Nuclear Fuel Requirements

A fission rate of 3.1 x 1010 fissions per second will provide 1W of power (thermal). For a power plant with a thermal output of 3000 MWt as heat or an electrical output of about 1000 MWe of electricity the number of 235U nuclei that must be fissioned per second is 9.3 x 1019.30 This corresponds to the fissioning of 3.1 kg of 235U per day. Uranium in spent fuel contains about 0.8 w % 235U.

29Zircaloys are alloys of zirconium and tin with minor constituents.

30MWe indicates the rate at which electrical energy is generated and MWt indicates the rate at which thermal energy is generated. MWe is about 30% of MWt for nuclear power plants with water-moderated, water-cooled reactors. (Note: in much of the literature, the symbols MWe and MWt are used without supscripts.)

FIGURE 14-11 (a) Fuel rod containing uranium dioxide pellets, (b) Assembly of fuel rods. From The Harnessed Atom, U.S. Department of Energy, DOE/NE-0073.

The fuel rods must be replaced periodically, not only because of partial depletion of the 235U, but also because of buildup of fission products that are neutron absorbers, distortion of the pellets as each 235U atom is replaced by two atoms of other elements, and stress cracking of the cladding. The extent to which fuel can be irradiated before being replaced is expressed in terms of "burnup": MWd/MTHM (megawatt-days per metric ton of heavy metal as LEU uranium before irradiation). Variations in expressing burnup include the use of MWDT (megawatt-days thermal), MTIHM (metric tons of initial fuel loading), MWd/t, GWd/t (t = metric ton), and TJ/kgU.

For light water reactors (described in Section 14.7.4.1.1), typical burnup in the past has been 30,000-40,000 MWd/MTHM with refueling (replacing one-third or less of the fuel) every 12-18 months. Increased burnup (e.g., 55,000 MWd/MTHM), resulting in longer time period between refueling shutdowns, has been achieved in many nuclear power plants in various ways, including improved fuel element design. Fewer and shorter outages (reduced from 30-50 days to about 20 days) for refueling and maintenance have improved the capacity factor and the economics of plant operation. The capacity factor for a specified period of time is the percentage of design (maximum) power output actually achieved.

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