At the University of Tennessee (where I have been a distance student for the past three years) they regularly have webcasts of nuclear colloquia that they offer on a variety of topics. In commemoration of Dr. Weinberg I wanted to post a set of links to broadcasts of four colloquia he gave over the last few years.
A Question and Answer Interview with Dr. Alvin Weinberg
Presented September 1, 2004
The Second Nuclear Era–Revisited
Presented September 4, 2002
Enrico Fermi and the World’s First Controlled, Self-Sustaining Chain Reaction
Presented September 5, 2001
People and Personalities in the Manhattan Project
Presented on September 20, 2000
Each colloquium is about an hour long and I really enjoyed hearing Dr. Weinberg’s voice and hearing him describe his experiences with the development of nuclear power.
Brian Wang and I have been having a good time lately talking to environmentalists on their blogs. On a blog from Australia recently, I took the opportunity to go after the old “nuclear emits as much CO2 as coal” canard. This is one that I seem to see all the time on blogs that have an anti-nuclear bent, and it’s so easy to refute. I took advantage of the fact that there’s an anti-nuclear site that has a very convenient calculator for figuring out the CO2 emissions of the conventional nuclear fuel cycle.
You just go in there, tell it you want to see the effect of 1000 MW of electricity produced for a year, and bam, it will tell you how much CO2 was emitted. I didn’t mess with any of the defaults (fuel enrichment, burn-up, CO2 emitted to produce the electricity to enrich the uranium) since I figured they were already set on some pretty pessimistic numbers. And they were, especially since the calculator assumes that all the electricity used to enrich uranium comes from dirty coal plants.
How does that stack up against alternatives? Well, coal took about 8 million tonnes of CO2 and natural gas about 4 million tonnes of CO2 to do the same thing. So there’s little doubt that, even with these assumptions, that conventional nuclear releases less CO2 than these baseload alternatives.
Which got me wondering, what would a steady-state thorium cycle look like in the same examination?
I have to make some extrapolations and projections to do the calculation, but I assumed that amount of thorium ore required to sustain power production is about 1/300th of what is currently needed for the conventional uranium reactors. This is based on a calculation I did recently that many of you have seen.
First of all, I’ll make the simplifying assumption that it takes about the same amount of parent ore, mining, and milling to get thorium as uranium. The main difference being that I only need about 1/300th the amount of thorium to produce the same amount of electricity.
But the big energy savings comes in the enrichment and fuel fabrication steps. That is where the calculator estimates that the bulk of the energy is consumed. Based on the assumptions that I need no enrichment for the thorium, and that it can be used in the reactor in metallic form, I will assume these number are zero.
(Metallic thorium will be added to the reactor’s blanket during the reductive extraction of protactinium, oxidizing to a fluoride even as protactinium reduces to a metal and is removed.)
Based on all these numbers, I would estimate that the CO2 production from 1000 MW-yr of electricity production would be about 100 tonnes of CO2, assuming that all liquid fuel is diesel and all electricity come from coal. That’s about 1/3000th of the value for the uranium fuel cycle that the program estimates, with most of the improvement coming from the lack of enrichment and fuel fabrication.
Alvin Weinberg passed away last night at the age of 91 at his home in Oak Ridge. He was the director of Oak Ridge National Lab from 1950 to 1970 and supervised the development and construction of two molten-salt reactors. In his book “The First Nuclear Era”, he stated that the reason he took the job in Oak Ridge was because he was enamored with the possibility of developing fluid-fueled thorium reactors to power the entire world, an interest that was stoked by his mentor, Nobel Laureate Eugene Wigner, during their association on the Manhattan Project.
Despite his poor health and advanced age, I spoke with him briefly on the phone about a year ago. I asked him what he thought of molten-salt reactors. He said to me:
“Molten-salt reactors were a great idea. Molten-salt reactors still are a great idea…”
This from the man who invented the light-water reactor.
The First Nuclear Era, 1994.
Bruce Hoglund has brought some very interesting information to my attention in recent months, that I wanted to talk about here.
Over the years, the strange beast of “proliferation” raises its head in various forms and fashions to afflict any reactor design. There are all different degrees in how it is interpreted and exercised. Some use the fact that fission bombs have fission reactions as an excuse to say that any nuclear reactor using fission material is a proliferation risk. On the opposite end of the spectrum, for years the nuclear community looked to a future “plutonium economy” where weapons-grade plutonium moved from fast breeder to reprocessing plant to sale on fuel markets.
Using thorium as the fertile material and U-233 as the fissile material has a secret advantage in the “proliferation” department–the inevitable formation of uranium-232. U-232 follows the same decay sequence as thorium-232, the only difference being that Th-232 takes 15 billion years for that first decay, whereas U-232 only takes about 78. The “4n” decay chain, of which U-232 and Th-232 both follow has a decay product (thallium-208) that emits a strong and penetrating gamma ray during its decay that makes it very unattractive in weapons use.
Weapons, especially those that want to be launched on ICBMs, can’t afford thick heavy gamma shielding around their fissile cores to protect the sensitive electronics that trigger detonation. So fissile material for weapons needs to emit easily-shielded radiation. Plutonium-239 and uranium-235 both fit the bill. Uranium-233 contaminated with uranium-232 does not.
So how do you get U-232 contamination? Well, in the core the predominant mechanism to form U-232 is an “n,2n” reaction with U-233, where U-233 is struck by a neutron and it then releases two neutrons, forming U-232.
But as I’ve talked about in other portions of this blog, in a two-fluid thorium reactor, thorium nuclei in the blanket will intercept neutrons and breed–first to protactinium-233, then to uranium-233. If the protactinium is isolated chemically after it is formed, then the U-233 in the protactinium decay tank will have little U-232 contamination.
The real question I’ve had is: how can we generate U-232 contaminated material, even in the Pa decay tank?
The answer may well be ionium.
Ionium isn’t really an element–it’s one of those names that was given to an isotope before it was recognized as an isotope. Ionium is thorium-230, which is part of the natural decay chain of uranium-238, which is rather abundant. Thus one could expect to find ionium in uranium tailing piles.
When ionium is exposed to neutrons, it first forms protactinium-231, and then another neutron absorption forms uranium-232. If the thorium in the blanket was “spiked” with ionium, it would be impossible to chemically separate the two forms of thorium (since they are chemically identical) but the ionium would preferentially absorb neutrons and form Pa-231. Another neutron absorption and you would have U-232. Both Pa-231 and U-232 would be collected by the reductive extraction technique used to remove protactinium from the blanket and would thus end up in the Pa decay tank. Now as Pa-233 decayed to U-233, there would be substantial U-232 contamination, and any weapons risk could be eliminated.
Collecting ionium from uranium tailings should not be particularly difficult since it is chemically distinct from uranium. It won’t take much ionium in the thorium-232 to “spike” it, and the small quantities should have little effect on the overall neutronics. Thorium reactors could certify that they were running on ionium-spiked thorium through a quick fuel examination in a mass spectrometer.
Lacing thorium with ionium could prove to be a much more attractive option than some of the others proposed, such as fueling the reactor on “denatured” uranium, which leads to the formation of transuranics and drastically compromises the ability of the reactor to breed and reprocess.