The Frisch-Peierls memorandum

As had others before him, Peierls had computed the critical mass of a natural uranium sphere to be of the order of tonnes, showing that a nuclear weapon constructed this way would not be practical. Following a suggestion by Bohr, Frisch then used Peierls' method to calculate the critical mass of a sphere of pure U235, getting a result of a few kilogrammes. Frisch and Peierls then wrote their famous memorandum, which begins

The attached detailed report concerns the possibility of constructing a ‘super-bomb’ which utilises the energy stored in atomic nuclei as a source of energy. The energy liberated in the explosion of such a super-bomb is about the same as that produced by the explosion of 1,000 tons of dynamite. This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city.

In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effects of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.

Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.

This memorandum was written in March 1940 and was the first clear demonstration that a fission weapon was practical.

The MAUD committee

As a result of the Frisch-Peierls memorandum, the MAUD committee¹ was formed, meeting first on the 10th April, 1940. Its initial members are reported in the public literature as Thomson, Chadwick, Cockcroft, Oliphant and Moon: there were, of course, others: I will not not mention them here.

Work was done under the supervision of the MAUD committee at five principal locations: the open literature mentions only four of these. The locations were

• Liverpool (Chadwick);
• Oxford (Simon);
• Cambridge (Bragg & Cockroft);
• Birmingham (Peierls);
• Glasgow (Innes & MacKay).

The work done at Glasgow is what concerns us here, of course, as its results lead to the foundation of the programme. The nature of the investigations will be well known to most readers but, in summary, it concerned the possibility of sustaining fission in a gas (uranium hexaflouride being considered at the time) under suitable dynamic compression, and the use of this technique to create rocket motors with very high specific impulse. This approach was rediscovered, perhaps independently, much later and is now known as a fission-fragment rocket². There is current research in this area being done by NASA. This approach is related to but distinct from a nuclear thermal rocket, which makes use of a fuel which is not itself a fissile material.

In June 1941 the MAUD committee produced a report in three parts, of which two are now known in the open literature: the first part on the possibility of a fission weapon and its details; the second on the possibilities of nuclear power generation; and the final part on the possible use of fissile rocket fuels for rocket propulsion. The final part has never been publicly released. The society has several original copies, but it is unknown whether copies exist in the national archives: certainly none are acknowledged.

Tube alloys

In response to the report of the MAUD committee the programme known as ‘Tube alloys’ was launched in the UK and Canada in September 1941, with its primary objective being the development of nuclear weapons. The history of that part of the programme is well-known, and I will only describe certain events important to the fuel-development programme in any detail here.

The Tube Alloys consultative council oversaw the project. Chaired by Anderson and also consisting of Lord Hankey, Lord Cherwell, Sir Edward Appleton and Sir Henry Dale. The technical committee was Akers, Chadwick, Halban, Peierls, Slade & Innes, with MacKay and others joining later.

Isotopic separation to produce U235 was investigated with a pilot plant opening in mid 1943 and another, under Innes, later in 1943 in Scotland.

Plutonium production was investigated and it was realised that Pu239 could be produced as a byproduct of nuclear reactors. The problems with gun-type weapons and plutonium were understood. In 1943 Chadwick learned from the Americans about the possibility of an implosion weapon: the type of weapon first demonstrated in the Trinity test and then later dropped on Nagasaki.

Heavy water was investigated, initially at Cambridge and then later at Montreal.

Innes and MacKay continued to work on what would become the fission-fragment rocket in Glasgow. It became increasingly clear that such a device could be fueled by enriched gaseous uranium hexaflouride, at a considerably lower level of enrichment than required for a bomb. The use or plutonium was also investigated although this seemed to be problematic.

It rapidly became clear that the focus of the Tube Alloys program would be the construction of one or more bombs, and that a fission-fragment rocket would not be needed to deliver such a weapon. Innes argued strongly that research and development of such a rocket should nevertheless be continued, both as a long-distance high-speed missile for delivery of conventional and perhaps nuclear devices, and also possibly for space exploration after the war which was, of course, the primary interest of both Innes and MacKay.

But Innes and MacKay, and the entire Scottish component of the programme fairly quickly became peripheral, and their funding, never really sufficient, became at best tenuous. In part this was for what must have seemed good reasons: although the specific impulse of a fission-fragment rocket is extraordinarily high, it was not then clear that the actual thrust was sufficient. Additionally jet engines were arriving and, although they are only useful within the atmosphere, they have effectively unbounded specific impulse as they do not carry their reaction mass with them. A jet-powered missile – something that would now be called a cruise missile – seemed a preferable option. The German V2 programme was not then known about, at least not by anyone involved in Tube Alloys. However it is clear that the sidelining of Innes & MacKay was at least in part due to prejudice by other people in the programme: the society has a number of documents in its possession in which this is made explicit.

Innes and MacKay, however, had other sources of funding, and their research continued, although increasingly separate from the rest of the programme. More on this below.

The Americans

Tizard went to the US in 1940, but shared very little information with the Americans at that point. In particular he did not share the Frisch-Peierls memorandum with them.

Lauritsen (Caltech) was invited to sit in on at least one MAUD meeting, and returned to the US where he briefed Vannevar Bush, of Memex fame.

Oliphant went to the US in 1941 and told Coolidge about the 10kg estimate for a device. Coolidge was very surprised by this and it later became clear that much of the information from MAUD had not been read in any detail by people in the US, even when it was available to them.

Oliphant, on the same trip, then told a number of other people about aspects of what would become the Tube Alloys programme. Significantly he withheld information about the Scottish fission-fragment rocket proposals from them at this time, and we believe throughout. Bush took this information to the President.

By early 1942 the Americans were actively working on the problem. Several British scientists, including MacKay, went to the US where information was shared. Again, we believe that MacKay did not share information on the fission-fragment rocket idea. By mid 1942 Leslie Groves was in charge of what was to become the Manhattan project, and there were increasing levels of paranoia on the American side. Information sharing slowed dramatically, and in October 1942 stopped altogether, after Bush and Conant persuaded Roosevelt that the British were not to be trusted.

Tube Alloys fell significantly behind the Americans by July 1942. British scientists no longer visited the US, which in turn hurt progress in the Manhattan project. Collaboration had effectively stopped by late 1942: this damaged both projects seriously, but the damage was terminal for Tube Alloys.

In March 1943 Conant persuaded people that help from the British would benefit the program: something which should have been extremely obvious well before that time. In July 1943 the Quebec agreement was signed. Under this agreement the US and UK agreed that they would pool their resources to develop nuclear weapons and that neither country would use them against the other, against other countries without mutual agreement, or would pass information about them to other countries. The US, starting from a position of strength as it did, also gained a veto over post-war commercial or industrial uses of nuclear energy in Britain.

Following the Quebec agreement, Tube Alloys was almost entirely subsumed into the Manhattan project, and those parts of it effectively came to an end at that point.

Notably absent from the Quebec agreement is any mention of the work being done in Scotland, of which the Americans were unaware and which the English side increasingly regarded as peripheral. Significantly the American veto only covered commercial or industrial use of nuclear energy and clearly this could be argued not to cover its use, for instance, for spaceflight. It is unclear if this omission was intentional: were the British representatives still taking account of the work of Innes and MacKay and wishing to protect it, or had they simply forgotten it was continuing? This is an area where further research is needed.

After Tube Alloys

Note. Some redactions have been made to the public version of this section. In particular the locations of the test and development sites have been redacted³. This is for three reasons:

• they are often still heavily contaminated and there would be considerable risk to anyone exploring them who did not understand the correct precautions to take;
• many are on private land and the owners of the land have given the society access on the understanding that we do not reveal the location of the areas in public;
• the sites contain irreplaceable and often fragile artefacts from the early days of the programme, which we do not wish to be damaged or removed.

There is no need to discuss the development of fission and later fusion weapons here: there are many good sources of this information. What is of interest to us is the work done in Scotland.

By early 1943 almost all official funding to the Scottish group had ceased. Innes and Mackay were no longer invited to meetings of the committee after December 1942. Although the details are unclear, it seems likely that the remaining committee members simply assumed that the work being done in Scotland would cease. However they were remarkably lax about this, especially given the existence of the pilot isotopic separation plant outside Glasgow. Perhaps they were simply unaware of how far the work had got by this point: certainly relations were bad enough that technical communication had all but stopped, on both sides. Almost certainly they were also unaware of the other sources of funding to which the Scottish group had access.

However work did, of course, continue. Developments after the war will be well-known to our readers. Here, then, is a sketch of what happened in the roughly two years from early 1943. Few documents survive from this period, so what we can provide is no more than a rough and approximate outline, which may well be incorrect in places.

After the separation from Tube Alloys, the group decided to focus on the original interests of its principals: the possibility of using fission-fragment rockets for space exploration. Scotland was considered an ideal location for a program like this: large parts of the country have a low population density, thus providing large areas for tests of systems which were inevitably radiologically rather dirty. The consequences of any accident would also be minimised. Needless to say, the cavalier attitude to safety demonstrated both at this time and later by the programme is not something that would be acceptable today: attitudes were very different then.

Enriched uranium was decided upon as the fuel, as plutonium production would require development of a fission reactor capable of producing it, an expensive proposition and one which would be hard to conceal or justify. The existing isotopic separation plant was moved away from Glasgow to [redacted] and considerably scaled up.

While this was happening work proceeded on the engineering design of the rocket engine. A number of prototypes were developed and tested, not using the special fuels. It was clear that the construction of a working engine was at best extremely challenging, and might be beyond the engineering and materials technology of the time.

In early 1944 there seems to have been a breakthrough in engine design. The details of this are not fully understood, although it seems to have involved the use of ceramic materials. However it is very unclear if this is the same approach that was used for post-war designs, or whether the approach was abandoned. We are hoping to recover debris from one of these engines from the test site at [redacted] which would allow us to understand the exact nature of the breakthrough. This is challenging, however: the site is sufficiently contaminated that remotely-controlled vehicles would be needed, and these vehicles would themselves then be too contaminated for further use.

Development of suitable fuels proceeded in parallel, based on a plant near the separation facility at [redacted]. It is unclear if the scheme of using a chemical propellant to spin the engine up before injecting the special fuel was being used at this time. It seems unlikely as this requires the use of chemical monoropellants of which there is no evidence at this stage of the programme. It is therefore unknown how these early engines were started.

These engines were tested three times in 1944 and 1945, using the special fuels. The first two tests resulted in destruction of the engine. The third test was a success however: the engine ran for several minutes at low power, and for approximately ten seconds at full power. The engine survived the test intact, although it was later disassembled for inspection (something that would not now be considered given the radiological state of an engine which has run for any length of time). Unfortunately the final fate of this engine, or its components, are not known: rumours persist that it was reassembled and used as the upper stage engine in the first flight test ten years later, although there is no good evidence for this.

Although the third test resulted in less contamination than either of the first two, it was clear even then that using these engines at ground level was too dangerous to consider. Given the questions about adequacy of available thrust (since resolved), the decision was made to undertake a two-stage design with a first, chemical, stage lofting a fission-fragment upper stage high enough into the atmosphere that the contamination from it would be widely dispersed.

It seems that at this point, in March 1945, active development of the engine was paused: it clearly could be made to work well, and the need both for a chemical lower stage engine design, the overall vehicle design, as well as increasing the output from the separation and fuel plant took precedence. It is remarkable how little time this early development took: from the initial breakthrough to a successful test was little over a year. The intention was that fuel production should continue at pace, with the fuels being stored until needed. We are still living with some of the consequences of the decision to store special fuels for long periods of time.

This brings us to mid 1945 and the end of our story. In June 1945 the following work was underway:

• construction of a full-scale isotope separation plant and fuel production facility, to be located at [redacted];
• fuel storage facilities at the same location;
• first-stage engine design (no engines, not even small-scale prototypes had been built by the end of the war);
• initial design for a vehicle.

It remains unclear whether it was, at this point, the intention that these vehicles should carry humans. Given the primitive state of automation in 1945, it seems likely that they understood that humans would be needed for any extended flight. If this was understood presumably at least some work was being done on life-support and other requirements to carry humans.

Trinity

On the 16th of July the Trinity test took place in New Mexico. Later the same day, Innes wrote a memorandum which circulated within the project. In it, she wrote

The reports from New Mexico today tell us, beyond all doubt, that our choice to work exclusively on the use of atomic fission for peaceful purposes is the correct one. Furthermore, given the appalling possibilities raised by the Trinity test, it seems likely that if mankind is to survive beyond the next century it will not do so on the planet which gave our species its birth, but elsewhere. We must proceed apace with our work, which I believe offers by far the best chance of establishing a presence on other planets in our solar system and, perhaps, elsewhere.

These words remain true today.