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.

Nuclear Proliferation: Devices and Weapons

Discussions of nuclear proliferation are tainted by some fundamental problems. Some academic "nuclear proliferation experts" appear to be unable to distinguish between nuclear weapons and nuclear devices. There are important differences. A nuclear weapon is deliverable by military means. That is it is possible to place a nuclear weapon at a particular spot and at a particular time, with a reasonable assurance that it will do what it is intended to do. A weapon must be storable, without short term deterioration. Finally a weapon should not kill the people who manufacture or handel it primary to delivery. A nuclear device on the other hand may be too heavy to deliver by conventional military means. Thus a device can be designed with radiation shielding materials that would be too heavy to use in a conventional nuclear weapon. A device may be constructed from material that immediately begins to damage parts of the device. A device might require immediate use in order to be effective. A device could be constructed in a special laboratory, which could be equipped with expensive and difficult to design and manufacture equipment intended to protect workers from high levels of radiation from fissionable and associated materials. Thus a device could contain materials that would be considered militarily useless or worse in a weapon.

On July 23, 1953 Edward Teller wrote a letter to Sterling Cole, who was then the
Chairman of the Joint Congressional Committee on Atomic Energy. Teller argued that there could be a

connection between power production and military application. . . . there is a great and increasing need for fissionable materials in the military field.

Teller suggested

It seems to be doubtful whether, on the basis of present technology, atomic energy can produce power in an economically profitable manner. Power production can, however, be conducted in such a manner as to produce militarily useful materials. It would seem to me reasonable to stimulate the construction of power-producing reactors by guaranteeing a price at which the Government will buy the militarily useful by-products. This price should of course be set lower than the price at which the Atomic Energy Commission is producing fissionable materials at the present time.

In fact both and Cole knew that this plan had not originated with Teller, in fact it had its origins in the United Kingdom. During the 1950's and 1960's the militant British Coal Miners Union, was a thorn in the flesh of the British Government, the British Economy, and the British public. Electrical power generation in the UK was dependent of coal mined by union miners, and thus the possibility existed that the British ability to generate electric power would be lost in the event of a long coal miners strike. Thus the British government proposed to build the Calder Hall Magnox Reactors the ability to generate electricity. The catch was that electricity produced by Calder Hall Magnox Reactors would be far more expensive than electricity produced by coal fired plants, but the British government had an answer to that. The real function of the Calder Hall reactors was the production of plutonium for nuclear weapons. Thus the British built a series of Magnox reactors for dual purpose use.

In October 1953, less than three months after the Teller letter to Cole, the British tested two devices made from some of the Calder Hall produced plutonium in a South Australian desert. The Totem devices both went off with quite a bang, but some thing about the test displeased the British mightily, for they never went through with their plan to weaponize Calder Hall Plutonium. These events have never been satisfactorily explained. In moderated reactors, if Uranium-238 absorbs a neutron, it is likely to be transformed into plutonium-239 rather than fission. Pu-239 is fissionable, but in moderated reactors 36% of the time Pu-239 undergoes transformation into Pu-240. Pu-249, like U-238 is not fissionable in moderated reactors.

Plutonium-240 does have one unusual quality. It undergoes spontaneous fission. Quite a lot of it as a matter of fact. Every pound of Pu-240 will produce something close to 200,000 fission events a second. And each fission event will produce neutrons, that can trigger fission in fissionable isotopes. Unlike U-235 and Pu-239, Pu-240 does not need to absorb a neutron prior to fission, and absorbing a thermal neutron does not trigger fission in a Pu-240 atom, which then becomes Pu-241. Pu-241 is fissionable in a moderated reactor.

A moderator is a material that, such as graphite, that decreases the energy level of neutrons. Moderators increase lower the criticality threshold of low enrichment nuclear fuel. With a graphite moderator it is possible to create a chain reaction in natural uranium. Since the first American reactors were built before the United States had the ability to produce enriched uranium, they had to use a moderator to create a chain reaction, and American reactor designers chose graphite. If a Neutron is absorbed by a U-235 atom, most of the time, the U-235 splits, and releases neutrons. If one or more of those neutrons is absorbed by U-235 atoms, we have a chain reaction. If a neutron is absorbed by a U-238 atom in a moderated reactor, the U-238 will undergo a nuclear transformation and become Plutonium-239. Now Pu-239 is fissionable, but it is more fissionable with fast neutrons, In a moderated reactor if a Pu-239 atoms absorbs neutrons, 64% split, and 36% become Pu-240.

For weapons designers, Pu-240 is a bad thing. If spontaneously fissioning Pu-240 is mixed with Pu-239, neutrons from the Pu-240, will start to trigger the fission of the Pu-239. If enough Pu-239 is precent, this can trigger a nuclear explosion, although one which might be of far less power than would be expected in an plutonium based nuclear weapon. In nuclear weapons the timing of the beginning of the nuclear explosion is very important. If the trigger goes off too soon, the explosion can loose most of its potential power.

If Pu-240 is present in a plutonium based bomb, its neutrons will trigger a premature explosion of a Plutonium based weapon, compromising its military effectiveness. The way weapons designers control this is to limit the mount of Pu-240 in nuclear weapons. Since 36% of Pu-239 will become Pu-240 in a moderated reactor, it is highly desirable to get it out of a moderated reactor almost as quickly as it is created. But power reactors need to be run very differently than weapons production reactors. A power reactor functions best when fuel is kept in the reactor as long as possible. This will produce Plutonium, but a mixed form of Plutonium called Reactor Grade Plutonium. RGP includes Pu-238, Pu-239, Pu-240 and Pu-241. From the weapons viewpoint this is very undesirable. In preparing Calder Hall plutonium for the Totem tests, the British pulled the nuclear fuel from the reactor quickly, and processed it. This lead to mix of about 90% Pu-239 and 10% Pu-240. The Totum experiment demonstrated that this a vigorous explosion could be produced with this Plutonium mix, but other results of the experiment proved less than satisfactory, and the British gave up the idea of using plutonium from the Calder Hall reactors to produce nuclear weapons.

The United States also ran a nuclear test with Calder Hall plutonium in 1962, probably with even less satisfactory results than the 1953 British experiments. In fact, by the mid 1960's the British thought the Calder Hall reactors were so useless for military purposes, that they declassified their plans, something they never would have done, if they believed that the Calder Hall could produce weapons useful Plutonium.

From the 1960's onward increasing amounts of discarded RGP, became available in both the West and in the Soviet Union, as a byproduct of civilian power reactors, yet no nation ever chose to weaponize RGP. Clearly then there are significant disadvantages to the use of RGP for weapons purposes. Some of these disadvantages are understood. Not only is a weapon built from RGP likely to be of limited explosive power, when compared with ordinary nuclear weapons, but heat and radiation from RGP are likely to damage the weapons triggering mechanism. Thus if kept on the shelf for too long, the RGP weapon is likely to disarm itself. How long is too long? it could be a period as short as a few days or even a few hours.

In addition there are the problems of building a nuclear weapon from highly radioactive materials like RGP.

It is possible for people in places like Los Alamos to design and build nuclear devices using "Reactor Grade Plutonium", and successfully trigger those devices. It does not appear possible to build triggerable nuclear weapons from RGP. Thus self styled academic nuclear experts who speek knowingly of nuclear proliferation using Reactor Grade Plutonium, simply don't know what they are talking about.

Alexander deVolpi's knols on nuclear non-proliferation

I have added a link on Nuclear Green to Alexander deVolpi's knols on nuclear non-proliferation. Dr DiVolpi's writings are required reading for anyone who who wants to claim expertise on nuclear proliferation. DiVolpi was a peer of the late J. Carson Mark of Los Alamos, and his knols correct the view that Mark thought reactor grade plutonium was a practical weapons materials. DeVolpi also parses statements on reactor grade plutonium from official sources, often cited by anti-nuclear experts. For example DiVolpi points to a statement by David Hafemeister of the United States DoE,

" [Advanced] nuclear-weapon states such as the United States and Russia, using modern designs, could produce weapons from reactor-grade plutonium having reliable explosive yields, weight, and other characteristics generally comparable to those of weapons made from weapon-grade plutonium."

DeVolpi comments,

"I suggest a discriminating reader would see that the quote is limited to “advanced nuclear-weapon states,” confined to “modern designs,” and qualified by terms such as “could produce,” “reliable yields,” and “comparable characteristics.” Since official declarations (hedges) are usually the product of a careful inter-agency vetting. His statement, thus, pretty much excludes reactor-grade plutonium as source material under a number of realistic circumstances: less-advanced nuclear-weapon states, less-sophisticated designs, less-than-assured yields, and other sub-marginal situations. In other words, neither advanced weapon states, nor less-advanced weapon states, nor threshold weapon states are likely to produce weapons from reactor-grade plutonium (for reasons validated by Hafemeister’s carefully chosen omissions)."

Needless to say nuclear critics do not engage in such carful reading of the documents that they draw on to make their case. But then nuclear critics are not interested in questions of truth or accuracy. They simply mine sources for supportive quotes, and hope that no one will note important qualifications. Such selective misreading of texts, such cherry picking turns sources into sock puppets on the hands of nuclear critics like Dr Frank Barnaby, who has become the new anti-nuclear wacko on scitizen.

Banaby, a sometimes associate of the infamous Jan Storm van Leeuwen in the Oxford Research group, appears to belong, like Storm van Leeuwen, to the Club of Rome wing of the anti-nuclear movement. A successful post carbon shift to nuclear power would definately put a crmp into the goal of Club of Rome plans to kill off most of the human race and return the economic basis of society to a medieval like peasant economy. In order to bully us into accepting this extremely unattractive scenario, Barnaby has to threaten us with nuclear proliferation, as if the die off of a few billion human beings would be a preferable consequence, and thetermination of modern society woiuld be a more acceptable outcome.

I previously called Dr, Barniby to task for ignoring DeVolpi's telling views on nuclear proliferation, but he continues to do so, no doubt because DeVolpi makes it quite clear that that reactor grade plutonium is not a practical material for the building of nuclear weapons, and that the danger of nuclear proliferation is not increased by building civilian power reactors that produce reactor grade plutonium as a byproduct. Needless to say, Dr. Barnaby ignored my comments, just as he ignores Alexanger DeVolpi's writings on reactor plutonium. Dr. Barnaby copes with criticism by ignoring it. By doing so he discounts himself as a serious intellectual. A serious intellectual acknowledges his critics, and tries to answer them. If he or she makes mistakes and they are pointed out he or she acknowledges them, at least to self, and learns to not make the same mistakes again. Since Barnaby does not even acknowledge mistakes when they are brought to his attention, his is not a rational voice.  

Waldo Cohn, nuclear and genetic pioneer

Waldo Cohn was both a remarkable scientist and a remarkable person. I have previously posted about his leadership in the school desegregation controversy in Oak Ridge, which lead to the peaceful desegregation of Oak Ridge Schools in 1955. Waldo's was a biochemist, but pioneering physical chemical research on plutonium during World War II made an important contribution of nuclear technology. The ion exchange method Cohn developed had wide applications including the processing of fission products, rare earth separation, and in nucleic acid research.
In 1963, the American Chemical Society honored Cohn for his "pioneering work in ion exchange chromatography which has made possible much of the progress that has been made in two completely different fields of chemistry since World War II."

Dr. Cohn pioneered the use of radioisotopes as tracers in medicine and was influential in starting the production of radioisotopes for scientific research and medicine at Oak Ridge. He set up a system for radioisotope distribution.pioneered the use of radioisotopes in medicine.

He applied his plutonium research methods to the study of the components of the nucleic acids DNA and RNA, and his research made a major contribution to the understanding information transfer from nucleic acids to protein molecules.

Alvin Weinberg stated about his long time friend Waldo Cohn:

"The main task (in 1943) was to produce gram quantities of the nuclear explosive, plutonium. The techniques developed there were transferred to the huge plutonium-producing nuclear reactors at Hanford, Wash.

"To manufacture plutonium, one had to 'cook' uranium in an atmosphere of neutrons in the nuclear reactor at Clinton Lab. In this process uranium atoms were split to create radioactive 'fission products.' Cohn set about to identify the chemical species of fission products. He applied to this process a technique known as 'ion exchange chromotography.'

"After the war Cohn realized that this technique could be applied to the characterization of the components of the nucleic acids, DNA and RNA. Cohn's technique ultimately led to Crick and Watson's structure of the genetic materials, DNA and RNA. For this achievement Cohn received the Chromotography Award of the American Chemical Society and he was named a fellow of the American Academy of Arts and Sciences.

"Cohn was also the first to organize and promote the use of radioactive radioisotopes produced in nuclear reactors. The widespread use of radioisotopes is perhaps the most important scientific byproduct of the Manhattan Project."

Dr. Cohn's obituary in the Oak Ridger noted his many civic, artistic and political contributions to the Oak Ridge community:

"Cohn was politically involved both nationally and locally. He was one of the organizers of a petition signed by a large number of scientists urging that a nuclear bomb first be detonated in a test blast before being used on human targets. Immediately after World War II he became active in urging international control of nuclear weapons."

"Locally he was one of the leaders of a group of local Democrats who worked to make basic reforms in the organization and operation of the party in Anderson County."

"He was a strong proponent of the development of nuclear power and often spoke out against what he thought were exaggerated fears about the dangers of radioactive materials to the public. , , ,"

"Less than two months after joining the staff at Clinton Laboratories, he placed a small notice in The Oak Ridge Journal, weekly newspaper published by the Manhattan Engineer District, inviting all Oak Ridgers interested in playing in an orchestra to a meeting. Nine other musicians responded, two other strings and seven woodwind players. In a 1983 interview he told Juanita Glenn of The Knoxville News-Sentinel, "I didn't want to join an orchestra, I was just looking for someone to play duets."

"Cohn had studied the cello since age 11, although at first he hated carrying the large instrument. Before his 16th birthday he had been invited to join the Berkeley Community Orchestra."

"That initial group of interested early Oak Ridge musicians grew into a string orchestra of 19. At first they rehearsed in the Cohn living room but soon moved to the auditorium of the original Oak Ridge High School, which was located on the knoll off Kentucky Avenue overlooking Blankenship Field and which, before it was demolished, had become Jefferson Junior High School."

"The early musician group named him their conductor and gave their first concert as the Oak Ridge Symphonette in June 1944. Then, within weeks, they became the 65-member Oak Ridge Symphony, which gave its first concerts on Nov. 3-4, 1944."

"Besides the Symphony and his service on the early Advisory Town Council, Cohn was a regular reader for Recording for the Blind and Dyslexic, reading primarily scientific texts. In more recent years he also volunteered regularly at the Oak Ridge Convention and Visitors Bureau, where he would answer visitors' questions about the wartime atomic bomb development and the later research at the ORNL Biology Division, from which he had retired in 1975."

Waldo stepped down as conductor of the Oak Ridge Symphony in 1955 when he received the first of his two Guggenheim Fellowships, this was for a year's study at Cambridge University in England. Dr. Cohn also received a Fulbright Research Scholarship.

Cohn continued to play the cello in the Oak Ridge Symphony until ill health forced him to stop a couple of years before his death in 1999.

I had a brief visit with Cohn about 1998, because I wanted to capture his memories of his 1954 recall election from Oak Ridge City Council. He was by then in poor health, but his stories of the recall period were lively and illuminating. Cohn was very charismatic. He was handsome, and had an air of distinction about him.

The Uranium Fuel Cycle

WASH-1097 remains an invaluable source of information on the thorium fuel cycle. It explains why the thorium fuel cycle creates such a small problem with transuranium isotope. First, however, it is important to understand why there is a problem in the uranium fuel cycle.

When U238 absorbs a neutron a transformation process is triggered. After a couple of sub-nuclear events (beta radiation), the two neutrons in the atom become protons. This process turns the uranium-239 atom into plutonium-239. Pu239 is fissile. But Pu239 has some characteristics that make it something less than a desirable fuel, in ordinary moderated thermal neutron reactors. Fission is most likely to occur with low energy neutrons. Yet, Pu239 has a healthy appetite for these low energy neutrons, while only fissions about 2 times out of 3 when it absorbs low energy neutrons. The net effect is that Pu239 doesn't "pull its weight" in the reactor when it is fissioned by low-energy neutrons. It doesn't produce enough neutrons per absorption to make up for the neutrons lost in absorption.

In the ideal uranium fuel cycle, a Pu239 nucleus absorbs a neutron, splits and emits three neutrons. One of them is absorbed by another fissile atom (U235 or Pu239) atom which splits. The other is absorbed by a U238 atom which is transformed into U239. As you get more and more of the absorption products of Pu239 building up in the nuclear fuel (Pu240, Pu241, etc), the neutronics become more and more unfavorable.

The heart of the problem is the fact that low energy neutrons split Pu239 atoms only about 2/3rds of the time. This is all laid out very nicely in WASH-1097. In the other 1/3rd of the time, Pu239 becomes Pu240. If Pu240 absorbs a neutron it becomes Pu241 and if Pu241 absorbs a neutron, 75% of the time it fissions. Thus by the WASH-1097 account, 25% of the time when Pu 241 absorbs a neutron it becomes Pu242. Thus after absorbing 4 neutrons, nearly 9% the atoms that started out as U238 are still plutonium. This is what is called a poor neutron economy. The neutron economy of fast breeders is better, because a neutron absorption in Pu239 is more likely to cause a fission with more energetic neutrons. Hence the desirability of fast breeder reactors in a transuranium reactor economy.

As we have seen conventional fast breeders use sodium as a coolant, and sodium is really nasty, dangerous stuff. In addition, as Kirk Sorensen points out, using liquid sodium as a coolant, limits the thermal efficiency of a reactor. Thus not only are LMFB reactors inherently dangerous, they
are also not as efficient as power producers as liquid fluoride reactors.

But here we must ask, why are we producing plutonium in breeder reactors? If we are producing it to go into conventional reactors, we are not producing very good nuclear fuel.

- Charles Barton

C.J. Barton, Sr. Molten Salt Chemistry

Shortly after my father, C.J. Barton, Sr., completed his Zarconium-Halfnium separation research, he was shifted administratively to ORNL, although he remained physically at Y-12 until 1959. In July 1950 he was reassigned to the Aircraft Nuclear Propulsion (ANP) Project. The Air Force had been thinking for some time about building reactor powered aircraft. It was quite frankly a hare brained scheme, but Alvin Weinberg knew how to play along with a joke. The ORNL approach was to build a small, very hot reactor, that would could provide enough heat to the aircraft's jet engine, to replace burning of jet fuel as a source of power for the engine. Light watter reactors were much to large to perform the task, and ORNL favored a molten salt reactor design.

My father also joined the new ORNL Reator Chemistry devision. My father's star among Oak Ridge chemists was assending due to his role in the Zarconium-Halfnium project. Molten Salt reactors are regarded as the playground of reactor chemists, and my father was assigned the role of group leader. His group of chemists was assigned the task of reviewing chemical literature on materials suitable for a molten salt reactor, and coming up with the formula for the ANP reactor. Research conducted under my father's supervision showed that a NaF, ZrF4, UF4 mix would make a satisfactory although in some respects not ideal Fluid. Zarconium was avaliable because my father's earlier research at Y-12 had lead to an industrial process for the seperation of Zarconium from Hafnium. Because of the danger of working with Berilium, it was decided to defer use of a superior formula involving the use of Lithium7. The more satisfactory salt mixture (LiF, BeF2, UF4) had already been idenitfied, but its use was delayed until the molten salt reactor experiment. My father and Warren Grimes held the patent on the NaF, ZrF4, UF4 salt mix. The LiF, BeF2, UF4 mix was not patentable, because the use of BeF2 had been suggested by an outside ccontractor prior to ORNL's involvement in the ANP project.

Much of the group's effort was directed to determining the melting point of various proposed Salt mixtures. These studies lead to triangular diagrams. My father was the leader of the reserch group assigned to come up with molten salt formuli.

Politics and cultural differences eventually lead to conflict between my father and his boss, Warren Grimes. My father was an East Tennessee "hillbilly," who had grown up in the mountain town of Jellico, Tennessee. He shared many cultural and religious attitudes native Tennesseans. Warren Grimes was not an East Tennessean. My father, like many East Tennesseans, believed that liquor was evil. He favored a prohibition of alcohol in the local county, and in fact became a leader of the prohibitionist faction in a local liquor refruendum. Grimes liked to drink. A disagreement over a liquor referendum lead to conflict between the two men. Grimes removed my father from leadership of the molten salt research group, and assigned him to another task. No doubt Grimes was able to get away with this because others in the ORNL leadership were upset with my fathers for his "nativist" views. Lab politics is always a part of science.

It had been propose that the molten salt reactor could use plutonium as a fuel. Some plutonium is generated and burned in light water reactors, but plutonium was, in the 1950's not included in the reactor fuel mix. However, if 238 was used in the molten Salt reactor, plutonium would be breed. As a consequence, the behavior of plutonium in a molten salt fuel mix needed to be understood. My father was assigned the task of determining the soluability of PuF3 salt in the molten salt fuel. Relationships were expresed in triangular phase diagrams. This project requited the use of glove boxes as research tools. My father traveled to Los Alomos to learn techniques for doing chemical research with plutonium. In addition, Los Alomos provided him with several kilograms of pure Pu 239 for his research.

A major aspect of my father;s research involved the development of a glove box design that could be used in the presence of high molten PuF3 and other salts. During the 1950's the glove box had a evil reputation with AEC researchers. There had been a history of glove box fires and other accidents. Part of my father's research task was to develop a glove box design that would be safe and would allow him to conduct what must have been a very difficult research task. He began this aspect of his research with a extensive literature review on glove box construction and research techniques. My father's literature review led him to write a report on glove box construction that is included in the Department of Energy's database. In addition, my father was asked to write a chapter on Glove Box Techniques, for Techniques of Inorganic Chemistry, Vol. III. Techniques was a manual for inorganic chemical research. My father's plutonium research produced an journal article, SOLUBILITY OF PLUTONIUM TRIFLUORIDE IN FUSED-ALKALI FLUORIDE—BERYLLIUM FLUORIDE MIXTURES, by CJ Barton in The Journal of Physical Chemistry 64:33, 306-309.

My father's Plutonium research caused him concern because Plutonium was regarded as highly dangerous. Y-12 supervisors were worried about it too. One day during my father's Pu 239 research there was a radiation incident at Y-12, and alarms went off. A supervisor rushed to my father's lab, geiger counter in hand, fearing that the incident involved my father's plutonium. My father was always extremely careful, and the incident was elsewhere.