In-core fuel management Gregory A. Moses Text: Nuclear Fuel Management Harvey W. Graves, Jr. pp. 269-278

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

Reactor core composed of fuel assemblies

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

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

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

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)

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.

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.

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

Single cycle refueling

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

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

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

Two cycle refueling

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

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.

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

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

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.

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

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

Comparison to single cycle change in multiplication factor

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.

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

Presenter: Production: Music: Software: Gregory A. Moses Brian Kiedrowski Travis Parisi Josh Kading Mike Litzkow University of Wisconsin - Madison © Gregory A. Moses, 2002 http://eteach.engr.wisc.edu