For many nuclear engineers, the moment a new reactor first achieves criticality is considered the “birth” of their creation, the moment it “comes alive”. Criticality is special, because it means that the reactor is generating new neutrons at precisely the rate at which they are consumed in fission.

Unlike a rocket launch, or a ship sliding into the harbor from its construction dock, the moment of criticality has no outward excitement attached to it. Were one to be looking at the reactor at that moment, it would be unlikely that one would even discern that criticality had been achieved, because usually the first criticality takes place at an absolutely insignificant power level. Think of the power of a fly’s wings beating. Even that is probably more power than a reactor generates at its first criticality. It would seem as though nothing at all had changed. But the instruments monitoring the reactor would make it clear that something very special had happened.

Fifty years ago today, the Molten-Salt Reactor Experiment, built at Oak Ridge National Laboratory, achieved criticality for the first time. My friend Paul Haubenreich, who passed away on Easter Sunday this year, wrote a paper describing the event in 1968. He had nothing exciting to say about the moment of first criticality, because to him and the other reactor engineers, it was just the beginning of a campaign of research and improvement that would go on to show that the MSRE was based on sound scientific principles, and that future molten-salt reactors could be built from the same principles. If they utilized the thorium fuel cycle, they would be some of the safest and most efficient nuclear reactors that the world had even known.

Unfortunately, even after the success of the MSRE, about a decade later the program was cancelled and the research and development team went on to other projects. Paul went on to work on a fusion reactor program. But he told me that he always considered the molten-salt reactor the best program on which he ever worked.

We’re forty years behind schedule in bringing to pass the thorium-powered world of the future. MSRE showed the way. Let us make haste to follow up on its success.

ORNL Molten Salt Reactor Goes Critical
June 2, 1965

By DICK SMYSER
Pictures By J. Ed Westcott, AEC

A new experimental reactor, for which Oak Ridge National Laboratory scientists have high hopes, went critical—began operating—at 6 p.m. Tuesday.

While much of the rest of the city was casting last minute ballots in the municipal election or eating dinner, an intent group of scientists gathered at the reactor facility in ORNL’s “Reactor Valley” for a very significant moment—the start up of the Molten Salt Reactor Experiment—the MSRE.

And “criticality” now means that ORNL will enter its most exciting reactor experimenting period since the start-up of the Homogeneous Reactor Experiment several years ago. However, they feel more confident of the ultimate success of the MSRE than they did for the HRE.

The MSRE will now be operated at gradually increasing power levels. Then it will be shutdown for examination. Then it will be started up again and, hopefully, sometime in September, it will be “cranked up” to full power and begin producing power.

And, when it begins producing power, then it will be studied closely over a long period of time. Hopefully, it will prove eventually that this particular type of liquid-fueled “breeder” reactor will answer many of the world’s energy needs of 25 to 30 years from now.

The MSRE is one of the advanced power reactor concepts. It is termed a “breeder” because it would actually produce more fuel than it will consume.

Currently much success is being realized with power reactors and many rosy predictions are being heard about nuclear power at very competitive prices. However, it is generally accepted that these current very successful nuclear power concepts will, within a decade or two, be replaced by better concepts—primarily because of the heavy fuel requirements of the present systems.

The MSRE is, therefore, aimed at the power market of the 1980s, 1990s and beyond.

H. G. MacPherson, assistant director of ORNL and the local scientist most closely associated with MSRE, expressed delight today with the final manipulations leading up to Tuesday evening’s criticality. He mentioned particularly the smoothness of the loading of the fuel which began on Friday.

MacPherson recalled that the original idea for the MSRE came nine years ago, although actual design on this particular reactor did not begin until 1961.

The MSRE is an outgrowth of the Air Nuclear Project—the effort to develop a reactor engine for an aircraft. ORNL had developed an experimental reactor using the basic molten salt concept, but the effort was abandoned in 1957. Then, in 1960, the whole aircraft-nuclear project was discontinued.

The MSRE is actually built on the same site as was the ANP reactor although at the time the ANP reactor was abandoned it was not expected that another reactor would be located there. “Reactor Valley,” where the MSRE is located, is in an area away from the main ORNL area.

Alvin M. Weinberg, ORNL director, hailed the MSRE’s success today also. “We’re very optimistic about the future of this system, although from past experience we know that a difficult and treacherous path lies ahead,” he commented.

MacPherson said he feels that the MSRE has much more chance of success than previous ORNL power experiments have had. And, he agreed, much has been learned from the past experiments, including the HRE, despite its eventual abandonment as a specific concept.

Commented MacPherson, “I personally think that the MSRE technology is better explored and more friendly than was the case with the HRE and we would be very surprised to run into comparable difficulties.”

The HRE operated successfully for a significant period of time but then developed leaks.

Weinberg also pointed out that this is only the first of three new reactors scheduled to go critical here this year. The others are the High-Flux Isotopes Research Reactor and the Experimental Gas-Cooled Reactor.

Following is the official ORNL announcement of the “criticality”.

“A pilot plant for a nuclear reactor system that someday may produce more fuel than it uses by transmuting thorium to uranium has begun operation at Oak Ridge National Laboratory,

“The Molten Salt Reactor Experiment (MSRE), a 10,000 thermal kilowatt reactor which uses uranium dissolved in a molten carrier salt for fuel, achieved its initial criticality at 6 p.m., June 1, 1965.

“The MSRE will demonstrate the molten salt fuel concept which ORNL scientists believe holds potential economic advantages for producing electrical power and for thermal breeding. Breeding generally relate to a reactor concept that produces more fissionable fuel than is consumed in the nuclear reaction.

“Other advantages include improved steam conditions and high thermal efficiency because of high operational temperatures and lower fuel fabrication and fuel reprocessing costs because the fuel is in a fluid solution.

“The reactor will be operated to provide the first large-scale, long-term, high-temperature tests in a reactor environment of the fuel salt, graphite moderator and the high-nickel-base alloy Hastelloy-N. Operating data from the MSRE should provide important information regarding the feasibility of larger scale molten-salt reactors.

“The MSRE is a circulating-fuel, graphite-moderated, single-region reactor employing a
molten mixture of lithium-7, beryllium and zirconium fluoride salts as a solvent for uranium salts as a solvent for uranium or thorium-uranium fluorides.

“The fuel is pumped through 1140 channels in the graphite core—4.5 feet in diameter by 5.5 feet high—where fissioning occurs, heating the fuel to 1225 degrees Fahrenheit. The heat is removed from the fuel through an intermediate heat exchanger that used lithium and beryllium fluorides as the secondary coolant.

“Since power recovery is not an objective of this experiment, no electric power equipment is utilized. Heat produced by the reactor will be released to the atmosphere through an air-cooled radiator.

“During later operation of the MSRE, thorium salts are expected to be added as a part of the fuel solution to demonstrate the potential of breeding uranium-233.

“Molten salt reactors were first investigated at ORNL as a means of providing a compact, high-temperature power plant for nuclear powered aircraft. In 1954, an Aircraft Reactor Experiment (ARE) was constructed at ORNL which demonstrated the nuclear feasibility of operating a molten-salt-fueled reactor at high temperature. Fuel entered the ARE core at 1200 degrees Fahrenheit and left at 1500 degrees Fahrnheit when the reactor power level was 2.5 megawatts.

“Immediately after the successful operation of the ARE, the Aircraft Reactor Test (ART) was started at ORNL as part of the Aircraft Nuclear Propulsion Program (ANP). This test was discontinued in 1957 when the ANP Program was revised, but the high promise of the molten salt reactor type for achieving low electric power generating costs in central station power plants led ORNL to continue parts of the basic study program.

The studies resulted in the construction of the Molten Salt Reactor Experiment to investigate remaining areas of uncertainty that could be resolved only by actually building and operating a molten salt reactor.”