Friday, June 13, 2008

A Primer on Nuclear Safety: 1.2.2 Heat, Water and Zirconium

1.2.2 Hear, Water and Zirconium

The United Sttes Navy had been buildig sips with boilers when Hyman Rickover arrived in Oak Ridge. They United States Nay is a very proud orgabization, and not the leastcause of its pride was its boiler technology. Hyman Rickover and his team were imerced in the Navy's boiler technology tradition.

Their challenge was to find a way to to transform Weinberg's tiny water moderated e Materials Testing Reactor mock-up into a  reactor powerful enough to power a submarine.

Before I discuss this challenge I need to explain a little more about Wienberg's toy reactor, the Low Intensity Test Reactor, which was what the original mock-up was called when it was converted into a real reactor.  Although the 1.7 billion year old Oklo mine reactors had not been discovered yet, Weinberg and company had designed a reactor that mimicked the major features of the Oklo reactors. Like the OKLO reactors, the Low Intensity Test Reactor contained uranium with a high enough U-235 ratio, that it would go critical in the presence of water. Thus far from being unnatural, Weinberg's little water cooled reactor unwittingly emulated nature. In both the Oklo reactor and Weinberg's toy reactor, water slows stray neutrons down enough to promote a chain reaction with fuel that had only modestly U-235 to U-238 ratios. This slowing down of neutrons is called moderation. Because the toy reactor generated so little heat, it was fueled by aluminum clad uranium metal plates.

So the Rickover's first step was to use a water moderator. The second step was to produce enough power to boil water. That meant that Rickover's reactors had to be more powerful in order to produce much more heat. But here Rickover encountered a potential road block. The uranium used in a reactor had to be kept separate from the water for a number of reasons. One was that the uranium would corrode. Also radioactive fission products could escape from the uranium into the the cooling-moderating water. This was true whether the uranium was in metal form, or if it had been oxidized and baked into a ceramic, and had undesirable consequences. Rickover's ship board reactor clearly required that the Uranium fuel have a cladding and safety was no small consideration.

Aluminum had some good features, and had been the preferred cladding material, but it had a corrosion problem at higher temperatures, and it was was too weak a metal to be be rupture proof in a high performance reactor. Aluninum only maintain strength up to 300°C, and melt at 650°C. This proved safe in Weinbergs toy reactor, but the short comings of aluminum as a fuel cladding were dramatically demonstrated over and over, beginning in 1952 during an accident in the Canadian water cooled NRX reactor ruptured. Due to an operator error, control rods were accidentally lifted. Further errors occurred when operators attempted to shut down the reactor. Shut-off rods failed to fully descend into the reactor core. Reactivity and heat shot up. Aluminum clad fuel elements ruptured. Hydrogen gas was generated, and exploded. The lid of an inner reactor containment dome was blown off, and radioactive materials leaked into the environment. Jimmy Carter, then one of Rickover's boys, participated in the clean up.

Failure of alunium fuel cladding in the Windscale reactor played a major role in the 1957 fire that released a great deal of radioactive material into the environment. An accident with the Canadian NRU reactor in 1958 further illustrated the dangers of aluminum cladding. Several aluminum clad fuel elements inside the reactor over heated and burst, One caught on fire. While being extracted from the reactor it was torn in two. The fragments fell into a pit, and continued to burn. The resulting release of radioactive materials created quite a mess.

Clearly then aluminum cladding would not be up to snuff as fuel cladding for Rickover's reactors.

Alvin Weinberg suggested to Rickover that Zirconium had potential as a reactor fuel cladding. Zirconium is stronger than aluminum, had good heat transfer properties, can tolerate 1,800° C heat and does not corrode in the presence of water and heat. There was one rub. Radiation testing of zirconium samples show them to be a neutron poison. Using zarconium as a reactor fuel element cladding would kill a chain reaction, or so it was thought. But Oak Ridge scientist Herbert Pomerance, Herb to those who cultivated his friendship, traced the problem to hafnium, a chemically similar element that was present as a natural contaminant in zirconium.

Thus if zirconium could be separated from the hafnium, it would give Rickover his safe reactor cladding. The task of industrial sale separation of zirconium from hafnium was given to a group of three Y-12 chemists under Warren Grimes. Grimes asked Lyle Overhoiser to work on the problem, but when Lyle needed some assistance from an analytic chemist, my father, C.J. Barton, Sr., was brought in to help. My father and Lyle had been Lab mates as chemistry graduate students at the University of Virginia, and they worked well together. At that point my father stopped being an analytic chemist, and became an industrial chemist. With the assistance of John W. Ramsey, George Parker, Cyrus Feldman, and many others, they were able to identify a method that could be scaled up to zirconium separation Industrial levels of production. While Rickover was said to be the father of the atomic submarine, Oak Ridge chemists, including my father, broke the prophylactic: for him.

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