Liquid Metal Fast Breeder Reactor Program. Volume III. Environmental Statement Page: 71 of 622
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as the gaseous diffusion process is used for this purpose. The fuel used in LWRs,
therefore, contains a mixture of about 3% U-235 atoms and 97% U-238 atoms, both in
the form of uranium dioxide (U02)* pellets encased (clad) in either stainless steel
or zirconium alloy (zircaloy) tubing (see Figure 6A.1-12). The fuel pellets are
right cylinders usually about one-half inch long with the diameter varying from
about 3/8 in. to over 1/2 in., depending on reactor core design,1 ground to close
dimensional tolerances. The voids in the fuel rod, especially the annulus (0.003-
in. to 0.005-in. diametral gap) between the pellet and the cladding, are filled with
helium under varying degrees of pressure, commensurate with the anticipated range
of external pressure forces during reactor operation.
Completed rods are inspected and assembled into fuel bundles. When inserted in the
pressure vessel of a LWR along with associated control rods and structures, the
fuel assemblies (bundles) collectively are called the nuclear core of the reactor.
Large PWR and BWR plants typically employ partial refueling annually; in BWRs, about
one-fourth of the fuel assemblies are removed and replaced with fresh fuel each
year, while in PWRs about one-third of the assemblies are replaced annually. Spent-
fuel assemblies are stored under water at the power plant for a period of five to
six months prior to shipment to a fuel-reprocessing plant.
6A.l.l.3.2 Nuclear Steam Supply Systems
BWR Description
The nuclear steam supply system (NSSS) of a BWR1 consists primarily of the reactor
vessel and equipment inside the vessel. (See Figures 6A.1-13 and -14.) The nuclear
fuel assemblies are arranged inside a core shroud in the reactor vessel. Water boils
in the core, and a mixture of steam and water flows out the top of the core and
through steam separators at the top of the core shroud. Steam from the separators
passes through dryers to remove all but traces of entrained water and then leaves
the reactor vessel through pipes to the turbine generator. Water from the steam
separators and water returned from the turbine condenser mix, flow downward through
the annulus between the core shroud and the reactor vessel, and return to the bottom
of the core. Because the energy supplied to the reactor coolant (water) from the
hot fuel is transported directly (as steam) to the turbine, the BWR system is
termed a "direct cycle" system. The pressure in a typical BWR is maintained at
*Prior to LMFBR operation, an appreciable fraction of the fuel in some replacement
fuel cores will contain plutonium (Pu02 in place of the U-235 oxide) in a mixed
oxide, U02/Pu02, pellet.6A.1-26
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Liquid Metal Fast Breeder Reactor Program. Volume III. Environmental Statement, report, December 1, 1974; Washington, District of Columbia. (https://digital.library.unt.edu/ark:/67531/metadc1019855/m1/71/: accessed August 15, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.