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| Fig. 1: Aerial view of the Hanford Site in southeastern Washington State, the primary plutonium production complex for the United States nuclear weapons program during the Manhattan Project and the Cold War. (Source: Wikimedia Commons) |
The Hanford Site in Washington State stores a large inventory of high-level radioactive waste. Waste generated during plutonium production for nuclear weapons was stored beginning in the 1940s in underground tanks. According to the U.S. Department of Energy (DOE), the site currently contains approximately 56 million gallons of radioactive waste stored in 177 underground tanks. [1]
A natural question is whether the radioactivity stored at Hanford represents an unusually large quantity when compared with normal nuclear power operations. This report examines that question using published values for the radioactivity of the principal fission products present in the waste. Fig. 1 shows the scale of the Hanford Site where these tank farms are located.
The tank wastes were produced during chemical processing used to separate plutonium from irradiated uranium fuel. The resulting acidic radioactive liquids were neutralized with sodium hydroxide before being stored in carbon-steel tanks. [1] Over time the waste separated into three major phases:
Supernate: Liquid solution containing dissolved salts
Saltcake: Crystallized salts formed by evaporation
Sludge: Dense insoluble solids that settle at the bottom of the tank
These phases are characteristic of stored high-level reprocessing waste. [1] Many radioactive isotopes are present in this waste. The U.S. Nuclear Waste Technical Review Board emphasizes that Cs-137 and Sr-90 are important because they produce substantial radioactivity and decay heat on multidecade timescales. [2]
Because Cs-137 and Sr-90 generate significant decay heat, these isotopes were partially separated from Hanford tank waste during operations at B Plant between 1967 and 1985. [2] The separated isotopes were converted into stable salts CsCl and SrF. These were sealed inside double-walled stainless steel capsules and placed in underwater storage at the Waste Encapsulation and Storage Facility (WESF). [2] The facility currently stores 1,335 CsCl and 601 SrF capsules. The combined radioactivity of these capsules is estimated to be on the order of 80 million Curies. [2]
To interpret this number it is useful to compare it with the production of fission products in a commercial reactor. Take a 1 GW(e) plant. If the thermal efficiency is about 1/3, the reactor runs at about 3 GW(th) = 3 × 109 J/s. Using 200 MeV ≈ 3.2 × 10-11 J released per fission, the fission rate is
| 3 × 109 J s-1 3.2 × 10-11 J fission-1 |
= | 9 × 1019 fissions s-1 |
If we assume that 10% of fissions yield Cs-137 or Sr-90 in the long-lived fission-product bucket, that each fission produces two fission fragments, and that the half-life is 30 years, then we obtain an activity production rate of
| ln(2) 30 y |
× | 2 × 0.1 × 9 ×
1019 Bq y-1 3.7 × 1010 Bq Ci-1 |
= | 1.12 × 107 Ci y-1 |
Thus a single 1-GW reactor produces roughly 11 million Curies of Cs-137 and Sr-90 each year. At this rate about eight years of reactor operation would produce the ~80 million Curies reported for the Hanford cesium and strontium capsules.
The Hanford Site contains approximately 56 million gallons of high-level radioactive waste stored in 177 underground tanks. [1] Much of the long-term radioactivity in this waste comes from the fission products Cs-137 and Sr-90, which have half-lives of about 30 years and therefore dominate the activity of nuclear waste for several centuries.
These isotopes were separated from some Hanford waste streams and sealed in capsules stored at WESF, where the total radioactivity is estimated to be on the order of 80 million Curies. [2] A comparison with reactor production shows that a single 1-GW reactor produces roughly 11 million Curies of these isotopes per year, meaning that the Hanford capsule inventory corresponds roughly to the fission product output of several years of reactor operation.
The Department of Energy is constructing the Waste Treatment and Immobilization Plant at Hanford to process and immobilize radioactive tank waste, a project estimated to cost more than $16 billion. [3]
© Baraa Abdelghne. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Draft Waste Incidental to Reprocessing Evaluation for the 200 West Area Tank Treatment Mission at the Hanford Site, Washington," U.S. Department of Energy, HFO-71486, January 2026.
[2] "Cesium and Strontium Capsules," U.S. Nuclear Waste Technical Review Board, June 2020.
[3] "Audit Report: Bechtel National, Inc's Subcontract Audit Program," U.S. Department of Energy, DOE-OIG-20-06, November 2019.
