Fuel Depletion And Burnable Poisons Flashcards
How does fuel depletion affect reactivity in the core over core life?
As fuel depletes, kexcess lowers
Factors of six factor formula affected over core life
Fast Fission Factor
Resonance Escape Probability
Thermal Utilization Factor
Reproduction Factor
How is Fast Fission Factor affected over core life?
As fuel depletes, U-235 concentration lowers which lowers the amount of “Fast” fissions this causes Fast Fission Factor to lower slightly over core life.
1.04@ BOL
1.03@ EOL
How does Resonance Escape Probability change over core life?
U-238 absorbs neutrons and B- decays to Pu-239 which then absorbs a neutron and becomes Pu-240 (assuming it doesn’t fission). Pu-240 has a higher microscopic cross section for neutron absorption at resonance peaks than U-238, therefore resonance escape lowers over core life.
How is Thermal Utilization Factor affected over core life?
Net effect is slight increase because of lower boron concentration over core life (lower competition with the moderator and fuel utilizes more neutrons).
How is Reproduction Factor affected over core life?
Pu-239 builds in and produces more neutrons per fission than U-235, however, the production of Pu-239 lags the depletion of U-235, therefore Reproduction Factor lowers slightly over core life.
Net effect on keff over core life
Without diluting, keff will lower.
Because we dilute, keff remains constant over core life.
What is a Fuel Cycle?
Core operation between refueling events.
Average fuel cycle results in 20,000 Mwd/MTU
What is Conversion Ratio?
Low enriched fuel creates more fuel.
As U-238 absorbs neutrons it produces Pu-239 (fissile).
Ratio of fuel production to fuel depletion is the Conversion Ratio.
Typical ratios are 50%-70% (0.5-0.7).
A 4.0% enriched core (at BOL) will typically be 0.8% U-235 and 0.6% Pu-239 enriched by EOL.
What is a Burnable Poison?
Installed poisons used to control large amounts of kexcess.
Deplete at a rate that adds approximately the same positive reactivity that will compensate for fuel depletion.
Also shape neutron flux for more uniform power density.
Allow boron concentration to be lower and MTC to be negative.
Types of burnable poisons
Burnable Poison Rod Assemblies (BPRA)
Poison integral to fuel itself.
What is Chemical Shim?
Use of Boron concentration to control reactivity.
Advantages of Chemical Shim.
Cost effective
Offsets effect is decreasing kexcess over core life
Flatter flux profile than rods produce
Finer reactivity control than rods
Allows for fewer control rods for kexcess control
Disadvantages of Chemical Shim
High concentration can result in +MTC
Additional costs for BA handling, storage, processing.
Metallurgical issues (corrosion)
Change in concentration is slow process and can’t be used to address faster transients.
Advantages of Fixed (installed) Burnable Poisons
Directly loaded in specific locations of the core to shape flux profile.
Allows boron concentration to be lower and more negative MTC.
