The Department of Nuclear Sciences and Engineering (DECN) results from the integration in IST, in March 2012, of the former state laboratory Instituto Tecnológico e Nuclear, ITN (Nuclear and Technological Institute, Decree-Law 29/2012). The DECN is located in the premises of Campus Tecnológico e Nuclear (CTN), IST Pole of Loures.

The origins of ITN can be traced to the “Laboratório de Física e Engenharia Nuclear-LFEN” established in December 1955 by the Junta de Energia Nuclear, JEN (Nuclear Energy Board) created the previous year under the direct supervision of the Prime-Minister of Portugal (Decree-Law 9580 of 29^th March 1954). The mission of LFEN was the development of research activities related to peaceful applications of nuclear energy. LFEN was the first, and for a long time the only large scale scientific investment of Portugal.

The construction of the first LFEN buildings and laboratories started in 1957 in land acquired by JEN the previous year near Sacavém. Among the main pieces of equipment of this new Laboratory were two particle accelerators – one 2 MeV Van de Graaff and one 0.6 MeV Cockroft-Walton – which arrived in Portugal at the end of the Summer of 1957, and a 1 MW swimming-pool type nuclear research reactor, ordered in 3rd July 1957, to AMF Atomics Inc., which become operational on 25th April 1961. Both the first accelerator and the reactor, after considerable modernization and upgrades, remain today as two main infrastructures still in service at the Campus.

In 1958 the building of a pilot plant for the production of metallic uranium was started. This became operational in 1960, regularly producing metallic uranium and compounds for uranium fuel until 1972.

On 1st October 1979 JEN was terminated and LFEN was integrated into the newly created Laboratório Nacional de Engenharia e Tecnologia Industrial - LNETI (National Laboratory for Engineering and Industrial Technology) under the authority of the Ministry of Industry. In October 1992, LNETI became the Instituto Nacional de Engenharia e Tecnologia Industrial - INETI (National Institute for Engineering and Industrial Technology). In spite of the reorganization and relocation of some groups to other departments, both under LNETI and INETI, the previous activities of LFEN in the field of nuclear science and technology continued to be carried out at Sacavém campus by Instituto de Ciências e Engenharia Nucleares, ICEN (Nuclear Sciences and Engineering Institute) one of the institutes of the new Laboratory.

The Instituto Tecnológico e Nuclear, corresponding to previous ICEN, was established in 1994 as an autonomous institution, under the authority of Ministry of Planning and Land Administration (Decree-Law 324-A/94 of 30^th December 1994). One year later, ITN was integrated into the Ministry of Science and Technology (Decree-Law 296-A/95 of 17^th November 1995). In October 1998, ITN absorbed additional personnel, services and competences, in its newly established Department of Radiological Protection and Nuclear Safety (Protection and Radiological Safety Unit, since 2008). Until the decision of integrating ITN within IST-Instituto Superior Técnico of the Lisbon University, taken by the government in the end of 2011, ITN remained as an autonomous institution under the authority of the Ministry of Science, Technology and Universities.