Configuration of a Molten Chloride Fast Reactor on a Thorium Fuel Cycle to Current Nuclear Fuel Cycle Concerns
A dissertation submitted in partial satisfaction of the requirements for the degree
Doctor of Philosophy in Engineering
by Eric Heinz Ottewitte
Pu, Proliferation,Safeguards, Security,and Waste Management
Much concern with the nuclear fuel cycle centers on Pu due to its carcinogenicity and its potential use in a weapon . One desires close control less it pollutes the environment or be di- verted (overtly or covertly) to weapons use. Once chemically separated (relatively easy to do for Pu) it can be put into a pre- assembled bomb within days. The main difficulty will be to properly implode the device . Large national or subnational groups might be able to practice that trick beforehand.
Present and prospective solid fuel cycles involve extensive fuel logistics including element fabrication, decladding, and post- operation storage. Molten salt reactors skip these steps, thereby reducing much of the pollution and diversion dangers.
A reactor on the Th fuel cycle minimizes production of Pu and heavy actinides by beginning much lower on the atomic weight scale and usefully consuming them. Fast reactors succeed even better by enhancing fission, especially for 23k This pertains especially to fast molten salt reactors with a very hard neutron spectrum .
All reactors use or produce weapons-grade fissile fuel. In the Th fuel cycle, the predominantly-233U fissile fuel discloses its presence by emitting a hazardous, characteristic highly- penetrating gamma ray. Any transformation of the reactor to the U/Pu fuel cycle will evidence a reduced gamma signal.
An MSR also allows one to continuously remove the principal (gaseous) radiation hazard from the fuel and store it away from the reactor . This fact plus the ability to transfer the whole core further underground at a moment’s notice should enhance security of the plant from attack .
Extensive evaluation of core salt properties favors cheap NaCl as the carrier salt, ThC14 for fertile material, and UCl3 for fissile material . High fissile salt proportion (Th/U = 0 .5, 30% NaCl) will yield high BG (up to 0 .4) and enhance actinide fission due to hard neutron spectrum . However, it also causes high doubling time and more frequent replacement of radiation- damaged core tubes.
Doubling time reduces to near 20 years at lower fissile contents (Th/U = 1 to 2, 30% NaCl). The optimum composition should lie in between these two points, depending on the assignment of priorities . In earlier years of operation one might operate with a low fissile content to achieve low DT, low-flux levels, and softer spectrum . Then in later years, when more is known about radiation damage and the press for available 233U is diminished, the plant could switch to a salt of higher fissile content so as to avoid actinide buildup and produce more surplus neutrons(higher BG) .
The blanket region could benefit from a thermalizing carrier salt like 7LiC1 or a fluoride instead of NaCl, but cost and high melting points, respectively, discourage those.
The optimum core configuration for BG and DT appears to be a few frequently-replaced, skewed, graphite tubes in a blanket bath 2 meters thick: a cost-effective solution to the high flux levels inherent to this concept . Inner blanket regions definitely detract: they decrease BG without decreasing the out-of-core inventory .
Technical and Economic Feasibility
Fast molten chloride reactors have been cursorily considered before but mainly for the U/Pu fuel cycle . The ORN`L MM program showed the feasibility of fuel salt circulation . The combination of that experience and MCFR research (out-of-pile experiments and theoretical studies, so far) provide a basis for believing the concept will work.
Chemical stability and corrosion of molten salts are fairly predictable . Low vapor pressure of the salts enhances safety and permits low pressure structural components .
Molten fuel state and cooling out-of-core simplify component design in a radiation environment . They forego complicated refueling mechanism, close tolerances associated with solid fuel, and mechanical control devices . Molten state and low vapor pressure of the salts also offer inherent safety advantages .
Graphite and Mo alloys and coatings appear as promising candidates for prinary salt containment, both in and out of core .
These high temperature materials may permit high fuel salt temperatures (above 1000°C). This can reduce fuel salt inventory in the heat exchanger and allow gas turbine cycles and/or process heat applications using helium as an intermediate and final coolant.
Use of the MCFR on the Th fuel cycle should greatly expand the amount of available fuel : three times as much Th exists as U .
The high BG accompanying very fast neutron spectra in some MCFR designs will enhance the economic retrievability of extensive low-grade U and Th ores.