III. Nuclear Fuel Cycles


This section provides background information about nuclear fuel cycles, which are the industrial operations necessary to operate nuclear reactors either for civil or military purposes. A primary motivation for this section is to assist a discussion of which facilities would be verified under a treaty.

Reprocessing facilities are key to military fuel cycles aimed at producing plutonium for nuclear weapons. They may or may not be important in civil nuclear fuel cycles.

Uranium enrichment plants are necessary to produce HEU for military programs. They may also be included in civil fuel cycles, for example, when reactors require enriched uranium fuel.

Civil Nuclear Fuel Cycles

The civil nuclear fuel cycle consists of all the industrial operations necessary to fuel nuclear reactors, whose main job is the production of electricity from heat produced when the nuclei of the atoms of heavy material are split. Although the heart of any nuclear fuel cycle is the reactor itself, many other types of facilities are also necessary. The fuel cycle includes the mining and milling of uranium ores, enrichment of uranium, fabrication and use of nuclear fuel, reprocessing of used nuclear fuel, and disposal or long-term management of radioactive wastes or unreprocessed spent fuel.

Civil nuclear fuel cycles may not involve reprocessing, leading to their characterization as "once-through" fuel cycles (see figure III.1). Those civil fuel cycles with reprocessing are often called "closed" fuel cycles (see figure III.2).

The material used as the fuel in current nuclear power reactors is mainly uranium. However, some power reactors use recycled plutonium or highly enriched uranium (HEU) fuels. Fuels involving thorium and uranium 233 have also been studied. This section considers those fuel cycles which involve uranium or plutonium fuels.

Front end of the Fuel Cycle (1). Uranium ore occurs naturally in the earth's crust and is mined by conventional mining techniques. Typically, only about a kilogram of uranium is contained in a tonne of uranium ore. As a result, uranium is concentrated in a uranium mill, which is typically located near the mine. Uranium concentrates are known commercially as "yellowcake."

Concentrates are shipped from the uranium mill to a uranium refinery or conversion plant. These facilities remove chemical impurities and convert the purified uranium into the chemical form needed for the next step in the fuel cycle. Much of it is processed into uranium hexafluoride gas, which is the form used in gaseous diffusion and gas centrifuge enrichment plants. Some of the purified uranium is processed into uranium metal or uranium oxide, depending on the specific reactor type.

Enrichment plants produce low-enriched uranium (LEU) to fuel light water reactors and other types of reactors, or HEU to fuel research reactors and a few power reactors. For commercial fuel cycles, the enrichment plants are typically large.

Enriched uranium hexafluoride, natural uranium metal, or natural uranium oxide are shipped to a plant for fabricating reactor fuel elements. Uranium hexafluoride is first converted into uranium oxide or other forms used in the reactor fuel. For light water reactors, the uranium oxide is pressed into pellets, which are heated or sinctured, and loaded into zirconium alloy tubing. Individual rods are then assembled into bundles, ready for shipment to the reactors.

The fuel is loaded into a reactor, and remains there until it is discharged. The length of time the fuel remains in the core depends on several factors, including reactor type, the initial enrichment of the fuel, and fuel management procedures. In a light water reactor, a typical fuel lifetime is three to five years.

Back end of the Fuel Cycle. Discharged fuel is intensely radioactive and requires special handling and storage at each reactor site. At the reactor, the fuel may be stored in special water pools or bays, or in heavy, air-cooled casks.

If the fuel is to be reprocessed, it is sent in heavily shielded casks to the reprocessing plant. At the reprocessing plant, the fuel cladding, or outer layer, is removed chemically or mechanically, the fuel material is dissolved in acid, and fissile and fertile materials are separated from fission products and from each other. For commercial power reactors, the most important materials are plutonium and uranium. The fission products and other nuclear wastes are stored at the reprocessing plant pending final disposal or shipment back to the original fuel owner.

Separated plutonium and uranium are sent to facilities for conversion into appropriate forms for use as reactor fuel. The most common type of plutonium fuel is called mixed oxide (MOX) fuel which is a mixture of natural uranium and plutonium oxide. It is principally used in light water reactors.

If the spent fuel is not to be reprocessed, it is either stored at the reactor site or at an "away from reactor" storage site. Under current assumptions, the spent fuel will eventually be sent to a geological repository for permanent storage.

Military Plutonium Nuclear Fuel Cycles

Plutonium for nuclear weapons has been produced in industrial complexes. In the United States and former Soviet Union, the production complexes were huge, involving many large nuclear reactors and reprocessing plants, and substantial uranium enrichment enterprises. Both countries have since dramatically downsized these complexes or converted these facilities to the production of civil materials. The production complexes in other states have been far smaller.

The following briefly describes a fuel cycle to make plutonium for nuclear weapons that existed in the United States in the 1980s, but has since been closed. A more complicated fuel cycle, operated by the United States at the Savannah River Site until the late 1980s produced both plutonium and tritium. It is represented schematically in the appendix to this section.

The fuel cycle of the N Reactor at Hanford Washington is illustrated in figure III.3. This reactor was converted to the production of weapon-grade plutonium in 1982. Its nominal output was about 600 kilograms of weapon-grade plutonium per year. It was shut down in the mid-1980s because of safety concerns.

Its fuel used slightly enriched uranium from the U.S. gaseous diffusion plants. After the fuel was irradiated in the reactor, it was sent for reprocessing at the nearby Plutonium-Uranium Extraction (Purex) plant.

The Z Plant, or Plutonium Finishing Plant, converted liquid plutonium nitrate from the Purex plant into disc-shaped metal buttons the size of hockey pucks. These metal pucks were shipped to the Rocky Flats Plant near Denver, Colorado where they were turned into nuclear weapon components.

(1) This section is drawn from Manson Benedict, thomas pigford, and Hans Levi, Nuclear Chemical Engineering (New York: McGraw-Hill Book Co., 1981)


Figure III.1:
"Once-Thru" Fuel Cycle for Low-enriched Uranium Reactors


Figure III.2:
"Closed" Fuel Cycle


Figure III.3:
N Reactor Nuclear Fuel Cycle


Figure III.A.1:
Savannah River Production Reactor Fuel Cycle