A new presidential speech on energy policy means it’s time again to trot out this historic overview of the subject:
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(Kirk’s note: I’d like to welcome Rick Martin as an author on Energy from Thorium. Rick is a prolific author whom many of you might remember from his WIRED article on thorium. Rick is writing a new book on advanced nuclear technology–keep your eye out for it! Welcome Rick!)
In the wake of the serial failures of cooling and containment systems at the Fukushima Daiichi nuclear power plant in Japan, the public is once again being asked to re-consider the dangers of nuclear power technology. Plenty of the responses have taken predictable courses. At U.S. PIRG (the association of state public-interest groups), the issue is closed: “Unacceptable Risk” is the title of the group’s latest report on nuclear power, which cites “Two Decades of ‘Close Calls,’ Leaks and Other Problems at U.S. Nuclear Reactors.”
Empirically, the actual risks of nuclear power – which tend to be minuscule and spread over thousands of years, as this New York Times story documents – are negligible compared to those of burning fossil fuels, which, based on current trends will unquestionably lead to a disastrous rise in average global temperatures over the next century. Six thousand people died in coal mining accidents in China last year; “In just the past year in the United States, the Deepwater Horizon blowout killed 11 people,” the Times noted, “the Upper Big Branch coal mine blast killed 29 and a natural gas pipeline explosion in California killed 8.” Deaths from accidents at nuclear power plants, 1990-2010: zero.
“Since air pollution from coal burning is estimated to be causing 10,000 deaths per year,” wrote Bernard Cohen, a physics professor at the University of Pittsburgh, in a study of the risks of nuclear power, “there would have to be 25 melt-downs each year for nuclear power to be as dangerous as coal burning.”
But people don’t perceive risks empirically: they respond more to the possibility of sudden catastrophe than to the certainty of prolonged, slow-motion havoc. It’s the plane-crash effect: fear of flying led to hundreds of additional deaths in automobiles in the months after 9/11, and the 32,000 or so people killed every year on U.S. highways rate far smaller headlines than an fatal airline accident that takes the lives of a couple of hundred people. A Chernobyl-sized nuclear accident is much more terrifying, though far less likely, than the gradual desertification of large swathes of the American West.
(It’s also the shark-attack effect: far more people drown in backyard swimming pools every year than die in shark attacks, but nobody ever made a scary movie about stumbling on your kid’s tricycle and hitting your head on the diving board.)
In a column with a familiar headline – “The True Costs of Nuclear Power” – Anne Appelbaum of Slate even trots out a cultural stereotype to justify irrational fears of nuclear power: “If the competent and technologically brilliant Japanese can’t build a completely safe reactor, who can?”
Appelbaum also recycles the hoary argument that the risks of nuclear power make it too expensive. “The cost of … a potential catastrophe is partly reflected in the price of plant construction.”
That’s confusing cause and effect: Building nuclear plants is not costly because of the risk of accident; it’s costly because public perceptions of risk enforce unreasonable licensing delays and add expensive, over-engineered containment systems, and investors price those risks into their funding models.
This sort of thing drives data-driven people – like me, and like many of the readers of this blog – crazy. If the public just had a “better,” more rational perception of risk, we fume, the obvious benefits of nuclear power would win out and the “true costs” of nuclear power would be understood, rather than exaggerated.
That in itself may be a fallacy, though. In 2004 George Gaskell, a professor at the Centre for Analysis of Risk and Regulation at the London School of Economics, wrote a paper along with six other authors looking at the perceptions of risk around genetically modified foods. What they found is that there’s an inverse relationship between the perception of benefits and the understanding of risk: for large numbers of people, “opposition to GM foods arises from a perception of the absence of benefits, a sufficient condition for rejection,” rather than a straightforward perception of risk. When perceived benefits go up, perceived risk goes down.
In other words, your perception of the risks of eating cheeseburgers depends not so much on your understanding of the health dangers of a diet of fatty red meat versus, say, steamed broccoli. It depends on how much you like cheeseburgers.
For supporters of alternative forms of nuclear power, that would seem to open up a public relations opportunity. Advocates of thorium power should focus more on benefits rather than comparative risks, in order to, in theory, shift the debate away from Chernobyl and toward clear skies, global climate stability and verdant forests. Everybody likes cheeseburgers.
Several people who have recently learned about thorium have contacted me with regards to a “fact sheet” about thorium issued by the Institute for Energy and Environmental Research (IEER) and
Physicians for Social Responsibility (PSR) called “Thorium Fuel: No Panacea for Nuclear Power.” The authors of this sheet were Arjun Makhijani and Michele Boyd.
Last year, Dr. Alexander Cannara wrote a letter to IEER/PSR pointing out errors and omissions in the “fact sheet” and requesting IEER/PSR to implement corrections. To the best of my knowledge no amendment or correction was ever issued.
I have taken it upon myself to write a more extended rebuttal to the claims made about thorium by Makhijani and Boyd and submit it to the online community for your consideration. The entirety of their original statement is included in the rebuttal and denoted by italics.
Thorium “fuel” has been proposed as an alternative to uranium fuel in nuclear reactors. There are not “thorium reactors,” but rather proposals to use thorium as a “fuel” in different types of reactors, including existing light-water reactors and various fast breeder reactor designs.
It would seem that Mr. Makhijani and Ms. Boyd are unaware of the work done at Oak Ridge National Laboratory under Dr. Alvin Weinberg from 1955 to 1974 on the subject of fluid-fueled reactors, particularly those that used liquid-fluoride salts as a medium in which to sustain nuclear reactions. The liquid-fluoride reactor was the most promising of these fluid-fueled designs, and indeed it did have the capability to use thorium as fuel. It was not a light-water reactor, nor was it a fast-breeder reactor. It has a thermal (slowed-down) neutron spectrum which made it easier to control and vastly improved the amount of fissile fuel it needed to start. It operated at atmospheric pressure rather than the high pressure of water-cooled reactors. It was also singularly suited to the use of thorium due to the nature of its chemistry and the chemistry of thorium and uranium.
Thorium, which refers to thorium-232, is a radioactive metal that is about three times more abundant than uranium in the natural environment. Large known deposits are in Australia, India, and Norway. Some of the largest reserves are found in Idaho in the U.S. The primary U.S. company advocating for thorium fuel is Thorium Power (www.thoriumpower.com). Contrary to the claims made or implied by thorium proponents, however, thorium doesn’t solve the proliferation, waste, safety, or cost problems of nuclear power, and it still faces major technical hurdles for commercialization.
Mr. Makhijani and Ms. Boyd may wish to update their document since “Thorium Power” is now called “Lightbridge” and no longer advocates for the use of thorium, whereas the community of supporters of liquid-fluoride thorium reactors (LFTR) still maintains strong support for the use of thorium because it is indeed a solution to the issues of proliferation, waste, safety, and cost that accompany the present use of solid-fueled, water-cooled reactors.
Thorium is not actually a “fuel” because it is not fissile and therefore cannot be used to start or sustain a nuclear chain reaction. A fissile material, such as uranium-235 (U-235) or plutonium-239 (which is made in reactors from uranium-238), is required to kick-start the reaction. The enriched uranium fuel or plutonium fuel also maintains the chain reaction until enough of the thorium target material has been converted into fissile uranium-233 (U-233) to take over much or most of the job. An advantage of thorium is that it absorbs slow neutrons relatively efficiently (compared to uranium-238) to produce fissile uranium-233.
On the contrary, thorium is very much a fuel because in the steady-state operation of a LFTR, it is the only thing that is consumed to make energy. Makhijani and Boyd are correct that any nuclear reactor needs fissile material to start the chain reaction, and the LFTR is no different, but the important point is that once started on fissile material, LFTR can run indefinitely on only thorium as a feed—it will not continue to consume fissile material. That is very much the characteristic of a true fuel. “Burning thorium” in this manner is possible because the LFTR uses the neutrons from the fissioning of uranium-233 to convert thorium into uranium-233 at the same rate at which it is consumed. The “inventory” of uranium-233 remains stable over the life of the reactor when production and consumption are balanced. Today’s reactors use solid-uranium oxide fuel that is covalently-bonded and sustains radiation damage during its time in the reactor. The fluoride fuel used in LFTR is ionically-bonded and impervious to radiation damage no matter what the exposure duration. LFTR can be used to consume uranium-235 or plutonium-239 recovered from nuclear weapons and “convert” it, for all intents and purposes, to uranium-233 that will enable the production of energy from thorium indefinitely. Truly this is a reactor design that can “beat swords into plowshares” in a safe and economically attractive way.
The use of enriched uranium or plutonium in thorium fuel has proliferation implications. Although U-235 is found in nature, it is only 0.7 percent of natural uranium, so the proportion of U-235 must be industrially increased to make “enriched uranium” for use in reactors. Highly enriched uranium and separated plutonium are nuclear weapons materials.
Since so many nuclear weapons have already been built and are being decommissioned, one might assume that Makhijani and Boyd would welcome a technology like LFTR that could safely consume these sensitive materials in an economically-advantageous way, beating swords into plowshares and using material that was once fashioned as a weapon as a material that can provide light and energy to billions. Enriched uranium or plutonium can’t simply be “thrown away”. LFTR puts these materials to productive use as they are destroyed in the reactor and uranium-233 is generated.
In addition, U-233 is as effective as plutonium-239 for making nuclear bombs. In most proposed thorium fuel cycles, reprocessing is required to separate out the U-233 for use in fresh fuel. This means that, like uranium fuel with reprocessing, bomb-making material is separated out, making it vulnerable to theft or diversion. Some proposed thorium fuel cycles even require 20% enriched uranium in order to get the chain reaction started in existing reactors using thorium fuel. It takes 90% enrichment to make weapons-usable uranium, but very little additional work is needed to move from 20% enrichment to 90% enrichment. Most of the separative work is needed to go from natural uranium, which has 0.7% uranium-235 to 20% U-235.
In a fluoride reactor, all of the fuel processing equipment will be located in a containment region containing the reactor and its primary heat exchangers, under very high radiation fields, and under the high heat needed to keep the fuel liquid. Once the system is properly designed to direct uranium-233 created in the outer regions of the reactor (the “blanket”) to the central regions of the reactor (the “core”) there will be no possibility of redirection of the material flow. Such a redirection would necessitate a rebuild of the entire reactor and would be vastly beyond the capabilities of the operators. Furthermore, the nature of U-233 removal and transfer from blanket to core involves the operation of an electrolytic cell that will allow very precise control and accountability of the material in question. Unlike solid-fueled reactors the uranium-233 never needs to leave the secure area of the containment building or come in contact with humans in order to continue the operation of the reactor. This is another important point that the authors have failed to distinguish as they have ignored the existence or implications of fluid-fueled thorium reactors.
To claim that uranium-233 is just as effective as plutonium-239 for nuclear weapons is gross simplification bordering on outright deception. They have similar values for critical mass, but this leaves out a very important point. The nuclear reactions that consume uranium-233 also produce small amounts of uranium-232, a contaminant that will later be mentioned by the authors but ignored at this stage of the criticism. U-232 has a decay sequence that includes the hard gamma-ray-emitting radioisotopes bismuth-212 and thallium-208. Indeed, the half-life of U-232 is short enough that this decay chain begins to set up within days of the purification of the uranium, and within a few months that gamma-ray flux from the material is intense. These gamma rays destroy the electronics of a nuclear weapon, compromise the chemical explosives, and clearly signal to detection systems where the fissile material is located. This is one of the key reasons why no operational nuclear weapons have ever been built using uranium-233 as the fissile material.
It has been claimed that thorium fuel cycles with reprocessing would be much less of a proliferation risk because the thorium can be mixed with uranium-238. In this case, fissile uranium-233 is also mixed with non-fissile uranium-238. The claim is that if the uranium-238 content is high enough, the mixture cannot be used to make bombs without a complex uranium enrichment plant. This is misleading. More uranium-238 does dilute the uranium-233, but it also results in the production of more plutonium-239 as the reactor operates. So the proliferation problem remains either bomb-usable uranium-233 or bomb-usable plutonium is created and can be separated out by reprocessing.
In my opinion, mixing uranium-238 with uranium-233 during the normal operation of a LFTR is a bad idea because it compromises the capability of the reactor to “burn” thorium to a degree that it then becomes necessary to add fissile material to keep the reactor running. This is because uranium-238 will absorb many of the neutrons that would otherwise convert thorium into uranium-233, instead converting uranium-238 into plutonium-239. Plutonium-239 is a poor fuel in a LFTR due to the limited solubility of plutonium trifluoride (PuF3) and the poor performance of plutonium in a thermal-neutron spectrum (only 2/3 of the plutonium-239 will fission when struck by a neutron).
But something is possible in the fluid fuel of a LFTR that is impossible in the solid fuel of a conventional reactor with regards to the “downblending” of uranium. Under extreme scenarios, it may be desireable to have a separate supply of uranium-238 inside the reactor containment that could be irreversibly mixed with the uranium-233 in the core. This would have the effect of making the reactor unable to restart, and despite the contention of Makhajani and Boyd, there is no feasible way to isotopically separate uranium-233 (contaminated with uranium-232) from uranium-238 because of the severe gamma radiation that would be emitted during any attempt to separate the isotopes. This approach to “just-in-time” downblending is only possible with fluid fuel, and its absence of consideration in the document again shows that the authors are unaware of the fluid fuel option and its implications.
Further, while an enrichment plant is needed to separate U-233 from U-238, it would take less separative work to do so than enriching natural uranium. This is because U-233 is five atomic weight units lighter than U-238, compared to only three for U-235. It is true that such enrichment would not be a straightforward matter because the U-233 is contaminated with U-232, which is highly radioactive and has very radioactive radionuclides in its decay chain. The radiation-dose-related problems associated with separating U-233 from U-238 and then handling the U-233 would be considerable and more complex than enriching natural uranium for the purpose of bomb making. But in principle, the separation can be done, especially if worker safety is not a primary concern; the resulting U-233 can be used to make bombs. There is just no way to avoid proliferation problems associated with thorium fuel cycles that involve reprocessing. Thorium fuel cycles without reprocessing would offer the same temptation to reprocess as today’s once-through uranium fuel cycles.
Makhijani and Boyd really betray a fundamental lack of understanding of the nature of uranium isotope separation facilities with their simplistic and cursory description of U-233 separation from U-238. Such a process would be so difficult due to the presence of U-232 that it simply would not be considered, even by the hypothetical “suicide” operators that they postulate. Anyone who had invested the large sums of money into a uranium isotope separation system would never risk permanently crippling its ability to operate by introducing U-232-contaminated feed into the system.
Proponents claim that thorium fuel significantly reduces the volume, weight and long-term radiotoxicity of spent fuel. Using thorium in a nuclear reactor creates radioactive waste that proponents claim would only have to be isolated from the environment for 500 years, as opposed to the irradiated uranium-only fuel that remains dangerous for hundreds of thousands of years. This claim is wrong. The fission of thorium creates long-lived fission products like technetium-99 (half-life over 200,000 years). While the mix of fission products is somewhat different than with uranium fuel, the same range of fission products is created. With or without reprocessing, these fission products have to be disposed of in a geologic repository.
Again, the authors make blanket statements about “thorium” but then confine their examination to some variant of solid thorium fuel in a conventional reactor. In a LFTR, thorium can be consumed with exceptionally high efficiency, approaching completeness. Unburned thorium and valuable uranium-233 is simply recycled to the next generation of fluoride reactor when a reactor is decommissioned. The fuel is not damaged by radiation. Thus thorium and uranium-233 would not enter a waste stream during the use of a LFTR.
All fission produces a similar set of fission products, each with roughly half the mass of the original fissile material. Most have very short half-lives, and are highly radioactive and highly dangerous. A very few have very long half-lives, very little radioactivity, and little concern. A simple but underappreciated truth is that the longer the half-life of a material, the less radioactive and the less dangerous it is. Technetium-99 (Tc-99) has a half-life of 100,000 years and indeed is a product of the fission of uranium-233, just as it is a product of the fission of uranium-235 or plutonium-239. Its immediate precursor, technetium-99m (Tc-99m), has a half-life of six hours and so is approximately 150 million times more radioactive than Tc-99.
Nevertheless, it might come as a surprise to the casual reader that hundreds of thousands of people intentionally ingest Tc-99m every year as part of medical imaging procedures because it produces gamma rays that allow radiography technicians to image internal regions of the body and diagnose concerns. The use of Tc-99m thus allows physicians to forego thousands of exploratory and invasive surgeries that would otherwise risk patient health. The Tc-99m decays over the period of a few days to Tc-99, with its 100,000 half-life, extremely low levels of radiation, and low risk.
What is the ultimate fate of the Tc-99? It is excreted from the body through urination and ends up in the municipal water supply. If the medical community and radiological professionals intentionally cause patients to ingest a form of technetium that is 150 million times more radioactive than Tc-99, with the intent that its gamma rays be emitted within the body, and then sees no risk from the excretion of Tc-99 into our water supply, where is the concern? It is yet another example of fear, uncertainty, and doubt that Makhijani and Boyd would raise this issue as if it represented some sort of condemnation of the use of thorium for nuclear power.
If the spent fuel is not reprocessed, thorium-232 is very-long lived (half-life:14 billion years) and its decay products will build up over time in the spent fuel. This will make the spent fuel quite radiotoxic, in addition to all the fission products in it. It should also be noted that inhalation of a unit of radioactivity of thorium-232 or thorium-228 (which is also present as a decay product of thorium-232) produces a far higher dose, especially to certain organs, than the inhalation of uranium containing the same amount of radioactivity. For instance, the bone surface dose from breathing an amount (mass) of insoluble thorium is about 200 times that of breathing the same mass of uranium.
Statements like this really cause me to wonder if Makhijani and Boyd understand the nature of radioactivity. Yes, thorium-232 has a 14-billion-year half-life, which means that the radioactivity of thorium is exceptionally low. It will rise as the decay chain of Th-232 begins to form, but it is still at a very low level. To be concerned with the radioactivity of thorium in spent fuel, while neglecting to mention the five billion kilograms of thorium contained in each meter of the Earth’s continental crust again appears to be another example of fear, uncertainty, and doubt levied unfairly against the use of thorium. The buildup of thorium-228 as part of the decay of thorium will happen on a scale within the Earth’s crust so titanically in excess of any activity on the part of man so as to render that point utterly immaterial to any discussion of thorium as a nuclear fuel.
Since both thorium and uranium are natural and common constituents of the Earth’s crust, discussing a bone surface dose obtained by breathing insoluble thorium—a very strange exposure pathway—and contrasting it with uranium is again utterly immaterial to the use of thorium as a nuclear fuel. Do Makhijani and Boyd mean to say that it would be preferable to be breathing uranium instead? The criticism seems to have no structure.
Furthermore, LFTR will not reject thorium to a waste stream nor generate “spent fuel” in the conventional sense. Thorium remains in the reactor until consumed for energy. At shutdown, unconsumed thorium is transferred to the next generation of reactor.
Finally, the use of thorium also creates waste at the front end of the fuel cycle. The radioactivity associated with these is expected to be considerably less than that associated with a comparable amount of uranium milling. However, mine wastes will pose long-term hazards, as in the case of uranium mining. There are also often hazardous non-radioactive metals in both thorium and uranium mill tailings.
Thorium is found with rare-earth mineral deposits, and global demand for rare-earth mining will inevitably bring up thorium deposits. At the present time, we in the US have the strange policy of considering this natural material as a “radioactive waste” that must be disposed at considerable cost. Other countries like China have taken a longer view on the issue and simply stockpile the thorium that they recover during rare-earth mining for future use in thorium reactors. In addition, the United States has an already-mined supply of 3200 metric tonnes of thorium in Nevada that will meet energy needs for many decades. The issues surrounding thorium mining are immaterial to its discussion as a nuclear energy source because thorium will be mined under any circumstance, but if we use it as a nuclear fuel we can save time and effort by avoiding the expense of trying to throw it away.
Research and development of thorium fuel has been undertaken in Germany, India, Japan, Russia, the UK and the U.S. for more than half a century. Besides remote fuel fabrication and issues at the front end of the fuel cycle, thorium-U-233 breeder reactors produce fuel (“breed”) much more slowly than uranium-plutonium-239 breeders. This leads to technical complications. India is sometimes cited as the country that has successfully developed thorium fuel. In fact, India has been trying to develop a thorium breeder fuel cycle for decades but has not yet done so commercially.
Thorium/U233 reactors like LFTR produce sufficient U-233 to make up for U-233 consumed in the fission process. This may be what the authors meant by “breeding more slowly”, but since they consider plutonium a dangerous substance and eschew the use of nuclear power, it is a wonder why they would consider a reactor that does not produce plutonium as having some sort of deficiency. They neglect to elaborate on what sort of “technical complications” this very attractive feature would entail.
The thorium effort in India has been centered around the use of thorium in solid-oxide form, and has suffered from the deficiencies of using this approach, which are transcended through the use of thorium in liquid fluoride form. This is further evidence that the authors are unaware of the implications of the liquid-fluoride thorium reactor.
One reason reprocessing thorium fuel cycles haven’t been successful is that uranium-232 (U 232) is created along with uranium-233. U-232, which has a half-life of about 70 years, is extremely radioactive and is therefore very dangerous in small quantities: a single small particle in a lung would exceed legal radiation standards for the general public. U-232 also has highly radioactive decay products. Therefore, fabricating fuel with U-233 is very expensive and difficult.
Previously I mentioned the implications of the presence of uranium-232 contamination within uranium-233 and its anti-proliferative nature with regards to nuclear weapons. U-232 contamination also makes fabrication of solid thorium-oxide fuel containing uranium-233-oxide very difficult. In the liquid-fluoride reactor, fuel fabrication is unnecessary and this difficulty is completely averted.
Thorium may be abundant and possess certain technical advantages, but it does not mean that it is economical. Compared to uranium, thorium fuel cycle is likely to be even more costly. In a once-through mode, it will need both uranium enrichment (or plutonium separation) and thorium target rod production. In a breeder configuration, it will need reprocessing, which is costly. In addition, as noted, inhalation of thorium-232 produces a higher dose than the same amount of uranium-238 (either by radioactivity or by weight). Reprocessed thorium creates even more risks due to the highly radioactive U-232 created in the reactor. This makes worker protection more difficult and expensive for a given level of annual dose.
The liquid-fluoride thorium reactor has an exceptionally simple and self-contained fuel cycle that has every promise of being less-expensive than today’s wasteful and complicated “once-through” approach to uranium fuel utilization. Makhijani and Boyd try to assign thorium to the wasteful “once-through” fuel cycle, point out deficiencies, and then condemn thorium as having no promise. This might analogous to putting diesel fuel in a gasoline-powered car and then pointing out how deficient diesel fuel is when the car will no longer operate. It is disingenuous and deceptive, and the kindest thing that can be said is that Makhijani and Boyd are ignorant of the implications of the liquid-fluoride thorium reactor and its fuel cycle, which they should not be if they presume to issue a “position paper” such as this.
Finally, the use of thorium also creates waste at the front end of the fuel cycle. The radioactivity associated with these is expected to be considerably less than that associated with a comparable amount of uranium milling. However, mine wastes will pose long-term hazards, as in the case of uranium mining. There are also often hazardous non-radioactive metals in both thorium and uranium mill tailings.
This is a repeat of the issue previously considered, as is immaterial as a factor for or against the use of thorium in nuclear powered reactors since thorium will be mined anyway during the mining of rare-earth minerals. The only question will be whether the mined thorium will be wasted or not.
In conclusion, Makhijani and Boyd fail to consider the implications of the liquid-fluoride thorium reactor on all aspects relating to the benefits of thorium as a nuclear fuel. They fail to consider its strong benefits with regards to nuclear proliferation, since no operational nuclear weapon has ever been fabricated from thorium or uranium-233. They fail to consider how LFTR can be used to productively consume nuclear weapons material made excess by the end of the Cold War. They fail to consider the reduction in nuclear waste that would accompany the use of LFTR. They fail entirely to account for the safety features inherent in a LFTR—how low-pressure operation and a chemically-stable fuel form allow the reactor to have a passive safety response to severe accidents. They fail to account for the improvement in cost that would be realized if LFTRs were to efficiently use thorium, reduce the need for mining fossil fuels, and increase the availability of energy.
Mr. Makhijani and Ms. Boyd should retract this statement in its entirety as flawed and deceptive to a public that needs clear and accurate information about our energy future.
Power has been restored to all six reactors at Fukushima-Daiichi. Now reactor coolant can be refilled and circulated and spent fuel pools can be refilled. Improvement in reducing radiation levels should be rapid now.
Welcome Telegraph readers!
In trying to answer the persistent question about LFTRs: “why wasn’t this done before?” I’ve obtained a report from 1962 made to President Kennedy where future development options for nuclear power were laid before him. Alvin Weinberg specifically references this report in his 1994 memoir (“First Nuclear Era”) and goes on to say that it recommends both the plutonium fast-breeder reactor and the liquid-fluoride thorium reactor as technologies that should be developed. Here’s what Weinberg had to say in his book:
“Until then I had never quite appreciated the full significance of the breeder. But now I became obsessed with the idea that humankind’s whole future depended on the breeder. For society generally to achieve and maintain a living standard of today’s developed countries depends on the availability of a relatively cheap, inexhaustible source of energy. (As I write these words, I realize that until recently I tended to dismiss solar energy as too expensive, and fusion as probably infeasible. I really don’t know whether this will always be the case.)
“The breeder became central in my thinking about nuclear-energy development. And, with Glenn Seaborg’s becoming the chair of AEC in 1960, the breeder acquired ever-increasing status with AEC—especially recognition as an essentially inexhaustible source of energy.
“In 1962, the AEC issued a report to the president on civilian nuclear power. Lee Haworth, a superbly responsible physicist-administrator, was in charge of drafting the report. He projected a nuclear deployment by 2000 of about 700 gigawatts (compared with the actual deployment in 1993 of 102 gigawatts), which seemed at the time quite reasonable. Both the fast breeder based on the 239Pu-238U cycle and the thermal breeder based on the 233U-232Th cycle figured prominently in the report. Indeed, the report implied that both systems should be pursued seriously, including large-scale reactor experimentation. It particularly favored molten uranium salts for the thermal breeder. But the molten-salt system was never given a real chance. Although the AEC established an office labeled “Fast Breeder,” no corresponding office labeled “Thermal Breeder” was established. As a result, the center of gravity of breeder development moved strongly to the fast breeder; the thermal breeder, as represented by the molten-salt project, was left to dwindle and eventually to die.”
At any rate, I have obtained a copy of this report and scanned it and made it available as a PDF. I think it is worthy of our study in an attempt to figure out why decisions were made that led us to the current situation.
Here are some interesting passages from the report:
In the thorium-uranium-233 cycle, the situation is quite different. U-233 emits more neutrons in thermal fission than does U-235; on the other hand, it is only slightly better in fast fission than in slow. Hence, thermal breeders offer greatest promise, minimizing as they do the power density and fuel endurability requirements. However, thermal breeders have a different complication in that fission products act as strong absorbers of slow neutrons, requiring that these products not accumulate too much. Among the most promising solutions of this difficulty is to use the fuel in fluid form, thus permitting continuous extraction and reprocessing to remove the fission products. Various fluid fuels have been studied for this purpose. The currently most promising approach is the use of fused uranium salts which can be circulated, both for reprocessing purposes and for heat transport. This technology is, however, in a fairly early stage.
Even when breeder reactors become economic and begin to be installed there will be a complication regarding fuel supplies. At least for some time to come, economic breeders will have breeding gains so low that they will produce not more than 3% or 4% of their fuel inventory each year. Hence, since the annual growth in energy consumption is about 6%, it will be necessary, if nuclear power increases its fractional share of the total load, to fuel some portion of the installations with fissionable uranium-235.
This leads to no great problem in the thorium-uranium thermal breeders. The fuel demand can be fulfilled simply by charging some of them, initially at least, with U-235, though at some sacrifice in economics and in the amount of U-233 that they produce.
On the other hand the “fast” reactors required to breed an excess of plutonium are economically attractive only when plutonium rather than U-235 is used to fuel them. Hence the most promising arrangement for incorporating them in a rapidly expanding nuclear power economy would undoubtedly be to use thermal converters to help provide the plutonium needed for added installations. This combination would continue until increases in the relative “yield” of plutonium from the breeders, together with a lower relative rate of growth of electrical energy consumption enabled the breeders to catch up and produce enough plutonium by themselves.
We get somewhat of an insight into the thinking of the Atomic Energy Commission with regards to breeder reactors. If they were to use uranium-plutonium, then plutonium supplies were crucial due to the fact that each fast breeder needed 10 to 15 times as much fissile material to generate a unit of power as a thermal reactor did. The light-water reactors at the time were producing plutonium as a byproduct. The fast-breeder needed and wanted that plutonium. Reactors like LFTR needed a tiny fraction of the fissile inventory as the fast breeder did and could be started on HEU.
Here’s an image of how the AEC envisioned light-water reactors running on enriched uranium and producing plutonium, and fast-breeder reactors needing that plutonium, working together.
The picture begins to become clearer, especially when we consider how Weinberg described what the AEC did with this report, establishing a “Fast Breeder” office but no “Thermal Breeder” office.
More thoughts on this later…
Kirk Sorensen’s note: I’d like to introduce my friend and colleague Kirk Dorius to the Energy from Thorium community. Kirk Dorius is a mechanical engineer and intellectual property attorney. I welcome his insights into the technology that underpins today’s solid-fueled uranium reactors.
A typical solid nuclear fuel rod includes a zirconium alloy tube or “cladding” encasing a single column of uranium fuel pellets. The cladding tube is smaller in diameter than your index finger, and is about 14 feet long. The uranium pellets are each about the size of the tip or your pinky finger, with the energy equivalent of 17000 cubic feet of natural gas, 1780 pounds of coal or 3.5 barrels of oil. The pellets are stacked in the tube with allowance for pellet expansion during fission and heating of the uranium. Once the uranium pellets are loaded into the cladding tube, zirconium end caps are welded in place to form a complete loaded fuel “rod.”
The fuel rods are then arranged in “bundles” or “fuel rod assemblies”, e.g., 14×14 or 17×17 arrays, which are then inserted into the core with a number of control rods being retractable from the bundle to initiate fission and insertable into the bundle to stop fission. Many rod bundles are oriented vertically in the reactor core with a substantial flow of water passing upward through the bundles to convey the fission reaction heat to a steam turbine for generation of electricity.
The zirconium cladding serves to hermetically isolate the uranium pellets and accumulated fission byproducts from exposure to the water flow in the core or cooling tank or to the atmosphere. The thin-walled cladding is transparent to radiation but is naturally affected by the high heat stresses and heat loading in the core. The rods are preemptively retired after a finite core cycle of several years to maintain cladding integrity even though only a very small fraction of the uranium is “spent.” This finite core cycle is also limited by accumulation of fission byproducts, particularly nuetron absorbers, inside the fuel rod.
A retired or spent nuclear fuel (“SNF”) rod is placed in a water cooling tank for an initial cool-down period during which the more highly radioactive (shorter half-life) isotopes rapidly decay. During this period, the rapid decay still generates substantial decay radiation and heat, albeit only a small fraction of the fission radiation and heat that is generated during reactor operation. After this initial cool-down period, the slower decay of the remaining longer-half-life isotopes generates a moderate amount of decay radiation and heat, which is readily absorbed by a concrete “dry cask” during long-term storage.
A typical nuclear plant can have hundreds of active fuel rod bundles in each core, thousands of SNF rods in short-term cool-down tanks and fuel from tens of thousands of SNF rods in long-term dry cask storage. The cooling tanks at the compromised Fukushima Daiichi nuclear plant collectively house around 11,000 SNF rods with a portion of those housed in the cooling tanks above reactors 1-4.
Water in the cool-down tanks acts as a neutron moderator, radiation shield and coolant, so long as the water level around the rods in the tank is maintained. If the SNF rods are left exposed and uncooled long enough, rapid oxidation (often called “burning”) and extreme heat stress can eventually compromise the cladding, expose the uranium, generate hydrogen, and release fission byproducts. Unmoderated and uncooled SNF rods can produce sufficient radiation and heat that even brief close proximity worker exposure is unacceptable. Should the cooling tank levels drop too low for too long, it could be challenging to restore the cooling tank water levels from a safe distance.
Hopefully, the cooling tank water levels at the Fukushima Daiichi nuclear plant will be restored and the situation stabilized soon.
Recent satellite images and helicopter crews flying over Fukushima-Daiichi indicate that the spent fuel pools contain boiling water (which is not surprising, and boiling is a very effective way to shed heat) but that there is water. This means that the fuel can’t be nearly as hot as feared. This is a Very Good Thing!
UPDATE: The IAEA is reporting that the temperatures in the spent fuel pools at units 5 and 6 measured on 3/17 are around 62 to 65 deg C, significantly below boiling temp of 100 deg C. The last measurement of the temperature in the spent fuel pool at unit 4 was on 3/13 and was 84 deg C.
From a nuclear expert:
Spent fuel in the pools in Units 3 and 4 is now uncovered. Unlike the releases from damaged fuel in the reactor cores of Units 1, 2, and 3, which were largely filtered by scrubbing in the containment suppression pools, releases of volatile fission products (Cs, I) from these pools have direct pathways to the environment.
Efforts to deliver water to these pools have proven to be very difficult, and fuel damage is occurring. The use of the evaporation of salt water as a heat sink over periods of more than a few days is not viable because the quantities of salt deposited as the water evaporates becomes large in volume and plugs the flow paths through the fuel, degrading heat removal. Fresh water supply is difficult to come by. It may be practical to bring fresh water by helicopter (this is being attempted), but the amounts needed imply a very large number of flights and radiation levels are extremely high above the pools making overflights hazardous. If radiation levels on the ground increase further, personnel access will become more challenging. Additional spent fuel is stored in pools in Units 5 and 6 and in a large centralized storage pool. A key issue is how to continue to make up water to these pools in the longer term, particularly if site access becomes more difficult or impossible.
In short, this accident is now significantly more severe than TMI. It resulted from a unique combination of failures to plant systems caused by the tsunami, and the broad destruction of infrastructure for water and electricity supply which would normally be reestablished within a day or two following a reactor accident.
My earlier belief that this would not lead to widespread radioactivity dispersal is based on the assumption that cooling to the spent fuel pools can be maintained. This is currently uncertain. Iodine-131 poses the most significant radiological risk to the surrounding populations, and access to potassium iodide or other iodine-rich foods would be prudent.
UPDATE: Commenter Zach Clayton pointed out something that I should have made very clear in this article–there is no iodine-131 in the spent fuel. It decayed away to harmless xenon-131 a long time ago. The xenon is also completely stable, and the krypton only contains krypton-85 which poses essentially no threat. The riskiest substance in spent nuclear fuel is likely the cesium, and potassium iodide pills will do little to counteract that threat. But cesium doesn’t “bioaccumulate” in the thyroid either.
In the mid-afternoon on Friday, March 11, 2011, the seismic sensors at the Fukushima-Daiichi nuclear power plant in the Fukushima Prefecture of Japan registered the earliest indications of the largest earthquake in modern Japanese history. They executed a preprogrammed response and began to drive all of the long control rods into the three reactors that were currently operating at the site. The control rods caused each generation of fission to produce fewer neutrons and fewer fission reactions. In three minutes the reactors were making 10% of their rated power from fission; in six minutes they were making 1%, and within ten minutes nuclear fission as a source of heat had ended in the first three units at Fukushima Daiichi. It would never begin again.
Each fission reaction splits the nucleus of an atom of uranium-235 or plutonium-239 into two smaller atoms and releases a great deal of energy. The energy release from nuclear fission is roughly a million times greater per unit weight than fossil fuels, which is why nuclear fission is such a compelling long term energy source. The two “fission products” that result are highly radioactive but decay towards stability very quickly. There are about 80 different sequences of decay that fission products can follow, and roughly a quarter reach a completely non-radioactive state within a day. Within a month, about three-quarters are stable, and within a year, about 80%. But in the first few hours after a nuclear reactor shuts down these fission products are producing significant amounts of heat and, unlike fission, this heat generation can’t be turned off. It has to run its course to completion. Therefore, managing what is called “decay heat” is one of the most important aspects of operating a nuclear reactor safely. To remove the heat, today’s reactors have an abundance of safety systems, all of which have the same mission—keep removing decay heat from the nuclear fuel. As the reactors at Fukushima-Daiichi cooled down, the tsunami hit.
The tsunami destroyed the diesel generators that provide power to drive the pumps that circulate the water coolant through the reactor that removes decay heat. Without an active removal of decay heat, the reactor was adding heat to the water faster than it was taking it out, and the temperature was rising. Because this was a reactor that operated on water that was already at its boiling point, this also meant that the pressure inside the reactor was rising as well.
The reactors at Fukushima-Daiichi are called boiling-water reactors (BWRs) and were manufactured by General Electric. They have a primary and a secondary containment structure, both made from thick reinforced concrete, to protect against the release of radioactive materials. Inside the primary containment are two vessels called a “drywell” and a “wetwell.” The drywell is a large steel pressure vessel that looks like a giant upside-down pear and holds the reactor and primary pumps, and the wetwell is a large toroidal vessel that looks like a donut. The wetwell is connected to the drywell by a number of wide pipes. Both the drywell and the wetwell are surrounded by a secondary containment vessel (or shield building) also built from reinforced concrete about a meter thick. This rectangular secondary containment building is the structure that most people have seen in pictures of the reactor. At the top of the secondary containment building is a steel frame structure with “blowout” panels that holds the crane used to remove solid nuclear fuel during fueling and refueling.
The designers of the reactors at Fukushima-Daiichi had anticipated situations where pressure was rising in the core. So long as power was available, pumps would circulate hot fluid from the reactor to the wetwell where it would be condensed. Heat removal could continue indefinitely in this way. But it all relied on a power source, and power had been lost due to the tsunami’s destruction of the diesel generators.
The water in the reactor is susceptible to damage from radiation, causing it to split into its components, hydrogen and oxygen. Normally, circulation would channel the hydrogen and oxygen to a recombiner where they would be restored back to water, but in the hours after the reactors were shut down, hydrogen was accumulating and separating in the wetwell and reached a point where it was vented into the sparse steel-frame structure at the top of the reactor building. It was only a matter of time before the hydrogen reached a level where it would detonate, and one after another, the first unit, then the third unit, and finally the second unit, suffered hydrogen explosions that blew off the steel panels and left the top of the reactor building exposed. The reactor vessels remained intact as did the reinforced concrete containment buildings, but each reactor building lost its hat due to the hydrogen explosions.
Initially there was hope of saving the reactors to generate power again after the crisis had passed. But as that hope faded and the need to remove the steadily-decreasing decay heat remained, operators at Fukushima-Daiichi took measures that would cool the reactors but would ruin them for future operation, such as the decision to try to cool the reactors with seawater. It will be necessary for some time to actively cool the reactors while the decay heat continues to decrease, but within a few months it will be possible to depressurize the reactors and assess their internal states. There may have been some melting and damage to the fuel—it is not known at this time.
What is known is that this is a situation very different than Chernobyl or Three Mile Island. There was no operator error involved at Fukushima-Daiichi, and each reactor was successfully shut down within moments of detecting the quake. The situation has evolved slowly but in a manner that was not anticipated by designers who had not assumed that electrical power to run emergency pumps would be unavailable for days after the shutdown. They built an impressive array of redundant pumps and power generating equipment to preclude against this problem. Unfortunately, the tsunami destroyed these systems.
There are some characteristics of a nuclear fission reactor that will be common to every nuclear fission reactor. They will always have to contend with decay heat. They will always have to produce heat at high temperatures to generate electricity. But reactors do not have to use coolant fluids like water that must operate at high pressures in order to achieve high temperatures. Other coolant fluids like fluoride salts can operate at high temperatures yet at the same low ambient pressures as the outside. Liquid fluoride salts are impervious to radiation damage, unlike water, and don’t evolve hydrogen gas which can lead to an explosion. Conventional solid nuclear fuel like that used at Fukushima-Daiichi can melt and release radioactive materials if not cooled consistently during shutdown and for a cool-down period thereafter. Liquid fluoride salts can carry fuel in a chemically-stable form that can be passively cooled without the need for pumps driven by emergency power generation. There are safe nuclear solutions even in extreme situations like those encountered at Fukushima-Daiichi, and it may be in our best interest to pursue them.