The Molten Salt Reactor Family: Uranium Fuel

In an earlier post, I stated that there were many different possible Molten Salt Reactor designs. I pointed to nuclear fuel as one possible source of reactor design variations. There are two potential nuclear fuel cycles that can be used in Molten Salt Reactors. Choice of fuel cycles can make a difference in reactor designs. Today, I want to focus on one of the two fuel cycle options, uranium. There are in fact several different types of uranium fueled Molten Salt Reactors.

The first type that I would consider could be called the ORNL technology uranium fueled MSR. The reactor could be a direct development of the technology used in the Molten Salt Reactor Experiment. Such a reactor would use LEU with up to 19.75% U-235, and could operate at temperatures of up to 704°C (1300°F), it would. Without nuclear proliferation concerns, the U-235 content could be raised higher, even 100% U-235 could be used. ORNL technologists preferred building their MSR core structure with Hastelloy ® N, an nickel alloy. This high operating temperature allows fos significant improvements in electrical generation thermal efficiency compared to conventional Light Water Reactors (LWRs).

In addition the UMSR would be simple and compact. It would not require massive steel pressure vessels, and massive concrete containment structures. These are two characteristics that could potentially lower reactor manufacturing costs. Thus the UMSR is likely to be less expensive to manufacture and less expensive to operate than conventional reactors. The ORNL Molten Salt Reactor Experiment (MSRE) proved to be highly reliable, thus the UMSR could compete with LWR as base load power sources. Advance reactor safety features, that are unique to Molten Salt Reactors could be included in the UMSR design. With low enriched uranium (LEU) the UMSR operate as a U-238 fuel cycle plutonium converter. A converter is a reactor that produces some new nuclear fuel, Unlike a nuclear breeder, a converter produces less than one atom of new nuclear fuel for every atom of old fuel it uses.

A U-238 cycle MSR converter would solve much of the nuclear waste problem that characterizes LWRs. MSRs are good plutonium burners, although they do not dispose of plutonium quite as efficiently as fast reactors. Because Xenon-135 can be continuously removed from the MSR core, thermal MSRs can convert U-238 into plutonium at a higher conversion ratio than LWRs can. And because Molten Salt Fuel can be easily reprocessed in its liquid form, any plutonium removed during reprocessing could be returned to the MSR core. Thus in MSRs plutonium and other actinides do not pose a long term nuclear wast problem. In fact faster MSRs are potentially so good at destroying nuclear waste, that both Russian and American reactor scientists have proposed using them to destroy the actinides waste from LWRs. What is left over from the nuclear waste destroying MSR process is fission products, much of which becomes useful for a variety of industrial uses very quickly, and all of which will be no more radioactive - and thus no more dangerous - than newly mind uranium within 300 years. (See this Google lecture by Kirk Sorensen, on the MSR solution to the so called nuclear waste problem.)

It has been a long standing contention of Nuclear Green that it is less expensive to build reactors in factories than to build them in the field. Although it is possible to factory manufacture large reactors in the form of kits containing several hundred large modules, smaller kits which contain as few as a half dozen modules are desirable. It would also be highly desirable if the modules could all be moved by truck or by train. The smallest practical size for such a reactor would be 100 MWe, although a 200-300 MWe size might be desirable.

Thus the UMSR, if it were to bew developed would be a transitional step in the evolution of reactors toward the Liquid Fluoride Thorium Reactor (the LFTR).

A second form of Uranium fueled MSRs would be what I call the Uranium Big Lots Reactor. The name Big Lots came from reactor design ideas I thought about while shopping in a Big Lots Store. The Big Lots Reactor was originally intended to be a LFTR, but it would work well with an all uranium fuel formula. The Big Lots idea was triggered by some comments by physicist David LeBlanc, who suggested MSRs cost could be lowered by building reactors from lower cost materials. What I realized during my Big Lots excursion was that for a small amount of the MSR performance sacrifice - say lowering operating temperatures from 700°C to 600°C - and by anticipating less capacity utilization - say a 15% to 25% capacity factor rather than the 90% capacity factor expected of base load generators. The Big Lots reactor was intended to load follow, to produce peak load and back up electrical generation. These were grid functions that both conventional nuclear power and renewable energy sources were not very good at. could be shifted from fossil fuels, probably without an increase in electrical price.

The Big Lots reactor would then be a discount store version of the Molten Salt Reactor. Not quite as good as the UMSR or the LFTR at pumping out full power 24 hours a day, 7 days a week, but very good for putting out power when the temperature runs to 103°F on a hot Texas Summer afternoon, or for providing quickly accessible nuclear power, if a wind farm looses its breeze or a base load nuclear plant unexpectedly shuts down. MSRs are superbly suited for backup role, but they can be designed to automatically shut down when they reach their top operating temperature. Fission product decay will keep the fluid salts in the core at peak temperature for some time. Power is transmitted to the electrical generating system by heated salt, and heated salt can be kept on tap for a week or so. Then the reactor will fire up again for a short while, only long enough to produce another few days worth of fission product decay heat.

I have recommended a number of steps to decrease nuclear costs in general and the costs of Molten Salt Reactors in particular. All of these steps could be applied to Big Lots Reactor cost lowering. Small size reactors represent a smaller risk to lenders and investors. Interest rates are tied to risks, and the lower the risk, the lower the interest rate. Thus building small reactors will quite likely lower interest rates on nuclear projects.

Big Lot reactors can be housed in underground silos, and thus would be invulnerable form attacks by large aircraft. Existing sites for natural gas fired power plants can be used to house Big Lot reactors. This would lead to further cost savings. For example the existing grid connection can be reused, saving the cost of building a new grid connection system.

Because they would only be expected to operate a small percentage of the time, and then frequently at less than full power, the Big Lot Reactor would have lower maintenance cost. Neutron radiation caused damage to reactor materials would be significantly less than in base load UMSRs.

Both the Big Lot and the base load UMSR could come in one and two fluid versions, although most would probably be one fluid reactors, because proliferation concerns would require mixing U-235 with U-238, and Plutonium involved in the nuclear process, should be kept in the same carrier fluid as the Uranium. Thus only one fluid would be required.

Thermal UMSRs would in all likelihood graphite moderated, although it has been proposed that thermal MSRs could also be heavy water moderated. This raises safety concerns.

If we decide to not use graphite as a moderator, and as I will indicate in a separate post on the use of graphite in MSRs, there are reasons why future MSR designers might decide to forego the use of graphite, we can still choose to moderate the nuclear process through carrier salt moderation. The primary carrier salt moderators are lithium fluoride (LiF) and beryllium fluoride (BeF2). These salts will slow neutrons to an epithermal speed range. More fissionable materials are required to sustain a nuclear reaction in an epithermal reaction than to sustain a chain reaction in a thermal reactor, and there ars some other issues as well, but if graphite concerns become to a major issue, epithermal may serve as a significant option.

Before we leave the world of graphite reactors behind, I would like to mention one more family of thermal Molten salt option, the Advanced High Temperature Reactor (AHTR) option being explored at the University of California Berkley and at ORNL. This reactor family might be considered a cousin of the MSR which uses liquid Salts are coolants but not as fuel carriers. The nuclear fuel for these reactors embeds the U-233, U-235 and/or Pu-239 in graphite, either in the form of graphite core structures or in the form of graphite pebbles. The AHTR is thus a hybrid of MSR and gas cooled graphite reactor technologies. ORNL is developing a small advanced high temperature reactor (SmAHTR), as a source of industrial process heat. The SmAHTR like Big Lots Reactor can serve as a source of peak demand electrical capacity, through the use of stored heated liquid salt.

Finally, it is possable to build fast Molten Salt Reactors, and they have a number of advantages over Liquid Metal Fas Breeder Reactors. Fast U-238 breeding MSRs can be designed to use either Fluoride or Chloride salts. Although it is possible to design a two fluid Fast Thorium Breeder of a hybred U-238/Th-232 fast breeder, it would certainly be possible to build a single fluid uranium cycle breeder. French physicists, working at Laboratoire de Physique Subatomique et de Cosmologie, of the University of Grenoble, (France), have proposed building a single fluid fast MSR which they intend to use as a thorium breeder, but which can be used as either a hybred breeder or a uranium cycle breeder. The French Reactor designers propose to eliminate beryllium from the salt formula. Lithium is a a moderator, but some what less so than beryllium, and there are some secondary safety advantages to removing beryllium from the reactor core.

The French molten salt fast breeder is primarily a thorium breeder, it offers some attractive features which I will discuss in a later post. In addition. Uranium/thorium hybred breeding cycles require firther discussion, and of course so does both thorium breeders and thorium converters.