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50th anniversary of U-233 start of MSRE

October 8, 2018, marks the fiftieth anniversary of the operation of the Molten-Salt Reactor Experiment (MSRE) using uranium-233 as a fuel. U-233 does not occur naturally; it is formed when thorium absorbs a neutron undergoes a double beta decay to form U-233. U-233 is a superior nuclear fuel, producing enough neutrons through its fission (whether by a fast or thermal neutron) to allow sufficient conversion of thorium to U-233 to replace its consumption. That makes it very unique and very valuable.

Alvin Weinberg and the other researchers on the Molten-Salt Reactor Program (MSRP) at Oak Ridge National Laboratory (ORNL) recognized this property of U-233 and sought to demonstrate its actual use in a real nuclear reactor. The MSRE was designed for this purpose. When the MSRE was first brought to criticality in June 1965, this was a great accomplishment, but the MSRP researchers had something more in mind. Therefore, after a few years of operation, they removed the initial uranium inventory from the reactor by fluorination and replaced it with uranium-233.

To commemorate this accomplishment, they invited Dr. Glenn Seaborg to ORNL to be the one to first take the MSRE to a significant power level on uranium-233 fuel. Seaborg’s participation was significant on several levels. At that time, Seaborg was the chairman of the Atomic Energy Commission (AEC) which had funded the development of the MSRE. But even more importantly, it was Seaborg who had a led of team of chemists at the University of California, Berkeley, to discover uranium-233 in the early days of the Manhattan Project. Seaborg was the first person to grasp the potential of thorium as an energy source when he received information about the performance of uranium-233 in the ORNL Graphite Reactor in late 1944.

At that time he recorded in his journal:

“These values are within the range to enable U-233 to be made from thorium by a chain reaction on the U-233—to make breeding possible— extremely important because it may make it possible to be independent of uranium once a supply of U-233 for starting purposes is on hand.”

Alvin Weinberg stood by Glenn Seaborg as he removed the control rod that allowed the MSRE to achieve significant power (100 kilowatts) on uranium-233 for the first time. Also at his side was Ray Stoughton, a chemist who had helped Seaborg to identify uranium-233 at Berkeley 26 years earlier.

The successful start of the MSRE on uranium-233 led to 14 months of productive research activity on this fuel in the MSRE. U-233 had a smaller delayed neutron fraction than the U-235 fuel that the MSRE had previously used. A circulating liquid fuel also meant that some of these delayed neutrons were released outside of the core region of the reactor, where they did not participate in the control of the reactor. Operation of the MSRE with U-233 showed that neither of these effects precluded successful operation of the reactor, and paved the way towards a future of thorium-powered reactors.

Indeed, Weinberg and others had very high hopes for the future of thorium after the U-233 start of the MSRE. But unfortunately, it was not meant to be. The AEC had funded a very public, very expensive effort using plutonium fast-breeder reactors with liquid sodium metal coolant, and the existence of the MSRP was viewed somewhere between a distraction and an embarrassment for that program. The AEC Director of the Reactor Division, Milton Shaw, ordered the shutdown of the MSRE by the end of 1969, and not many years after the reactor was shut down, the entire program was cancelled.

In his memoirs, Glenn Seaborg conceded that pursuing the plutonium fast-breeder rather than Weinberg’s thorium molten-salt reactor might have been a mistake, and that most of the arguments that the AEC had put forward in favor of the fast-breeder were in error. Weinberg and others always viewed with regret bordering on bitterness the cancellation of their marvelous work on the thorium molten-salt reactor.

But fifty years later, there is a substantial development effort towards thorium molten-salt reactors in China and a strengthening effort towards the technology in the United States. Weinberg’s dream will yet come to pass, even if it has been delayed by fifty years. Weinberg called this thorium-fueled, molten-salt-powered future the “Second Nuclear Era”, and in his autobiography he stated that his greatest regret was that he would not live to see it. Weinberg died in 2006, and Seaborg died in 1999.

8 thoughts on “50th anniversary of U-233 start of MSRE

  1. Kirk Sorensen,
    You need to contact Taylor Wilson, you two guys are on the same page, and need to talk if you have not already

    Mike Kertesz MSEE

  2. Hi Kirk
    Glad to have come across your blog and recent articles here and so to know that your heart is still beating to promote and the LTF reactor as the energy solution for the future ! I’ve been fascinated by your presentations and your argumentations for Thorium e.g. TED Talks, TEAC4A, which with my limited technical knowledge sounds incredible convincing. However, it also make me wonder WHY things have moved faster forward towards buildings Thorium based reactors. Is it because there’s still some technical difficulties to overcome as the below commentor to your TEAC4 video argues [1], or is it more a question of politics, of resistance from the traditional reactor industry and their stakeholders ?

  3. MSTR are the future of nuclear power in the USA. But money needs to be applied for pure research. The USA should lead the world on this research !!

  4. I’m onboard.
    What can we do to get ahead of the Chinese ?
    It’s been a long time, I haven’t spent the night in a Holiday Inn Express, but I did study Nuclear Engineering at the University of Arizona.

  5. I have heard that Protactinium (233Pa) has a tendency to capture neutrons if left in the core, and then becomes “something else” instead of the fissile 233U. 233Pa is also very radioactive and difficult to handle.
    Has there been any progress on a method of removing 233Pa from the core and storing it for the required 27-days, and then separating the 233U back into the core?

  6. Hi Kirk,
    I too have followed your work for quite a while, and I am impressed by how far you have taken the idea, and your passion for leading this development to a working prototype.
    When I do an internet search for companies trying to develop a thorium based product for power generation, Flibe Energy, is often not referenced. Is there a reason for this?

  7. I have a couple of questions about safety. I’ve recently been reading about the Chernobyl disaster, which prompts the following questions.

    In the article above, it says that U233 has a smaller fraction of delayed neutron production. Since it is the delayed fraction that provides the ability to control the reactor, does this make a LFTR reactor less stable than a U235 reactor? Is there still enough of a fraction of delayed neutrons to allow sufficient control plus a generous margin of safety? The RBMK reactor in Chernobyl was inherently unstable in low-power regimes.

    In most discussions about LFTR safety, emphasis has been given to the LFTR’s ability to shutdown in event of external power failure, but Is it also immune to exceeding its designed maximum power output if control rods cannot be inserted for some reason? Can a failure of this type cause a prompt-neutron surge, thus able to take the reactor outside effective control? Although the operating pressure is low, could any overpower event result in a spike in pressure or rapid breakdown of the reactor vessel walls due to accelerated corrosion or the like?

    Are there scaling effects that need to be taken into account with a LFTR? For example, if the reactor is scaled up significantly does it become less controllable? It is my understanding that scaling was one of the complicating factors in the RBMK reactor accident. In that accident, nuclear activity was not uniform throughout the reactor, but because of the scaled up size some sections of the reactor could be supercritical while other areas could be critical or sub-critical. I would think that with a liquid fuel, a LFTR’s nuclear activity would be fairly uniform, is that a valid assumption?

    Since all nuclear power is dependent on the physical arrangement of the fuel particles within the reactor, are there possible design ‘gotchas’ that could render a reactor unsafe? If so, is there any possibility that an event outside ordinary design limits can modify the internal arrangement so that the reactor becomes more dangerous? (Could graphite voids or excessive wear within flow channels cause a problem?)

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