Question

Describe nuclear fuel burnup. What causes it? How does increasing the burnup effect the case at which a reactor is controlled in transients? How does fuel burnup affect overall reactivity in a reactor?

Answer 1

Increase burnup places additional demands on fuel cladding, which must withstand in the reactor environment for longer period.

The longer residence in the reactor requires a higher corrosion resistance.

Higher burnup leads to higher accumulation of gaseous fission products inside the fuel pin resulting in significant increase in internal pressure.

Higher burnup leads to increased radiation induced growth, that can lead to undesirable changes in core geometry (fuel assembly bow or fuel rod bow). FA bow can result in an increased 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 volume of fuel for reprocessing, but with a higher specific activity.(also known as fuel utilization) is a measure of how much energy is extracted from nuclear fuelfuel and a measure of fuel depletion. The most commonly defined as the fission energy released per unit mass of fuel in megawatt-days per metric ton of heavy metal of uranium (MWd/tHM), or similar units. Fuel burnup defines energy release as well as it defines isotopic composition of irradiated fuel. Since during refueling, every 12 to 18 months, some of the fuel – usually one third or one quarter of the core – is replaced by a fresh fuel assemblies.

effects of increasing burnup

• 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 fuel cycle.
• Reducing duration of refueling outage.
• High burnup results in a lower mass of spent fuel discharged per unit of electricity generated. This can reduce spent fuel handling and transportation.
• A benefit (crutial aspect for some operators) is that loading of 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.
• Increase burnup places additional demands on fuel cladding, which must withstand in the reactor environment for longer period.
• The longer residence in the reactor requires a higher corrosion resistance.
• Higher burnup leads to higher accumulation of gaseous fission products inside the fuel pin resulting in significant increase in internal pressure.
• Higher burnup leads to increased radiation induced growth, that can lead to undesirable changes in core geometry (fuel assembly bow or fuel rod bow). FA bow can result in an increased 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 volume of fuel for reprocessing, but with a higher specific activity.