Whether or not to use thorium is the “Debate of the Week” over at “Nuclear Townhall”, which is rapidly turning into one of my favorite nuclear news sites. Stop by and share your thoughts. Here’s their intro:
You don’t have to be involved in nuclear very long before you start hearing about thorium. It’s the other naturally occurring radioactive element that exists in large supplies and can produce nuclear fission.
The story is that Eugene Wigner, Alvin Weinberg and other pioneers of the Manhattan Project era believed thorium offered a much better way to tapping nuclear energy. We went the uranium route instead because uranium was the more practical option for the immediate task of building a bomb.
Nevertheless, thorium is three-to-four times as abundant as uranium. It doesn’t require isotope separation – a huge cost saving. When bombarded by neutrons, thorium doesn’t fission but converts to uranium-233 — which does. With U-233, the production of transuranics is orders of magnitude lower. This obviates any proliferation issues. (U-233 can be used to make a conventional weapon but is consumed all along within the reactor.). Depending on the reactor, the spent fuel can be much easier to handle. India has large supplies and is developing a thorium-based nuclear cycle.
While it might be a potentially appealing package for the U.S. — and was actually pursued to some extent in the 1990s — there are significant hurdles. The U.S. is obviously fully committed to the uranium fuel-cycle — as is the balance of the world — for the Renaissance. We are heavily invested in the status quo, both to meet U.S. demands and to compete internationally.
Can or should a thorium fuel cycle play a side-by-side role in Renaisssance Rev 1.0? Is there a plausible business case for the massive investment necessary? Or do public acceptance and first-of-a-kind licensing issues make it impractical? Are there other more appealing Generation IV options? In short, what’s the best way to proceed — if any — with the Thorium option?
I left a comment (still in moderation as I write this) that said:
Yes, we should use thorium, but it is important that you use the right machine to do it.
The “magic” of thorium is between it and U-238 (the two abundant nuclear isotopes) it’s the only one that can be consumed in a thermal spectrum reactor. That’s because its fissile product (U-233) produces more than two neutrons per thermal neutron absorption. Here’s the picture of how it works:
http://energyfromthorium.com/images/thoriumCycleNielsen.gif
Uranium-238 only can be completely consumed in a fast spectrum reactor. And fast reactors require 5-10 times more fissile material to produce the same amount of power, because the cross-sections are so much smaller in the fast spectrum. Here’s another picture:
http://energyfromthorium.com/images/fissionXSgraphic.gif
So you have only one choice if you want a thermal-spectrum breeder: thorium.
That said, you have to account for thorium’s (and U-233’s) “idiosyncrasies”. It takes about a month for Pa-233 (the intermediate product in thorium’s conversion to U-233) to decay. If it absorbs a neutron during that time you lose it as fuel. So you either need to remove Pa-233 or you need to have a low core power density. In solid fuel, the second option is the only one available to you. That’s how the Shippingport reactor successfully bred on thorium in its last core, but running a reactor at low power density isn’t very economic.
With fluid fuel, you have a lot of new options. That’s what Wigner advocated, but he didn’t know what fluid fuel to use. He thought it might be one based on water, and that’s what Weinberg started investigating at ORNL in the early 1950s. But there was no chemical form of thorium that was soluble in water in the conditions needed in the reactor. The better answer came out of left field from the folks who were working on the Aircraft Reactor project at ORNL in the early 1950s. They showed that a nuclear reactor based on liquid fluoride salts was feasible and had a lot of performance and safety advantages.
Weinberg was clever enough to realize that because there was a soluble form of thorium in fluoride salts (thorium tetrafluoride) that he had the basis for a potentially winning combination: fluoride salts and thorium. He worked for the remainder of his time at the ORNL to try to bring that dream to fruition, but the AEC was against him. They thought that his “molten-salt breeder reactor” threatened the agency’s position behind the plutonium-breeding sodium fast breeder. So they fired Weinberg and canned the molten-salt thorium research.
But we ought to reconsider that today, because safety and waste concerns are a lot more important now than they were in 1970, and fluoride reactors running on thorium excel in both categories.
I’m going to turn comments off on this post so that comments can be directed to Nuclear Townhall.