Experimental Breeder Reactor II
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Experimental Breeder Reactor-II (EBR-II)is a sodium cooled reactor with a thermal power rating of 62.5 megawatts (MW), an intermediate closed loop of secondary sodium, and a steam plant that produces 19 MW of electrical power through a conventional turbine generator. The original emphasis in the design and operation of EBR-II was to demonstrate a complete breeder-reactor power plant with on-site reprocessing of metallic fuel. The demonstration was successfully carried out from 1964 to 1969. The emphasis was then shifted to testing fuels and materials for future, larger, liquid metal reactors in the radiation environment of the EBR-II reactor core. It operated as the Integral Fast Reactor prototype. Costing more than $32 million, it achieved first criticality in 1965 and ran for 30 years. It was run to produce about one megawatt of heat and 300 kilowatts of electricity, but was capable of producing much more.
[edit] Design
The fuel consists of uranium rods 5 millimeters in diameter and 13 inches long. Enriched to 67% uranium-235 when fresh, the concentration dropped to approximately 65% upon removal. The rods also contained 10% zirconium. Each fuel element is placed inside a thin-walled stainless steel tube along with a small amount of sodium metal. The tube is welded shut at the top to form a unit 29 inches long. The purpose of the sodium is to function as a heat-transfer agent. As more and more of the uranium undergoes fission, it develops fissures and the sodium enters the voids. It extracts an important fission product, cesium-137, and hence becomes intensely radioactive. The void above the uranium collects fission gases, mainly krypton-85. Clusters of the pins inside hexagonal stainless steel jackets 92 inches long are assembled honeycomblike; each unit has about 10 pounds of uranium. All together, the core contains about 680 pounds of uranium fuel, and this part is called the driver.
The EBR-II core can accommodate as many as 65 experimental subassemblies for irradiation and operational reliability tests, fueled with a variety of metallic and ceramic fuels - the oxides, carbides, or nitrides of uranium and plutonium, and metallic fuel alloys such as uranium-plutonium-zirconium fuel for the IFR. Other subassembly positions may contain structural-material experiments.
[edit] Safety advantage
The IFR design gains safety advantages through a combination of metal fuel (an alloy of uranium, plutonium, and zirconium), and sodium cooling. By providing a fuel which readily conducts heat from the fuel to the coolant, and which operates at relatively low temperatures, the IFR takes maximum advantage of expansion of the coolant, fuel, and structure during off-normal events which increase temperatures. The expansion of the fuel and structure in an off-normal situation causes the system to shut down even without human operator intervention. In April of 1986, two special tests were performed on the EBR-II, in which the main primary cooling pumps were shut off with the reactor at full power (62.5 megawatts, thermal). By not allowing the normal shutdown systems to interfere, the reactor power dropped to near zero within about 300 seconds. No damage to the fuel or the reactor resulted. This test demonstrated that even with a loss of all electrical power and the capability to shut down the reactor using the normal systems, the reactor will simply shut down without danger or damage. The same day, this demonstration was followed by another important test. With the reactor again at full power, flow in the secondary cooling system was stopped. This test caused the temperature to increase, since there was nowhere for the reactor heat to go. As the primary (reactor) cooling system became hotter, the fuel, sodium coolant, and structure expanded, and the reactor shut down. This test showed that an IFR type reactor will shut down using inherent features such as thermal expansion, even if the ability to remove heat from the primary cooling system is lost.
EBR-II is now defueled. The EBR-II shutdown activity also includes the treatment of its discharged spent fuel using an electrometallurgical fuel treatment process in the Fuel Conditioning Facility located next to the EBR-II
The cleanup process for EBR-II includes the removal and processing of the sodium coolant, cleaning of the EBR-II sodium systems, removal and passivating of other chemical hazards and placing the deactivated components and structure in a safe condition.
