- #36
Astronuc
Staff Emeritus
Science Advisor
2023 Award
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The neutron fluence is neglible out at the containment. The iron in the pressure vessel and core structures has a very good scattering (fast removal) cross-section for fast neutrons, and the water moderates the fast/epi-thermal neutrons to thermal energies. Furthermore, the reactor vessel is located down in a cavity, which is surrounded by a lot of interior concrete walls. The cavity is empty during operation, but it is flooded during refueling while the reactor head is removed.
Neutron fluence is certainly a major issue for the core baffle plates, core barrel and structures which surround the core. Together, the pressure vessel and the core barrel form the downcomer (in PWR) or annulus (in BWR) for the coolant returning to the core.
Starting back in the 1980's, utilities implemented the so-called 'low-leakage' loading patterns, whereby high burnup fuel is located toward the periphery of the core. The fuel toward the periphery is operation at about 0.1-0.3 of core average power, depending on depletion of U-235 and conversion of U-238 to Pu-239/240/241.
A problem with high burnup fuel though is the dimensional stability (bowing of the assembly) which has caused some recent headaches in the industry. In addition, grid-to-rod fretting in PWR fuel became a problem during the mid-1990's.
One thing to consider about any advanced reactor design - will it require a new manufacturing facility and proof of concept program?
Gen 3 plants can use existing LWR fuel designs/products.
Some Gen IV concepts call for higher temperatures or different coolants (e.g. gas, or liquid metal). Higher temperatures may severely limit some existing LWR fuel designs.
Certainly, pebble bed, gas-cooled reactor fuel (carbide fuel in carbide/carbon spheres) would require a new plant to manufacture fuel, unless its manufactured in an existing plant. Liquid metal-cooled, fast reactors would require a different manufacturing plant than current LWR designs. If gas reactor or fast reactor fuel used UO2, this could be handled in existing conversion plants, but spherical fuel would obviously require a different manufacturing route than cylindrical pellet fuel, although in the case of fast reactors, existing pellet lines could be used, assuming the fast reactor fuel uses pellets in cylindrical tubing.
Neutron fluence is certainly a major issue for the core baffle plates, core barrel and structures which surround the core. Together, the pressure vessel and the core barrel form the downcomer (in PWR) or annulus (in BWR) for the coolant returning to the core.
Starting back in the 1980's, utilities implemented the so-called 'low-leakage' loading patterns, whereby high burnup fuel is located toward the periphery of the core. The fuel toward the periphery is operation at about 0.1-0.3 of core average power, depending on depletion of U-235 and conversion of U-238 to Pu-239/240/241.
A problem with high burnup fuel though is the dimensional stability (bowing of the assembly) which has caused some recent headaches in the industry. In addition, grid-to-rod fretting in PWR fuel became a problem during the mid-1990's.
One thing to consider about any advanced reactor design - will it require a new manufacturing facility and proof of concept program?
Gen 3 plants can use existing LWR fuel designs/products.
Some Gen IV concepts call for higher temperatures or different coolants (e.g. gas, or liquid metal). Higher temperatures may severely limit some existing LWR fuel designs.
Certainly, pebble bed, gas-cooled reactor fuel (carbide fuel in carbide/carbon spheres) would require a new plant to manufacture fuel, unless its manufactured in an existing plant. Liquid metal-cooled, fast reactors would require a different manufacturing plant than current LWR designs. If gas reactor or fast reactor fuel used UO2, this could be handled in existing conversion plants, but spherical fuel would obviously require a different manufacturing route than cylindrical pellet fuel, although in the case of fast reactors, existing pellet lines could be used, assuming the fast reactor fuel uses pellets in cylindrical tubing.
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