Saturday, 1 October 2011

On the train through K-25

I'm in Oak Ridge, Tennessee. It's an important place in the history of nuclear physics, being built in the second world war for the Manhattan Project. One of the main jobs that Oak Ridge had was to separate the two main isotopes of Uranium that are found in Uranium ore, Uranium-235 and Uranium-238. U-235 is the one that is needed for nuclear reactors and bombs, but makes up a little under 1% of natural Uranium. For fission in either bombs or a reactor, a much higher concentration of U-235 is needed - the Little Boy bomb that was dropped on Hiroshima consisted of two lumps of enriched Uranium, which were pushed together by a chemical explosion in the bomb to create one lump exceeding critical mass. The average enrichment of those lumps of Uranium was around 80% U-235.

Three different enrichment techniques were developed at Oak Ridge: Gaseous diffusion, electromagnetic separation, and liquid diffusion, with gaseous diffusion taking place at the K-25 plant. The Uranium ore, which came from mines in the Belgian Congo, and bought by the US on the open market, was processed into Uranium Hexaflouride, which is is gaseous at about 55°C. The plant works by repeatedly allowing the gas (containing both isotopes of Uranium) to diffuse through a porous membrane, with the lighter U-235 finding it easier to do so, and so being more concentrated after diffusion. To get high concentrations, the process must be repeated many times, and a huge cascade of diffusing membranes was built, making the plant building enormous.
The picture on the right shows the main building. Each arm of the "U" is half a mile long, and it was reportedly the biggest building under a single roof at the time of completion.

The building is now part-way through being demolished, but today I took a train journey through the site, and saw some of what was left. The Secret City Scenic Excursion Train is a volunteer-run railway that does occasional trips over about a 7 mile distance and back, starting from the edge of the old K-25 plant, through the plant, and then on through some East Tennessee countryside, before getting to the junction with a freight line it's not allowed to use. It's been in existence for coming up for 10 years, so just young enough that it wasn't here when I lived in Oak Ridge, and I was glad I found out about it while I'm over visiting. It would be even better to have really got to look inside the K-25 plant while it was still operational. During decommissioning, the plan had been to preserve the top of the U-shaped building, but it turned out to be too corroded and contaminated to make it viable. A shame...