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?