About a month ago, an article appeared in the Telegraph entitled, “Obama could kill fossil fuels overnight with a nuclear dash for thorium.” It’s central claim was that,
If Barack Obama were to marshal America’s vast scientific and strategic resources behind a new Manhattan Project, he might reasonably hope to reinvent the global energy landscape and sketch an end to our dependence on fossil fuels within three to five years.
This article (which should be read) was intriguing, but I am no expert on nuclear power generation, so I asked Randy Brich, who is. He in turn asked the expert’s expert, James Mahaffey, author of the highly regarded Atomic Awakening: A New Look at the History and Future of Nuclear Power.
Dr Mahaffey was kind enough to reply. Here is his take on thorium.
Ah yes. Thorium.
Thorium is 4 times more abundant in the Earth’s crust than uranium. It occurs in only one isotope, thorium-232. There is therefore no isotope separation, and 100% of the mined thorium is useful in a power reactor. Very little of the mined uranium is useful. You can’t make a bomb out of thorium.
BUT, thorium is not fissile. So, how do you burn it in a reactor? Thorium activates under neutron bombardment into uranium-233, which is quite fissile. All a thorium reactor needs is an aggressive neutron start-up source Once it gets going, it “breeds” U-233 in situ, and burns it. You can even run a converter operation in a thorium blanket, making U-233 for later use. No isotope separation is necessary for the uranium fuel. It’s pure, fissile U-233.
About 20% of the world’s thorium is in India. Putting two and two together, India has developed a thorium fuel cycle, and is currently working to build a thorium power economy. They’re building a full-scale power reactor right now. I think 30% of the thorium is in Australia. There’s plenty of it in the US. Brazil has a lot of thorium (the black sand on the beaches is thorium oxide) and they have phased in and out of a try for energy independence over the past 40 years.
So, why have we not built any thorium reactors in this country? Actually, we have. The Shippingport Power Station, the first commercial nuke plant in the US, used a thorium blanket and became a thermal breeder reactor with its second fuel loading. Fort St. Vrain was a thorium reactor. The molten salt reactor experiments, including the one at Oak Ridge, were thorium reactors. But, it has never caught on because the nuke industry is so utterly committed to uranium fuel. Everything, from core designs to fuel fabrication, depends on the fuel being uranium.
An advantage of the molten salt thorium reactor: there’s no fuel fabrication. The fuel is dissolved in the coolant. Refueling is maybe once every 10 years. The entire primary loop is full of fuel. It only goes critical in the reactor vessel, where the shape and volume make it critical. The molten salt runs at atmospheric pressure. No explosions in the primary.
How did this happen? The first criticality experiments were with U-235 as the fissile component. Everything had to fall into place behind U-235, under the frantic war-time conditions, and there was no diversion into other, more promising avenues. Imagine, if the B-reactor1 had been configured a little differently, it could have made U-233 instead of Pu-239. There would have been no need for the tricky implosion setup. U-233 has no spontaneous fission problem. No need for tedious isotope separation. They could have turned out cheap “gun-assembly” bombs by the hundreds.
The Alsos mission turned in a big scare in 1944. They found that the retreating Germans has taken with them a big load of thorium out of Paris. Were the Germans short-cutting to a U-233 assembly weapon? No, they weren’t. The Germans were anticipating a post-war craze for radioactive cosmetics.
Why didn’t we at least divert into thorium development for weapons after the war? Excellent question. In the 50’s, the Army wanted its own nuclear arsenal, separate and specialized. The MK 33 240mm artillery shell was developed. The Air Force hit the ceiling. The small, 8-inch shell would have to be a gun-assembly weapon, and that meant it would use the preciously small supply of bomb-grade high-enriched uranium (HEU). The Air Force got there first with its weapons needs, and they were not willing to share. Fine, said the Army, we’ll use U-233. They started work on the chemical processing and handling facilities for U-233, which had never been built before.
Los Alamos National Lab stepped in and nixed the concept of using U-233. I’m not exactly sure why. They cited “handling problems.” The MK 33 became the W-33/M-422, using fully enriched uranium.
You will hear that we can’t make bombs out of U-233 because it is a virulent gamma-ray emitter. This is not true, and I find it curious that it is used as a reason. U-233 has a 158,000-year half-life. What they are referring to is the protactinium-233 contaminant, which has a 27-day half-life, beta-decaying into U-233 with gamma-ray involvement. Chemically scrub the protactinium, of course, or just wait a year and it will be gone.
The US has a moderately large stockpile of U-233 in Oak Ridge. They are presently arguing that it should be buried along with the rest of the fission waste. Why? Why ruin a potential energy resource? Fear of theft? We sit on a lot of HEU, so why the worry over U-233? It’s a bit odd. There’s a lot they aren’t saying.
Don’t get me started about thorium!
1 Update: The B-reactor was the first of several plutonium production reactors built at the Hanford Works in Richland, WA, in 1943. It was the first reactor built to run at high power. Graphite, natural uranium, and water cooled. It used excess neutrons to convert U-238 into Pu-239. The same load of excess neutrons could have been used to convert Th-232 into U-233.