In the process of studying natural radioactivity - studying the radiation of radioactive elements and radioactive transformations, new natural radioactive elements were discovered, systematized in radioactive groups - uranium and thorium, which include the third, so-called actinium family - actinides (this name was proposed by S. A. Shchukarev). F.Soddy's discovery of the law of radioactive displacements made it possible to assume that the final stable decay products of elements of all three families are three isotopes of the same element - lead.
The Bohr model of the atom is based on the study of natural radioactivity, which showed the complexity of the structure of the atom, the decay of which produces atoms of other elements, which is accompanied by three types of radiation: α, β и γ.
The neutron-proton theory of the structure of the atomic nucleus owes its origin to the discovery of new elementary particles that make up the nucleus: the neutron (n) and the proton (p), which became possible by the artificial splitting of the atom under the influence of α-particles (1919):  N+He→O+H,  accompanied by the release of a proton (soon experiments were carried out with a number of other light elements).
Further fundamental research in this area showed that in light elements the number of neutrons in the nucleus is equal to the number of protons; and as we move to heavy elements, neutrons begin to dominate over protons, and the nuclei become unstable - they become radioactive.
As part of the atomic project, he was a member of the technical council and was responsible for the activities of the radium institute. Through the efforts of V.G. Khlopin and the First Secretary of the Leningrad Regional Committee and City Committee of the All-Union Communist Party of Bolsheviks, Alexey Kuznetsov, the Radium Institute received additional premises. The decision to allocate space was made by the Special Committee in November 1945, carried out by the chairmen of the Operations Bureau of the Council of People's Commissars of the RSFSR A. N. Kosygin and the representative of the State Planning Committee in the Special Committee N. A. Borisov.