In 1969, Nebraska buried much of its experimental nuclear plant beneath a 1.4-acre grassy mound: The reactor vessel remains underneath today

Workers finished dismantling an experimental nuclear reactor near Hallam, Nebraska in 1969, but they did not remove everything from the site. The reactor vessel and other radioactive components were sealed below ground, covered with concrete and soil, and eventually hidden beneath a flat-topped grassy mound.

The Hallam Nuclear Power Facility had operated for only a short period. The U.S. Atomic Energy Commission ran the sodium-cooled, graphite-moderated reactor from 1962 to 1964 as part of its Power Demonstration Reactor Program. Nebraska Public Power District then decommissioned and dismantled the facility between 1967 and 1969. Today, the buried reactor remains under a roughly 1.4-acre mound at the former site, about 19 miles south of Lincoln. The U.S. Department of Energy, which assumed long-term stewardship of the location in 2003, continues to inspect and monitor it.

Hallam was built to test a different kind of reactor


The Hallam project began during the early years of commercial nuclear development in the United States. The facility was designed around a sodium-cooled, graphite-moderated reactor rather than the water-cooled designs that later became more common in the American nuclear industry. The reactor produced heat with a core containing fuel and graphite moderator elements. Liquid sodium circulated through the reactor as coolant, and heat from the radioactive primary system was transferred through intermediate heat exchangers before steam was produced for a turbine generator.

Hallam was integrated with the existing Sheldon Station, which is a coal-fired power plant operated by the Consumers Public Power District of Nebraska. The nuclear reactor supplied steam to the same general power-generation system rather than operating as an entirely separate station. A technical report preserved by the Department of Energy's Office of Scientific and Technical Information says construction was completed in 1961. The reactor achieved criticality in August 1962 and produced electricity for the first time in May 1963. Its design capacity was 240 megawatts thermal and 76 megawatts electric.The reactor was shut down in September 1964 after problems developed in the system. The Atomic Energy Commission eventually ended its agreement with the utility, and the utility declined an option to purchase the facility, and by 1966, the federal government had announced plans to decommission Hallam.

The reactor was dismantled but not completely removed

Decommissioning began in the late 1960s, when workers removed the reactor's fuel, bulk sodium and many of the radioactive materials from the site. Other components were decontaminated or disposed of separately. The reactor vessel was different, since the large steel vessel remained in the belowground portion of the reactor building. The remaining radioactive components were sealed in place, rather than excavating and transporting the entire structure. The underground areas were then isolated and covered.

A 1975 report by researchers from Atomics International and the Energy Research and Development Administration described the Hallam reactor as having been decommissioned in 1969. It states that fuel, coolant and support systems were removed, while the reactor vessel was entombed at the site. The remaining systems were sealed in underground vaults and covered with six feet of earth. The approach was known as entombment, and was used at several early demonstration reactors because the facilities were relatively small and much of their remaining radioactive material could be isolated within existing structures. Hallam was one of only a small number of U.S. reactors to use onsite entombment during that period. The NRC later identified Hallam, the Piqua Nuclear Power Facility in Ohio and the Boiling Nuclear Superheater Power Station in Puerto Rico as early demonstration reactors that used the approach.

A grassy mound now covers the former reactor

The Hallam site no longer resembles the nuclear facility that once operated there, since the aboveground portion of the reactor building was demolished during decommissioning, except for the structure containing the intermediate heat exchanger. The belowground sections remained. DOE later weatherproofed the concrete ceiling and added a sand layer, a plastic membrane and soil. That cover now forms the roughly rectangular, flat-topped mound visible at the site.

DOE's current fact sheet identifies three belowground areas where radioactive material remains. One contains the reactor vessel and its containment structures, another contains components associated with a fuel storage pit, while a third contains moderator-element storage cells. The remaining material is largely made up of activation products. These are materials that became radioactive after exposure to the neutron environment inside the reactor. DOE lists isotopes including cesium-137, cobalt-60, iron-55, nickel-63, strontium-90 and tritium among the contaminants remaining at the site. Approximately 300,000 curies of radioactive material were entombed when the facility was closed.

DOE says investigations began after concerns were raised in the early 1990s about the possibility of shallow groundwater contacting radioactive material near the former reactor structure, and monitoring has continued since then. DOE reports that no radioactivity above background levels has been detected in the groundwater samples collected to date. The agency's calculations indicate that the decommissioned reactor could be released for unrestricted use around 2070. For now, the mound remains part of a managed federal cleanup site. DOE's Office of Legacy Management is responsible for long-term stewardship, including inspections, maintenance and monitoring. The reactor that once operated beside a Nebraska coal plant is no longer visible above ground. Its vessel, however, remains inside the sealed underground portion of the former facility, beneath the 1.4-acre grass-covered mound that now marks the site.