Liquid Fluoride Thorium Reactors

David Berryrieser
March 22, 2012

Submitted as coursework for PH241, Stanford University, Winter 2012

Fig. 1: A molten salt reactor. A LFTR design would have a blanket chamber for breeding surrounding the main reactor. (Courtesy of the U.S. Department of Energy.) Source: Wikimedia Commons

A LFTR implements the MSR concept as a breeder reactor, breeding the fertile Th-232 into fissile U-233. The main reactor chamber would contain the the U-233 in the form of uranium tetra-fluoride at an appropriate concentration in a carrier salt. Surrounding the main reactor chamber would be a blanket chamber of thorium tetra-fluoride in a carrier salt. [1] Excess neutrons from the main reactor would get absorbed by thorium atoms in the blanket, which then transmute into U-233. This U-233 is then chemically separated from the blanket salt and used as fuel. [2]

Introduction

Changing priorities regarding world energy consumption, in particular rising concerns about global warming, have let to renewed interest in nuclear power generation. Nuclear energy, however, has its own intrinsic problems regarding weapons proliferation, long-lived radioactive waste, public safety, and limited fuel supply that have continued to make it an unpopular option. There is thus a need to look beyond traditional light water reactors (LWR) that can address these problems. The Generation IV reactor designs are attempts to do just that. This paper will focus on the Liquid Fluoride Thorium Reactor (LFTR) design, an implementation of one such Gen IV idea, the Molten Salt Reactor. The goal here is to present the basics of a LFTR design and the inherent advantages and problems with such a design.

Reactor Design

In a MSR, the nuclear fuel, the so called fissile isotope, is contained within a liquid salt solution. Fission reactions heat the salt, which is then circulated out of the main reactor and into a heat exchanger, where the thermal energy is carried away to produce electricity, as shown in figure 1. The salt can also be continuously run through a chemical processing plant that can remove fission products, thus increasing the neutron efficiency of the reactor. [2] This is in contrast to LWRs where the fissionable elements are in solid rods. The heat must be carried away by a coolant (water) and the fission products are trapped inside the fuel rods. A MSR can burn any of the three fissionable isotopes U-233, U-235, or Pu-239 but we will focus on U-233 here. [3]

Safety Features

Molten Salt Reactors, and by extension LFTRs, have several very attractive safety features. First, and most importantly, is the negative coefficient of reactivity. This means that as the temperature in the reactor increases, the rate at which the fission reactions proceed decreases. This will self-regulate the temperature in the fuel salt and prevent the reactor from going prompt critical (i.e. blowing up). [2] It is worth noting that the coefficient of reactivity for the reactor shown in figure 1 would actually move from negative to positive due to heating of the graphite moderator. [2] LFTR designs generally do not have a graphite moderator.

In most MSR designs, there is a freeze plug safety mechanism built into the reactor plumbing. If the plug were removed, the reactor salt would flow down into holding tanks. A freeze plug needs to be continuously cooled to prevent it from melting and thus allowing the salt to flow out of the reactor. If power to the MSR facility were removed, say due to some natural disaster, the reactor would power down without the need for any human intervention.

Nonproliferation

The LFTR is a breeder design and like any breeder reactor, it can be used to create fuel for nuclear weapons in addition to civilian power. One advantage of using the thorium to breed fissile U-233 is that some U-232 is produced along with U-233. [4,5] Any U-233 withdrawn from the reactor for weapons use will be contaminated with small amounts of U-232. The advantage here is that U-232 is highly radioactive and would pose a severe radiation hazard to any personel attempting to handle the bred uranium. This feature is not completely fool-proof. By using continuous chemical processing on the blanket salt it would be possible to extract relatively pure U-233 for weapons use.

Conclusion

The LFTR implementation of the MSR design presents an attractive alternative to existing reactors. There additional features of LFTRs regarding the reduction of transuranic waste and the large availability thorium resources in the Earth's crust not fully discussed here. [1] However, the route chosen for the future of nuclear energy will be a come from a political decision, not a technical one. With hope, technical wisdom will be called upon to make the in the process so as to arrive at a decision for the best possible reasons.

© David Berryrieser. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] R. Hargraves and R. Moir, "Liquid Fluoride Thorium Reactors," Am. Sci. 98, 304 (2010).

[2] D. LeBlanc, "Molten Salt Reactors: A New Beginning for an Old Idea," Nucl. Eng. Design 240, 1644 (2010).

[3] H. G. MacPherson, "The Molten Salt Reactor Adventure," Nucl. Sci. Eng. 90, 374 (1985).

[4] M. Kazimi, "Thorium Fuel for Nuclear Energy," Am. Sci. 91, 408 (2003).

[5] J. Kang and F. N. von Hippel, "U-232 and the Proliferation-Resistance of U-233 in Spent Fuel," Sci. Global Security 9, 1 (2001).