seade, mis sisaldab piisaval hulgal lõhustuvat materjali, mis on paigutatud nii, et kontrollitud iseseisev tuumalõhustumiste ahelreaktsioon on võimalik, A device containing fissionable material in sufficient quantity and so arranged as to be capable of maintaining controlled, self-sustaining nuclear fission chain reaction., device in which a self-sustaining nuclear fission chain reaction can be maintained and controlled (fission reactor)