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Decades later, the story changed. “Th[orium]-based fuels and fuel cycles have intrinsic proliferation resistance,” according to the IAEA in 2005. Mainstream media have repeated this view ever since, often without caveat. Several scholars have recognized the inherent proliferation risk of protactinium separations in the thorium fuel cycle, but the perception that thorium reactors cannot be used to make weapons persists. While technology has advanced, the fundamental radiochemistry that governs nuclear fuel reprocessing remains unchanged. Thus, this shift in perspective is puzzling and reflects a failure to recognize the importance of protactinium radiochemistry in thorium fuel cycles. 

Protactinium turns 100. The importance of protactinium chemistry for obtaining highly attractive fissile material from thorium has been recognized since the 1940s. However, the story really begins 100 years ago during the earliest research on natural radioactivity. In 1918, Austrian-Swedish physicist Lise Meitner and German chemist Otto Hahn were on a quest to discover the long-lived isotope of “eka-tantalum” predicted to lie between thorium and uranium in the periodic table. The isotope they sought would decay to actinium, which was always found with uranium but was known to be the parent of an unknown natural radioactive decay chain distinct from that of uranium 238, the most common isotope of uranium found in nature.

Meitner and Hahn discovered that treating pitchblende with nitric acid yielded an insoluble fraction of silica that associated with tantalum and eka-tantalum. After many years, they purified enough eka-tantalum for identification and measured its properties. As discoverers of eka-tantalum’s longest-lived isotope, Meitner and Hahn named this new element protactinium. They had isolated protactinium 231, a member of the uranium 235 decay chain. In 1938, they discovered that protactinium 233 could be produced by neutron irradiation of thorium 232, the most abundant isotope in naturally occurring thorium.

For the next several decades, protactinium was shrouded in “mystery and witchcraft” due to its scarcity in nature and its perplexing chemical properties. We now know that protactinium’s peculiar chemistry is due to its position in the periodic table, which lends the element vastly different chemical properties than its neighbors. Protactinium behaves so differently from thorium and uranium that, under many conditions, their separation is inevitable.
Scientists did not investigate the macroscopic chemistry of protactinium until the Manhattan Project. In 1942, Glenn T. Seaborg, John W. Gofman, and R. W. Stoughton discovered uranium 233 and observed its propensity to fission. Compared with naturally occurring uranium 235, uranium 233 has a lower critical mass, which means that less material can be used to build a weapon. And compared with weapons-grade plutonium 239, uranium 233 has a much lower spontaneous fission rate, enabling simpler weapons that are more easily constructed. A 1951 report by the Manhattan Project Technical Section describes extensive efforts devoted to the production of uranium 233 via neutron irradiation of thorium 232. Because the initial thorium feed material was often contaminated with natural uranium 238, the scientists obtained pure uranium 233 by using a variety of methods for separating the intermediate protactinium 233.

By this time, advances in technology and projections of uranium shortages stimulated interest in developing a breeder reactor, which produces more fissile material than it consumes. In the late 1960s, a team at Oak Ridge National Laboratory designed a Molten Salt Breeder Reactor fueled by thorium and uranium dissolved in fluoride salts, but it could only breed uranium 233 by continuously removing impurities—including protactinium 233—from the reactor core. To improve breeding ratios, the researchers investigated methodsfor removing protactinium from the molten fluoride salts.

In 1977, President Jimmy Carter banned commercial reprocessing of spent nuclear fuel, citing concerns with the proliferation of technology that could be used to make nuclear weapons. And with the high startup costs of developing new reactors, there would be no place for the Molten Salt Breeder Reactor in the energy market. With the end of research on thorium reactors came the end of ambitious research on protactinium separations. Over time, the role of protactinium in obtaining weaponizable uranium 233 from thorium was largely forgotten or dismissed by the thorium community.

Thorium reactors born again. Fast forward to 2018. Several nations have explored thorium power for their nuclear energy portfolios. Foremost among these is India. Plagued by perennial uranium shortages, but possessing abundant thorium resources, India is highly motivated to develop thorium reactors that can breed uranium 233. India now operates the only reactor fueled by uranium 233, the Kalpakkam Mini reactor (better known as KAMINI).

Thorium reactors have other potential advantages. They could produce fewer long-lived radioactive isotopes than conventional nuclear reactors, simplifying the disposal of nuclear waste. Molten salt reactors offer potential improvements in reactor safety. Additionally, there is the persistent perception that thorium reactors are intrinsically proliferation-resistant.

The uranium 233 produced in thorium reactors is contaminated with uranium 232, which is produced through several different neutron absorption pathways. Uranium 232 has a half-life of 68.9 years, and its daughter radionuclides emit intense, highly penetrating gamma rays that make the material difficult to handle. A person standing 0.5 meters from 5 kilograms of uranium 233 containing 500 parts per million of uranium 232, one year after it has been separated from the daughters of uranium 232, would receive a dose that exceeds the annual regulatory limits for radiological workers in less than an hour. Therefore, uranium 233 generated in thorium reactors is “self-protected,” as long as uranium 232 levels are high enough. However, the extent to which uranium 232 provides adequate protection against diversion of uranium 233 is debatable. Uranium 232 does not compromise the favorable fissile material properties of uranium 233, which is categorized as “highly attractive” even in the presence of high levels of uranium 232. Uranium 233 becomes even more attractive if uranium 232 can be decreased or eliminated altogether. This is where the chemistry of protactinium becomes important.

Protactinium in the thorium fuel cycle. There are three isotopes of protactinium produced when thorium 232 is irradiated. Protactinium 231, 232, and 233 are produced either through thermal or fast neutron absorption reactions with various thorium, protactinium, and uranium isotopes. Protactinium 231, 232, and 233 are intermediates in the reactions that eventually form uranium 232 and uranium 233. Protactinium 232 decays to uranium 232 with a half-life of 1.3 days. Protactinium 233 decays to uranium 233 with a half-life of 27 days. Protactinium 231 is a special case: It does not directly decay to uranium, but in the presence of neutrons it can absorb a neutron and become protactinium 232.

Neutron absorption reactions only occur in the presence of a neutron flux, inside or immediately surrounding the reactor core. Radioactive decay occurs whether or not neutrons are present. For irradiated thorium, the real concern lies in separating protactinium from uranium, which may already have significant levels of uranium 232. Production of protactinium 232 ceases as soon as protactinium is removed from the neutron flux, but protactinium 232 and 233 continue to decay to uranium 232 and 233, respectively.

The half-lives of the protactinium isotopes work in the favor of potential proliferators. Because protactinium 232 decays faster than protactinium 233, the isotopic purity of protactinium 233 increases as time passes. If it is separated from its uranium decay products a second time, this protactinium will decay to equally pure uranium 233 over the next few months. With careful attention to the relevant radiochemistry, separation of protactinium from the uranium in spent thorium fuel has the potential to generate uranium 233 with very low concentrations of uranium 232—a product suitable for making nuclear weapons.
Scenarios for proliferation. Although thorium is commonly associated with molten salt reactors, it can be used in any reactor. Several types of fuel cycles enable feasible, rapid reprocessing to extract protactinium. One is aqueous reprocessing of thorium oxide “blankets” irradiated outside the core of a heavy water reactor. Many heavy water reactors include on-power fueling, which means that irradiated thorium can be removed quickly and often, without shutting the reactor down. As very little fission would occur in the blanket material, its radioactivity would be lower than that of spent fuel from the core, and it could be reprocessed immediately.

Myriad possibilities exist for the aqueous separation of protactinium from thorium and uranium oxides, including the commonly proposed thorium uranium extraction (THOREX) process. Alternatively, once dissolved in acid, protactinium can simply be adsorbed onto glass or silica beads, exploiting the same chemical mechanism used by Meitner and Hahn to isolate protactinium from natural uranium a century ago.

Another scenario is continuous reprocessing of molten salt fuel to remove protactinium and uranium from thorium. Researchers at Oak Ridge explored the feasibility of online protactinium removal in the Molten Salt Breeder Reactor program. Uranium can then be separated from the protactinium in a second step.

Sensible safeguards. Protactinium separations provide a pathway for obtaining highly attractive weapons-grade uranium 233 from thorium fuel cycles. The difficulties of safeguarding commercial spent fuel reprocessing are significant for any type of fuel cycle, and thorium is no exception. Reprocessing creates unique safeguard challenges, particularly in India, which is not a member of the Nuclear Non-Proliferation Treaty.

There is little to be gained by calling thorium fuel cycles intrinsically proliferation-resistant. The best way to realize nuclear power from thorium fuel cycles is to acknowledge their unique proliferation vulnerabilities, and to adequately safeguard them against theft and misuse.