universities of Lancaster, Manchester, Liverpool, Leeds and Sheffield. GREEN aims to develop and deliver the research and skills required to address key challenges in the field of nuclear energy across the entire fuel cycle.