Characterisation of reactor graphite to inform strategies for the disposal of reactor decommissioning wasteAndrew Hetherington (presented by Dr Paul Norman)University of Birmingham
UNTF April 2010
EC CARBOWASTE Project
CARBOWASTE: Treatment & Disposal of
Irradiated Graphite & Carbonaceous Waste
• Co-ordinator: DR WERNER VON LENSA, Forschungszentrum Juelich GmbH (FZJ-ISR), Germany
• PhD project contributes to Work Packages 3 & 6 : Characterisation and Modelling and Disposal Behaviour
Context of work
• Reactor decommissioning in the UK will give rise to some 90,000 tonnes of graphite
• Major source is core moderator and reflector from decommissioning stage 3 but also fuel element components
• Baseline plan to package and consign to deep geological disposal
• Packaging and disposal costs >£2bn• Not yet shown that this represents the optimum solution• NDA commitment to ‘explore management/treatment options for
graphite waste taking account of worldwide developments’
Inventory
• UK has largest irradiated graphite inventory of any country• Magnox
• ~56,000 tonnes• ~20% LLW, 80% ILW
• AGR• ~22,000 tonnes• 30% LLW, 70% ILW
• 100,000 m3 of packaged material • 25% by volume of the total waste inventory destined for
geological disposal
Overall View of Issues for Graphite Wastes• Graphite has characteristics that make it different from other
radioactive wastes
• Radioactivity arises from activation of impurities
• Significant amounts of long-lived radionuclides• 14C from 14N, nitrides and absorbed N2
• 36Cl from 35Cl left behind on purification of graphite from neutron poisons
• Wigner energy• Stored energy – function of neutron flux, exposure time and
irradiation history• Potentially releasable
Management options
• No internationally accepted solution for dealing with graphite waste
• Most plans involve burial as the favoured option
• A proportion of graphite is LLW but waste acceptance criteria precludes disposal of large quantities to the LLWR near Drigg
• Direct disposal (Baseline)
• Disposal following treatment/cleaning to reduce long-lived radionuclide content
• Gasification followed by discharge to atmosphere or CO2 sequestration
• In principle LLW-type disposal is a possibility
Context of Issues – 14C
• 14C occurs in a number of waste streams, around 80% of the inventory is in graphite (on basis of analysis of 2007 National Inventory) • Half-life 5730 years
• Could be transported to the biosphere either as a gas or by groundwater
• Risk is very low from groundwater
• Gas potentially significant during post-closure phase
Routes of 14C generation in nuclear graphite
• Nitrogen route dominates production, for example - 60% for a Magnox reactor
Reaction Capture Cross-Section (barns)
Abundance of Isotope in Natural Element (%)
14N(n,p)14C 1.8 99.63
13C(n,γ)14C 0.0009 1.07
17O(n,α)14C 0.235 0.04
Why is 14C Important?
• Need to improve confidence in disposal inventory for this radionuclide
• If it is transported as a gas, possible forms are: • carbon dioxide (14CO2) or • methane (14CH4).
• If 14CO2, we assume the gas will react with the cement materials in the repository and form a low solubility carbonate phase (e.g. CaCO3)
• If 14CH4, there would be no reaction, and 14CH4 could be transported to the soil, metabolised by microbes and enter the food chain.
Context of Issues – 36Cl
• The current reference case based on the 2007 Inventory has a total 36Cl inventory of 31 TBq of which approximately 75% (23 TBq) arises in graphite from Final Stage Decommissioning Wastes• Half-life 301,000 years
• Transported to the biosphere by groundwater
• One of the key radionuclides in post-closure performance assessments
• Believe we can meet the regulatory target in an appropriate geological environment
Radiological characterisation of graphite waste
• Modelling production of radionuclides requires knowledge of:• Neutron flux levels in the graphite• Operational history of the reactor• Any incidents which occurred during operation• Type and concentrations of impurities in the original graphite
and coolant
• Work underway to progress understanding of uncertainties in the 14C content of graphite calculated by waste producer.
• Emerging evidence to suggest that operational factors may reduce 14C content.
Reactor modelling
• Aim to use multiple models to give diversity of approach• Modelling based on “Pippa” reactor type at Chapelcross
• WIMS • TRAIL• FISPIN
- Preliminary results indicate 14C levels of ~25 kBq/gram- 36Cl levels of ~500 Bq/gram
• MCNP whole core model under development
• Tracking the reactions which are of interest
Pin-cell model
Moderator
CladdingFuel
Validation of results
• Results of predictive methods need to be backed up by analysis of representative samples
• Samples of Magnox and AGR graphite available from NNL’s graphite handling facility in B13 at Sellafield
• Spectral gamma scanning inappropriate for the long-lived nuclides of interest
• Method of Beta-counting will be used in sample analysis
Summary
• Graphite treatment/disposal a major challenge to the nuclear industry
• Research required in order to move forward with strategy development
• Accurate characterisation of graphite waste is very important for interim storage and disposal safety cases
• But…..can predictive methods deliver results that are representative of the true radiological inventory?