2009 Report to Government on National Research and

than that for uranium-235. Likewise, plu<strong>to</strong>nium-241 (fissile; half life 14.3 years)
decays <strong>to</strong> americium-241 (non-fissile; half life 432 years).
A.69 Several transuranic elements decay by spontaneous fission <strong>to</strong> some degree, and
while this is often a minor pathway (e.g. 5.75 x 10 -6 % for plu<strong>to</strong>nium-240), it can
be very significant in some cases (e.g. 8.39% for curium-248). The presence of
such iso<strong>to</strong>pes is important in criticality because spontaneous fission maintains an
elevated neutron flux and the higher the neutron flux, the easier it will be <strong>to</strong>
initiate a self-sustaining criticality. The inven<strong>to</strong>ry of spontaneously fissioning
iso<strong>to</strong>pes will therefore be important. These are relatively low in AGR spent fuel
but greater in higher burn-up fuels. There are uncertainties in the likely
inven<strong>to</strong>ries of heavy actinides because the physical data for their formation are
not well known.
A.70 The criticality hazard of a GDF depends on fac<strong>to</strong>rs such as the facility design, the
nature and distribution of wastes within the GDF, and the composition and
geometry of the wasteforms. It may be possible <strong>to</strong> reduce the hazard, for
example by including neutron poisons in waste forms or backfill, and hazard
assessments are an important input <strong>to</strong> GDF design.
A.71 The assessment of criticality hazard is demanding, requiring knowledge of
wasteform performance, nuclear physics data, fissile material migration in the
GDF environment, and neutron transport modelling. In the absence of specific
information on the GDF, wasteforms and inven<strong>to</strong>ry, the primary need for R&D
in<strong>to</strong> criticality is <strong>to</strong> preserve and develop capability for use as more details of the
GDF are resolved. While we do not yet know the scope of the work undertaken, it
is nevertheless encouraging that NDA has apparently recognised this need and
has supported Serco Assurance and Imperial College <strong>to</strong> carry out some
research.
Multiphase Fluid-Solid Interactions in Perturbed Geochemical Environments
“Over a wide range of scenarios, the thermodynamic properties of complex
geological fluids and solids, and the reaction rates among phases and
species, must be known <strong>to</strong> define the critical environmental parameters that
control migration or immobilization of wastes.” (USDOE, 2008a)
A.72 It is accepted (USDOE, 2008a) that the emplacement of radioactive waste in<strong>to</strong> a
geochemical environment that has previously reached a steady or equilibrium
state over a relatively long time will lead <strong>to</strong> reactions among minerals, pore
waters, and the wastes or their engineered container/buffer system. The
reactions and interactions triggered by perturbations <strong>to</strong> the original in situ
conditions may be both sudden and gradual, because the changes in
temperature and gas pressure, chemical gradients, oxidation-reduction
conditions, or radiation fields are non-linear and subject <strong>to</strong> feedback effects.
A.73 Most fluid in fine-grained and/or low permeability rocks (e.g., shales, ben<strong>to</strong>nite,
many igneous rocks), is present as nanometre-scale fluid films and in submicronscale
pores (USDOE, 2008a). This leads <strong>to</strong> mesoscopic reaction and transport
rates that are governed by microscopic fluid-mineral surface environments.
Transport of contaminants in<strong>to</strong> these microenvironments is therefore an
important first step leading <strong>to</strong> immobilisation on mineral surfaces or incorporation
CoRWM Document 2543, Oc<strong>to</strong>ber <strong>2009</strong> Page 117 of 151