Presentation on theme: "Characterisation of reactor graphite to inform strategies for disposal of reactor decommissioning waste Andrew Hetherington University of Birmingham UNTF,"— Presentation transcript:
Characterisation of reactor graphite to inform strategies for disposal of reactor decommissioning waste Andrew Hetherington University of Birmingham UNTF, April 2011
EC CARBOWASTE Project CARBOWASTE: Treatment & Disposal of Irradiated Graphite & Carbonaceous Waste Co-ordinator: WERNER VON LENSA Forschungszentrum Juelich GmbH (FZJ-ISR), Germany
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 but not yet shown that this represents the optimum solution Packaging and disposal costs >£2bn 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 m 3 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 14 C from 14 N, nitrides and absorbed N 2 36 Cl from 35 Cl 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 CO 2 sequestration In principle LLW-type disposal is a possibility
Context of Issues – 14 C 14 C 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 Readily assimilated in living organisms Could be transported to the biosphere either as a gas or by groundwater Gas potentially significant during post-closure phase Need to improve confidence in disposal inventory for this radionuclide
Routes of 14 C generation in nuclear graphite Nitrogen route dominates production, for example - 60% for a Magnox reactor ReactionCapture Cross- Section (barns) Abundance of Isotope in Natural Element (%) 14 N(n,p) 14 C1.899.63 13 C(n,γ) 14 C0.00091.07 17 O(n,α) 14 C0.2350.04
Context of Issues – 36 Cl Current reference case based on the 2007 Inventory has a total 36 Cl inventory of 31 TBq of which approximately 75% (23 TBq) arises in graphite from Final Stage Decommissioning Half-life 301,000 years Highly mobile Transported to the biosphere by groundwater One of the key radionuclides in repository post-closure performance assessments
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 Concentrations of impurities in the original graphite and coolant Dialogue underway to progress understanding of uncertainties in the 14 C content of graphite calculated by waste producer. Emerging evidence to suggest that operational factors may reduce 14 C content.
Reactor modelling Multiple models used to give diversity of approach Modelling based on “Pippa” reactor type at Chapelcross WIMS TRAIL FISPIN -Preliminary results indicate 14 C levels of ~25 kBq/gram - 36 Cl levels of ~500 Bq/gram MCNP whole core model under development Tracking the reactions which are of interest
Pin cell fuel graphite block control rod channel fuel cladding coolant ~ 1.2m
Outputs Aim to determine activity of whole core Map of flux across core showing proportions with activation of ‘x’ Understand degree of segregation of graphite according to activation levels Help inform NDA strategy
Validation of results Results of predictive methods need to be backed up by analysis of representative samples Major question marks over errors on impurity levels in virgin material 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
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?