bei48482_FM

456 Chapter Twelve

The fuel for a water-moderated reactor consists of uranium oxide (UO 2 ) pellets

sealed in long, thin tubes. Control rods of cadmium or boron, which are good absorbers

of slow neutrons, can be slid in and out of the reactor core to adjust the rate of the

chain reaction. In the most common type of reactor, the water that circulates around

the fuel in the core is kept at a high pressure, about 155 atmospheres, to prevent

boiling. The water, which acts as both moderator and coolant, is passed through a heat

exchanger to produce steam that drives a turbine (Fig. 12.23). Such a reactor might

contain 90 tons of UO 2 and operate at 3400 MW to yield 1100 MW of electric power.

The reactor fuel must be replaced every few years as its^235 U content is used up.

Breeder Reactors

Some nonfissionable nuclides can be transmuted into fissionable ones by absorbing

neutrons. A notable example is^238 U, which becomes^239 U when it captures a fast

neutron. This uranium isotope beta-decays with a half-life of 24 min into^23993 Np, an

isotope of the element neptunium, which is also beta-active. The decay of^239 Np has

a half-life of 2.3 days and yields^23994 Pu, an isotope of plutonium whose half-life against

Fuel rods being loaded into the core of a 1,129-MW reactor at the William McGuire Nuclear Power

Plant in Cornelius, North Carolina.

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