Liquid Fuel Reactors and Thorium David H. Lester, Ph. D. Introduction Any nuclear power plant has the following basic needs:
Means of attaining a critical mass for a sustained nuclear fission reaction Control of the fission reactor to attain a desired power level and for shutdown Means of transferring the energy produced to a system for electricity generation Means of refueling as nuclear materials are used up Safety-protection of the plant, the workers and the public from accidents
The critical mass is attained by use of fissile materials and moderators. Fissile materials are atoms which capture neutrons, split into smaller atoms and generate heat and more neutrons. Moderators slow down the neutrons so that they have the correct energy for capture by the fissile material. There are also fertile materials which capture neutrons and become fissile. In today’s light water reactors (LWRs) the fissile material is Uranium-235 and the moderator is water. The water also acts as the coolant that carries heat to a steam generator and transfers energy to the electricity generating plant. The fuel in these reactors consists of zirconium tubes in which are placed solid pellets of uranium. The uranium in the pellets is mainly U-238, the most abundant natural uranium isotope, with 1.8% U-235, the fertile isotope of uranium. A key problem with water is risk of steam or hydrogen explosion if the reactor’s pressure boundary or cooling fails. Thus a great deal of engineering and conservatism is construction is devoted to these risks. Another potential fission fuel is thorium. Natural thorium is 100% thorium-232. Thorium is a fertile material, i.e., it cannot support fission but does become fissile when it captures a neutron. When thorium captures a neutron it becomes protactinium-233 which decays to uranium -233 (half-life is 27 days). Uranium-233 is fissile. Reprocessing thorium fuel to remove the Protactinium-233 has a number of technological challenges many of which have been conquered (see item below on India). The use of thorium in water reactors requires the use of heavy water (enriched in deuterium) as a moderator. Use of liquid fuel reactors offers several advantages for the thorium cycle as well as some advantages for the uranium cycle. The major advantages of thorium over uranium are the abundance and accessibility of thorium when compared with uranium. Thorium has about 4 times greater abundance in the earth’s crust than Uranium1 and a great deal of thorium currently exists already mined as a by-product of rare-earth mining (rare earths are important in the manufacture of electronics). Liquid Fuel Reactors
In a molten salt reactor, a radioactive fuel such as uranium or thorium is dissolved into fluoride or chloride salts to form a solution referred to as “fuel salt.” The fuel salt is normally an immobile solid material, but when heated above approximately 500°C, it becomes a liquid that flows. Thus it is the liquid fuel salt, rather than water, that carries the heat out of the reactor. The plant can operate near atmospheric pressure with a coolant that returns to a solid form at ambient temperatures. Atmospheric pressure operation eliminates the threat of explosions and the need for expensive pressure containment structures. The basic liquid fuel reactor plant would contain a pump to circulate the liquid salt (primary loop), a reactor vessel, a heat exchanger to transfer heat to a second salt cycle (secondary loop), and a steam generator to make steam from the heat in secondary salt. The steam is then used to make electricity in the generating plant. The reactor vessel would contain a solid moderator and be of sufficient volume to allow a sustained fission reaction to occur. The moderator could be graphite (such as in the MSRE discussed below) or another material (see the Transatomic Power discussion below). The plant would also include a catch basin isolated from the primary salt loop with a freeze plug. The freeze plug would be maintained by circulating air over the freeze plug area keeping it cool. In the event of a power failure the freeze plug cooling would fail, the primary loop would drain into the catch basin with no moderator and everything would stop. This is a simple, passive safety system requiring no intervention. The Molten Salt Reactor Experiment (MSRE) was an experimental molten-salt nuclear reactor at the Oak Ridge National Laboratory. It was operated successfully from 1965 to 19692. The MSRE was a 7.4 MW test reactor that was an example of a LFTR, a liquid fluoride thorium reactor. The first fuel was a highenriched (33% U-235) uranium dissolved as UF4 in a LiF-BeF2-ZrF4 salt. The moderator was graphite. The secondary coolant was LiF-BeF2. The reactor operated at temperatures up to 650 oC and operated for an equivalent of about 1.5 years at full power. This experiment demonstrated the practicality of such a reactor and provided important information concerning design and materials of construction. Recent Developments Thorium power in India India is moving ahead with a thorium power program based on solid fuels in a heavy water reactor. In the reactor a thorium blanket is exposed to neutrons. The neutron capture of the thorium breeds uranium-233 which can then be recycled to fuel the reactors. India claims to have solved many of the difficult fuel reprocessing problems. One important safety advantage is that the melting point of thorium dioxide is 500 degrees Celsius higher than that of uranium dioxide. This difference provides an added margin of safety in the event of a temporary power surge or loss of coolant in a reactor. Transatomic power Transatomic Power has proposed an advanced design for a molten salt reactor using uranium. Key advances include use of a LiF salt instead of a LiF-Be2 salt and use of a zirconium hydride moderator instead of graphite. The LiF system avoids the use of beryllium, a substance to which a large segment of the population is sensitive to it as a toxin. Also the LiF salt can contain higher concentrations of (heavy
metal)F4 which allows use of a 1.8% enriched uranium rather than the 33% enriched uranium used in the MSRE. This enables this reactor to burn spend fuel from the old LWR reactors consuming most of the existing high-level nuclear waste as fuel. The use of the zirconium hydride moderator solves materials degradation (shrinking) and occupies much less space allowing more volume for fuel. This also aids the use of low-enriched uranium and spent fuel. Thorcon Thorcon is a concept which incorporates a modular design for easy change out of components and packaged mini-reactors. The package reactor modules can be shipped to the site and changed out with installed, packaged reactors which can then be sent as a unit to the reprocessing facility. The focus of the design is to use proven technology and currently available materials so that no new advancements are required. The design includes the use of a lower temperature melting salt NaF-BeF2 in the primary (fuel salt) and secondary loops. The fuel salt would contain UF4 and ThF4 so thorium would be converted to protactinium-233. The salt does not use lithium which eliminates problems with Li-6. Li-6 transmutes to tritium in a slow neutron environment. Natural lithium contains about 7% Li-6. Use of LiF salts pose problems with evolution of excessive tritium unless the lithium is refined to mostly Li-7, a technology problem. In the process of transmutation the Li also consumes neutrons and acts as a poison to sustained fission process. Fuji MSR The Fuji MSR is a design for a 100-200MW molten-salt-fueled thorium fuel cycle breeder reactor using technology similar to the Oak Ridge MSRE. See links below for more details. Chinese thorium MSR China’s thorium project was launched as a high priority by Jiang Mianheng, son of former leader Jiang Zemin. He estimates that China has enough thorium to power its electricity needs for “20,000 years”. The project began with a start-up budget of $350m and the recruitment of 140 PhD scientists at the Shanghai Institute of Nuclear and Applied Physics. It then had plans to reach 750 staff by 2015, but may be accelerating the project. The reactor concept would be a molten salt reactor – or a liquid fluoride thorium reactor (LFTR) . Flibe Energy Flibe refers to the LiF-BeF2 salt. A company founded by Kirk Sorensen, a former NASA scientist, called Flibe Energy is seeking to promote LFTR technology for small modular power reactors for military bases. TEG Thorium Energy Generation Pty. Limited (TEG) is an Australian research and development company dedicated to the worldwide commercial development of LFTR reactors, as well as thorium acceleratordriven systems. In November 2011, TEG announced the formation of a joint venture with Czech Republic scientists intended to develop a 60MW pilot plant in Prague.
References 1. Hargraves, Robert; Moir, Ralph (July 2010). "Liquid Fluoride Thorium Reactors". American Scientist 98 (4): 304–313. doi:10.1511/2010.85.304 2. Briggs, R. B. (1964). "MSR Program Semiannual Progress Report for the period ending July 31, 1964". (ORNL-3708) (66.3 MB PDF), Oak Ridge National Laboratory, U.S. AEC (published November 1964). Pp.373-390. Links Liquid Fluoride Thorium Reactor http://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactor Molten Salt Reactor Experiment http://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment Flibe Energy: http://nextbigfuture.com/2014/09/kirk-sorensen-describes-liquid-thorium.html Fuji MSR http://www.the-weinberg-foundation.org/2013/03/22/a-plan-to-turn-japans-nuclear-past-intoits-future-with-molten-salt-reactors/ Chinese MSR http://blogs.telegraph.co.uk/finance/ambroseevans-pritchard/100026863/china-going-for-brokeon-thorium-nuclear-power-and-good-luck-to-them/ Thorium Power http://en.wikipedia.org/wiki/Thorium-based_nuclear_power India’s Thorium Program http://www.pocket-lint.com/news/129913-world-s-first-thorium-reactor-ready-to-be-built-forcheaper-safer-nuclear-energy http://en.wikipedia.org/wiki/India's_three-stage_nuclear_power_programme Transatomic Power http://transatomicpower.com/ Thorcon http://www.c4tx.org/thorcon/pub/exec_summary.pdf http://www.thoriumenergycheaperthancoal.com