The Molten-Salt Reactor Demonstration Reactor

Ed Bettis, who was one of the inventors of the Molten Salt Reactor concept, and was a pioneering champion, went on to become lead an ORNL design group during the 1960's and 1970's. Bettis designed both one and two fluid reactors, including a pioneering ORNL two fluid 250 kWe Modular MSR. Bettis also realized that a practical MSR, based on MSRE tested technology and in ORNL-TM-3832 (DESIGN STUDIES OF A MOLTEN-SALT REACTOR DEMONSTRATION PLANT) offered a practical design for such a reactor. The purpose of the Demonstration Reactor project was to

represents a molten-salt reactor plant which is feasible to build, will produce a significant amount of electrical power, and will be a major step toward a useful family of breeder reactors.

The abstract of ORNL-TM-3832 reads

The MSDR, a 350-MW(e) Molten-Salt Reactor Demonstration Reactor, is based on technology much of which was demonstrated by the MSRE. The cylindrical vessel (26 ft diam by 26 ft high) houses a matrix of graphite slabs forming salt passages having 8 volume fraction in the core of 10%. . . In the secondary exchanger, heat is transferred to a stream of Hitec salt (in at 800 F, out a t 1000F). The Hitec oxidizes tritium to tritiated water which is removed and disposed of. The Hitec generates steam at 9OO F, 2400 psi in a boiler, super- heater, and reheater. Electricity is produced a t an overall efficiency of 36.6%. Soluble fission products are removed by discarding the carrier salt every 8 years after recovery of the, uranium by fluorination. Volatile fission products are removed by sparging the fuel salt with helium bubbles in the reactor primary system. The fuel cycle cost was estimated t o 0.7 mill/kWhr for inventory, 0.3 mill/kWhr for replacement, and 0.1 mill/kWhr for processing, giving a total of 1.1 mills/kWhr.

Although ORNL's primary reactor development focus during the early 1970's was on the development of a Molten Salt Breeder Reactor, Bettis suggested:

An alternative approach to the development of a commercial MSBR has also evoked interest. This approach emphasizes more rapid attain- ment of commercial size but more gradual attainment of high performance. The step beyond the MSRE is construction of a 300-MW(e) Molten-Salt Demonstration Reactor(MSDR). The purpose of the MSDR would be to demonstrate the molten-salt reactor concept on a semi-commercial scale while requiring little development of basic technology beyond that demonstrated in the MSRE.

We see from the abstract that many improvements would be possible with the 1972 design. Compared to the MSBR, Bettis and his associates proposed,

First, the MSDR has only such chemical processing as was demonstrated in the
MSRE and has no provision for removing fission product poisons on a short time cycle. Thisresults in a much less complicated chemical processing plant, although it means that the reactor has a breeding ratio less than one and i s therefore a converter. The second major simplification i s that the power density was made low enough for the graphite core to last the 30-year design lifetime of the plant, thus simplifying the reactor vessel and eliminating the equipment for replacing the core.

At the time ORNL MSBR plans called for the periodic removal and replacement of the MSBR Graphite core, as a solution to the problem of core swelling caused by neutron bombardment. This involved design complexities, and so Bettis proposed an alternate scheme to deal with graphite swelling, a scheme that involved core enlargement.

The ORNL-TM-3832 design although interesting is flawed. The MSDR designers, in an effort to solve core a graphite problem increased the amount of core graphite, this in turn increased the size of the core. But a large core increases reactor construction costs. From a cost viewpoint, it is probably better to replace a small core every few years, than to build a very large core, that will last for 30 years.

While ORNL-TM-3832 represents a serious attempt to simplify MSR design, it hardly represents the last word in MSR simplification. While we may appreciate the ingenuity of Bettis' 1972 design, a revolutionary innovation in core design by Dr. David LeBlanc has greatly simplified MSR core concepts. If the MSDR was altered by substituting Dr. LeBlanc's two tube core for the original core design the entire reactor design would require great alteration. The LeBlanc tube core would almost certainly lower MSR costs, compared to all ORNL core designs of the 1960's and 1970's.

Other problematic features of the Demonstration Reactor involved the use of LiF-BeF2 salts. Since the purpose is not producing nuclear fuel in the breeding range, other salts might carry significant advantages including lower costs, and the elimination of the tritium problem. By switching to another salt combination the tritium problem associated with LiF-BeF2 salts. The MSDR included a third heat transfer loop as part of its tritium control system, and that loop decreased thermal efficiency, increased reactor complexity and costs. If the goal of MSR design is breeding, LiF-BeF2 are the preferred carrier salts, but when breeding ceases to be the objective, then the possibility of using other salts comes into play.

The MSDR was designed to generate power through the medium of superheated steam turbines. Gas turbines, and particularly CO2 turbines would be preferable, if available, but they are not an option yet. As it is the use of superheated steam would make the MSDR more efficient than Light Water Reactors.

Thus it would appear that development of the original MSDR design is not warranted, but that development of the design concept could be. Compared to a MSBR (a LFTR), an advanced MSDR would be a design slam dunk, because of the reliance on tested technology and because of the simplicity of the design. Not only would the MSDR cost less to manufacture than LFTRs, it would probably cost significantly less than LWRs and IFR SMRs such as the ARC-100.

In our current energy situation, a MSDR type reactor would be highly desirable. Not only would it serve as a route to a LFTR type molten salt thorium breeders, but it would offer a potential low cost alternative to the Light Water Reactor, that would be both safe, and would reduce the nuclear waste problem.