1. In-core fuel management
Gregory A. Moses
Text: Nuclear Fuel Management
Harvey W. Graves, Jr. pp

2. Learning objectives
Understand variables and constraints of in-core fuel management
Understand units of fuel burnup
Understand a linear reactivity model of fuel burnup

3. Reactor core composed of fuel assemblies

4. Fuel loading variables
Fuel enrichment
Re-load batch size–number of assemblies
Fuel loading pattern of fresh and partially spent fuel assemblies
Control mechanisms

5. Fuel loading constraints
Lead time requirements of fuel fabrication
System power demand schedule
Discharge burnup of the fuel
Capacity of reactivity control system

6. Fuel burnup units Energy generated per unit of fuel consumed MWD/MTU
Megawatt days per metric ton of Uranium
Burnup is limited by ability of fuel to sustain chain reaction (reactivity) and by radiation damage
30,000-35,000 MWD/MTU is limit for thermal reactor fuel

7. Simple model of fuel reload
On each cycle (1/n)th of the fuel is replaced
Each fuel batch experiences a discharge burnup of Bd
Each fuel batch on each cycle experiences a burnup of Bd/n
kreactor is the uncontrolled multiplication factor (excess reactivity)
ki is the infinite multiplication factor of a fuel batch (excess reactivity)

8. Simple model of fuel reload
The uncontrolled fuel reactivity is the average of the infinite multiplication factors of the n batches of fuel that reside in the reactor. The newer batches have greater ki and the older batches have lower ki, perhaps ki<1.

9. Uniform fuel depletion
k0 is the uncontrolled infinite multiplication factor of the fuel batch when it is fresh.
Bn is the burnup of the batch in a single cycle. The n refers to the number of batches that the reload scheme includes.
is a constant of proportionality with units of 1/Bn.
kF is the uncontrolled infinite multiplication factor necessary to sustain a chain reaction at the end of an operating cycle.

10. In core fuel management 2
Gregory A. Moses
Text: Nuclear Fuel Management
Harvey W. Graves

11. Single cycle refueling

12. Single cycle refueling
For a single cycle refueling
And the fuel burnup capability is

13. Summary
Fuel cycle decisions depend on many engineering factors, some of which are not related to the core itself
To a pretty good approximation the linear burnup model or linear reactivity model is valid for slightly enriched water reactors

14. Learning objectives Analyze multi cycle fuel management
Compare to single cycle schemes

15. Two cycle refueling

16. Two cycle refueling
At the end of each cycle one batch of fuel has been burned for one cycle and the other batch has been burned for two cycles. Thus
and

17. 1/3 more burnup per batch in the two cycle reload
Each batch in the two cycle reload scheme is burned for 2B2. Thus in terms of the single cycle reload burnup
The two cycle reload scheme gives 1/3 more burnup of the fuel for the same initial and final multiplication factors k0 and kF, i.e. for exactly the same fuel.

18. n cycle fuel reload scheme
The relation between end-of-cycle core multiplication factor kF and the fresh fuel batch infinite multiplication factor k0 and the batch burnup in general is

19. Discharge fuel burnup of n-cycle reload scheme
Batch burnup
in single cycle
Discharge fuel burnup is

20. Limiting case of continuous refueling
With continuous refueling , the burnup obtainable for a given initial fissile loading enrichment is twice that obtainable by refueling an entire core at each refueling.

21. Reduction of excess reactivity for n-cycle reload scheme
Initial multiplication factor at start of cycle
Change in multiplication factor in one cycle

22. Comparison to single cycle change in multiplication factor
for single
cycle in n-
cycle reload
for single
cycle in 1-
cycle reload

23. Comparison to single cycle change in multiplication factor

24. Number of cycles--tradeoffs
Shorter cycles have less excess reactivity to control but have more refueling downtime.
Longer cycles have more excess reactivity to control but have less refueling downtime.

25. Summary
Multi cycle fuel reloads are compared to single cycle fuel reload as a reference
Limits of multi cycle reloads are computed using linear reactivity model

26. Presenter:
Production:
Music:
Software:
Gregory A. Moses
Brian Kiedrowski
Travis Parisi
Josh Kading
Mike Litzkow
University of Wisconsin - Madison
© Gregory A. Moses, 2002