Development of nuclear criticality safety controls on intermediate-level waste packages

Abstract

Intermediate-level wastes (ILW) include substantial quantities of fissile material and controls are required to ensure that its storage, transport and disposal does not present a nuclear criticality hazard. This paper describes the Radioactive Waste Management Directorate's research to develop package fissile material limits (in the form of screening levels) for four different categories of ILW, defined according to uranium or plutonium composition: (1) irradiated natural and slightly enriched uranium (uranium containing up to 1.9 wt.% 235 U); (2) low-enriched uranium (uranium containing up to 4 wt.% 235 U); (3) high-enriched uranium (uranium containing up to 100 wt.% 235 U); and (4) separated plutonium (plutonium containing up to 100 wt.% 239 Pu).The derivation of package screening levels was supported by neutron transport calculations that addressed conditions during waste package transport to a geological disposal facility (GDF), during the GDF operational phase and after GDF closure. The analysis included consideration of combinations of events and processes that could result in fissile material accumulation and concentration after GDF closure, when waste packages have deteriorated sufficiently for fissile material to be mobilized. The results of the calculations have provided input to Radioactive Waste Management Directorate's decision making on setting waste package screening levels.

A Review of Zirconolite Solid Solution Regimes for Plutonium and Candidate Neutron Absorbing Additives

“…A key aspect of the safety case is an assessment of waste package criticality during transport, GDF operation, and post closure [6]. Generic Criticality Safety Assessments (CSAs), as described by Hicks (2007), are considered for waste packages containing one or more of the following four waste components: irradiated natural and slightly enriched uranium; low enriched uranium; high enriched uranium and; separated plutonium [7]. In the interest of brevity, this report will focus only on separated Pu.…”

“…In summary, the advantage of Cd 2+ within the formulation may be similar to that of Hf 4+ insofar as Cd 2+ may be able to effectively replace Ca 2+ as the A-site cation in zirconolite on the basis of ionic radii, though the phase CdZrTi 2 O 7 has not yet been reported. Furthermore, Cd is an attractive additive due to its relatively low cost index in relation to Gd/Hf/In.…”

Geological disposal of radioactive waste: underpinning science and technology

“…The first major section on wasteforms, containment and criticality begins with an introduction to package evolution and criticality research studies relevant to the UK disposal programme by Cristiano Padovani and co-workers of the NDA (Padovani et al, 2012). There follow papers which describe: waste container durability by Nick Smart (AMEC) and co-workers (Smart et al, 2012); the corrosion and expansion of grouted Magnox waste by James Cronin and Nicholas Collier (NNL) (Cronin and Collier, 2012); the durability of potential plutonium wasteforms under repository conditions by Guido Deissmann (Brenk Systemplanung) and co-workers (Deissmann et al, 2012), the chemical durability of vitrified wasteforms and the effects of pH and solution composition by Stephen Swanton (AMEC) and co-workers (Utton et al, 2012); three-dimensional imaging of inhomogeneous lithologies using X-ray computed tomography in the characterization of drill core from the Borrowdale Volcanic Group by Dirk Engelberg (The University of Manchester) and co-workers (Engelberg et al, 2012); the release of uranium from candidate wasteforms by Nicholas Collier (National Nuclear Laboratory) and co-workers (Collier et al, 2012); the development of criticality safety controls on intermediate-level waste packages by Tim Hicks (Galson Sciences Ltd) and co-workers (Hicks et al, 2012b); an experimental study to evaluate the effect of polymeric encapsulants on the corrosion resistance of intermediate-level waste packages by Robert Winsley (AMEC) and co-workers (Winsley et al, 2012); the site-bond modelling of porous quasi-brittle media by Andrey Jivkov (The University of Manchester) and co-workers (Jivkov et al, 2012); the break-up testing of waste-form materials by Martin Metcalfe (NNL) and co-workers (Metcalfe et al, 2012); and initial studies on the effects of radiation, thermal ageing and aqueous environments on the stability and structure of candidate polymeric encapsulant materials by John Dawson (AMEC) and coworkers (Dawson et al, 2012).…”

“…Discussion of gas begins with an overview of gas research in support of the UK geological disposal programme by Steve Williams of the NDA (Williams, 2012). There follow contributions which describe: gas migration experiments in bentonite and their implications for numerical modelling by Caroline Graham (BGS) and coworkers (Graham et al, 2012); the rate and speciation of volatile carbon-14 and tritium releases from irradiated graphite by Graham Baston (AMEC) and co-workers (Baston et al, 2012d); gas flow in Callovo-Oxfordian clay including results from laboratory and field-scale measurements by Jon Harrington (BGS) and coworkers (Harrington et al, 2012a); the comparison of alternative approaches to modelling gas migration through a higher strength rock by Andrew Hoch and Martin James (AMEC) (Hoch and James, 2012); evidence for gas-induced pathways in clay using a nanoparticle injection technique by Jon Harrington (BGS) and coworkers (Harrington et al, 2012b); multicomponent gas flow through compacted clay buffer in a higher activity radioactive waste geological disposal facility by Shakil Masum (Cardiff University) and co-workers (Masum et al, 2012); the migration and fate of 14 CH 4 in subsoil: tracer experiments to inform model development by George Shaw (The University of Nottingham) and co-workers (Atkinson et al, 2012); a data analysis toolkit for long-term, largesc ale experiments by Daniel Benn ett (Geoenvironmental Research Centre) and coworkers (Bennett et al, 2012); and understanding the behaviour of gas in a geological disposal facility by George Towler (Quintessa Ltd) and coworkers (Towler et al, 2012).…”