Thebismuth-phosphate process was usedto extractplutonium from irradiated uranium taken fromnuclear reactors.[1][2] It was developed duringWorld War II byStanley G. Thompson, a chemist working for theManhattan Project at theUniversity of California, Berkeley. This process was used to produce plutonium at theHanford Site. Plutonium was used in theatomic bomb that was used in theatomic bombing of Nagasaki in August 1945. The process was superseded in the 1950s by the REDOX andPUREX processes.
DuringWorld War II,plutonium was used to make both the firstatomic bomb ever to be detonated (near Alamogordo, New Mexico) and the atomic bomb that was dropped on Nagasaki in Japan. Plutonium had only been isolated and chemically identified in 1941, so little was known about it, but it was thought thatplutonium-239, likeuranium-235, would be suitable for use in an atomic bomb.[3]
Plutonium could be produced by irradiatinguranium-238 in anuclear reactor,[4] but developing and building a reactor was a task for theManhattan Project physicists. The task for the chemists was to develop a process to separate plutonium from the otherfission products produced in the reactor, to do so on an industrial scale at a time when plutonium could be produced only in microscopic quantities,[5] and to do so while working with dangerously radioactive chemicals likeuranium—the chemistry of which little was known—andplutonium, the chemistry of which almost nothing was known.
Chemists explored a variety of methods for separating plutonium from the other products that came out of the reactor:
While the chemical engineers worked on these problems, Seaborg askedStanley G. Thompson, a colleague at Berkeley, to have a look at the possibility of aphosphate process because it was known that the phosphates of manyheavy metals were insoluble in an acid solutions.
Thompson tried phosphates ofthorium, uranium,cerium,niobium andzirconium without success. He did not expectbismuth phosphate (BiPO
4) to work any better, but when he tried it on 18 December 1942, he was surprised to find that it carried 98 percent of the plutonium in solution.[9] The crystalline structure of bismuth phosphate is similar to that of plutonium phosphate, and this became known as the bismuth phosphate process.[10][11]
Cooper and Burris B. Cunningham were able to replicate Thompson's results, and the bismuth phosphate process was initially adopted as a fallback in case the lanthanum fluoride process could not be made to work. The processes were similar and the equipment used for lanthanum fluoride could be adapted for use with Thompson's bismuth phosphate process.[9] In May 1943, the DuPont engineers decided to adopt the bismuth phosphate process for use in the Clinton semiworks and the Hanford production site.[7]
As Brown, Hill, and other chemists explored plutonium chemistry,[12] they made the crucial discovery that plutonium has two oxidation states, atetravalent (+4) state and ahexavalent (+6) state, which have different chemical properties that could be exploited.[13] (This work was performed at the Manhattan Project's Radiation Laboratory at the University of California,Metallurgical Laboratory at theUniversity of Chicago andAmes Laboratory atIowa State College.)
The bismuth phosphate process involved taking the irradiated uranium fuel slugs and removing their aluminium cladding. Because there were highly radioactive fission products inside, this had to be done remotely behind a thick concrete barrier.[14] This was done in the "Canyons" (B and T buildings) at Hanford. The slugs were dumped into a dissolver, covered withsodium nitrate solution and brought to a boil, followed by slow addition ofsodium hydroxide. After removing the waste and washing the slugs, three portions ofnitric acid were used to dissolve the slugs.[15][16]
The second step was to separate the plutonium from the uranium and the fission products.Bismuth nitrate andphosphoric acid were added, producing bismuth phosphate, which was precipitated carrying the plutonium with it. This was very similar to the lanthanum fluoride process, in which lanthanum fluoride was used as the carrier.[17] The precipitate was removed from the solution with a centrifuge and the liquid discharged as waste. Getting rid of the fission products reduced thegamma radiation by 90 percent. The precipitate was a plutonium-containing cake which was placed in another tank and dissolved in nitric acid.Sodium bismuthate orpotassium permanganate was added to oxidize the plutonium.[15] Plutonium would be carried by the bismuth phosphate in the tetravalent state but not in the hexavalent state.[17] The bismuth phosphate would then be precipitated as a by product, leaving the plutonium behind in solution.[15]
This step was then repeated in the third step. The plutonium was reduced again by addingferrous ammonium sulfate. Bismuth nitrate and phosphoric acid were added and bismuth phosphate precipitated. It was dissolved in nitric acid and the bismuth phosphate was precipitated. This step resulted in reducing the gamma radiation by four more orders of magnitude, so the plutonium-bearing solution now had 100,000-th of the original gamma radiation. The plutonium solution was transferred from the 221 buildings to the 224 buildings, through underground pipes. In the fourth step, phosphoric acid was added and the bismuth phosphate precipitated and removed; potassium permanganate was added to oxidize the plutonium.[18]
In the "crossover" step, the lanthanum fluoride process was used. Lanthanum salts and hydrogen fluoride were added again and lanthanum fluoride was precipitated, while hexavalent plutonium was left in solution. This removedlanthanides like cerium,strontium [sic] andlanthanum, that bismuth phosphate could not. The plutonium was again reduced withoxalic acid and the lanthanum fluoride process was repeated. This timepotassium hydroxide was added tometathesize the solution. Liquid was removed with a centrifuge and the solid dissolved in nitric acid to form plutonium nitrate. At this point, a 330-US-gallon (1,200 L) batch sent would have been concentrated to 8 US gallons (30 L).[18]
The final step was carried out at the 231-Z building, where hydrogen peroxide, sulfates andammonium nitrate were added to the solution and the hexavalent plutonium was precipitated asplutonium peroxide. This was dissolved in nitric acid and put into shipping cans, which were boiled in hot air to produce a plutonium nitrate paste. Each can weighed about 1 kg and was shipped to theLos Alamos Laboratory.[18] Shipments were made in a truck carrying twenty cans and the first arrived at Los Alamos on 2 February 1945.[19] The plutonium was used in theFat Man bomb design tested in theTrinity nuclear test on 16 July 1945, and in thebombing of Nagasaki on 9 August 1945.[20]
In 1947, experiments began at Hanford on a new REDOX process usingmethyl isobutyl ketone (codenamed hexone) as the extractant, which was more efficient. Construction of anew REDOX plant commenced in 1949 and operations began in January 1952, the B plant closing that year. Improvements to the T plant resulted in a 30 percent increase in productivity and improvements were made to the B plant. There were plans to reactivate the B plant but the new PUREX plant that opened in January 1956 was so efficient that the T plant was closed in March 1956 and plans to reactivate the B plant were abandoned.[21] By 1960, the PUREX plant's output had surpassed the combined output of the B and T plants and the REDOX plant.[22]
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