Thorium Reactors – Advantages and Disadvantages
It is very difficult to explain the possible advantages and disadvantages. Some of the following points can be valid for one reactor design and another point can be invalid for another thorium-based reactor. Therefore, be careful when you argue for or against thorium reactors.
- Abundance of Natural Thorium. Although the uranium resources may be very large, it is recognized that the thorium content of the earth’s crust (0.0006% vs. 0.00018%) is about three times larger, reflecting its longer half-life. On the other hand, due to the low demand for thorium, the known reserves for uranium and thorium are both nearly identical.
- Neutronic and Thermal Parameters. For a thermal neutron spectrum (E < 1 eV) and the thorium-based fuel, the reproduction factor is considerably larger than for uranium-based fuel. Due to very low capture-to-fission ratio, the reproduction factor for uranium 233 is about η = 2.25. From this point of view is thorium fuel cycle is very promising. Another advantage can be favorable physical and chemical properties of thorium dioxide. Compared to the predominant reactor fuel, uranium dioxide (UO2), thorium dioxide (ThO2) has a higher melting point, higher thermal conductivity, and lower coefficient of thermal expansion. In particular the thermal conductivity is important, since it leads to a lower fuel pellets temperature.
- Lower Production of Transuranic Elements. The transuranics elements (plutonium and other minor actinides) are the major health concern of long-term (on the order of roughly 103 to 106 years) nuclear waste. In uranium-based fuels, only a single neutron capture in uranium 238 is sufficient to produce these transuranic elements, whereas five captures are generally necessary to do so from thorium 232. Therefore, thorium is a potentially attractive alternative to uranium-based fuels, because the production of transuranic elements is significantly lower.
- Proliferation Resistance. With regard to proliferation significance, thorium-based fuels are generally accepted as proliferation resistant compared to uranium-based fuels. This comes mostly from the fact that almost no plutonium is produced. In fact, if plutonium is used as the fissile component of thorium fuel, the plutonium is efficiently consumed. Moreover, the uranium 233 produced in thorium fuels is significantly contaminated with uranium 232 in proposed power reactor designs. 232U has a relatively short half-life of 68.9 years, and therefore the specific activity of 232U is much higher than specific activity of the isotope 238U. In addition the decay chain of 232U produces very penetrating gamma rays. The most important gamma emitter, accounting for about 85 percent of the total dose from 232U after 2 years, is thallium 208, that emits gamma rays of 2.6 MeV which are very energetic and highly penetrating. These intense radiations make handling of fissile 233U or reprocessed uranium contaminated with 232U far more dangerous than conventional fuels.
- Material Buckling. As was written, naturally occurring thorium is effectively mononuclidic of thorium 232, which is a fertile isotope. Therefore to initial fuel load another fissile material must be added to achieve criticality. This fact must be taken into account during considerations about possible advantages.
- Half-life of 233Pa. Thorium 232 is “only” a fertile material and the main problem can be directly in the breeding of fissile uranium 233. If 232Th is loaded in the nuclear reactor, the nuclei of 232Th absorb a neutron and become nuclei of 233Th. The half-life of 233Th is approximately 21.8 minutes. 233Th decays (negative beta decay) to 233Pa (protactinium), whose half-life is 26.97 days. 233Pa decays (negative beta decay) to 233U. Therefore, proposed reactor designs must attempt to physically isolate the protactinium from further neutron capture before beta decay can occur. 233Pa is a significant neutron absorber and, although it eventually breeds into fissile 235U, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.
- Radiation Protection. 232U is produced from 232Th via specific (n,2n) reactions in which an incoming neutron knocks two neutrons out of a target nucleus. 232U has a relatively short half-life of 68.9 years, and therefore the specific activity of 232U is much higher than specific activity of the isotope 238U. In addition the decay chain of 232U produces very penetrating gamma rays. These gamma rays are very hard to shield, requiring more expensive spent fuel handling and/or reprocessing.
- Delayed Neutrons. Another important aspect for reactor safety is the delayed neutron fraction. Despite the fact the number of delayed neutrons per fission neutron is quite small (typically below 1%) and thus does not contribute significantly to the power generation, they play a crucial role in the reactor control and are essential from the point of view of reactor kinetics and reactor safety. The delayed neutron fraction is significantly lower for uranium 233 than for uranium 235. In result, a reactor in which uranium 233 is the predominant isotope responds more rapidly and puts increased demands on the design of control system.