sign in sign up
<<
Home , Alvin Radkowsky, Thorium

A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society

This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Fuel for Nuclear

An unconventional tactic might one day ease concerns that spent fuel could be used to make a bomb

Mujid S. Kazimi

ow might a determined terrorist fuel taken from their nuclear reactors— The accommodation that Carter Hgroup get hold of the or far too little to make even one bomb— helped to work out alleviated many needed to make an atom but they refused to allow inspectors to worries: In return for mothballing their bomb? That question has been weigh- verify this claim. graphite-based reactors, Pyongyang re- ing heavily on many people’s minds. Why was North Korea reprocessing ceived assistance from Washington in The easiest way is probably to buy it, nuclear fuels in the first place? After obtaining plants of the perhaps from North Korea, which, ac- all, the peaceful pursuit of civilian nu- type used in the , along cording to intelligence reports, may clear power does not require any re- with a generous aid package. The solu- have the means to produce a modest processing, as the United States and tion was, at least in part, a technical fix, stockpile. Although the nuclear aspira- several other countries have demon- offering the North Koreans a way to de- tions of Pyongyang have been much in strated. (At U.S. power plants, the velop a peaceful program of nuclear the news this year, experts also worry spent fuel materials are simply stored energy. They could then continue to op- about other “nations of concern” ob- in dry casks or in cooling ponds, in erate nuclear power plants without cre- taining these terrifying weapons. The preparation for their eventual disposal ating so much concern abroad that in the North Korean example is, however, at the Yucca Mountain Repository in course of reprocessing spent fuel they rather clear-cut, and the details illumi- Nevada, which should begin operation might extract plutonium for bombs. nate a longstanding problem of inter- around 2010.) Was not North Korea’s Of course, without adequate over- national security, one that nuclear en- decision to reprocess spent nuclear fu- sight the North Koreans could conceiv- gineers like myself would dearly like els prima facie evidence that it intended ably use their newer reactors for breed- to help solve. to extract the plutonium generated ing plutonium, by reprocessing the Almost a decade ago the world within its reactors and use it to fabri- spent fuels at some secret site. Indeed, breathed a sigh of relief when diplo- cate nuclear bombs? their penchant to work clandestinely to matic efforts, including those of former Not exactly. North Korea’s nuclear obtain bomb-making materials became President Jimmy Carter, defused what power reactors are quite different in de- obvious last year, when it was reported then threatened to become a violent sign from the ones now operating in that Pakistan had sent North Korea conflict: At the time, North Korea was the United States, which use water both high-speed centrifuges—equipment interfering with the monitoring work as the coolant and the moderator, the for making weapons-grade uranium— of the International Atomic Energy substance that slows the neutrons re- in return for missile technology. Such Agency, in clear breach of that coun- leased during , allowing apparatus is growing increasingly easy try’s obligations as a signatory to the them to initiate further fission reactions. to obtain, and thus efforts to transform nuclear nonproliferation treaty. In par- The North Korean “magnox” reactors ordinary uranium into the highly en- ticular, the North Koreans were assert- (a name derived from the riched form suitable for bombs are be- ing that they had produced just a tiny that encloses the uranium coming harder and harder to police. So amount of plutonium from the spent fuel) use gas as the coolant and the world will probably always face graphite as the moderator, having a de- that threat. But what of the problem of Mujid S. Kazimi received his Ph.D. in nuclear engi- sign similar to one long in use in the spent nuclear fuels being used for neering in 1973 from the Massachusetts Institute of United Kingdom. It turns out that the bomb making? Technology, where he is now the Tokyo Electric Pow- spent fuel from magnox reactors can- One of the important barriers to er Company Professor of . Kaz- not safely be stored: It must be re- such a diversion of spent fuel is that it imi directs MIT’s Center for Advanced Nuclear En- processed to a form that is less suscep- remains highly radioactive for cen- ergy Systems (CANES), which studies new tible to oxidation in air or water. So the turies after discharge, thus requiring concepts for nuclear power with the aim of making it fact that the North Koreans were repro- remote handling and facilities with ad- more economical and safe, lessening its environmen- tal impact and raising the barriers to the prolifera- cessing their spent fuels could not in it- equate shielding for extracting the plu- tion of nuclear weapons. Address: CANES, Room self be taken as evidence of ill intent. tonium. Might there be effective tech- 24-215, Massachusetts Institute of Technology, Their interference with international in- nical solutions to further limit the Cambridge, MA 02139. Internet: [email protected] spectors was, however, quite troubling. problem of spent nuclear fuels being

© 2004 Sigma Xi, The Scientific Research Society. Reproduction 408 American Scientist, Volume 91 with permission only. Contact [email protected]. Figure 1. Uranium fuels most of the world’s 439 nuclear reactors (103 in the United States), which together provide 16 percent of the planet’s electricity. In most installations, uranium oxide pellets fill slender fuel rods, which are typically arranged in a square array within each fuel as- sembly. A modern power reactor houses roughly 200 such assemblies in its core. These are shuffled about and replaced periodically, the spent fuel being either reprocessed or stored. A troubling feature of this system of electricity generation is that it produces plutonium, which can be chemically extracted from the spent fuel and used in nuclear weapons. Using thorium as well as uranium in the fuel can diminish that threat: Thorium-based nuclear fuels produce substantially less plutonium, and the isotopic composition of the plutonium that they do create is un- suitable for bomb making. The ability to thwart the proliferation of nuclear weapons, coupled with the great abundance of thorium in the ’s crust, has lately spurred nuclear engineers to reconsider this approach, which was abandoned in most parts of the world decades ago. (Photograph courtesy of British Energy.)

© 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 September–October 409 up more than 95 percent of most nu- clear fuels. A conventional reactor breeds various of plutonium pressurizer steam generator from uranium-238, and some of that plutonium in turn undergoes fission in turbine the reactor, adding to the power the uranium-235 provides. The hitch with using thorium as a cooling fuel is that breeding must occur be- fuel generator tower fore any power can be extracted from it—and that requires neutrons. Some control engineers have proposed using parti- rods cle accelerators to generate the need- ed neutrons, but this process is costly, reactor vessel condenser and the only practical scheme at the containment structure moment is to combine the thorium with conventional nuclear fuels (made up of either plutonium or en- riched uranium or both), the fission- ing of which provides the neutrons to control rods start things off. steam The breeding of uranium-233 from generator thorium is more efficient than the breeding of plutonium from uranium- 238, because less of various nonfissile isotopes is created along the way. De- turbine signers can take advantage of this effi- ciency to decrease the amount of spent fuel per unit of energy generated, cooling which reduces the amount of waste to generator tower be disposed of. There are some other pluses as well. For example, , the form of thorium used for graphite fuel balls nuclear power, is a highly stable com- pound—more so than the uranium containment structure condenser dioxide typically employed in today’s fuel. So there is less concern that the Figure 2. Thorium-based fuels are appropriate both for pressurized-water reactors, which use or- fuel pellets could react chemically with dinary water to transfer heat from the core and to slow the neutrons generated in the fission re- the cladding around them or actions (top), and for high-temperature gas reactors, which use a gas such as helium to transfer with the cooling water should there be heat and graphite to slow the neutrons (bottom). Several pressurized-water reactors (in- a breach in the protective cladding. cluding the very first reactor built for commercial power generation) were run on thorium during Also, the of thori- various early trials. And some high-temperature gas-cooled reactors have operated with thorium- um dioxide is 10 to 15 percent higher based fuels as well, including the German THTR-300, a 300-megawatt reactor built near Ham- burg in the 1980s. This reactor was of the “pebble-bed” design indicated above, wherein fuel in than that of , making it the form of many small balls is placed in a hopper-like vessel. This arrangement allows refueling easier for heat to flow out of the slender to take place continually, avoiding the costly outages that periodically take place at most rods used inside a reactor. What is power plants, where reactors must be shut down for refueling. more, the of thorium dioxide is about 500 degrees Celsius exploited for military ends? That is a than uranium. Unfortunately, thori- higher than that of uranium dioxide, question that the designers of nuclear um atoms cannot themselves be easi- and this difference provides an added fuels have asked themselves over and ly induced to split—the basic require- margin of safety in the event of a tem- over. Here, I would like to explore one ment of a fission reactor. But when a porary power surge or loss of coolant. possible answer that has garnered quantity of thorium-232 (the common Knowledge of such advantages has much recent interest: thorium. of that element) is placed repeatedly spurred nuclear engineers to within a , it readily ab- conduct experiments, and some groups Now You’re Cooking with Thorium sorbs neutrons and transforms into have even gained experience running The use of thorium in power reactors uranium-233, which, like the urani- commercial power reactors on thorium- has been considered since the birth of um-235 typically used for generating based fuels. For example, a gas-cooled, nuclear energy in the 1950s, in large nuclear power, supports fission chain graphite-moderated reactor called part because thorium is considerably reactions. Peach Bottom Unit One, located in more abundant than uranium in the Thorium is thus said to be “fertile” southeastern Pennsylvania, used a com- Earth’s crust. Roughly speaking, there rather than fissile. In this respect it is bination of thorium and highly en- is about three times more thorium similar to uranium-238, which makes riched uranium in the mid-1960s. An-

© 2004 Sigma Xi, The Scientific Research Society. Reproduction 410 American Scientist, Volume 91 with permission only. Contact [email protected]. other gas-cooled reactor at Fort St. Vrain strained by the provisions that com- (something that is currently discour- in Colorado was run on a similar thori- mercial uranium suppliers in countries aged because of worries over prolifera- um-based fuel between 1976 and 1989. such as require: They demand tion) and presupposed that the spent Tests with relatively simple mixtures of that purchasers of their ore allow fuel would be reprocessed for the ex- thorium oxide and highly enriched ura- enough oversight to ensure that the traction of its fissile contents. Neither nium oxide also began with water- fuel (or the plutonium spawned from practice is now envisaged. The thori- cooled reactors during the 1960s, at the it) is not used for nuclear weapons. um-based fuel assemblies currently be- “BORAX” (Idaho) and Elk River (Min- Previous work on thorium else- ing designed are different from past ex- nesota) facilities and at the Indian Point where in the world did not to its amples in other ways too. For example, (New York) power plant. And between adoption, largely because its perfor- they can withstand greater exposure to 1977 and 1982, more complicated com- mance in water reactors, such as the the heat and radiation experienced in- binations of thorium and either urani- first core at the Indian Point power sta- side the core of a reactor, which allows um-235 or uranium-233 were also em- tion, did not live up to expectations. more of the fertile thorium-232 to be ployed in a water-cooled reactor at Given this history, it may come as converted into fissile uranium-233. So Shippingport, Pennsylvania, in an ex- something of a surprise that thorium- what’s being talked about now is defi- perimental program seeking to develop based nuclear fuels are once again be- nitely not your father’s thorium-based a fuel that produces more fissile materi- ing considered, this time as the means nuclear fuel. al than it consumes. Interestingly, Ship- to stem the potential proliferation of pingport, which began operation in nuclear weapons. Using thorium to Averting Proliferation 1957, was the very first nuclear power prevent the buildup of plutonium re- As I mentioned, the lack of uranium- plant built in the United States for the quires that the fuel be configured dif- 233 in necessitates using a dif- commercial generation of electricity. ferently than in most of the experi- ferent , such as urani- Work with thorium-based nuclear ments of years past. Those trials um-235 (or perhaps plutonium-239), to fuels has by no means been restricted incorporated highly prime a reactor running on thorium. to the United States. German engi- neers, for example, have used combi- nations of thorium and highly enriched conventional fuel uranium, or thorium and plutonium, in both gas- and water-cooled power reactors. Thorium-based fuels have also been tried in the United Kingdom, France, Japan, , Canada and . But despite these considerable 235 235 early efforts, most nations long ago U U abandoned the notion of using thori- neutron fission um to power their nuclear generating product stations. One country that has main- 239 tained interest is , which began 238U Pu fueling some of its power reactors in the mid-1990s with bundles containing thorium. Although one of the reasons for employing thorium was simply to thorium-based fuel even out the distribution of power within the cores of these reactors, Indi- an engineers also took the opportunity to test how well thorium could func- tion as a fuel source. The positive re- sults they obtained motivated their current plans to use thorium-based fu- 235U 235U els in more advanced reactors now un- der construction. neutron fission India’s attraction to thorium-based product fuels stems, in part, from its large in- 233 232Th U digenous supply. (With estimated tho- rium reserves of some 290,000 tons, it ranks second only to .) But that nation’s pursuit of thorium, which 235 helps bring it independence from over- Figure 3. Conventional nuclear fuel (top) contains both a fissionable isotope of uranium ( U) and a nonfissionable isotope (238U). Sparked by an incoming neutron, the fissioning of a 235U seas uranium sources, came about for a nucleus releases two or three more neutrons. These in turn can cause another 235U nucleus to reason that has nothing to do with its split, or they can induce atoms of 238U to change into plutonium-239, which, being itself fis- balance of trade: India uses some of its sionable, then helps to power the reactor. Thorium-based nuclear fuels (bottom) operate in reactors to make plutonium for atomic much the same fashion, except that instead of breeding plutonium from 238U, they breed a fis- bombs. Thus India refuses to be con- sionable isotope of uranium, 233U. © 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 September–October 411 Corbis Corbis

Figure 4. was a pioneer in the use of thorium-based fuel and was perhaps its greatest proponent until his death in 2002. Rad- kowsky served as chief scientist for the U.S. Naval Reactors Program under the direction of Admiral Hyman Rickover, who is often remembered as the father of the nuclear Navy. In 1954, their program produced the first nuclear powered submarine, the USS Nautilus (bottom left). Here the bespectacled Radkowsky is seen receiving an award for technical achievement from Rickover (top left). Behind them is a diagram of the first nu- clear reactor designed specifically for commercial power generation, which was constructed near Shippingport, Pennsylvania, in the late 1950s (right). (Photograph of Radkowsky and Rickover courtesy of Thorium Power, Inc.)

Given the present-day proscription duced is roughly halved. But mixing amount of nuclear waste created. In against commercial fuels that are too them uniformly is not the only way to 1992 he helped to found a private highly enriched in uranium-235, a con- combine the two elements. company, Thorium Power, Inc., to siderable amount of (nonfissionable) Indeed, the approach undergoing commercialize this technique. Sadly, uranium-238 would clearly need to be the most investigation now is a combi- Radkowsky would not live to see his included in the primer; current stan- nation that keeps a uranium-rich vision materialize: He died last year, dards require at least 80 percent, and “seed” separate from a thorium-rich at the age of 86. more is typical. As is the case with “blanket.” The chief proponent of this Radkowsky’s idea was to construct conventional reactors, this would concept was the late Alvin Rad- special fuel assemblies that could be make it impossible to use the fresh fuel kowsky, a nuclear pioneer who, under used in typical water-cooled reactors for a bomb without first having to go the direction of Admiral Hyman Rick- with very little modification. These through the technically difficult step of over, helped to launch America’s nu- units are made up of a central seed re- isotopically enriching the uranium. clear Navy during the 1950s as chief gion containing fuel rods filled with re- The main advantage of using a com- scientist of the U.S. Naval Reactors actor-grade uranium (that is, having no bination of thorium and uranium is the Program. Radkowsky went on to more than 20 percent uranium-235). significant reduction in plutonium con- make significant contributions to the Surrounding the seed is a blanket re- tent of the spent fuel compared with commercial nuclear industry during gion with fuel rods containing thorium what comes out of a conventionally fu- the 1960s and ’70s. Then, at the urging and a small amount of uranium. Hav- eled reactor. Just how much less pluto- of (one of his former ing uranium-238 in the blanket pre- nium is made? The answer depends on teachers) to find a way to reduce the vents anyone from withdrawing these exactly how the uranium and thorium threat of nuclear weapons getting into rods and using only chemical means to are combined. For example, uranium the wrong hands, Radkowsky turned separate out the fissionable uranium- and thorium can be mixed homoge- his attention to the use of thorium- 233 that is created over time. neously within each fuel rod. In this based fuels, which he had already rec- With support from the U.S. Depart- case the amount of plutonium pro- ognized as a means of lessening the ment of Energy and technical assis-

© 2004 Sigma Xi, The Scientific Research Society. Reproduction 412 American Scientist, Volume 91 with permission only. Contact [email protected]. tance from Brookhaven National Labo- ratory, Thorium Power is now work- ing with the Kurchatov Institute in Moscow to investigate this strategy more fully. Their concept calls for using a metallic alloy as the seed fuel and for keeping the seed units in a Russian re- actor for three years before replacing them but leaving the blanket rods in the reactor for 10 years. Their results are not going to be directly applicable to the nuclear power stations in most other parts of the world, however, be- cause the fuel material is not in the form of an oxide (as preferred in the West) and because the Russian reactors involved in these tests use a hexagonal array of rods for each fuel assembly, seed fuel pin whereas most facilities operating in blanket fuel pin Western countries use a square array. reactor core Radkowsky and his colleagues had calculated that their scheme would re- duce the amount of plutonium pro- duced by 80 percent compared with what goes on in a conventionally fu- eled reactor of the same energy output. What is more, they found that the mix of plutonium isotopes generated, most- ly in the seed fuel, would not be partic- ularly desirable for military use, be- cause a bomb made from it would be extremely unlikely to give much explo- sive yield—in the slang of weapons de- signers, it would probably “fizzle.”Also, the plutonium has such a high content of the 238Pu isotope that its may be sufficient to melt or damage the other materials used in constructing a weapon. Even if a terrorist wanted to use the blanket plutonium for making a terrifying (if not terribly powerful) fresh seed once-burned seed twice-burned seed blanket bomb, extracting it from Radkowsky’s thorium fuel—indeed from any thorium Figure 5. Thorium-based nuclear fuels can be designed in different ways. One general scheme, fuel used in a reactor—would be more first conceived by Radkowsky, is to have each nuclear fuel assembly (squares) composed of difficult than removing it from today’s uranium-rich “seed” rods surrounded by thorium-rich “blanket” rods (top). The uranium, spent fuel. The spent blanket fuel con- which includes up to 20 percent of the fissionable isotope 235U, produces enough neutrons to tains uranium-232, which in the course transform the “fertile” thorium around it into another fissionable isotope of uranium, 233U. of a few months decays into isotopes This mixing of fuel types within an assembly complicates the refueling of a nuclear reactor, be- that emit high-energy gamma rays. cause the seed rods need to be replaced much more frequently than the blanket rods. An al- ternative approach, called the whole-assembly seed-and-blanket core (bottom), utilizes fuel as- Thus pulling out the plutonium would semblies that each contain only uranium-rich seed rods or thorium-rich blanket rods. These require significantly beefed-up radiation assemblies can be more easily shuffled or replaced at prescribed intervals. shielding and a more widespread use of remotely operated equipment within the reprocessing facility, further com- Reality Check larly eager to foster research activities plicating an already challenging task. In light of the potential advantages for in this area. In addition to funding And the abundance of uranium-232 reducing the quantity of nuclear waste Radkowsky’s company and its part- and its highly radioactive products in and preventing the dissemination of ners in their tests with Russian reac- the spent fuel would probably thwart bomb-making materials, it is not sur- tors, the DOE has lent support to three any effort to separate uranium-233 prising that interest in thorium-based other recent efforts. One involves a (which, being fissionable, could also be fuels has recently undergone some- consortium made up of two national used for a bomb) from uranium-238. thing of a renaissance. The U.S. De- labs (the Idaho National Engineering partment of Energy has been particu- and Environmental Laboratory, and

© 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 September–October 413 14%

13%

8.3%

14%

15% 24%

6%

8% 28%

10% 25%

Figure 6. India is the only country actively pursuing thorium-based fuels at this time, in part because this tactic offers that nation a degree of independence from foreign uranium suppliers. India claims almost a quarter of the world’s established thorium reserves, whereas it has comparatively little uranium. Countries with at least 5 percent of the global uranium (pink) or thorium (green) reserves are indicated. Tho- rium reserves are not as well known as those of uranium, because the current uses of thorium are limited. (Reserve estimates from the World Nuclear Association.) Argonne National Laboratory), two University examined the use of pluto- The Bottom Line private companies in the business of nium-primed thorium as fuel for boil- Even with a whole-assembly seed- fabricating nuclear fuels (Framatome ing-water reactors: These designs are and-blanket core, where each type of Technologies and Westinghouse) and distinct from the more common pres- fuel assembly is of homogenous con- three universities (the University of surized-water variety, which keep the struction, it is clear that the manufac- Florida, Purdue University and my cooling water under high pressure so ture of the fuel and its management own institution, the Massachusetts In- that it always remains a liquid. The within the reactor would be more stitute of Technology). The goal has idea behind this program is that it may complicated than usual. In a typical been to come up with a scheme for us- provide an economical means to con- power reactor, the fuel assemblies are ing thorium in reactors without the sume surplus weapons plutonium— shuffled at intervals so that each will added complication of dealing with without producing yet another genera- be exposed, on average, to the same separate types of fuel arrays (from the tion of plutonium waste, as would conditions of heat and radiation. In a seed and blanket units), as is required happen with the leading plan currently seed-and-blanket core, the seeds must in Radkowsky’s design. being contemplated, something known sustain power levels that are signifi- In another program that brought in- as the mixed oxide option. In this re- cantly above average, while the blan- vestigators at Brookhaven National spect, the Brookhaven-Purdue research ket assemblies experience far less Laboratory together with the Center on plutonium-seeded thorium fuel is stressful conditions. Thus the fuel in for Advanced Nuclear Energy Systems similar to some of the work that Thori- the seed rods reaches higher tempera- (CANES) at MIT, the objective is to um Power and its Russian partners are tures, releases more of the gaseous fis- look at practical ways to simplify the hoping soon to engage in. sion products into the limited space al- design of the separated seed and blan- My CANES colleagues and I have lowed for them within the fuel rods ket units. This could be done by as- devoted considerable effort over the and requires more cooling than does signing entire fuel assemblies to be ei- past few years to evaluating the details the fuel used in the blanket regions. ther seeds or blankets. Although the of various designs, including ways of These demands can be accommodat- terminology of “seeds” and “blankets” combining uranium and thorium with- ed in various ways—for example, by al- has been kept (we name this arrange- in individual fuel rods. As might be ex- lowing more coolant to flow through ment the whole-assembly seed-and- pected, our conclusions about the tech- the seeds and by making the fuel mate- blanket core), the metaphor is less ap- nical and economic feasibility vary rials less resistant to the flow of heat. In plicable in this case, which calls for depending on the particular design un- the Radkowsky-Kurchatov approach, these assemblies to be arranged, more der consideration. Here I would like to the seed rods are made from a metallic or less, in a checkerboard array within describe just a few of our results for the uranium alloy (following designs that the core of a reactor. seed-and-blanket arrangements, the have been tested in Russian sub- In a third research thrust, nuclear en- strategy that in my view has the best marines), which improves their thermal gineers at Brookhaven and Purdue chances of commercial success. conductivity. In the MIT-Brookhaven

© 2004 Sigma Xi, The Scientific Research Society. Reproduction 414 American Scientist, Volume 91 with permission only. Contact [email protected]. 100 100 93.8 weapons-grade plutonium spent-fuel plutonium thorium-based (seed) 75 75 thorium-based (blanket) typical all-uranium 50 50

25 25

isotopic content (percent) 5.8 isotopic content (percent) 0.012 0.35 0.022 0 0 238Pu 239Pu 240Pu 241Pu 242Pu 238Pu 239Pu 240Pu 241Pu 242Pu

Figure 7. Nuclear power reactors produce plutonium as a byproduct, which raises concerns that this material could be diverted to the pro- duction of nuclear weapons. But not all plutonium is alike. Weapons-grade plutonium must be rich in the isotope 239Pu and should have lit- tle of the highly radioactive isotope 238Pu (left). The plutonium left over from thorium-based nuclear fuels is thus much less suitable for use in bombs than is the plutonium from a conventional water-cooled reactor, because the highly radioactive isotope, 238Pu, makes up a larger proportion of the resultant plutonium (right). Thorium-fired reactors are also attractive because they produce less plutonium overall. For ex- ample, a thorium-fueled reactor using a whole-assembly seed-and-blanket core (the type considered for the chart at right) would produce a total of 92 kilograms of plutonium per gigawatt-year of electricity generated, whereas a conventional water-cooled reactor would result in 232 kilograms. scheme, the uranium oxide pellets with- make it quite difficult if not impossible to the existing commercial infrastruc- in the seed rods are hollow, which low- to fabricate and maintain a nuclear ture would clearly be needed, but no ers their temperature. Although the weapon. fundamentally new technology is re- blanket rods are less problematic in this The production of such large quired. And the fact that the relevant regard, they too must be carefully engi- amounts of plutonium-238 comes materials (thorium and enriched ura- neered so that the exterior cladding about because more of the fuel is con- nium) have a long record of experi- holds up well, the working lifetime of sumed (or “burned up,” in the lingo of mental use in reactors lends credibility these rods being in some designs as nuclear engineers) than is the case in to the notion that this scheme could long as 13 or 14 years. conventional uranium-fueled reactors. one day find widespread application, In addition to examining these vari- An equivalent amount of plutonium- should policymakers push the nuclear ous engineering concerns, investigators 238 could be created using an all- industry in that direction. at CANES have also quantified the ad- uranium fuel, but this would require a vantages of the seed-and-blanket de- higher initial amount of fissile urani- Bibliography signs in terms of their contribution to um (235U) than is typical in today’s Garwin, R. L.,and G. Charpak. 2002. Megawatts averting the proliferation of bomb- practice, and the economic projections and Megatons. Chicago: University of Chica- making materials, and we have also for that are discouraging. go Press. tried to evaluate their economics. Thus our recent work amply con- Mark, C. 1992. Explosive properties of reactor- grade plutonium. Science and Global Security We found that the seed-and-blanket firms that the various engineering con- 3:1–13. arrangements produce less plutonium cerns can be met and that running reac- Radkowsky, A., and A. Galperin. 1998. The than competing schemes in which ura- tors on thorium could indeed forestall nonproliferative light water reactor: A new nium and thorium are mixed at finer clandestine efforts to use the spent fuel approach to light water reactor core tech- scales. But our results are not quite as for making bombs. But the results of nology. 124:215–222. optimistic as Radkowsky’s earlier work our investigation into the economics of Shwageraus, S. , X. Zhao, , M. Driscoll, P. Hej- zlar, M. S. Kazimi and J. S. Herring. In had indicated: We calculate a reduction thorium are less clear-cut. We estimate press. Micro-heterogeneous thoria-urania of only 60 percent (for the whole-as- that thorium-based fuels could cost fuels for pressurized water reactors. Nuclear sembly system) or 70 percent (if both anywhere from 10 percent less to about Technology. seed and blanket rods are used within 10 percent more than conventional nu- each assembly), compared with Rad- clear fuels. The wide range stems from kowsky’s estimate of an 80-percent re- fundamental uncertainties about the duction for the latter. cost of the seed uranium (which must Our calculations of plutonium pro- be four times more enriched in urani- duction do, however, support Rad- um-235 than is the case with typical nu- kowsky’s assertions that the spent fuel clear fuels), the cost of fabricating the For relevant Web links, consult this issue of would contain appreciable amounts of fuel assemblies and the savings that American Scientist Online: plutonium-238, a highly radioactive might accrue in the future as a result of isotope, which thus produces a lot of the reduction in the amount of spent http://www.americanscientist.org/ heat. Indeed, the plutonium-238 con- fuel in need of disposal. template/IssueTOC/issue/400 tent would be three to four times high- Although it seems unlikely that eco- er than with conventional uranium fu- nomics alone could drive the adoption els. As Radkowsky pointed out, the of thorium fuels, there are no technical heat given off by this isotope would “show-stoppers” here. Modifications © 2004 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 September–October 415