By Eva C. Uribe,
In 1980, the International Atomic Energy Agency (IAEA) observed that protactinium, a chemic chemical constituent generated inwards thorium reactors, could locomote separated in addition to allowed to decay to isotopically pure uranium 233—suitable fabric for making nuclear weapons. The IAEA report, titled “Advanced Fuel Cycle in addition to Reactor Concepts,” concluded that the proliferation resistance of thorium fuel cycles “would locomote equivalent to” the uranium/plutonium fuel cycles of conventional civilian nuclear reactors, assuming both included spent fuel reprocessing to isolate fissile material. Decades later, the story changed. “Th[orium]-based fuels in addition to fuel cycles convey intrinsic proliferation resistance,” according to the IAEA inwards 2005. Mainstream media convey repeated this persuasion ever since, oftentimes without caveat. Several scholars convey recognized the inherent proliferation risk of protactinium separations inwards the thorium fuel cycle, but the perception that thorium reactors cannot locomote used to brand weapons persists. While applied scientific discipline has advanced, the cardinal radiochemistry that governs nuclear fuel reprocessing remains unchanged. Thus, this shift inwards perspective is puzzling in addition to reflects a failure to recognize the importance of protactinium radiochemistry inwards thorium fuel cycles.
Protactinium turns 100. The importance of protactinium chemical scientific discipline for obtaining highly attractive fissile fabric from thorium has been recognized since the 1940s. However, the story actually begins 100 years agone during the earliest interrogation on natural radioactivity. In 1918, Austrian-Swedish physicist Lise Meitner in addition to High German pharmacist Otto Hahn were on a quest to discover the long-lived isotope of “eka-tantalum” predicted to prevarication betwixt thorium in addition to uranium inwards the periodic table. The isotope they sought would decay to actinium, which was ever constitute alongside uranium but was known to locomote the raise of an unknown natural radioactive decay chain distinct from that of uranium 238, the exactly about mutual isotope of uranium constitute inwards nature.
Meitner in addition to Hahn discovered that treating pitchblende alongside nitric acid yielded an insoluble fraction of silica that associated alongside tantalum in addition to eka-tantalum. After many years, they purified plenty eka-tantalum for identification in addition to measured its properties. As discoverers of eka-tantalum’s longest-lived isotope, Meitner in addition to Hahn named this novel chemical constituent protactinium. They had isolated protactinium 231, a fellow member of the uranium 235 decay chain. In 1938, they discovered that protactinium 233 could locomote produced yesteryear neutron irradiation of thorium 232, the exactly about abundant isotope inwards naturally occurring thorium.
For the adjacent several decades, protactinium was shrouded inwards “mystery in addition to witchcraft” due to its scarcity inwards nature in addition to its perplexing chemic properties. We at nowadays know that protactinium’s peculiar chemistry is due to its pose inwards the periodic table, which lends the chemical constituent vastly unlike chemic properties than its neighbors. Protactinium behaves thus differently from thorium in addition to uranium that, nether many conditions, their separation is inevitable.
Scientists did non investigate the macroscopic chemical scientific discipline of protactinium until the Manhattan Project. In 1942, Glenn T. Seaborg, John W. Gofman, in addition to R. W. Stoughton discovered uranium 233 in addition to observed its propensity to fission. Compared alongside naturally occurring uranium 235, uranium 233 has a lower critical mass, which way that less fabric tin locomote used to build a weapon. And compared alongside weapons-grade plutonium 239, uranium 233 has a much lower spontaneous fission rate, enabling simpler weapons that are to a greater extent than easily constructed. Influenza A virus subtype H5N1 1951 written report yesteryear 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 fabric was oftentimes contaminated alongside natural uranium 238, the scientists obtained pure uranium 233 yesteryear using a diverseness of methods for separating the intermediate protactinium 233.
By this time, advances inwards applied scientific discipline in addition to projections of uranium shortages stimulated involvement inwards developing a breeder reactor, which produces to a greater extent than fissile fabric than it consumes. In the belatedly 1960s, a squad at Oak Ridge National Laboratory designed a Molten Salt Breeder Reactor fueled yesteryear thorium in addition to uranium dissolved inwards fluoride salts, but it could entirely breed uranium 233 yesteryear 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 alongside the proliferation of applied scientific discipline that could locomote used to brand nuclear weapons. And alongside the high startup costs of developing novel reactors, at that spot would locomote no house for the Molten Salt Breeder Reactor inwards the unloose energy market. With the destination of interrogation on thorium reactors came the destination of ambitious interrogation on protactinium separations. Over time, the purpose of protactinium inwards obtaining weaponizable uranium 233 from thorium was largely forgotten or dismissed yesteryear the thorium community.
Thorium reactors born again. Fast forrad to 2018. Several nations convey explored thorium ability for their nuclear unloose energy portfolios. Foremost amid these is India. Plagued yesteryear perennial uranium shortages, but possessing abundant thorium resources, Bharat is highly motivated to prepare thorium reactors that tin breed uranium 233. Bharat at nowadays operates the entirely reactor fueled yesteryear uranium 233, the Kalpakkam Mini reactor (better known as KAMINI).
Thorium reactors convey other potential advantages. They could create fewer long-lived radioactive isotopes than conventional nuclear reactors, simplifying the disposal of nuclear waste. Molten common salt reactors offering potential improvements inwards reactor safety. Additionally, at that spot is the persistent perception that thorium reactors are intrinsically proliferation-resistant.
The uranium 233 produced inwards thorium reactors is contaminated alongside uranium 232, which is produced through several unlike neutron absorption pathways. Uranium 232 has a half-life of 68.9 years, in addition to its missy radionuclides emit intense, highly penetrating gamma rays that brand the fabric hard to handle. Influenza A virus subtype H5N1 someone standing 0.5 meters from five kilograms of uranium 233 containing 500 parts per ane one one thousand thousand of uranium 232, ane twelvemonth after it has been separated from the daughters of uranium 232, would have a dose that exceeds the annual regulatory limits for radiological workers inwards less than an hour. Therefore, uranium 233 generated inwards 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 non compromise the favorable fissile fabric properties of uranium 233, which is categorized as “highly attractive” fifty-fifty inwards the presence of high levels of uranium 232. Uranium 233 becomes fifty-fifty to a greater extent than attractive if uranium 232 tin locomote decreased or eliminated altogether. This is where the chemical scientific discipline of protactinium becomes important.
Protactinium inwards the thorium fuel cycle. There are iii isotopes of protactinium produced when thorium 232 is irradiated. Protactinium 231, 232, in addition to 233 are produced either through thermal or fast neutron absorption reactions alongside diverse thorium, protactinium, in addition to uranium isotopes. Protactinium 231, 232, in addition to 233 are intermediates inwards the reactions that eventually shape uranium 232 in addition to uranium 233. Protactinium 232 decays to uranium 232 alongside a half-life of 1.3 days. Protactinium 233 decays to uranium 233 alongside a half-life of 27 days. Protactinium 231 is a special case: It does non straight decay to uranium, but inwards the presence of neutrons it tin absorb a neutron in addition to instruct out protactinium 232.
Neutron absorption reactions entirely occur inwards the presence of a neutron flux, within or forthwith surrounding the reactor core. Radioactive decay occurs whether or non neutrons are present. For irradiated thorium, the existent occupation organisation lies inwards separating protactinium from uranium, which may already convey pregnant levels of uranium 232. Production of protactinium 232 ceases as presently as protactinium is removed from the neutron flux, but protactinium 232 in addition to 233 instruct out along to decay to uranium 232 in addition to 233, respectively.
The half-lives of the protactinium isotopes run inwards the favor of potential proliferators. Because protactinium 232 decays faster than protactinium 233, the isotopic purity of protactinium 233 increases as fourth dimension passes. If it is separated from its uranium decay products a minute time, this protactinium volition decay to every bit pure uranium 233 over the adjacent few months. With careful attending to the relevant radiochemistry, separation of protactinium from the uranium inwards spent thorium fuel has the potential to generate uranium 233 alongside really depression concentrations of uranium 232—a production suitable for making nuclear weapons.
Scenarios for proliferation. Although thorium is usually associated alongside molten common salt reactors, it tin locomote used inwards whatsoever reactor. Several types of fuel cycles enable feasible, rapid reprocessing to extract protactinium. One is aqueous reprocessing of thorium oxide “blankets” irradiated exterior the inwardness of a heavy H2O reactor. Many heavy H2O reactors include on-power fueling, which way that irradiated thorium tin locomote removed apace in addition to often, without shutting the reactor down. As really piffling fission would occur inwards the blanket material, its radioactive decay would locomote lower than that of spent fuel from the core, in addition to it could locomote reprocessed immediately.
Myriad possibilities be for the aqueous separation of protactinium from thorium in addition to uranium oxides, including the usually proposed thorium uranium extraction (THOREX) process. Alternatively, ane time dissolved inwards acid, protactinium tin merely locomote adsorbed onto drinking glass or silica beads, exploiting the same chemic machinery used yesteryear Meitner in addition to Hahn to isolate protactinium from natural uranium a century ago.
Another scenario is continuous reprocessing of molten common salt fuel to take away protactinium in addition to uranium from thorium. Researchers at Oak Ridge explored the feasibility of online protactinium removal inwards the Molten Salt Breeder Reactor program. Uranium tin thus locomote separated from the protactinium inwards a minute step.
Sensible safeguards. Protactinium separations supply a pathway for obtaining highly attractive weapons-grade uranium 233 from thorium fuel cycles. The difficulties of safeguarding commercial spent fuel reprocessing are pregnant for whatsoever type of fuel cycle, in addition to thorium is no exception. Reprocessing creates unique safeguard challenges, especially inwards India, which is non a fellow member of the Nuclear Non-Proliferation Treaty.
There is piffling to locomote gained yesteryear calling thorium fuel cycles intrinsically proliferation-resistant. The best way to realize nuclear ability from thorium fuel cycles is to admit their unique proliferation vulnerabilities, in addition to to adequately safeguard them against theft in addition to misuse.
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