There are safety concerns associated with FFR: "Possible fluctuations of reactivity caused by density or concentration changes in the fuel, e.g., bubbling." For MSRs this concern is primarily the coalescence of dissolved gas into large bubbles and their collapse, or in some concepts, such as the MSBR, the expansion of bubbles. To assure that this does not occur, continuous removal of gaseous (fission products) must be employed, usually through sparging.
L. M. Toth, U. Gat, G. D. Del Cul, S. Dai, and D. F. Williams
Oak Ridge National Laboratory, Oak Ridge, TN 37831
Abstract. The current status of molten salt reactor development is discussed with reference to the experience obtained from the Oak Ridge Molten Salt Reactor Experiment. The assessment of the future for this reactor system is reviewed with both consideration of advantages and disadvantages. Application of this concept to ADTT needs appears to be feasible by drawing on the experience gained with the MSRE. Key chemical considerations remain as: solubility, redox behavior, and chemical activity and their importance to ADTT planning is briefly explained. Priorities in the future development of molten salts for these applications are listed with the foremost being the acceptance of the 2LiF-BeF2solvent system.
The molten salt reactor experiment, MSRE, was operated from 1965-1969 and was shut down by draining the homogeneous fuel from the reactor circuit into two drain tanks located at a lower level in the MSRE facility. The purpose of this review is to identify the ORNL position with respect to MSR’s; review the pertinent MSR chemistry and the significant understanding gained since the MSRE operation era; and, finally, to relate these two elements to present accelerator driven transmutation technology, ADTT,interests.
If one surveys the current attitude at ORNL with regard to MSR’s, he would observe that there is no official position. In parallel with the current national and DOE attitudes toward advanced nuclear programs, it is difficult to discern a definite position or proactive attitude toward the MSR status and technology. Rather, there is, at best, a “wait and see” attitude in spite of the fine history of development associated with this reactor concept and this attitude is consistent with the current DOE attitude toward all advanced reactor concepts. When viewed from outside, however, it is possible to become dismayed even though interaction with individuals or groups is still taking place.
The most recent expression of a ORNL interest in MSR’s came in 1992 with documents written by F. J. Homan (then, director of reactor technology programs, ORNL) and U.Gat. While this expressed position is dated[l],it served to demonstrate the attitude toward MSR’s that still prevails with some individuals. At that time, Homan and Gat identified the MSR advantages as:
(1) Simplicity: External cooling allows optimization of the core design to maximize efficiency. Molten salts have low vapor pressures at high temperatures, reducing the need for thick walled pipes and vessels.
(2) Ease of fuel handling: Fluid fuel can be moved with pumps and pipes eliminating the need for complex fuel handling machinery.
(3) Simple fuel cycle: No head end preparation for reprocessing, or complex waste management. The fuel is in fluid form, amenable to chemical manipulation. Molten salts can be purified using the very simple fluoride volatility process.
(4) Simplified waste disposal: The waste which comes out of the reprocessing step contains little or no uranium or actinide elements. This simplifies waste management and results in better resource utilization than other reactor fuel cycles.
(5) Versatility: A single, basic design can be operated as modules, or in varying sizes; temperature control assures efficiency. One design can operate on any fuel. The MSR can burn actinides, burn plutonium from dismantled weapons, or breed.
(6) Ease of shutdown and maintenance: Fuel can be drained for maintenance, reducing occupational exposure.
(7) Economy: MSRs have been shown in various studies to have an excellent economic potential.
From today’s stand point we might qualify waste disposal and economy realizing that these issues are now seldom simple or economical. Relative to other reactor systems, however, these points could be valid.
On the negative side of the issue, Homan and Gat identified disadvantages and unresolved issues as:
(1) Possible reactivity fluctuations due to density variation or bubble formation.
(2) Necessity for a large external (to the core) fuel inventory,
(3) High radiation levels in primary system due to presence of fuel throughout.
(4) Reliability of components which contain and circulate the molten fuel.
(5) A not fully integrated (combined) chemistry.
(6) Lack of utility and industrial support.
(7) Lack of licensing experience.
In addition to these listed issues, we might also add graphite swelling and undeveloped plutonium chemistry.
As a result of this analysis, Homan and Gat identifiedthe following position reflecting the ORNL attitude of the time:
(1) The MSR has promise.
(2) The economic climate is not right to recommend the rebirth of MSR’s.
(3) A small program should be established to retain MSR experience lest retirement and/or death cause it to be lost.
(4) M S R s might even be more attractive than LMFBR’s in today’s climate.
Since the 1969 shutdown of the MSRE, several evaluations of MSR technology have occurred[2, 3, 4]. While many evaluations have been and will continue to be made, the primary requirements for the molten salt in a homogeneous fluid-fueled reactor still remain as expressed earlier.
The salt should have:
(1) Low neutron cross section for the solvent components.
(2) Thermal stability of the salt components.
(3)Low vapor pressure.
(4) Adequate solubility of fuel and fission product components.
(5) Adequate heat transfer and hydrodynamic properties.
(6) Chemical compatibility with container and moderator materials.
(7) Radiation stability
(8) Low fuel and processing costs.
It is readily seen from Table 2 or Ref. 5 that several of the fluoride salts satisfy the
In considering the application of these molten salts to a MSR situation, three important and interrelated chemical concepts must be controlled. These are solubility,redox chemistry, and chemical activity.
Solubility as applied to molten salts involves more than what we ordinarily understand for queous systems. In the case of the pure fluoride materials, solubility is determined by phasediagramsofthemixtures. These temperature versus composition determinations give the temperature at which components might fall from solution (as determined by the liquidus line on the plot) and the identity of the salt phase which precipitates out. Because this information was of such vital importance to the control of the molten salt solution, much attention was given to the determination of phase diagrams in the early stages of the ORNL/MSRprogram. While much has been learned already about this phase diagram behavior, comparable information relating to PuF, is sorely lacking. . . .
(What follows is a technical discussion of the application of Molten Salt Reactor technology to accelerator driven transmutation technology. I will skip this discussion because it adds little to our understanding of the MSR development task and move on to the conclusion.)
Molten salt reactors are a proven concept and merit future development for ADTT applications. The molten fluoride chemistry for the 2LiF-BeF2system is well established and can be applied with great confidence. However, other less understood solvent systems must be considered with caution lest subtle solvent effects such as those presented here cause severe problems with process operations. The chemistry of plutonium, as PuF,, needs further study and testing, especially in corrosion loop studies for redox control. Processing of the fuel salt needs further development with continuous on-line processing as the ultimate objective. Nevertheless, the future for systems utilizing this high-temperature fluid can be very promising based on the fine record established over the past several decades.
Homan,F. J. and Gat, U., “Status Position on the Molten Salt Reactor,” June 1992m (unpublished).
MacPherson, The Molten Salt Reactor Adventure, Nuclear Science and Engineering, 90,(1985).
Rosenthal, M. W.,Haubenreich, P. N., McCoy, H.E., and McNeese, L. E. , “Recent Progress in Molten Salt Reactor Development”, Atomic Energy Review, IX,3,601,(1971).
L. M. Toth, G. D. Del Cul, S. Dai, D. H. Metcalf, “Molten Fluoride Fuel Salt Chemistry”, First International Conference on ADTT and Applications, Las Vegas, Nevada, USA, July 25-29, 1994.
Gilpatrick, L. O., and Toth, L. M., “The Hydrogen Reduction of UF, in Molten Fluoride Solutions,” J. Inorg. and Nuclear Chemistry, 39 (10), 1817, (1977).