Wednesday, 6 February 2013

Wigner Energy In Irradiated Graphite

On the 26th January, I posted details of a question, that I had submitted to the Nuclear Decomissioning Agency (NDA). The question was in relation to a underground nuclear waste repository:
Could you please inform me of the steps that would be taken to ensure in a (possible) future geological repository, in West Cumbria, that there will be no release of Wigner Energy from irradiated graphite? As I understand it, annealing is carried out to remove, and prevent Wigner Energy, but I believe that some graphite from *** has not undergone the annealing process - is this correct?

If there is a risk from Wigner Energy in a future repository, surely the common sense approach would be to dispose of irradiated graphite by incinerating?

Also, what steps would be taken to ensure that Uranium Hydride does not ignite?
Today, I'm pleased, after many weeks of asking these questions to various people/organisations, to announce that I have received a response to my questions:

Response from NDA-RWMD
Wigner Energy
Neutron irradiation in a nuclear reactor modifies the graphite moderator, displacing atoms from their original positions in the crystal structure. This results in energy being ‘stored’ in the lattice. The types and concentrations of displaced atoms and associated changes that are created in the graphite depend on various factors, such as the irradiation energy of the neutrons and the temperature of the reactor during irradiation. These displacements and changes can be reversed through a thermally activated annealing process, releasing the stored energy as heat, also known as Wigner energy.

Some annealing will occur during irradiation, depending on the temperature of the graphite in the reactor. This auto-annealing will affect the retained stored energy release characteristics. It is common practice to characterise the stored energy in irradiated graphite samples using calorimetry to determine both total stored energy and stored energy release characteristics.

Stored energy release is a thermally activated process, and experiments have shown that the rate of release of energy is negligible unless the temperature is raised above that at which the graphite was irradiated. The rate of release of energy increases with temperature, but if the temperature is held constant energy release falls off rapidly with time.

Graphite from UK reactors has been irradiated over a wide range of temperatures. Some graphite, notably that from the Windscale Piles, was irradiated at temperatures as low as 30°C, although all commercial power reactors (Magnox and AGR) operate at much higher temperatures. More importantly, in early development and operation of graphite moderated nuclear reactors, it was recognised that a self-sustaining release of stored energy could occur provided that three conditions are met:
  • there is a source of heat large enough to raise the temperature of the graphite above its irradiation temperature
  • there is a high level of stored energy available in the graphite
  • the rate of loss of heat by cooling is not too large.

The Windscale Piles were subject to annealing cycles during their operation and it was during one of these in 1957 that Pile 1 suffered the runaway excursion that led to the ‘Windscale Fire’. Neither of the two Piles were operated after that incident, and it is believed that the graphite in Pile 2 contains residual amounts of low temperature Wigner energy.

Graphite from the Windscale Piles is stored at Sellafield, either as the moderator blocks still in the Piles, or as numerous ‘boats’ and ‘dowels’ associated with fuel channels, which are stored in the Pile Fuel Cladding Silo. Under current waste management plans this graphite will be subject to geological disposal.

It is not possible to say that there will be no energy release from any irradiated graphite in a disposal facility. Indeed, the temperature in such a facility could exceed the minimum original irradiation temperature of graphite from the Windscale Piles, and therefore it would be expected that unannealed graphite from the Windscale Piles would release some stored energy during disposal. However, with appropriate package and facility design, this released heat will not be significant to safety.

The strategy for ensuring the safe disposal of graphite containing residual Wigner energy will take account of:
  • the amount of residual stored energy in low temperature irradiated graphite
  • the irradiation temperature of the graphite
  • the expected temperature of waste packages in the disposal facility
  • losses of heat to other materials (heat sinks) in waste packages.

The outcome of these analyses is not yet known, and it is possible that some annealing would be required as part of preparing the graphite for disposal, or that packages of irradiated graphite may need special emplacement in modified disposal facility designs. There are currently no plans to incinerate irradiated graphite.

The current understanding of Wigner energy in irradiated graphite can be summarised as follows:
  • The mechanisms of energy accumulation and subsequent release are well understood
  • Only the stored energy in graphite that was irradiated at low temperatures could be released after disposal
  • Other waste materials, mixed with graphite in waste packages, will absorb any released heat and prevent runaway energy release
  • The strategy for dealing with bulk ‘low temperature’ graphite from Pile 2, including the potential to anneal it before disposal, remains under review.

Uranium Hydride
Uranium hydride only forms from metallic uranium in the presence of a source of hydrogen (i.e. in anaerobic conditions). Hydrogen can be created in significant quantities from the corrosion of reactive metals, principally uranium, aluminium and Magnox alloy. Rapid exposure of uranium hydride to oxygen (in air) will result in an exothermic reaction converting the hydride to oxide forms and releasing hydrogen. The exothermic reaction can release large amounts of energy, potentially igniting the released hydrogen and mobilising volatile species that may be associated with the uranium.

NDA-RWMD is aware of the risks associated with the potential formation of uranium hydride within waste packages, and a defence-in-depth approach is taken. Best practice for waste package design is to ensure that metallic uranium is minimised, that packages are robust, and that wastes are effectively immobilised (usually by encapsulation) in a wasteform. This is particularly important for two reasons: robust packages will mitigate against package failure in the event of an accident, preventing exposure of any uranium hydride to air encapsulated wastes will provide a passive wasteform, providing conditions favourable to conversion of hydride to stable and safe uranium oxides, and restricting the ingress of air.

Implementation of these measures would significantly reduce the possibility of a uranium hydride exothermic event. Ongoing design work for the geological disposal facility is looking at the control of credible accident scenarios and is aiming to prevent the accidental dropping of packages in underground operations.

The current understanding of uranium hydride can be summarised as follows:
  • The conditions required for formation of uranium hydride, and its subsequent exothermic reaction, are well understood.
  • Best practice package design will prevent exposure of uranium hydride to air during storage.
  • Design of the disposal facility to prevent impact accidents will eliminate the potential for rapid hydrogen release from any uranium hydride in waste packages.

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