Posted by David Martin

General View of the UK MSFR. UK National Archives.

Deep in the vaults of the UK National Archives at Kew, South West London, lie a series of extraordinary documents which few have examined since they were composed in the 1960s and 1970s. Now declassified, the documents are the remaining records of Britain’s Molten Salt Reactor (MSR) programme, relics of a time when nuclear fission R&D was considered a priority by the British government.

While Alvin Weinberg’s more famous Oak Ridge National Laboratory (ORNL) MSR work progressed in the US, the documents reveal that Britain’s Atomic Energy Research Establishment (AERE) were developing an alternative MSR design across its National Laboratories at Harwell, Culham, Risley and Winfrith. AERE decided that there would be little to gain from replicating ORNL’s work, so they opted to focus on a lead-cooled 2.5GWe Molten Salt Fast Reactor (MSFR) concept using a chloride salt, as opposed to ORNL’s thermal spectrum Molten Salt Breeder Reactor (MSBR) which employed a fluoride salt.

The UK MSFR would be fuelled by plutonium, a fuel considered to be ‘free’ by the programme’s research scientists, who viewed the UK’s plutonium stockpiles as an asset rather than a liability. Despite developing quite different designs, ORNL and AERE seem to have had close contact during this period with information exchange and expert visits taking place across the pond.

Theoretical work on the concept was conducted between 1964 and 1966, and experimental work was ongoing between 1968 and 1973. Extensive experiments on molten salt fluid dynamics were undertaken in large test rigs, the results of which are compiled in the documents. Substantial work was also carried out on heat exchanger design and the lead coolant system. The programme appears to have been receiving government funding of around £2-3 million annually[1].

Correspondence between senior programme researchers reveals that the programme came to an end in 1974 when the government withdrew funding, in part as a response to the 1973 cancellation of ORNL’s MSBR work, and in part because the Prototype Fast Reactor at Dounreay, which went critical in 1974, was considered the priority for funding.

Tantalisingly, the documents show that in 1975 AERE were approached by Swiss and French nuclear researchers to form a joint European MSR programme. Without domestic funding in place, Britain was unable to join, leaving us to wonder about what might have been.

While it’s disappointing to see that the UK MSR programme ran aground after going so far, it’s also inspiring to learn about the level of commitment backed up with resources that enabled this work to be carried out over a ten year period. The documents are a sobering reminder of how much we’ve lost in the UK in terms of advanced fission expertise and political will.

The scale of the 2.5GWe MSFR, equivalent in output to two Sizewell Bs, demonstrates the huge ambition of the project. If the programme had continued and a commercial MSFR had started supplying the grid in 1995, removing the need for 2.5GWe of coal-fired power, this would have prevented 340 million tonnes of CO2 emissions — or nearly three quarters of the UK’s entire annual CO2 emissions in 2013. A single MSFR would have reduced UK carbon emissions by approximately 5% a year, every year, since 1995! A fleet of six MSFRs could have completely replaced coal in the UK’s energy mix.

The National Archives contain all of the experimental data and research findings. Despite this reactor’s role in reducing carbon and eliminating plutonium, those research papers gather dust. The relevant authorities must digitise and make freely available these papers, as soon as possible.

There is always hope that the UK MSFR work was not completed in vain. Current reactor programmes freely mine the work of the “golden age” of nuclear innovation in the ’60s and ’70s. For instance, the promising U-Battery reactor, designed by Anglo-Dutch company Urenco, is a modern iteration of the Dragon Reactor, designed, built and operated at Winfrith in the 1960s. The Chinese MSR programme has extensively mined the treasure trove of research findings accumulated during ORNL’s 1960s MSR programme.

Could a 21st century nuclear innovator make use of the UK MSR programme data, breathing new life into Britain’s forgotten molten salt reactor?

The AWF is presently in the process of digitising the MSFR papers which we hope to release for public benefit. This is a time-consuming process, so please bear with us.

The physical AERE MSFR papers are accessible to all, for free, at the National Archives, Kew. Search the National Archives at

Did you work on the UK Molten Salt Fast Reactor programme? If so, we’d love to hear from you. Email us at

[1] 2005 equivalent.

UK MSFR, Cutaway of Core and Intermediate Heat Exchangers.  UK National Archives.

UK MSFR – Cutaway of Core and Intermediate Heat Exchangers. UK National Archives.

UK MSFR - Size Comparison with CFR & MSBR.  UK National Archives.

UK MSFR – Size Comparison with CFR & MSBR. UK National Archives.


  1. Martin Kral says:

    Wow! I didn’t need to read this. It saddens me to know that this project was cancelled also.

  2. Keith Hawkins says:

    This could be a real contribution to Thorium reactor engineering. Chloride salt should be less corrosive and less hazardous than fluoride. (In the Petro-chem industry we phased out HF in favour of H2SO4 for processes such as alkilation for these very reasons.)
    …And, of couse we currently have some 110 tons of Plutonium to dispose of!

    • David Martin says:

      Thanks Keith. Yes, we believe the old research is a major new data source for MSR researchers worldwide. The data could help MSR projects “leapfrog” and avoid reinventing the wheel.

      AERE Harwell had some of the best chemists in the world at the time: the reprocessing flowsheets for this reactor are probably 20 years ahead of their time. It was an short-sighted act of scientific vandalism to close down Harwell, Winfrith, and the other AERE sites.

    • Khan says:

      excuse me a sec but fkng obsidian won’t penervt the explosion? O.op.s. regarding the solar flowers. You get half output with that interception cause current gets halved each deviation (in 1 or more cables); if that’s not changed since IC1. You should put at least batpack at each interception.

    • Charles Beaudry says:

      Yes, indeed, one of the important issues with the thorium MSR design is the need for the initial “start-up” fuel to get the U233 breeding process going. Plutonium could easily supply the initial charge, after which the reactor would run for a long period subject to topping off the Th232 supply from time to time. The issue of the moderator also needs to be addressed as most designs using graphite have concluded that a practical life span of about 8 years is predicted before the reactor would need to be stripped down and the moderator replaced, obviously a notion that is at odds with the whole concept of a “maintenance free” reactor.


  3. John Preedy says:

    I’ve never seen anyone write about this project before. Presumably it was classified, so that it was never discussed apart from with security cleared personnel in the UK and overseas. Fantastic that the AWF is going to publish a digitized version. What a shame that like ORNL’s MSRE it was closed down.

  4. Mark A says:

    Dear Weinberg Foundation,

    Please scan and openly post design details, research, and data for the MSR world communities and the private firms pursuing such endeavors.

    MSR Advocate

  5. Charles Booth says:

    Below is a draft letter to George Osborne. I believe he has the power to do something about it. This article indicates we have quite a lot of the know-how, but it is likely to reside in the minds of people who are quite old and may not be around for much longer. This needs to be released before it is too late.
    Please copy it to your M.P.

    Dear Mr Osborne,

    The subject is clean electric power.
    The only way to generate it on demand without using fossil fuel in the present state of knowledge, is to use nuclear fission.
    Present nuclear fission reactors are based upon old designs whose objective was to co-produce Plutonium for military use
    Research into other designs was suspended in order that we could win the Second World War and maintain our deterrent in the subsequent Cold War.
    Amongst these other designs, was the molten salt reactor, which consumes its nuclear fuel much more completely than traditional designs and could be capable of using up our stockpile of “nuclear waste”.
    I believe you have the power to create a financial climate which stimulates research into such a reactor.
    Success of such a project would not only enable us to generate power both cleanly and cheaply and also rid us of an embarrassing pile of nuclear waste, but also give Britain a saleable technology.
    Failure do follow such a course of action, would leave India and China to become experts in the field and enable them to sell the technology to us rather that the other way round
    Baroness Worthington in her capacity as patron of the Weinberg Foundation has access to much of the technology which needs to be developed
    Can you please do something about it

  6. Timok says:

    Has anyone considered that the UK effort in this area of research may have been stopped for good technical and commercial reasons? The technological challenges making a MSR work commercially are very considerable, and reading around on this Weinberg Foundation web-site makes me appreciate that much of the material presented is merely old material which has been reformatted. There seems to be little true innovation over original ORNL work. There are mentioned discussion groups, folders of documents, historical perspectives and so forth which merely superficially scratch the surface of the volume of work required to make a commercial MSR a reality. Note the UK MSR effort was focused on Plutonium P239, the most radiotoxic element of all.

    The technical challenges are so great for MSR (LFTR), that it is unlikely that MSR will become a commercial reality to assist with anthropogenically-forced climate change issues. In the interim period, renewable energy system will develop considerably, and then other promising technologies such as LENR will then evolve, which seem to have none of the horrendous hazards associated with MSR (LFTR).

    The reality is that MSR (LFTR) is outdated old technology from the 1950’s which was never pursued, other than an experimental ORNL reactor which identified a whole range of issues, one of which is corrosion. The ORNL reactor did not function long enough for the issues of neutron embrittlement to be properly researched, which would be a severe problem for a commercial MSR designed to operate over several decades.

    It is worth bearing in mind that there are 77000 tonnes of high-level nuclear waste in the USA, and 17000 tonnes of such waste in Japan, some at the stricken Fukushima Dai’ichi site (which is rapidly becoming a radioactive swamp as coriums of reactors 1 to 3 continue to bore into the ground towards the water table there (with horrendous eventual consequences)).

    I respectfully suggest that Thorium LFTR may have a potential role in transmuting present nuclear waste to less harmful materials, but Thorium LFTR for power generation is unlikely to be commercially competitive in comparison to LENR when evolved. There is mention of small modular Thorium LFTR:
    (i) where are the highly skilled staff to run these units going to come from?
    (ii) bomb making materials can be potentially extracted from the continuous chemical processing needed for a Thorium LFTR – who will police that these do not end up in dirty nuclear bombs (note: the USA in the Manhatten project exploded a U233 bomb)
    (iii) who will insure a Thorium LFTR? – a major accident with a small Thorium LFTR would be particularly hazardous due to the hard Gamma-radiation involved? – in practice, a Thorium LFTR in serious failure would simply have to be abandoned for 300 yeas, because it would be too hazardous to try to clean up the mess,

    and the issues go on. To say that Thorium LFTR will never suffer a breakdown in operation is simply unrealistic. A Thorium LFTR is not “divinely blessed”, such that, by some sort of magic, it will never go wrong and never break down in operation. Accidents will happen with Thorium LFTR, mututis mutandis for example as at Fukushima Dai’ichi, in adverse conditions.

    Above may be disturbing and uncomfortable for naive advocates of Thorium LFTR. However, to ignore these aforementioned issues would be grossly unprofessional and misleading the public. You must present the other side of the argument for and against Thorium LFTR, and MSR in general.

    • Charles Beaudry says:

      ” horrendous hazards associated with MSR (LFTR)”.

      I find this line of reasoning to be suspect, simply because molten salts are used everyday, all over the world, in numerous industrial processes. Take for example the solar-thermal power plant, which uses arrays of solar reflectors to concentrate solar radiation onto a tower which heat up – get what? – a molten salt which is then passed through a heat exchanger to produce steam and electricity. Environmentalist are fine with this technology but they don’t like it if the MSR is part of a nuclear process. Very strange logic indeed. It is more likely that you just hate nuclear. Forget the data, forget the reality that renewables simply cannot replace the coal-fired power plant; anyone who has done the math knows this as a self-evident truth. Who care about data when we can live with the certainty that something which cannot happen will in fact occur.
      All the potential problems of the thorium MSR are nothing compared to the impossible mathematics presented by the environmental lobby in support of their cherished dream of a sustainable future.


    • Mark Pawelek says:

      Worldwide, there’s over a ¼ million tonnes of ‘high-level nuclear waste’. Over 95% of this is suitable for reuse as fuel since only 3.5% of the original energy has been used. The whole point of a fast spectrum MSR is that is burns up ‘high-level nuclear waste’ leaving only fission products, which are radiotoxically safe after 300 years. Instead of using 3.5% a LFTR will use 99%. A combination of thorium, uranium and plutonium will work in the fast spectrum. The Transatomic Power WA-MSR is specifically designed as a “waste annihilating” reactor. It’s a mystery to me why so many people pontificate over the danger of ‘nuclear waste’ but get all snooty when someone proposes to deal with the ‘problem’. I wish someone would explain this paradox to me.

      The whole point of a “thorium LFTR (sic) breaking down in reality” is that is has no conceivable potential for wide-scale radioactive contamination. Everything will be contained locally (within metres!, not within miles).

    • daniel jackson says:

      Skillful liar “USA in the Manhatten project exploded a U233 bomb” totally wrong, in fact first thing noted, before Fermies Pile December 1942, was associated U232 isotope that was so highly radioactive, as to make the Thorium route to a nuclear bomb not possible using 1940’s technology……Of course you knew that …

  7. Alex J. says:

    Often in the news, there are billionaires who are knocking on a bit.
    Wont just one opt for immortality by mega funding this research?
    Remember that Carnegie said, ”A man who dies rich, dies disgraced”!

  8. David Meigh says:

    It’s hard to understand how the LFTR research was abandoned at the same time as at Oakridge where they had a 10MW reactor working for 4 years before Richard Nixon terminated the research in favour of a system generating bomb material. It offers a possibility for cheap electricity to challenge coal, gas and oil. But, it is not the only British inventor option that was terminated. There was John Seales electromagnetic generator, Fleishmans unfortunately called cold fusion experiments and in the US focused fusion and Stan Meyer’s hydrogen generating car. Could it be that vested interests are preventing us from generating power and heating at home and using vehicles that don’t use their expensive and highly taxed fuel?

  9. Ian Darley says:

    This technology needs to be developed and implemented quickly. If China pips the West to the post with and patents the I.P. rights we will be forever slaves.

    • David Buchanan says:

      Whilst wholly supportive of basic science and research and the value in at least trying to achieve a working design, I would argue that we shouldn’t worry too much about China getting there first. They have never respected the patents of other countries, so why should we respect theirs.

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