Atomic spies

Atomic spies or atom spies were people in the United States, the United Kingdom, and Canada who are known to have illicitly given information about nuclear weapons production or design to the Soviet Union during World War II and the early Cold War. Exactly what was given, and whether everyone on the list gave it, are still matters of some scholarly dispute. In some cases, some of the arrested suspects or government witnesses had given strong testimonies or confessions which they recanted later or said were fabricated. Their work constitutes the most publicly well-known and well-documented case of nuclear espionage in the history of nuclear weapons. At the same time, numerous nuclear scientists wanted to share the information with the world scientific community, but this proposal was firmly quashed by the United States government. 
Confirmation about espionage work came from the Venona project, which intercepted and decrypted Soviet intelligence reports sent during and after World War II. These provided clues to the identity of several spies at Los Alamos and elsewhere, some of whom have never been identified. Some of this information was available to the government during the 1950s trials, but it was not usable in court as it was highly classified. Additionally, historians have found that records from Soviet archives, which were briefly opened to researchers after the fall of the Soviet Union, included more information about some spies.
Importance.
Before World War II, the theoretical possibility of nuclear fission resulted in intense discussion among leading physicists world-wide. Scientists from the Soviet Union were later recognized for their contributions to the understanding of a nuclear reality, and won several Nobel Prizes. Soviet scientists such as Igor Kurchatov, L. D. Landau, and Kirill Sinelnikov helped establish the idea of, and prove the existence of, a splittable atom. Dwarfed by the Manhattan Project conducted by the US during the war, the significance of the Soviet contributions has been rarely understood or credited outside the field of physics. According to several sources, it was understood on a theoretical level that the atom provided for extremely powerful and novel releases of energy, and could possibly be used in the future for military purposes. 
In recorded comments, physicists lamented their inability to achieve any kind of practical application from the discoveries. They thought that creation of an atomic weapon was unattainable. According to a United States Congressional joint committee, although the scientists could conceivably have been first to generate a man-made fission reaction, they lacked the ambition, funding, engineering capability, leadership, and ultimately, the capability to do so. The undertaking would be of an unimaginable scale, and the resources required to engineer for such use as a nuclear bomb, and nuclear power were deemed too great to pursue.
At the urging of Albert Einstein and Leo Szilard in their letter of August 2, 1939, the United States — in collaboration with Britain and Canada — recognized the potential significance of an atomic bomb. They embarked in 1942 upon work to achieve a usable device. Estimates suggest that during the quest to create the atomic bomb, an investment of $2 billion, 86,000 tons of silver and 24,000 skilled workers drove the research and development phase of the project. Those skilled workers included the people to maintain and operate the machinery necessary for research. The largest Western facility had five hundred scientists working on the project, as well as a team of fifty to derive the equations for the cascade of neutrons required to drive the reaction. The fledgling equivalent Soviet program was quite different: The program consisted of fifty scientists, and two mathematicians trying to work out the equations for the particle cascade. The research and development of techniques to produce sufficiently enriched uranium and plutonium were beyond the scope and efforts of the Soviet group. The knowledge of techniques and strategies that the Allied programs employed, and which Soviet espionage obtained, may have played a role in the rapid development of the Soviet bomb after the war.
The research and development of methods suitable for doping and separating the highly reactive isotopes needed to create the payload for a nuclear warhead took years, and consumed a vast amount of resources. The United States and Great Britain dedicated their best scientists to this cause and constructed three plants, each with a different isotope-extraction method. The Allied program decided to use gas-phase extraction to obtain the pure uranium necessary for an atomic detonation. Using this method took large quantities of uranium ore and other rare materials, such as graphite, to successfully purify the U-235 isotope. The quantities required for the development were beyond the scope and purview of the Soviet program.
The Soviet Union did not have natural uranium-ore mines at the start of the nuclear arms race. A lack of materials made it very difficult for them to conduct novel research or to map out a clear pathway to achieving the fuel they needed. The Soviet scientists became frustrated with the difficulties of producing uranium fuel cheaply, and they found their industrial techniques for refinement lacking. The use of information stolen from the Manhattan Project eventually rectified the problem. Without such information, the problems of the Soviet atomic team would have taken many years to correct, affecting the production of a Soviet atomic weapon significantly.
Some historians believe that the Soviet Union achieved its great leaps in its atomic program by the espionage information and technical data that Moscow succeeded in obtaining from the Manhattan Project. Once the Soviets had learned of the American plans to develop an atomic bomb during the 1940s, Moscow began recruiting agents to get information. Moscow sought very specific information from its intelligence cells in America, and demanded updates on the progress of the Allied project. Moscow was also greatly concerned with the procedures being used for U-235 separation, what method of detonation was being used, and what industrial equipment was being used for these techniques.
The Soviet Union needed spies who had security clearance high enough to have access to classified information at the Manhattan Project and who could understand and interpret what they were stealing. Moscow also needed reliable spies who believed in the communist cause and would provide accurate information. Theodore Hall was a spy who had worked on the development of the plutonium bomb the US dropped in Japan. Hall provided the specifications of the bomb dropped on Nagasaki. This information allowed the Soviet scientists a first-hand look at the set up of a successful atomic weapon built by the Manhattan Project.
The most influential of the atomic spies was Klaus Fuchs. Fuchs, a German-born British physicist, went to the United States to work on the atomic project and became one of its lead scientists. Fuchs had become a member of the Communist Party in 1932 while still a student in Germany. At the onset of the Third Reich in 1933, Fuchs fled to Great Britain. He eventually became one of the lead nuclear physicists in the British program. In 1943 he moved to the United States to collaborate on the Manhattan Project. Due to Fuchs's position in the atomic program, he had access to most, if not all, of the material Moscow desired. Fuchs was also able to interpret and understand the information he was stealing, which made him an invaluable resource. Fuchs provided the Soviets with detailed information on the gas-phase separation process. He also provided specifications for the payload, calculations and relationships for setting of the fission reaction, and schematics for labs producing weapons-grade isotopes. This information helped the smaller under-manned and under-supplied Soviet group move toward the successful detonation of a nuclear weapon.
The Soviet nuclear program would have eventually been able to develop a nuclear weapon without the aid of espionage. It did not develop a basic understanding of the usefulness of an atomic weapon, the sheer resources required, and the talent until much later. Espionage helped the Soviet scientists identify which methods worked and prevented their wasting valuable resources on techniques which the development of the American bomb had proven ineffective. The speed at which the Soviet nuclear program achieved a working bomb, with so few resources, depended on the amount of information acquired through espionage. During the Cold War trials, the United States emphasized the significance of that espionage.