Thirty years of fuels and materials information from EBR-II

The Experimental Breeder Reactor-II (EBR-II) was a 62.5 MWt±20 MWe sodium cooled fast reactor that was operated successfully for 30 years. Over its period of operation a wealth of fuels and materials information originated from EBR-II. Several missions were conducted in EBR-II, all of which yielded new and valuable additions to the world's knowledge base for nuclear materials. Some of the ®rst pioneering experiments on irradiation e€ects in stainless steels were conducted in EBR-II. Later, practical manifestations of enhanced irradiation creep, swelling, and loss of ductility were experienced on EBR-II components. In addition, for a period of more than 15 years, the EBR-II reactor would become the primary irradiation facility for all fast reactor fuels and materials research and development. Both the initial mission and ®nal mission for EBR-II (the Integral Fast Reactor Concept, IFR), involved the remote reprocessing and irradiation of fast reactor metallic fuels. The fuels and materials information gleaned from these missions will be summarized with the intent of portraying a sample of the valuable legacy that EBR-II contributed to the world's store of nuclear fuels and materials knowledge. Ó 1999 Elsevier Science B.V. All rights reserved. 1. Brief history of EBR-II/description The Experimental Breeder Reactor II (EBR-II) went critical in 1964. In 1951, its predecessor, EBR-I, generated the ®rst electrical power using a nuclear reactor. Both reactors are located on the high desert near Idaho Falls, Idaho. EBR-II is a liquid sodium cooled fast reactor with the reactor core immersed in a 90,000 gallon pool of sodium. Driver fuel for the reactor has always been variations of metallic fuel. The core of the reactor was originally surrounded by a blanket of depleted uranium, both axially and radially. Later, for neutron economy, some inner rows of blankets were replaced with stainless steel assemblies. The axial blankets were also removed. The reactor generated 62.5 MW of thermal power and 20 MW of electrical power. Over the years the reactor consistently achieved a capacity factor of 70% or better despite the continual use of the reactor for experimental programs. Of course the reactor did not stand alone. It was surrounded by all the facilities and capability required for the development of fast reactors and the accompanying fuel cycle. Fig. 1 shows the EBR-II reactor and associated facilities. Well equipped hot cells for the examination of components, an analytical chemistry laboratory for analysis of radioactive materials, and metallurgical laboratories for use in the characterization and fabrication of fuels and materials all grew around the reactor. As well, two other reactors existed at the site; one called the TREAT reactor for the overpower transient testing of fuels, and the other being the Zero Power Physics Reactor for use in generating physics data for any fast reactor core con®guration. This capability in Idaho was well supported by analytical and experimental e€orts at the Argonne National Laboratory site in Chicago, Illinois. The EBR-II reactor assumed several missions over its 30 years of operation with some overlap as the transition was made from one mission to another. At the beginning, the goal of the EBR-II installation was to demonstrate the viability of a closed fuel cycle. EBR-II would generate electricity, breed plutonium from depleted uranium, and the fuel would be reprocessed and returned to the reactor in a closed fuel cycle [1]. This mission was completed by 1969 with 35 000 fuel pins (about seven cores) having been reprocessed, refabriJournal of Nuclear Materials 270 (1999) 39±48 * Corresponding address. Tel.: +1-208 533 7809. 0022-3115/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 3 1 1 5 ( 9 8 ) 0 0 7 6 0 0 cated remotely, and returned to the reactor for further irradiation. Although the demonstration was incomplete because plutonium was never removed from the fertile blanket for reirradiation nor were waste forms developed for containment of ®ssion products, the reprocessing demonstration was valuable. The early reprocessing demonstration provided the basis for the return of the concept some 15 years later. At the end of the reprocessing demonstration, signi®cant decisions were made in the Atomic Energy Commission (AEC) that set the course for all fast reactor development throughout the 1970s. Fast reactors were thought to be the power source for the future, but an interim period was envisioned where light water reactors (LWR) fueled with uranium oxide fuel would assume the nation's electrical power needs. The LWR technology emanated from the submarine reactor development. Utilities felt comfortable with water coolant, but not with sodium. Uranium oxide fuel was compatible with water, metallic fuel was not. Thus, a worldwide industry was created around the oxide fueled LWR reactors. This choice for the LWR industry in̄uenced the direction to be taken for fast reactors. Even though little was known about the performance of oxide fuel in a fast reactor, the mixed uranium oxide± plutonium oxide fuel was chosen for fast reactors. Three reasons were given for the oxide option. First, the sodium coolant outlet design temperatures for fast reactors in the late 1960s were very high, too high for the steel structural components. Metallic fuels could not achieve these temperatures without the possibility of deleterious fuel±cladding interaction. It appeared that oxide fuels could. Second, at that time of decision in the late 1960s, metallic fuels could achieve only limited burnup due to fuel swelling. A design breakthrough was only months away from demonstration in EBR-II, but oxide fuels were believed to be capable of much higher burnup even though no irradiation data Fig. 1. The EBR-II reactor in the background with the power plant on the left and the fuel cycle hot cells on the right. 40 L.C. Walters / Journal of Nuclear Materials 270 (1999) 39±48