Thorium Reactors

Naturally occurring thorium, atomic mass number 90, exists entirely as Th-232 which is not a fissile material for thermal neutrons. It is, however, fertile: upon neu­tron capture it is converted into U-233 which is fissile (Equation 9.10).

232 1 neutroncapture 233 betadecay 233 0 betadecay 233 0

90 th + 0n——————– > 90 th—————- > 91 Pa + _jp————— > 92 U + _jp

(9.10)

Thus, a thorium reactor requires a “seed” fissile material (U-233, U-235, or Pu-239) to initiate the production of neutrons, but once the product U-233 is formed, the reaction is off and running. This is the basis of the thorium-uranium fuel cycle.

There are many advantages to using thorium-232 as a nuclear fuel, including its abundance (it is 3-4 times more abundant than uranium), relative availability (it can be mined from the mineral monazite, Sm0.2Gd0.2Th0.15Ce0.15Ca0.1Nd0.1(PO4)09, as the phosphate or from thorite, Th(SiO4) as the silicate), it does not require enrich­ment, and there is no opportunity for nuclear proliferation of Pu-239. In fact, the waste generated from a thorium reactor, while still hazardous, is lower in volume and less radioactive compared to that from a conventional uranium reactor (Cooper et al. 2011). Given these advantages and the fact that the nuclear capabilities of thorium have been known since the 1950s, why did uranium become the fuel of choice for nuclear energy? It is the result of history: because thorium reactors do not provide a source of Pu-239 for nuclear weapons programs, they were set aside as nuclear energy developed under the shadow of the Cold War and the nuclear arms race. The uranium fuel cycle with its production of Pu-239 allowed for the development of a nuclear stockpile.

As a result of being rejected in favor of uranium, thorium does not have decades of research and development to facilitate its entry into commercialization. Nevertheless, progress on thorium-based nuclear energy is continuing. A heavy water BWR in Halden, Norway has recently begun a thorium fuel burn using pellets consisting of a thorium oxide ceramic matrix with approximately 10% plutonium oxide to drive the fission reaction (World Nuclear News 2013). And thorium energy proponents are encouraging continued development of the liquid fluoride thorium reactor (LFTR). This reactor is a variation of the molten salt reactor (Table 9.2) and uses a combina­tion of two molten salts as the fuel: a core of 233UF4 dissolved in a lithium-beryllium fluoride surrounded by a “blanket” of 232ThF4 in the same solvent. As the 233UF4 generates neutrons, the blanket of 232ThF4 captures them to generate more U-233. This uranium fuel is converted into gaseous UF6 and fed back into the core as fresh thorium fuel is supplied to the blanket (Jacoby 2009). Not only does the LFTR pres­ent the advantages noted above due to the use of thorium fuel, it also presents greater safety features, primarily because the fuel is molten. Should a catastrophic event occur, the escape of the molten fuel will quickly cease as the fuel solidifies upon escape from the high-temperature core. It is also run at low pressure, thus obviat­ing the need for housing that can withstand high pressures and therefore reducing construction costs. The high temperature of a molten salt reactor also means that thermal efficiencies are increased (from about 35% to 50%) (Cooper et al. 2011; Jacoby 2009).

In addition to the reactors described above, some of which are in operation, some under construction, and some planned for construction, the next generation of nuclear reactors—Generation IV—has been evolving and is discussed in Section 9.4.3. However, progression to Generation IV is not likely for many years to come.

Updated: September 25, 2015 — 12:58 pm