As a World War rages, humanity stands on the precipice of a new era in warfare. The atomic age is dawning and Earth's great powers are determined to secure their place in it. In these episodes of True Spies, Sophia Di Martino meets the spies who shared nuclear secrets that still influence global geopolitics today. In Part 1, Professor Emeritus of Theoretical Physics at Oxford University, Frank Close, tells the story of the scientist who helped Soviet Russia build the bomb: Klaus Fuchs.
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True Spies, Episode 160 - Atomic Spies, Part 1: The Klaus Fuchs Effect

NARRATOR: This is True Spies, the podcast that takes you deep inside the greatest secret missions of all time. Week by week, you’ll hear the true stories behind the operations that have shaped the world we live in. You’ll meet the people who live life undercover. What do they know? What are their skills? And what would you do in their position? I’m Sophia Di Martino, and this is True Spies, from SPYSCAPE Studios.

FRANK CLOSE: As a weapon, the super bomb would be practically irresistible. There was no material on earth that would be able to resist the force of the blast. And the only defense against such a weapon would be to have one yourself. 

NARRATOR: Atomic Spies, Part 1: The Klaus Fuchs Effect. This story is about the race to build an atomic weapon. It’s also a story about morality, and why some of the finest scientific minds of their generation became involved in Soviet espionage, leading double lives and risking it all. Perhaps the most prolific of all those spies was a scientist called Klaus Fuchs.

FRANK CLOSE: Eugenia ‘Genia’ Peierls regarded him as one of the most honorable men that she ever knew, which is quite astonishing. 

NARRATOR: A spy and a gentleman. This is Professor Frank Close. He’s been piecing together Fuchs’ incredible story in his book Trinity. 

FRANK CLOSE: Trinity was the codename for the first test of an atomic bomb in New Mexico in 1945. 

NARRATOR: Frank is professor emeritus of theoretical physics at the University of Oxford and a nuclear physicist by training. Notably - part of that training was under the infamous Rudolf Peierls.

FRANK CLOSE: Those were the days when we as students knew that Peierls had been involved in the development of the atomic bomb during the war. But it was still very secret. 

NARRATOR: If we think of this story in terms of an atom, then Rudolf Peierls could very well be seen as its nucleus. Peierls is thought of by many as the ‘father’ of the atomic bomb, but it was also Peierls who first invited a young German emigre, Klaus Fuchs, to join him in Birmingham, England, to work on the top-secret British atomic program in 1941. A decision that changed the course of history - and broke Rudolf Peierls’ heart.

FRANK CLOSE: I do recall way back at a Christmas party, I was talking to him and he said to me with a sort of sad voice, “You never can tell.” And that was the moment I began to realize the effect that Fuchs had had on him and the family. 

NARRATOR: But let’s go back. Who exactly was Klaus Fuchs? Emil Julius Klaus Fuchs was born in 1911 in Rüsselsheim, near Frankfurt. Named after his father, he decided as a boy to go by his third name, Klaus. It was an unusual childhood in which politics and tragedy each played their part. 

FRANK CLOSE: The family were Lutheran Social Democrats, strongly anti-Nazi. His mother committed suicide by drinking hydrochloric acid. His grandmother, it turned out, also committed suicide. 

NARRATOR: His father was a staunch pacifist. A very principled man, he had a favorite mantra that he instilled in the children from an early age. 

FRANK CLOSE: To do what you believe is right, whatever the cost. And I think Fuchs probably took that mantra to heart.

NARRATOR: When Klaus was 13, he showed immense academic promise and three years later was even awarded a prize for ‘best student in the school’. He was quiet and bookish with a razor-sharp mathematical brain that eventually led to him being enrolled at the University of Leipzig to study mathematics and science. He transferred to the University of Kiel following his mother’s death. Soon after, he joined the Communist Party. By the time Adolf Hitler was appointed chancellor of Germany in January 1933, Klaus Fuchs and his family were already in danger, owing to their engagement in left-wing politics. 

FRANK CLOSE: He was a student activist and was quickly on the Gestapo hit list. And he was tipped off that the Gestapo were after him and that his life was in danger so he fled first to France and then to Bristol in England.

NARRATOR: In the UK, Fuchs was sponsored by a family called the Gunns, whom he lived with when he enrolled as a student of physics at the University of Bristol, soon to become known to Peierls. 

FRANK CLOSE: Rudolf Peierls was only four years older than Fuchs, but when you’re very young in science, four years is quite a significant amount. 

NARRATOR: Like Fuchs, Peierls’ destiny was also determined in the 1930s by the rise of fascism. 

FRANK CLOSE: He had also been born in Germany, in Berlin. He was Jewish, and that was the reason why he had fled and he was then stationed at the University of Birmingham. 

NARRATOR: Remember how I told you Peierls is the nucleus of this story? Well, it was Fuchs’ arrival in the UK that started off the chain reaction.

FRANK CLOSE: He was a leading theorist and he visited the University of Bristol to consult with one of the professors there on a regular basis. And in the course of this met Fuchs, who was a very good mathematical calculator. Fuchs was doing some work on a particular problem that interested Peierls and it turns out that Fuchs solved this particular problem in a way that impressed Peierls.

NARRATOR: This encounter would eventually spark a close working relationship between Peierls and Fuchs. Now, this is 1934. At this point, there’s no conception of an atomic bomb. History is still waiting to be shaped. In 1938, while Hitler was annexing Austria and expanding the Third Reich, two German scientists, Otto Hahn, and Fritz Strassmann, discovered ‘fission’ when experimenting with firing neutrons at uranium. 

FRANK CLOSE: Uranium is the heaviest naturally occurring element and it is the largest nucleus of all. It is only just able to hold itself together and the impact of a single neutron will split that nucleus in two. And it turns out when it does that, it releases a huge amount of energy and that process is known as nuclear fission. 

NARRATOR: But, at this moment, the scientific community’s vision is focused on nuclear power; to see whether it is possible to liberate energy from uranium in a heat engine - what we’d now call a nuclear reactor. But nobody yet has explosives on their mind. The reason being that there are two different forms of uranium - uranium 238, which is completely harmless, and uranium 235. Only seven atoms in every 1,000 in naturally occurring uranium are of the seriously explosive sort. 

FRANK CLOSE: Surprisingly, nobody seems to have asked the question, but what would happen if somehow I was given a lump of pure uranium 235, the nasty stuff if you like, the one that can release energy very rapidly. 

NARRATOR: But as events in Europe take a turn for the worse, a startling discovery is made. 

FRANK CLOSE: Rudolf Peierls and another Jewish emigre, Otto Frisch, both working in Birmingham early in 1940 had an astonishing discovery, which was that you only needed a few kilograms. I mean, the size of a grapefruit, made of uranium 235 would be able to produce an explosion ‘equivalent to 1,000 tons of dynamite’ was the phrase that they actually used. And it would also emit radiation they were aware of. And they wrote a top-secret memorandum called the Frisch-Peierls memorandum, which was sent to the British government. 

NARRATOR: In just 1,428 words, this world-changing memorandum forecasts just how devastating such a weapon could be.

FRANK CLOSE: As a weapon, the super bomb would be practically irresistible. There was no material on earth that would be able to resist the force of the blast. And the only defense against such a weapon would be to have one yourself. And that was really what drove them. 

NARRATOR: The British government was worried Hitler's scientists may have been further along this road of discovery than their own and that the Germans were already building such a device. So a top-secret research and development program was set up, taking place at different universities in the UK. The ambitious atomic project was given the name Tube Alloys - a name so bland as to go unnoticed by anyone not working for it. Rudolf Peierls was asked to take the lead on the project. 

FRANK CLOSE: Now by April 1941, progress is being made and the problems to solve were piling up so fast that Peierls needed an assistant and that is when he remembered his meeting with Klaus Fuchs and realized that, for many reasons, Fuchs would be the ideal person to work with. First of all, he was very technically competent. Secondly, and very important for Peierls, he was German like Peirles was. And he hated Hitler as Peierls did. And he was clear that he was very much there for the cause. 

NARRATOR: Fuchs had proved himself to be an outstanding pupil. He received his Ph.D. from the University of Bristol in 1937 and his Doctoral of Science from the University of Edinburgh in 1939, where he worked as an assistant to world-renowned German mathematical physicist and one of the pioneers of quantum mechanics, Max Born. But on May 10, 1941, something happened that would prove to be a fork in the road for Fuchs. Peierls wrote to Fuchs inviting him to Birmingham for theoretical work involving mathematical problems of considerable complexity, the purpose of which he could not disclose.

FRANK CLOSE: The first great irony of this whole story then happens because Fuchs is now about to be enrolled in this top-secret project. And to do that he has got to be vetted. See Fuchs, as an emigre, his passport was still German and the passport was about to expire and the Germans wanted him back. And they said, “Oh, the only way you can get a new passport is to come back to Kiel.” Well, Fuchs was no fool. He wasn't going to do that. But there's a lot of pressure being put. And this letter arrived from the German Embassy, and it was passed on to the Chief Constable of Bristol in the hope that this would provide leverage to get their hands on Fuchs. And in this letter, there are two phrases that say, "Fuchs joined the Communist Party" and then later on, "Books of the Communist Party were found in Fuchs' house". So information that Fuchs had a communist link was already present and known to British authorities. What did they do? Well, they said, “Who sent this letter? It was the Gestapo.” And because it was the Gestapo, they didn't trust it. And so it was discounted. And through the whole of Fuchs' career, the fact that this evidence, as we now understand it, had originated with the Gestapo was enough for it to be disregarded. 

NARRATOR: Because of the worry about Birmingham being bombed, Peierls and his wife Genia had decided to send their two young children to stay with family friends in North America. 

FRANK CLOSE: And so Peierls and his wife were living in an empty home into which Fuchs comes and lives with them - almost metaphorically, like a son into this empty nest. And they forged a very close relationship, not just in physics, but personally.

NARRATOR: Genia even affectionately referred to Fuchs as ‘penny in the slot’ - in that if you put a penny in, he would respond. But he didn't spontaneously do anything.

FRANK CLOSE: I would say that he has some of the classic features of mathematicians and mathematical physicists in the sense that they are very focused. He was very much liked by women. It was, however, that they wanted to mother him in the sense that he was clearly a very brilliant professor who had the sort of classical imagery that you often associate with such people as being in a world of their own and that they need to be taken care of. 

NARRATOR: But Klaus Fuchs was more than just a benign and brilliant Professor. He was a spy, remember? So when did Klaus Fuchs begin his journey into espionage? Nobody can be 100 percent certain, but Frank Close found some important clues. Let’s go back. Back to just one month before Fuchs received that invitation from Peierls to work on the top-secret atomic project. At the time, Fuchs had been repatriated to Edinburgh following a period of internment in Canada. 

FRANK CLOSE: Basically anybody with any German background was regarded as a potential member of the Fifth Column and was interned. 

NARRATOR: In early 1940, there was a real fear of a German invasion of Britain. Ironically, people who had come to the UK to seek refuge were now regarded as ‘enemy aliens’ and sent to internment camps. 

FRANK CLOSE: And he was interned initially in the Isle of Man, and then, with many others, shipped across to a camp in Canada. And it was probably while he was in the camp that he first really began to get involved in his communist activities.

NARRATOR: Because of his scientific standing, Fuchs was among the first group of people to be repatriated to the UK. 

FRANK CLOSE: The fact that Fuchs had been repatriated and was working in Edinburgh for me turned out to be one of the interesting breakthroughs because as an enemy alien, he was not allowed to travel overnight without special permission. And as a result of that, I was able to find the permission that he was given to travel from Edinburgh to London in April 1941. And it showed that he was in London for [around] the order of a week or so. And it was during that period that he was invited to a party, a gathering of intellectuals who met regularly in the Lawn Road Flats in Hampstead. 

NARRATOR: Home to intellectuals and strongly leftist German emigres, the Lawn Road Flats became a hub of Soviet espionage in Britain. Fuchs was probably invited to the party through the contacts that he had made back in the Canadian camp. 

FRANK CLOSE: And at that party, he met a man who gave his name as Johnson, but actually was the Russian military attaché from the Soviet embassy, one Colonel Kremer. 

NARRATOR: At the gathering in Hampstead the group discussed aspects of nuclear science and politics. ‘Johnson’ or rather Colonel Simon Kremer found Fuchs interesting enough to report his name back to the Soviets and asked the scientist to keep in touch. It’s likely that Fuchs started making plans to pass top-secret information regarding the bomb on to the Soviets as early as June 1941.

FRANK CLOSE: Because something significant happened in the world that month. Stalin had been in a non-aggression pact with Hitler until June when that broke down and Hitler foolishly invaded Russia. And Winston Churchill, the PM at that time, spoke on the radio that night and basically said, “The Soviet Union are now our friend and we will do everything we can to help them.” Now, imagine yourself as Klaus Fuchs. You are a committed communist. You happen to be working by total chance on a project which could change the nature of warfare. And the Prime Minister of your newly adopted country has just said that we will give every help we can to your Soviet colleagues. So he did. And that, I think, is when he began spying. 

NARRATOR: This assessment certainly adds up when we consider the fact that in August of the same year, Fuchs had a meeting with his contact, Colonel Kremer. He passed on six sheets of data to Kremer which were then transmitted back to Moscow in encrypted code. This was to be his first transmission of many over the next few years. 

FRANK CLOSE: These codes were intercepted, but they were not possible to track them. The key one, sent on August 8, says that basically, Colonel Kremer has met with Dr. Fuchs, his earlier acquaintance. And in this, it also refers to 1,000 tons of dynamite, the phrase straight out of the Frisch-Peierls memorandum. So it is clear that by August 8, Fuchs was passing the first information to the Soviet Union. 

NARRATOR: Churchill had always been reluctant to share information on the atomic bomb with the Americans. Fuchs and the Tube Alloys team were now making good progress in working out the best way to enrich uranium 235 via a method called ‘diffusion’. However, the US had begun its own bomb-building mission once it was brought into the war following the attack on Pearl Harbor. By 1942, the Tube Alloys manager, Wallace Akers, felt that a merger with the American project would be necessary for such a mammoth industrial endeavor. It was during this time in 1942 that a new Soviet contact came into Fuchs’ orbit. Her name was Ursula Kuczynski codename: Agent Sonya. 

FRANK CLOSE: She was, I think it's fair to say, the Soviet Union's top agent, a genuinely secret agent, merged into the British countryside. 

NARRATOR: Sonya was living in a village called Great Rollright in the heart of the Oxfordshire countryside. 

FRANK CLOSE: And she would go to Banbury with a bicycle and Fuchs would come down by train from Birmingham to Banbury and they would meet and go for a walk so that if they were sort of seen it would just appear to be like a loving couple going for a walk. And he passed information to her, it would appear, from the receipt dates that I found in the Soviet Union files on five occasions. 

NARRATOR: By walking together, arm in arm, they could give the illusion of a courting couple. But it also gave Sonya time to finely tune Fuchs’ spycraft. 

FRANK CLOSE: She had a long history already before she was involved with Fuchs. And this, I think, was very important for Fuchs who continued spying, it became clear, for eight, nine years. And he had had first-class training on how to check he wasn't being followed and all the other things. 

NARRATOR: Ursula even hid a radio transmitter inside one of her children's Teddy Bears. Over the space of 12 months, Moscow received almost the entirety of the scientific data produced by the Tube Alloys team, including large books filled with blueprints, drawings, and equations, one book totaling over 100 pages long. Stalin gave the Allies’ project the codename Enormoz (Enormous). While all of this vital information was being smuggled out in plain sight, Churchill and Roosevelt were unaware that the Soviets had any real knowledge of the atomic bomb project at all. In fact, thanks to Klaus Fuchs, they now had enough information to begin work on their own bomb. 

FRANK CLOSE: By 1944, when the Brits had been working on this for two to three years, various reasons made it clear that everything had to move across to North America.

NARRATOR: One being the fear of German aircraft and damage they could inflict on labs here in Britain. Remember, this wasn’t long after the Blitz. And then there was the need for infrastructure, equipment, and of course space. 

FRANK CLOSE: And so the British delegation of a couple of dozen people, including Peierls and Fuchs, moved across initially to New York, where they now basically completed this work on uranium diffusion by designing what became the Oak Ridge National Laboratory, which was a vast industrial enterprise that would enrich the uranium 235 which would eventually be the fuel for the bomb. 

NARRATOR: Here is the second chance that Britain has to sideline Fuchs. Again, he is vetted. 

FRANK CLOSE: So there is a letter that I found in the files at the National Archive, written at the time when Fuchs was about to go across to North America for the first time. It was written by a man called Major Garrett, who was an MI5 officer, and it was sent to Michael Perrin, who was the administrator in charge of the whole British nuclear effort. And basically, Garrett was saying it would not appear to be wise to mention Fuchs’ proclivities to the authorities in the US. By this, he meant his communist background. "And I don't think it's at all likely that he'll make political contacts in that country while he's there". Well, I mean, how wrong could you be in that? But again, Fuchs passed through the net on this occasion by actually a manifest serious error by the Brits.

NARRATOR: Once again, Klaus had slipped through the net and was deemed safe enough to move across to New York with the British delegation. And although he’ll never see Agent Sonya again, she does help set him up with his new courier.

FRANK CLOSE: Sonya had spent some time in New York and she knew of a sort of dwelling house on the Lower East Side in Henry Street. And so she told Fuchs to use that as a meeting place. And she arranged through the Russians for their courier in North America to use that also. 

NARRATOR: That man was Harry Gold, the son of a Russian Jewish immigrant who’d turned to communism during the Great Depression. He introduced himself to Fuchs upon their first meeting, on February 5, 1944, as Raymond. 

FRANK CLOSE: He was by training a chemist who lived in Pennsylvania and Fuchs and he met for the first time in Henry Street. I don't think anybody else in their right mind would go to this particular part of New York unless they had to. It was pretty sleazy. 

NARRATOR: Gold and Fuchs would meet in the cold, wearing overcoats, sometimes they’d even go for dinner but would make sure only to hand over the vital papers at the very end of their meeting.

FRANK CLOSE: What Fuchs was doing, he was working on mathematical problems in connection with uranium enrichment. And he had every right to have notes on these problems in his briefcase. And as long as he wasn't caught having handed them over to somebody else, it was fine. If any FBI person came in, Fuchs would say, “This is my stuff.” So you saw all the classic things of walking backward and forward, checking that you weren't being followed by going down side streets and so on. This was reality, not just Hollywood. 

NARRATOR: So far, it seems Fuchs - or ‘Charles’ as the Russians would later codename him - had not put a foot wrong. But there was a flaw in the Russian operation that would come back to bite him. 

FRANK CLOSE: The weak spot was when he was in North America he would meet his courier, like Harry Gold. He would pass information to Gold. Gold would then go off and pass it on to somebody from the Soviet Embassy and it would go back to the Soviet Embassy. So far, perfect. Now the Soviet Embassy has to transmit this information to Moscow, and they do this by coding it up and sending it by telegraph. Now, all of these telegraphs were read and recorded, but the coding was done using what's called a ‘one-time pad’. 

FRANK CLOSE: The bottom line is if you use a one-time pad once and only once, it is impossible to crack it. But for some reason, the Russians use one of their pads more than once, and it turns out that that is sufficient to allow an expert cryptographer gradually to start working their way in and breaking the codes. 

NARRATOR: There was, perhaps, another personal misjudgment during this time. Agent Sonya had helped set Fuchs up in New York. In exchange, Fuchs gave her instructions for contacting his sister Kristel in case anyone should need to. Kristel had also fled Germany and was now living in Boston, Massachusetts. Embroiling his only surviving sister in his espionage activities had not been part of his original plan, especially as Fuchs became even more deeply involved with the atomic project when he was transferred to Los Alamos. Here in a vast industrial complex in the New Mexico desert, Fuchs would be placed at the very heart of the US bomb mission, famously named the Manhattan Project. 

FRANK CLOSE: Fuchs was originally not expected to go to Los Alamos. It was thought that he would return to the UK to start working on the British nuclear reactor program that the Harwell laboratory was being built for. But Pieirls had been across to Los Alamos from New York and learned that in addition to the uranium bomb, which they had been working on for the last two to three years, there were now discoveries that you could use plutonium. And plutonium, it turns out, was much more fissile. Now, Peierls knew that there were some very tricky technical problems to be solved. But he also realized that the mathematical approach that he and Fuchs had been working on back in the days in Birmingham was very much relevant to the sort of questions that they would have to solve. And so because of that, Peirles insisted that Fuchs also be allowed to join the delegation in Los Alamos. And so the decision for Fuchs to return to Britain was overruled and he went to Los Alamos. 

NARRATOR: And from the Russian’s perspective, he disappears off the radar. Now, Harry Gold knew from their conversations that Fuchs had a sister who was living in Boston, and he also knew that this sister could be used as an ‘emergency contact’. 

FRANK CLOSE: I do now know, from having read translations of the KGB files, that Fuchs' sister also had a codename in the Soviet system and that she certainly wasn't an active spy but she was certainly a ‘letter box’ for them, if you like to call it. Because she would leave the curtains open or closed to let Harry Gold know whether Fuchs was living there at the time or not. So it was through the sister that Gold managed to re-establish contacts with Fuchs in Los Alamos. 

NARRATOR: Los Alamos, where Fuchs arrived by train in August 1944, was 7,500 feet above sea level and one the most security-conscious places on the Earth at that time. J. Robert Oppenheimer, a theoretical physicist from Berkley, was placed in charge of the entire operation overseen by General Leslie Groves. One thing to keep in mind is that by 1944 tensions were growing over future cooperation between the British and Americans. Fuchs and others would argue later that there was a growing fear among some of the scientists working on the project - a fear that America would monopolize the bomb after the war was over. 

FRANK CLOSE: Fuchs always claimed and gave the impression that he felt that this knowledge should not be retained by a single power. He wanted to share it around. If I'm sympathetic to him, I would say I can understand that. As I said, [in the] 1930s, the intellectuals, by and large, had grown up as anti-fascist or even communist. And to the scientists, the fact that Stalin had been frozen out of the project for political reasons was not understood by them.

NARRATOR: Accommodation at Los Alamos was put in place for the families of the scientists - although any discussion of the project was strictly forbidden - and to the Peierls’ two children. Fuchs became like a big brother. For two years, Fuchs was able to carry out some of the most extensive espionage in history. On Wednesday 21 February 1945, Gold met Fuchs at his sister Kristel’s house. After lunch, Gold and Fuchs went upstairs, where Fuchs handed over a full report on their research into plutonium bombs. Such a bomb would be dropped on the Japanese city of Nagasaki by the Americans just a few months later. This would prove to be the most significant in all of Fuchs’ espionage and armed with this new knowledge, Igor Kurchatov, leader of the Soviet bomb program, chose the plutonium implosion bomb as his primary goal. 

FRANK CLOSE: The Soviets exploded their first atomic test in August 1949. And from the fallout and details of that, we now know it was essentially a replica of the bomb that was tested in the Trinity Test in Los Alamos in 1945. 

NARRATOR: Trinity was the codename for the first successful test of an atomic bomb in New Mexico in 1945 by the United States. 

FRANK CLOSE: So that was all thanks to the information they got from Fuchs. 

NARRATOR: Fuchs also handed over information on the newly emerging Hydrogen or ‘Super’ bomb as it was dubbed. A device of almost limitless destructive power. 

FRANK CLOSE: Enrico Fermi was one of the great scientists of the 20th century who was working at Los Alamos. It turns out that over the period of four months, between August and December ‘45, Fermi gave six lectures on the idea of developing a hydrogen bomb. It was clear that he was working these things out in his office and every few weeks will give a seminar to this closed group of scientists on his progress. We do know from the records that on September 19, 1945, Fuchs met Harry Gold in Santa Fe, a town near Los Alamos, and passed him information. And I have no doubt that Fuchs passed him the information about Fermi's lectures. The proof being that from the Freedom of Information, we have the original notes of Fermi's lectures, and they contain identical diagrams and identical formulae that appear in the Russian volume on the history of their hydrogen bomb. 

NARRATOR: On September 5, 1945, Japan formally surrendered. But two weeks before Fuchs was due to meet with Gold again, something rather shocking happened. A cipher clerk at the Soviet Consulate in Ottawa defected and exposed a Soviet spy ring in Canada. Partly as a result of the discovery of one spy, Alan Nunn May - who we’ll meet in the next episode of True Spies - the McMahon Act was introduced in August 1946. The Act prohibited the US from collaborating with foreign nations on nuclear weapons development. A huge blow to the UK. Fuchs returns to the UK at the end of that year to work at the Atomic Energy Research Establishment at Harwell, Oxfordshire, as Britain decides to go it alone. 

FRANK CLOSE: In addition to developing nuclear reactors, which is what Harwell is primarily there for, they're also going to build their own atomic bomb. And this was so secret that not even anybody in the Cabinet was aware of it. 

NARRATOR: The bomb’s all-important trigger device was being designed at Fort Halstead, near Severn Oaks in Kent, in a sleepy Victorian armaments store. This was under the leadership of a British mathematician called William Penny, who’d also been at Los Alamos leading the British team. Fuchs was a regular visitor.

FRANK CLOSE: At this stage, I'm not overstating it. I think that Fuchs probably knew more about the atomic bomb and the emerging hydrogen bomb than all but a small handful of people in the world. And so he was absolutely essential to the Brits having to discover this all for themselves. In addition to his regular career at Harwell, he, in secret, would periodically commute to London to advise on the British Atomic Bomb Project. And I found in these travel records interesting coincidences with dates [on which] we know he passed information. He obviously used those trips also to take diversions and meet his courier in London and tell them what was going on. So the British Atomic Bomb Project was an open book in Moscow, thanks to Fuchs. And I regard this as the most serious part of his career because during the war, after all, the Soviet Union was technically our ally. But by 1950, the Cold War was beginning and the Soviet Union was very much not our ally. And so Fuchs was telling them basically what ability Britain and also the United States had to counteract the Soviet threat. 

NARRATOR: But remember those encrypted codes that Harry Gold had sent back to Moscow when he was Fuchs' courier? 

FRANK CLOSE: The decryption of the codes was the key element that revealed that Fuchs had been spying. 

NARRATOR Klaus Fuchs was unmasked as a spy in the summer of 1949 when British authorities received information from the American Federal Bureau of Investigation that decoded Soviet messages in their possession.

FRANK CLOSE: It wasn't until 1949 that these messages were decoded sufficiently to the point where one of them revealed that there was a spy in the British delegation in New York working on his specialty of diffusion. And putting all this together, immediately, it was clear this must be either Fuchs or Peierls. But the key thing at the bottom of this decryption was that this particular spy was expected to be returning to Britain in six weeks' time, and that was the very time when Fuchs was discussing with the head of the British delegation about returning to Britain, and that was what proved it was Fuchs and not Peierls. 

NARRATOR: And it all came back to that mistake of Russia using their ‘one-time pad’ twice. Once MI5 had Fuchs in their crosshairs, the chase was on.

FRANK CLOSE: The irony was the fact that we were able to decrypt these codes was so secret that this could not be used. And the only way to proceed with Fuchs was to catch him in the act. And so for four months, he was watched 24 hours a day. His telephone was monitored. His apartment where he lived was bugged. The phone in his office was bugged. And also the phone of his boss Herbert Skinner was bugged. 

NARRATOR: But nothing of note was discovered. Fuchs must have been aware that he was coming under increasing suspicion. After all, he’d been trained by the legendary Agent Sonya, so he knew when he was being followed - which at this point was most of the time.

FRANK CLOSE: They followed him and Erna in his car up to London and back one day in September ‘49, and they reported they stopped for an hour at the [Bull on Bell] in Henley, drinking double gins before driving to Harwell. 

NARRATOR: It turns out Fuchs was engaged in an altogether different type of subterfuge than the one MI5 was expecting. When they followed his car to London, they discovered that he was having an affair with his boss's wife, Erna Skinner.

FRANK CLOSE: There are remarks in the MI5 files which naively say Fuchs is not a very good driver. Sure, he's looking in the mirror at who's following him and taking diversionary tactics. It’s quite astonishing. The pressure eventually got to him. He obviously knew he was being followed and he went off for three days with Erna. And during those two nights, it seems that he confessed to her. Now, exactly the nature of his confession we don't know. But it's clear that the pressure was getting to him, and within a few days, he then confessed. And MI5's chief interrogator - a man called Jim Skardon - went back to Fuchs' apartment at Harwell and wrote out Fuchs his confession. And the document is there with Fuchs formally signing and agreeing that is the state of affairs. 

NARRATOR: When this came out, the authorities' minds were blown. All that they knew was that Fuchs had apparently, on one occasion, passed some information about diffusion. But under interrogation in Harwell, Fuchs confessed to having spied from 1941 right the way through to 1949. He admitted to handing the closest guarded secrets of the atomic bomb, the plutonium bomb, the hydrogen bomb, everything, to the Russians. Why confess everything? 

FRANK CLOSE: My theory is, that it was clear to Fuchs that the FBI seemed to have been putting a lot of pressure on Harry Gold. Fuchs knew that he hadn't made a single mistake and he began to worry that his sister in Boston had somehow been compromised. Now, go right back to the start of our story when I said that Fuchs' mother committed suicide drinking hydrochloric acid. His grandmother committed suicide and one of his sisters had. I think that he was now worried that his other sister, the one that was being used to this end, might also be because she - we now have on the record - was, by this stage in 1950, she was in a sanitarium. She'd had breakdowns, depression, and so forth. And I think Fuchs was very concerned for her. I think that this was when he decided that he didn't want his sister to go the same way as the rest of the family. And he confessed, if you like, to save her.

NARRATOR: Fuchs was prosecuted and was convicted on March 1, 1950, of four counts of breaking the Official Secrets Act by communicating information to a potential enemy. Justice Lord Goddard sentenced Fuchs to 14 years imprisonment and this, in turn, set off a chain of arrests. 

FRANK CLOSE: The result of Fuchs’ arrest led to the arrest of his courier Harry Gold. This, in turn, led to others being arrested and the Soviet spy network in North America was decimate

NARRATOR: The fallout was disastrous for the USSR. And what about Rudolf Peierls, the nucleus of all this? Unsurprisingly, the news of Fuchs’ arrest came as both a shock and a personal dagger to the heart. 

FRANK CLOSE: Just imagine if somebody that you have known for many, many years, one of your closest friends, is suddenly revealed, has been passing all of your work to the enemy. 

NARRATOR: But it was a letter sent to Fuchs whilst in prison, written by Genia Peierls, that seems to have really hit home. 

FRANK CLOSE: She says, "I am writing to you in front of our sitting room fire where we so often talked about so many things. This is a hard letter to write, perhaps even a harder one to read. But you know me well enough not to expect me to mince my words." And then she carried on. "I'm taking it all much easier than everybody else because my Russian childhood and youth taught me not to trust anybody and to expect anyone and everyone to be a communist agent. Twenty years of freedom in England softened me. I learned to like and trust people. Or at any rate, some of them. I certainly did trust you. Even more, I considered you the most decent man I knew. I do that even now.” And this really got through to him. But the tragedy of this is that this really shows how much the Peierls' family loved him. 

FRANK CLOSE: Fuchs never had any contact with the Peierls for the rest of his life. I think that letter that Genia had written to him really exposed the nature of their relationship, and he had broken that. Probably the only people who had ever really loved him in his life - he'd betrayed them. And he couldn't handle it.

NARRATOR: In all of his efforts, Fuchs had been living by his father’s mantra. ‘Do what is right, whatever the cost.’ In this instance, the cost was the defining friendship of his life. And he would have to live with the consequences. I’m Sophia Di Martino. Join me next week when we hear about a very British atomic spy, Alan Nunn May.

Guest Bio

Frank Close is a Professor Emeritus of Theoretical Physics and Fellow Emeritus at Exeter College at the University of Oxford. He is also the author of 20 books about science. The latest - Trinity, The Treachery and Pursuit of the Most Dangerous Spy in History - is about the relationship between Oxford Professor Rudolf Peierls and his assistant, atomic spy Klaus Fuchs.

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