The last days of Hallam
The Hallam nuclear power plant, about 25 miles southwest of Lincoln, Neb., was an important part of the Atomic Energy Commission’s Reactor Power Demonstration Program. But in the end, it operated for only 6,271 hours and generated about 192.5 million kilowatt-hours of electric power during its short, 15-month life.
Looking back at nuclear history, the June 1962 issue of Nuclear News reported that dry criticality was achieved in January 1962 “to verify physics calculations on the reactor core.” Full power was achieved in July 1963, and then the reactor was shut down in September 1964 due to moderator element defects in the reactor core. The proposed repair was deemed too costly, and so ultimately, that was the end of the line for Hallam, as it never resumed power.
The reactor, however, did successfully complete the testing program, demonstrating that a sodium-cooled reactor could safely provide high-quality steam for electrical generation while also showcasing the excellent thermal absorption properties that liquid metal sodium coolant possesses.
The June 1970 issue of Nuclear News covered the decommissioning of Hallam in its cover story. The AEC in August 1966 issued a letter to the Consumers Public Power District, operators of the Hallam plant, to proceed with the reactor’s “retirement.”
The seven basic requirements of decommissioning, as dictated by the AEC, were as follows, according to the article “Decommissioning: Four Case Histories” (June 1970 NN, p. 40):
Remove all fuel from the site.
Remove all bulk sodium that can be drained from the systems.
Chemically react all residual sodium remaining in the systems.
Dispose of radioactive residues by burial within the reactor cavity, or in other adequately contained subsurface areas of the plant, with controlled release to the environment, or by removal from the site.
Decontaminate all components of the plant above the operating floor of the reactor building; except that any parts that are not to be further used or that cannot be readily decontaminated shall be buried in the reactor cavity or in other adequately contained subsurface areas of the plant.
Prevent release of contamination from, and physical access to, any subsurface portion of the plant by permanently sealing all operating floor level penetrations of the entire facility, and by removal of loose contamination on the surface of such areas, as determined necessary on the basis of radiation surveys, before permanent sealing of these below-operating-floor areas.
Isolate the reactor vessel by capping and sealing all pipes and pipeways thereto; provide a permanent barrier against access from the operating-floor level to the reactor vessel, the fuel storage cells, and the IHX [intermediate heat exchanger] cells; and decontaminate and seal the surface access to the underground tanks of the separate radioactive waste disposal building, or remove these tanks for burial.
The process of removing irradiated fuel was a complicated task as the fuel assemblies were transferred via the fuel handling machine from the fuel storage vault to the maintenance cell. Once the fuel assemblies were in the maintenance cell, the fuel rod bundles were removed and secured in stainless steel canisters. The fuel was then loaded into spent fuel shipping casks and shipped to the Savannah River Plant in South Carolina for planned reprocessing.
Grid plate with 141 moderator can supports placed in preparation for securing to the grid plate. (Photo: J. E. Mahlmeister/public domain)
About 550,000 pounds of sodium from the primary cooling system were shipped via railroad to Hanford Works in Richland, Wash., for planned reuse. “After the primary system was drained, there remained on the bottom of the reactor vessel a sodium ‘heel’ about 2 inches deep,” the NN article states. “This heel was reduced to less than 1/4 inch by pumping sodium into the primary sodium fill tanks by means of pressurizing the reactor vessel to 10 psig while pulling a vacuum on the intermediate drain tank. This pumping arrangement required passing a pipe through a fuel plug hole in the reactor loading face shield down to the bottom of the reactor vessel.”
About 220,000 pounds of sodium from the secondary cooling system were removed and shipped to Atomics International for reuse. “Residual sodium remaining in the secondary sodium system components was disposed of by removing system components and reacting the sodium in a specially constructed HNPF [Hallam Nuclear Power Facility] steam cleaning facility. No portion of the secondary sodium system remain[ed] in the HNPF’s remaining structure.”
The Hallam turbine generator that was supplied steam from the conventional coal boiler and the nuclear reactor plant. (Photo: U.S. AEC/public domain)
The final issue was the isolation structure’s ability to contend with the environmental factors during entombment. External factors such as damaging winds, groundwater, and surface water were considered. In addition, internal factors such as degradation of remaining reactor components and sodium were considered. “There remains in the reactor vessel and attached sodium lines a total of 840 pounds of fused sodium hydroxide following reaction residual sodium,” the article states. “This sodium hydroxide residue was evaluated to determine its potential for contributing to the loss of integrity of the isolation structure, specifically from the standpoint of the degree of completion of the reaction of the residual sodium, the effects of corrosion by the residual sodium hydroxide, and potential hydrogen evolution from radiolytic decomposition of sodium hydroxide and contained water. On the basis of this evaluation, it was determined that there is no mechanism internal to the isolation structure capable of causing loss of integrity of the structure within its design lifetime.”
To this day, various reactor components and materials, including the reactor vessel, are entombed on-site in three storage areas. According to the Department of Energy, “Documents describing the layout and dimensions of the former reactor building, location of the buried reactor vessel, and detailed engineering information are sealed in stainless steel boxes that are secured in two locations at the site.”
The DOE further estimates that “the decommissioned reactor can be released for unrestricted use around the year 2070,” according to decay and dose calculations, 101 years after Hallam’s decommissioning was completed. The DOE regularly collects groundwater samples from the site to ensure there is no groundwater contamination. Estimates indicate that by 2071, groundwater monitoring can be discontinued.
Jeremy Hampshire is an ANS member whose avocation is writing about nuclear science and technology’s history. His experience includes time as a lead nuclear quality assurance auditor and a senior nuclear technical advisor.
