Fuel management

The removal of spent fuel is an important step in the decommissioning of research reactors. Benefits of early defuelling include decreased radiological hazards, timely implementation of dismantling, downgrading of the operational licence, shutdown of some systems (e.g. surveillance), and reduced safeguards requirements. In addition, as long as fuel remains in the fuel storage pools, continuous manning of the unit with shift workers may be required, albeit with a reduced number. If consideration is given to adopting shorter refuelling cycles towards the end of the plant’s life the period required for cooling the fuel in the fuel storage pool is reduced. Thus the pool can be emptied earlier than would otherwise be the case. Costs could thus be reduced accordingly.

As long as all infrastructure and provisions are in place, final defuelling can be done in the same way as during plant operation. However, if removal of

fuel is delayed for a very long time, loss of qualified staff and necessary equipment could create a problem. Also, the costs of surveillance and maintenance may increase dramatically.

In some research and prototype reactors defuelling is not a routine operation and requires special planning during the operation to decommissioning transition period. Even in projects where a transport container is available, some adaptation may be needed, e.g. to couple the container to the reactor [64]. Typical handling issues include:

(a) Reactors where refuelling is a one-off operation;

(b) No fuel storage pond is available;

(c) Lifting equipment may not be capable of carrying fuel transport containers;

(d) Space for loading fuel elements into transport containers may not be available.

Timely removal of reactor fuel is an important prerequisite of any decommissioning strategy. Recent international developments have highlighted this. Many research reactors were provided with fuel from another country. Reactor operators planned to return the spent fuel to the supplier when necessary, but in many cases this has now become impractical or difficult for a variety of reasons. As a result, spent fuel has been accumulating in at-reactor storage facilities for some time with an indefinite future storage period, complicating practical completion of decommissioning projects.

In the USA, the University of Illinois chose in 1999 to put its TRIGA reactor into a safe storage mode for at least a decade because they were unable to dispatch its spent fuel. The estimated cost of keeping the reactor in safe storage was US $23 000 per year, not including the salaries of the engineers responsible for periodic surveillance and monitoring. Even with a computerized monitoring system to allow remote monitoring of the entire reactor facility it was estimated that at least two individuals would spend 25%

of their time on monitoring and surveillance. That brought the estimated cost of the project to nearly US $50 000 per year [65].

As another example the Egyptian ETTR 1 reactor, which uses Russian EK-10 fuel, has recently built a new spent fuel storage/fuel encapsulation facility to protect the fuel from corrosion prior to the Russian Federation making a decision on when it will take back the fuel. Repatriation of spent fuel of Russian origin will hopefully solve this major issue soon [2, 45, 66].

The USA has extended to 2016 its programme to take back US origin research reactor fuel (the deadline was recently extended by 10 years due to delays in developing new high density low enriched uranium (LEU) fuel to

replace the HEU fuels currently in use around the world [67]). The US spent fuel repatriation policy has been in place for a few years and has contributed to solving this major issue for many research reactors worldwide [45, 66]. For reactors that operate with non-US or non-Russian origin (e.g. indigenous) fuel, however, disposition of used fuel is very problematic. Other technical aspects related to the removal of spent fuel from research reactors have been documented by the IAEA in Ref. [4].

In addition to the removal of nuclear fuel it is highly desirable to eliminate the potential for criticality during the transition period. If the spent fuel and other nuclear materials cannot be moved outside the nuclear installation, decommissioning cannot be fully completed. One example of the avoidance of this problem was the removal of spent fuel from the JASON reactor in late 1998. Special procedures were required to remove the spent fuel from the reactor hall without jeopardizing building integrity (Fig. 4).

In order to comply with the acceptance criteria for the safe transport of nuclear fuel, the precise unirradiated and irradiated characteristics of each fuel assembly have to be verified and be in full compliance with all safety rules at

FIG. 4. Spent fuel removal from the JASON reactor, UK.

every step of transport, reception, unloading, storage and treatment of the spent fuel in the reprocessing plants, if any. COGEMA’s experience [68] has shown that the first verification stages are based on documents, long before the fuel is shipped. General criteria for the acceptance of sound spent fuel were developed long ago and are well consolidated. They relate to geometry and mechanical integrity after chopping, non-leaking fuel and, for damaged and leaking fuel, to supplementary criteria having to be met.