Experimental Breeder Reactor II
Experimental Breeder Reactor-II (EBR-II) is a reactor designed, built and operated by Argonne National Laboratory in Idaho. It was shut down in 1994. Custody of the reactor was transferred to Idaho National Laboratory after its founding in 2005.
It 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 US$32 million, it achieved first criticality in 1965 and ran for 30 years. It was designed to produce about 62.5 megawatts of heat and 20 megawatts of electricity, which was achieved in September 1969 and continued for most of its lifetime. Over its lifetime it has generated over two billion kilowatt-hours of electricity, providing a majority of the electricity and also heat to the facilities of the Argonne National Laboratory-West.
In controlled testing in 1986, with the EBR-II reactor running at full power and the emergency shutdown systems disabled, the reactor's supply of electricity was intentionally turned off, causing the coolant pumps to stop. This is a worse scenario than what happened in the Fukushima Nuclear Disaster. (At Fukushima, which began operation in 1971, the emergency shutdown system turned off the reactor as soon as it detected the earthquake. However the tsunami destroyed the electric generators powering the coolant pumps, which needed to continue running after the reactor shutdown. Subsequently, the core overheated and meltdown occurred.) EBR-II, in contrast, handled the event without creating a dangerous situation. EBR-II had a negative thermal coefficient of reactivity that shut down the reactor when the temperature increased due to loss of the coolant pumps; the time required to heat the large pool of sodium surrounding the reactor provided a sufficient time buffer for the passive decay heat removal system to prevent the EBR-II reactor from melting down. The safe shutdown of the EBR-II relied only on the laws of physics and did not require operator or control system intervention.
The fuel consists of uranium rods 5 millimeters in diameter and 33 cm (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 73 cm (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, caesium-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 234 cm (92 inches) long are assembled honeycomb-like; each unit has about 4.5 kg (10 lbs) of uranium. All together, the core contains about 308 kg (680 lbs) of uranium fuel, and this part is called the driver.
The EBR-II core can accommodate as many as 65 experimental sub-assemblies 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. Other sub-assembly positions may contain structural-material experiments.
The pool-type reactor design of the EBR-II provides passive safety: the reactor core, its fuel handling equipment, and many other systems of the reactor are submerged under molten sodium. By providing a fluid which readily conducts heat from the fuel to the coolant, and which operates at relatively low temperatures, the EBR-II 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 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 it 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 clean-up 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.
The objective of the EBR-II was to demonstrate the operation of a sodium-cooled fast reactor power plant with on-site reprocessing of metallic fuel. In order to meet this objective of on-site reprocessing, the EBR-II was part of a wider complex of facilities, consisting of
- Fuel Conditioning Facility: facility for reprocessing and treating spent fuel from the EBR-II and other reactors, using an electrorefiner for electrometallurgical treatment of spent fuel
- Fuel Manufacturing Facility: facility for the manufacturing of metallic fuel elements
- Hot Fuels Examination Facility: a "hot-cell" complex for handling and examining highly radioactive materials remotely
- Sodium Processing Facility: facility for processing of reactive sodium into low-level waste
Integral Fast Reactor
The EBR-II has served as prototype of the Integral Fast Reactor (IFR), which was the intended successor to the EBR-II. The IFR program was started in 1983, but funding was withdrawn by U.S. Congress in 1994, three years before the intended completion of the program. The Nuclear Energy division of General Electric, which was involved in the development of the IFR, has presented a design for a commercial version of the IFR: the S-PRISM reactor.
Control room of the EBR-II in 1986
Schema of the EBR-II
Schema of the spent fuel treatment process
- Till, Charles; Chang, Yoon Il (2011). Plentiful energy : the story of the integral fast reactor, the complex history of a simple reactor technology, with emphasis on its scientific basis for non-specialists. ISBN 1466384603.
- EBR-II at "Reactors designed by Argonne National Laboratory" web site.
- Experimental Breeder Reactor-II (21 MB) Leonard J. Koch
- Westfall, Catherine (Feb 2004). "Vision and reality: The EBR-II story" (PDF). Nuclear News: 25–32.
- Passively safe reactors rely on nature to keep them cool