Reactor control

Control of a nuclear reactor is accomplished through control of the rate of fission reactions in the reactor. Power output is directly proportional to the rate of fission reactions. When a reactor is critical, it has a sustained power output. When the reactor is supercritical, the power output is increasing, and when it is subcritical, power decreases until the reactor shuts down. Reactivity is way of describing the criticality condition of the reactor. Positive reactivity means a supercritical reactor, zero reactivity means a critical reactor, and negative reactivity means a subcritical reactor. The rate of fission in a reactor is controlled through the insertion and withdrawal of neutron-absorbing material, such as boron, in the form of control rods which are interspersed with the fuel rods. (In pressurized water reactors boron can also be added chemically to the water.) By lowering and raising the control rods, which absorb neutrons available for fission reactions, the rate of fission reactions, and hence the reactor's power output, can be controlled. A particular property of fission makes it possible to achieve reactor control. While most neutrons emitted from the fission process are released immediately, (known as prompt neutrons), some are emitted seconds to minutes later. These are known as delayed neutrons. In uranium-235 fission in a thermal reactor, the proportion of delayed neutrons is about 0.65 percent. In the case of plutonium-239 fission, the proportion is only 0.2 percent. If the reactivity stays below the proportion of delayed neutrons, the reactor can be controlled. But if it increases above this proportion, control is lost and there will be a runaway nuclear chain reaction until the reactor is destroyed, as happened at Chernobyl. A smaller fraction of delayed neutrons can affect reactor control during emergencies, unless the reactor has appropriate control equipment. Plutonium tends to absorb neutrons efficiently not only at the neutron energies for which light water reactors are designed, but also at a somewhat higher neutron energy. Heating of the fuel above normal operating temperatures tends to increase the rate of plutonium fission, which in turn increases the temperature. This phenomenon, called a positive temperature coefficient of reactivity (a positive feedback loop of reactivity and temperature) can cause problems for reactor control. The problem can be addressed by adding neutron absorbers, like erbium, with the ability to absorb neutrons at particular thermal energies. For more information on technical issues related to nuclear power in the United States, see Arjun. Makhijani and Scott Saleska, The Nuclear Power Deception, (Takoma Park, MD: Institute for Energy and Environmental Research, April, 1996.)