Nuclear Fuel Steam Plant

In a nuclear power plant, many of the components are similar to those in a fossil-fueled plant, except that
the steam boiler is replaced by a Nuclear Steam Supply System (NSSS). The NSSS consists of a nuclear
reactor and all of the components necessary to produce high pressure steam, which will be used to turn
the turbine for the electrical generator.

Like a fossil-fueled plant, a nuclear power plant boils water to produce electricity. Unlike a fossil-fueled
plant, the nuclear plant’s energy does not come from the combustion of fuel, but from the fissioning

(splitting) of fuel atoms.

The most common fuel for the electrical producing reactor plants in the United States is uranium. The
uranium starts out as ore, and contains a very low percentage (or low enrichment) of the desired atoms
(U-235). The U-235 is a more desirable atom for fuel, because it is easier to cause the U-235 atoms to
fission (split) than the much more abundant U-238 atoms. Therefore, the fuel fabrication process
includes steps to increase the number of U-235 atoms in relation to the number of U-238 atoms
(enrichment process).

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REACTOR FUEL ASSEMBLIES

Both boiling water reactor and pressurized water reactor fuel assemblies consist of the same major components. These major components are the fuel rods, the spacer grids, and the upper and lower end fittings. The fuel assembly drawing on page 1-11 shows these major components (pressurized water reactor fuel assembly). The fuel rods contain the ceramic fuel pellets. The fuel rods are approximately 12 feet long and contain a space at the top for the collection of any gases that are produced by the fission process. These rods are arranged in a square matrix ranging from 17 x 17 for pressurized water reactors to 8 x 8 for boiling water reactors. The spacer grids separate the individual rods with pieces of sprung metal. This provides the rigidity of the assemblies and allows the coolant to flow freely up through the assemblies and around the fuel rods. Some spacer grids may have flow mixing vanes that are used to promote mixing of the coolant as it f...

Boiling Water Reactors (BWRs)

US interest in water-cooling of reactors stemmed from the Hanford reactors and was furthered by the submarine PWRs. It was known that water allowed to boil is more effective in removing heat, but boiling was thought likely to trigger instabilities in a reactor core. The water in such a core serves also as moderator; if a steam bubble forms, the local effect on reactivity is swift and its consequences difficult to predict. But experiments in the mid-1950s demonstrated that water could indeed be allowed to boil in a reactor core. Accordingly, a new design of reactor was developed, which is by far the simplest in concept of all power reactors: the boiling water reactor, or BWR. BWRs and PWRs are often mentioned in the same breath, as 'light water reactors' or LWRs. In a BWR the water serves as moderator, reflector and coolant - and in addition, when boiled, produces steam which is ducted directly to drive a turbo-generator. Once through the turbines, the coolant water...

Light Water Reactors

Pressurized Water Reactors (PWRs) Like the first British power reactors, which were built to produce weapons-plutonium, the first US power reactors also began under military auspices, albeit specifically as power plants. The US Navy realized after the Second World War that a submarine powered by nuclear fuel would not need to resurface to replenish oxygen supply, since the 'burning' of nuclear fuel - unlike that of oil - does not require oxygen. Spurred by this idea, and constrained by the space limitations in a submarine, US designers developed a reactor using a core of relatively high power density, with fuel elements immersed in a tank of ordinary water - called 'light water' to distinguish it from heavy water - under sufficient pressure to keep it from boiling. The 'first power reactor ever built', according to its builders, went critical on 30 March 1953 in a land-based mock-up of a submarine hull at the National Reactor Testing Station in I...

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