Nuclear Green has in the past offered sketches of the early history of reactor design. The classic reactor design was created by Enrico Fermi, and featured a solid core. Fermi was a physicist, and in a way designed his first reactor as a physics experiment. From the view point of process, materials were placed in the nuclear core and then mechanically removed. What happened to the materials after their removal was not a part of the physicists business. There is no doubt that Fermi was the god father of the Sodium cooled fast breeder reactor. The late World War II Manhattan Project New Piles Committee, of which Fermi was a member discussed breeder options.

The World War II Metallurgy Lab of the University of Chicago was the nursery for both Argonne National Laboratory and Oak Ridge National Laboratory. Argonne basically was formed from Fermi's staff, and was lead by a long time Fermi protegee, Walter Zinn. As early as 1944, Fermi who was convinced of the importance of the breeder reactor project as a future source of energy, suggested to Zinn that he and his subordinates begin to develop sodium cooled breeder reactor technology. Alvin Weinberg described Zinn as the
gray eminence of nuclear development.
Argonne National Laboratory under Zinn was originally intended to be the center of national reactor design, although ORNL was to emerge as its rival during the 1950's. Weinberg notes,
WALTER (“WALLY”) HENRY ZINN was Enrico Fermi’s close associate during the Manhattan Project. After World War II he became the leading U.S. figure in the earliest development of nuclear energy. So pervasive was his stamp on nuclear development that a proper obituary to Walter Zinn must be nothing short of an account of the origins of nuclear energy and how Zinn profoundly affected its development.
Weinberg, who was himself an important figure in the history of nuclear developments, thus points out the importance of Zinn's role. While Weinberg was responsible for the suggestion to Hyman Rickover that the Light Water Reactor would prove more suitable for submarine propulsion than a sodium cooled reactor would, it was to Argonne and its director, Walter Zinn that Rickover turned to superintend its development. Zinn had a Rickover size ego, and when Rickover tried to control the Argonne group managing submarine reactor development. Zinn threw Rickover out of Argonne, and Rickover retaliated by moving the submarine reactor project to Bettis Laboratory, controlled by Westinghouse.

Zinn was to leave Argonne in 1956 after pushing through the Experimental Breeder Reactor-1 (EBR-1) project. Zinn's departure from Argonne followed a serious accident with the EBR-1 and Zinn's future focus was on Light Water Reactors. Argon researchers continued to investigate liquid metal breeder technology.

Thus the initial prestige of the fast breeder concept was to rest primarily on Fermi's shoulders, with Walter Zinn playing an important role. Yet the Fast Breeder was problematic from the start. A report issued by Sandia Laboratory in 2007 focused on Liquid Metal Fast Breeder sodium related safety issues/ the report notes numerous safety hazards for Sodium cooled reactors. Among notable sodium related safety hazards are sodium fires, and the positive void coefficient reactivity hazard of sodium cooled reactors. Sodium firers can be caused by sodium contact with
* air
* water
* and concrete
The Sandia Report focuses on the void problem
A fundamental difference between water and sodium-cooled reactors is the void reactivity coefficient. If the water around the core is voided (boiled, drained) in a water-cooled (thermal) reactor during operation, the power level will automatically drop. The reactor is therefore said to have a negative void reactivity coefficient. In contrast, if sodium is voided in certain sodium-cooled fast reactors (particularly large reactors), it will cause the power level of the reactor to rapidly increase. This reactor is said to have a positive void reactivity coefficient. When the reactor power increases, it can lead to additional boiling and voiding until fuel melts. This positive feedback can lead to extremely rapid surges in reactor power, potentially damaging or melting fuel and cladding.
Multiple events can lead to core voiding during operation, and great care is taken in the proposed new reactors to ensure that these events are prevented. They include sodium boiling, loss of coolant accidents (LOCA), and gas bubble entrainment within the sodium. Sodium fires could lead to sodium boiling if an undercooling event is initiated without scram (reactor shutdown). A severe leak in the secondary system, perhaps coupled with cable fires could lead to this situation. A large leak in the primary system could also disrupt flow enough to induce sodium boiling in the core. A sodium leak in the primary system could also lead to either a LOCA or gas bubble entrainment event. A large primary leak could potentially uncover a portion of the core. If gas is pulled back into a leak in the primary system, the resulting bubbles could also reach the core.
Many of the problems of sodium cooled reactors were still unknown to Ed Bettis and his associates in 1947 when K-25 Physicist Cecil Ellis assigned them the task of developing a sodium cooled reactor for a nuclear powered aircraft. But as Ed Bettis later explained even then enough was known to understand that a sodium cooled reactor would be difficult to control as well as potentially dangerous.

There were significan problems with sodium cooled reactors, as Ed Bettis was to later explain:
By 1950, at various places in the country, work had progressed on the handling of high- temperature sodium metal to the point that it was being seriously considered as a coolant for nuclear reactors. Accordingly, a group of engineers and physicists at ORNL started design work on a solid-fuel-pin sodium-cooled reactor, with the fuel consisting of 235U (as UO2) canned in stainless steel. It was decided to make this a thermal reactor and to use BeO blocks as the moderator. The circulating sodium was to extract heat from the fuel pins and at the same time to remove heat from the moderator blocks. . . .
The solid-fuel-pin thermal reactor design was found to possess a serious difficulty when the design concept was projected to cover a relatively high-power reactor. The problem was the positive temperature coefficient of reactivity associated with the cross section of xenon at elevated temperatures.. . .
Th Xenon problem was serious enough to foce Bettis and his associates to look at an alternative.
This xenon instability was considered to be serious enough to warrant abandoning the solid-fuel design concept, because of the exacting requirement placed on any automatic control system by this instability.
But what sort of alternative reactor would solve the Xenon issue?
An obvious way to avoid the control problem would be to incorporate a liquid fuel that would have a large density change for a given change in temperature. If, upon heating and expanding, a portion of the fuel could, in effect, be made to leave the critical lattice, a self- stabilizing reactor would result.
Bingo! Ed bettis and his associates had discovered one of several MSR advantages, its self stabilization.

In 1950 the K-25 aircraft nuclear propulsion program was turned over to Fairchild Aircraft. which decided to move it to Ohio. The program staff was given a choice of following the program to Ohio, or to remain in Oak Ridge, where a new nuclear powered aircraft program was to emerge superintended by ORNL. A Brilliant Chemist, Raymond Clair Briant was to be the new Program manager, and Bettis approached Briant about the Molten Salt Reactor concept, and so the ORNL Molten Salt Reactor adventure was born.

In an often noted 1957 paper, "Molten Fluorides as Power Reactor Fuels"Alvin Weinberg proposed the construction of a liquid fluoride salts based thermal breeder reactor. The very concept than Weinberg announced was revolutionary. In 1959 the AEC commissioned an evaluation of three potential fluid fuel reactor technologies capable of breeding. They were"
* Aqueous Homogeneous Reactors
* A Liquid Metal Fast Breeder with a slurry rather than a solid fuel core
* The Molten Salt Breeder Reactor
Of the three the committee commissioned to write the report concluded that the MSR represented the smallest developmental challenge. Unfortunately the AEC did not also commission a direct comparison between the MSR and the standard Liquid Metal Fast Breeder Reactor. Had they done so they would have found that the Molten Salt Reactor was a far more practical reactor concept than the Liquid Metal Fast Breeder Reactor. This statement can be tested by comparing the developmental problems of the two MSR prototypes with the developmental histories of the early liquid sodium cooled breeders. While both MSR's performed as expected, with no major accidents this was not the case with early sodium cooled fast breeders. Compared to the Molten Salt Reactor, the LMFBR faced daunting and expensive to fix safety challenges. A list of major LMR accidents will be sufficient to make this point.
* The EBR-1 suffered a partial core melt down in 1955.
* The Fermi 1 suffered a partial core meltdown in 1966
* The Sodium Reactor Experiment suffered a partial core meltdown in 1959
In addition to these major accidents, LMFBRs have suffered numerous lessor accidents including sodium leaks with fires, and fuel cladding ruptures. I am not going to argue that these accidents mean that LMFBR are unsafe, or that safety progress has not been made in LMFBR design. Rather my point is that the MSR posed far fewer safety challenges than the LMFBR did in 1959, and a direct comparison of the two breeder technologies would have revealed this fact.

But safety was hardly the only area in which the MSR held advantages. In terms of materials problems, ORNL was able to come up with solutions quickly once problems were known. Thus in the early 1960's ORNL possessed a potential breeder technology that was safer than the more conventional LMFB and a technology that was likely to pose far fewer developmental challenges. There is evidence that the AEC was interested in the development of MSR technology. But that began to change with the arrival of LMFBR fan Glenn Seaborg as AEC Chairman, Milton Shaw, another LMFBR supporter, as AEC reactor Czar, and with the emergence of another LMFBR supporter, Congressman Chet Holifield, as a controlling influence over AEC policy. None of these people people would have favored a point by point comparison of the prospects of LMFBR and MSR technology. The decision to favor the LMFBR was thus political, and was not based, nor was it justifiable, on scientific or rational grounds.