Alvin Weinberg and the Molten Salt Reactor

After Alvin Weinberg's death in October 2006, I began a study of his writings that were available on the internet. This was to lead me to several brief papers on the Molten Salt Reactor, and to the Molten Salt Breeder (the LFTR). My interest in Weinberg's Molten Salt Reactor vision, was facilitated by the emergence of Kirk Sorensen's blog Energy from Thorium. Kirk had started his blog about six months before my rediscovery of Weinberg's vision. Kirk had done an amazing job of documenting the history of Molten Salt Reactor Research in Oak Ridge. Kirk's Document archive is perhaps the single most powerful rhetorical tool on nuclear energy available on the Internet. Most people who bother to work their way through Kirk's Documents end up being MSR believers.

In addition to Kirk's Documents a number of important Weinberg papers were posted on the Office of Science and Technology Information web page. One of the most important of those papers was “Towards an Acceptable Nuclear Future" in which Weinberg had

re-examine man's long-term energy options, in particular solar energy and the breeder reactor.

In Acceptable Nuclear Future Weinberg had discussed the cause of the collapse of the first Nuclear Era:

At that time, with Oyster Creek being contracted at a little over $100 per kilowatt of electricity [kW(e)], it seemed plausible to expect nuclear energy to be extremely cheap, as well as inexhaustible. Our dreams of nuclear-powered agro-industrial complexes seemed like legitimate extrapolations from what we thought was demonstrated technology. That the technology turned out to be much more expensive for reasons that few could foresee, or that other sources of energy have also become very expensive, is beside the point: disillusionment with our predictions made it difficult for the nuclear community to retain the confidence of some of the public.

But then Weinberg pointed to a potential low cost nuclear future,

I am unprepared to give up: reactors with an intrinsically low fuel-cycle cost, such as CANDU-Th or molten salt, may yet be realized.

During his OrNL days Weinberg wrote a number of short papers intended to introduce Molten Salt Reactor technology. One was titled WHY DEVELOP MOLTEN-SALT BREEDERS? it was a an introduction too ORNL-TM-1851 (posted in Kirk's document archive). "Why develop" summarized Weinberg's thinking about the Molten Salt reactor, and was offered Weinberg's case to make a persuasive case for the Molten Salt Breeder Reactor:

Nuclear power, based on light-water-moderated converter reactors, seems to be an assured commercial success. This circumstance has placed upon the Atomic Energy Commission the burden of forestalling any serious rise in the cost of nuclear power once our country has been fully committed to this source of energy. It is for this reason that the development of an economical breeder, at one time viewed as a long-range goal, has emerged as the central task of the atomic energy enterprise. Moreover, as our country commits itself more and more heavily to nuclear power, the stake in developing the breeder rises—breeder development simply must not fail. All plausible paths to a successful breeder must therefore be examined carefully.

To be successful a breeder must meet three requirements. First, the breeder must be technically feasible. Second, the cost of power from the breeder must be low; and third, the breeder should utilize fuel so efficiently that a full-fledged-energy economy based on the breeder could be established without using high-cost ores. The molten-salt breeder appears to meet these criteria as well as, and in some respects better than, any other reactor system. Moreover, since the technology of molten-salt breeders hardly overlaps the technology of the solid-fueled fast reactor, its development provides the world with an alternate path to long-term cheap nuclear energy that is not affected by any obstacles that may crop up in the development of the fast breeder.

The molten-salt breeder, though seeming to be a by-way in reactor development, in fact represents the culmination of more than 17 years of research and development. The incentive to develop a reactor based on fluid fuels has been strong ever since the early days of the Metallurgical Laboratory. In 1958 the most prominent fluid-fuel projects were the liquid bismuth reactor, the aqueous homogeneous reactor, and the molten-salt reactor. In 1959 the AEC assembled a task force to evaluate the three concepts. The principal conclusion of their report was that the “molten-salt reactor has the highest probability of achieving technical feasibility.”

This verdict of the 1959 task force appears to be confirmed by the operation of the Molten-Salt Reactor Experiment. To those who have followed the molten-salt project closely, this success is hardly surprising. The essential technical feasibility of the molten-salt system is based on certain thermodynamic realities first pointed out by the late R.C. Briant, who directed the ANP project at ORNL. Briant pointed out that molten fluorides are thermodynamically stable against reduction by nickel-based structural materials; that, being ionic, they should suffer no radiation damage in the liquid state; and that, having low vapor pressure and being relatively inert in contact with air, reactors based on them should be safe. The experience at ORNL with molten salts during the intervening years has confirmed Briant’s chemical intuition. Though some technical uncertainties remain, particularly those connected with the graphite moderator, the path to a successful molten-salt breeder appears to be well defined.

We estimate that a 1000 MWe molten-salt breeder should cost $115 per kilowatt (electric) and that the fuel cycle cost ought to be in the range of 0.3 to 0.4 mill/kWh. The overall cost of power from a privately owned, 1000-MWe Molten-Salt Breeder Reactor should come to around 2.6 mills/kWh. In contrast to the fast-breeder, the extremely low cost of the MSBR fuel cycle hardly depends upon sale of byproduct fissile material. Rather, it depends upon certain advances in the chemical processing of molten fluoride salts that have been demonstrated either in pilot plants or laboratories: fluoride volatility to recover uranium, vacuum distillation to rid the salt of fission products, and for highest performance, but with somewhat less assurance, removal of protactinium by liquid-liquid extraction or absorption.

The molten-salt breeder, operating in the thermal Th-233U cycle, is characterized by a low breeding ratio: the maximum breeding ratio consistent with low fuel-cycle costs is estimated to be about 1.07. This low breeding ratio is compensated by the low specific inventory* of the MSBR. Whereas the specific inventory of the fast reactor ranges between 2.5 to 5 kg/MWe the specific inventory of the molten-salt breeder ranges between 0.4 to 1.0 kg/MWe. The estimated fuel doubling time for the MSBR therefore falls in the range of 8 to 50 years. This is comparable to estimates of doubling times of 7 to 30 years given in fast-breeder reactor design studies.

From the point of view of long-term conservation of resources, low specific inventory in itself confers an advantage upon the thermal breeder. If the amount of nuclear power grows linearly, the doubling time and the specific inventory enter symmetrically in determining the maximum amount of raw material that must be mined in order to inventory the whole nuclear system. Thus, low specific inventory is an essential criterion of merit for a breeder, and the detailed comparisons in the next section show that a good thermal breeder with low specific inventory could, in spite of its low breeding gain, make better use of our nuclear resources than a good fast breeder with high specific inventory and high breeding gain.

The molten salt approach to a breeder promises to satisfy the three criteria of technical feasibility, very low power cost, and good fuel utilization. Its development as a uniquely promising competitor to the fast breeder is, we believe, in the national interest.

It is our purpose in the remainder of this report to outline the current status of the technology, and to estimate what is required to develop and demonstrate the technology for a full-scale thermal breeder based on molten fluorides.

Papers like this sent me to the Internet, where i found Bruce Hoglund's Molten Salt Interest Pages. Bruce was not actively adding to his Pages when i discovered them, so by far the most significant web site on Molten Salt Reactor Technology was Energy from Thorium. My interest in EfT eventually lead to my encounter with Kirk Sorensen.

Weinberg's interest in Molten Salt Reactor technology extended well beyond writing a few brief papers. He was passionately involved in the Progress of the MSR project.

In a brief 1987 talk, R. G. Wymer of the ORNL Chemical Technology Division, described Weinberg's passion for fluid fueled reactors.

At Oak Ridge we were concentrating on the thorium fuel cycle. Alvin Weinberg, for many years the Director of the Oak Ridge National Laboratory, is a great phrase maker. His dream was to "burn the rocks", in "a pot, a pipe, and a pump". By those rather fanciful phrases he meant that the thorium present in many granitic rocks throughout the world could, in a sense, be "burned" by incorporating it in a fluid fuel reactor. The fluid was to be pumped around and around in a loop. The loop was the pipe. As the liquid fuel was pumped through an enlargement in the pipe (the enlargement was the pot), there was enough of it present in the right configuration to go critical. The heat of fission was to be taken off by heat exchangers in the loop and converted to electrical energy. Also, as the fuel circulated around the loop, it was to be reprocessed continuously by equipment located in a side stream through which a fraction of the fuel was diverted. In principle that was a pretty good idea. One of the major reasons for the poor capacity factors of present-day power reactors is that it takes a long time to refuel them. Weinberg's concept avoided that refueling problem. Of course it created a few problems too.

Two major programs were carried out at ORNL based on the pot, pipe, and pump concept. One of them was the Aqueous Homogeneous Reactor, and the other was the Molten Salt Reactor Experiment. Needless to say, these projects were of major importance to Dr. Weinberg. Occasionally he would engage me in conversation. Invariably the conversation would turn to the current ORNL pot, pipe, and pump reactor project. This was perfectly reasonable from his point of view because my division, Chem Tech, had a major role in both of those reactor projects.

The interesting thing about Weinberg's encounters with Wymer was that Wymer was never directly involved in the development of fluid core reactors. Wymer knew what was going on in the Chemical Technology Division and Weinberg was pumping him for information that he was not being given by other division leaders. And Heaven help the division leader who Weinberg was not satisfied with after Weinberg pumped members of their staff for information.

Kirk had posted on EfT a number of short Weinberg papers on Molten Salt Reactors and thorium breeding . Thus my interest in Weinberg lead in turn to an interest in the Molten Salt Reactor, that became foundational to my views on the future of energy. That is, however, another story.