In depth – History

Why have we not already built thorium-MSR’s?

A valid question that is often asked when discussing molten salt reactors is ‘if this technology is so promising, why was it not developed in the past?’ This question is often answered with the assertion that thorium or the derived 233U is less suitable for nuclear weapon production, but this claim has not been supported by historical evidence. Hargraves (2012) and Macpherson (1985) explain that the molten salt reactor program at Oak Ridge was stopped by the Atomic Energy Commission (AEC) in favour of the Liquid Metal Fast Breeder Reactor (LMFBR) program. This type of reactor would be able to optimize or even ‘close’ the U-Pu fuel cycle and at the time the assumption was that the LMFBR was much closer to market than the thorium MSR, the latter being a new and generally unknown ‘peculiar’ system, that would involve a new nuclear fuel cycle. Both the thorium MSR and the LMFBR are breeder reactors. These have a positive neutron economy, which allows them to produce more fissile material than is put in: they breed fissile fuel. A fuel has to be able to produce more than 2 neutrons to sustain the fission reaction and fissile material generation. Fast breeder reactors on plutonium are much better suited for breeding than thorium molten salt breeder reactors as they emit more neutrons. On the other hand, a high breeding ratio is not always required or an advantage. Designs have been proposed for MSR’s that can ‘iso breed’: these will have the ability to breed exactly the amount of fuel they need to operate.

The LMFBR was also seen at the time as a good complementary system for the light water reactor, that had been the first nuclear reactor to leave the laboratories. It had been built for the nuclear submarine Nautilus in the mid-50s. Two decades later, the concept had been successfully introduced for civilian power generation, in a world that wanted nuclear. At the time, the light water reactor was already on its way to become the standard nuclear reactor for the energy market. Once established, a technology becomes difficult to change, as a whole industry is built around it. This is even more true for nuclear power, where the switching costs are high, due to the high cost of nuclear research and development.

Many commenters, including the LWR’s inventor Alvin Weinberg, have considered the LWR as humanity’s prototype reactor, ‘our first try’. In the course of nuclear history, several efforts have been made to move past LWR’s, that many have regarded as a choice that had more to do with historical circumstances than with the technology being optimally suited for the purpose of civilian power production. France for instance started nuclear development with high temperature gas-cooled system development, but later moved to LWR’s. Germany developed the HTR in the 1980s; this is now a Chinese concept.

The LMFBR has been abandoned by most countries in the Western world, with the exception of France. But one of the reasons for building LMFBR’s is still as relevant as ever: the implementation of a technology that is able to create a closed nuclear fuel cycle. Thorium MSR’s offer the possibility to realise this closed cycle on the basis of a thermal spectrum reactor, thus eliminating the need to overcome the substantial challenges of building safe large-scale fast reactors.

The Molten Salt Reactor Experiment at Oak Ridge

Molten salt reactors were first proposed in the years after WOII as a means to power an aircraft. In 1959 an assessment of high potential civilian reactor designs was held by the United States Atomic Energy Commission (AEC) which cumulated in efforts of the now famous Oak Ridge National Laboratories (ORNL) in designing a functional molten salt reactor. The experiment came to be known as the MSRE (Molten Salt Reactor Experiment) and was led by Dr. Alvin Weinberg.

Design of the MSRE started in 1960 and construction started in 1962. By mid-1965 the reactor went critical for the first time. A 30 days continuous operation run was completed by 1966 and continued to operate for 15 months whilst experiments were held. The MSRE was also the first reactor to be fuelled with 233U instead of 235U. The MSRE was judged a very successful experiment by its engineers as it posed little problems. The only obstacle encountered by the ORNL staff was the radiation hardening of the Hastelloy-N material used in the reactor. Tritium reaching the steam turbines was another concern of the engineers but it was ultimately resolved by an intermediate salt coolant of sodium fluoride and sodium fluoroborate which would capture the tritium. Lastly tiny cracks in the Hastelloy-N piping were discovered due to tellurium (a fission product) attacks. The solution to this problem was altering the chemistry so that 2% of the uranium was made of UF3 instead of UF4, which is a simple solution with modern analytics and beryllium metal additions.

It is also important to note that commercial interests and accompanying studies proved successful at the time.

The demise of the MSR experiment

Despite successful tests and reworked designs of the MSRE the proposed design of a second test was rejected by the AEC in 1970 and again in 1972 due to budgetary reasons.

The demise of the MSR was not due to technological reasons nor the often misquoted necessity to build more nuclear weapons for which vast amounts of plutonium were needed. Instead former director of ORNL MacPherson states:

1. “The political and technical support for the program in the United States was too thin geographically. Within the United States, only in Oak Ridge, Tennessee, was the technology really understood and appreciated.

2. The MSR program was in competition with the fast breeder program, which got an early start and had copious government development funds being spent in many parts of the United States. When the MSR development program had progressed far enough to justify a greatly expanded program leading to commercial development, the AEC could not justify the diversion of substantial funds from the LMFBR (liquid metal fast breeder reactor) to a competing program – Macpherson -1985”

Alvin Weinberg later said: “It was a successful technology that was dropped because it was too different from the main lines of reactor development”.

In the 1970s, nuclear increasingly became the subject of societal criticism. The bigger fast reactors turned out to have safety issues that were more difficult to solve than originally thought. Budgets went down and alternatives could no longer be supported.

The LMFBR which ended the MSRs career prematurely also underwent a tragic fate after president Carter on April 7, 1977 deferred from commercial fuel reprocessing in the USA. The LMFBR was also deemed too expensive compared to a LWR. Ironically the MSR was much cheaper with an estimated 1% of capital cost compared to the LWR in 1970. However the cost of machining tools, remote maintenance of radioactive primary systems and decommissioning were still unsure at the time (MacPherson, 1985)(Weinberg, 1994)(Mahaffey, 2014).

Also, the uncertainty of a new development program in combination with large investments has often proved to be an effective stopper for promising new technologies, especially when there is also the conviction that the existing technologies, the LWR in this case, are perceived as adequate. The accidents at Three Mile Island and Fukushima have changed this perception for many. The conviction that in the long run we need a better technology has recently revived the interest in MSR’s.