Current light water reactors are typically designed to achieve burnup of about 50 GWd/tU. With newer fuel technology, and particularly the use of advanced burnable absorbers, these same reactors are now capable of achieving up to 60 GWd/tU. Some studies show that soon, even with the present enrichment limit (5 wt %), fuel burnup could be extended near to 70 MWd/kg. Further increase in fuel burnup is impossible without the relaxation of the present enrichment limit (5%).
As can be seen, there has been a clear trend over the last decades toward increasing fuel burnup in light water reactors. In nuclear power plants, high fuel burnup is desirable for:
- Reducing the number of fresh nuclear fuel assemblies required and thus reducing spent nuclear fuel assemblies generated. Both aspects lead to improvements in the economics of the fuel cycle.
- Reducing the duration of refueling outage.
- High burnup results in a lower mass of spent fuel discharged per unit of electricity generated, reducing spent fuel handling and transportation.
- A benefit (crucial aspect for some operators) is that loading the “high” burnup assemblies in the periphery reduces the neutron flux on the pressure vessel.
- Reducing the potential for diversion of fissile material from spent fuel for non-peaceful.
On the other hand, there are signals that increasing burnup above 50 or 60 GWd/tU leads to significant engineering challenges, and even it does not necessarily have to lead to economic benefits.
See also: Etienne Parent. Nuclear Fuel Cycles for Mid-Century Deployment. MIT. 2009.
Higher burnup fuels require higher initial enrichment to sustain reactivity. Since the amount of separative work units (SWUs) is not a linear function of enrichment, it is more expensive to enrich higher enrichments. For instance, to produce one kilogram of uranium enriched to:
- 5% U-235 requires 8.9 SWU if the tails assay is 0.20%
- 4% U-235 requires 6.3 SWU if the tails assay is 0.20%
But there are also operational aspects of high burnup fuels associated especially with the reliability of such fuel. The main concerns associated with high burnup of fuels are:
- Increased burnup places additional demands on fuel cladding, which must withstand in the reactor environment for a longer period.
- The longer residence in the reactor requires a higher corrosion resistance.
- Higher burnup leads to a higher accumulation of gaseous fission products inside the fuel pin resulting in a significant increase in internal pressure.
- Higher burnup leads to increased radiation-induced growth, which can lead to undesirable changes in core geometry (fuel assembly bow or fuel rod bow). FA bow can increase control rods drop time due to friction between control rod and bowed guide tubes.
- These factors must be considered because they can lead to a decrease in operational reliability.
- High burnup fuel generates a smaller fuel volume for reprocessing but with a higher specific activity.
Strategic management and decision-making regarding the middle part of nuclear fuel cycles is a very specific problem of power engineering. Such evaluation must also contain nuclear calculations and business-economic evaluation; thus, they cannot be carried out separately. Detailed nuclear calculations are necessary because they can exclude many promising strategies. On the other hand, they do not provide any information about benefits or costs. Strategic decision-making must be carried out based on business-economic evaluations.
Special reference: Technical and economic limits to fuel burnup extension, Proceedings of a Technical Committee meeting, IAEA, 11/2009