Types of Reactors
Reactors are generally purpose-built, and reactors built and operated for plutonium production are less efficient for electricity production than standard nuclear electric power plants because of the low burnup restriction for production of weapons grade plutonium. The types nuclear fission reactors which have been found most suitable for producing plutonium are graphite-moderated nuclear reactors using gas or water cooling at atmospheric pressure and with the capability of having fuel elements exchanged while on line. Several distinct classes of reactor exist, each optimized for one purpose, generally using fuel carefully chosen for the job at hand. These classes include the following:
Research reactors. Usually operates at very low power, often only 1-2 MW or less. Frequently uses high-enriched uranium fuel, although most newer models use no more than 20-percent enrichments to make the theft of fuel less attractive. Fertile material ( 238 U for Pu, 6 Li for tritium) can be encapsulated in elements known as "targets" for insertion into the reactor core. The reactor can also employ a fertile blanket of 238 U in which plutonium can be bred. Cooling requirements and shielding requirements are relatively modest. Some research reactors can be refueled while operating, and such reactors are of special concern for plutonium production because they can limit fuel burnup, which enhances the quality of the plutonium compared to that obtained from reactors that require high burnup before shutdown and refueling. Research reactors using nearly 100-percent enriched material produce almost no plutonium in their fuel because the fertile species, 238 U, has been removed. These reactors can, however, be built with a surrounding "blanket" of natural or depleted uranium in which plutonium can be bred efficiently. The Osirak reactor built in Iraq and destroyed by Israeli aircraft was of this type.
Propulsion reactors. Primarily found on submarines and large-surface combatant ships, nuclear reactors have given new operational freedom to the underwater navy and deliver increased time on station combined with high speed for both the submarine service and the surface navy. The United States and Russia have built most of the world's shipboard reactors. The world's first nuclear powered cargo ship was the U.S.N.S. Savannah; however, nuclear propulsion power has not been particularly successful in the commercial world. Today, the only operating commercial vessels using nuclear propulsion are Russian icebreakers. To keep the core size small, propulsion reactors generally use highly enriched uranium as fuel. In principle, a propulsion reactor core could be surrounded with a fertile blanket and used to produce plutonium. In practice, this has never been done.
Space reactors and mobile power systems. Nuclear reactors have been used from time to time, usually by the former Soviet Union, to provide on-orbit electrical power to spacecraft. In principle, they will use HEU as fuel to keep the core mass and volume small. Other spacecraft have been powered by the heat released by the radioactive decay of 238 Pu.
Power reactors. These are used to generate electric power. Few use fuel enriched to greater than 5-7% 235 U. Practical power levels range from a few hundred MW(e) (three times that in terms of thermal power output) to 1,000 or 1,500 MW(e)-meaning 3,000-4,000 MW(t). Power reactors designs have included water cooled-graphite moderated (the Soviet RBMK used at Chernobyl), boiling (light) water, pressurized (light) water, heavy water-moderated and cooled, graphite-moderated/helium cooled, and liquid metal-moderated. Most power reactors operate under pressure and cannot be refueled in operation. The RBMK and CANDU reactors are notable exceptions to this rule. The CANDU reactor was developed for the Canadian nuclear power program and is a deuterium oxide (heavy water) moderated reactor which can operate on natural uranium fuel.
Breeder reactors. These reactors generate plutonium at a rate greater (numbers of nuclei per unit time) than they burn their fissile fuel (numbers of nuclei per unit time). Normally, breeders use fast neutrons and irradiate a fissile 238 U blanket. Plutonium produced in the fuel generally has a higher fraction of 240 Pu than that produced in other reactors, but the Pu made in the blanket of uranium surrounding the core is usually of a high quality, containing very little 240 Pu.
Production reactors. These are used to make plutonium (and often tritium) efficiently. Production reactors are frequently graphite-moderated and either air-, CO 2 -, or helium-cooled. The longer a given sample of fuel is irradiated, the greater the build-up of 240 Pu, an isotope which decays by spontaneous fission and which should be minimized in weapon fuel. Consequently, plutonium production reactors usually are designed to be refueled while operating (on-line refueling) so that relatively little 240 Pu is found in the "spent" fuel.
|Join the GlobalSecurity.org mailing list|