These Regulations commence on exit day, which is when the United Kingdom will cease to be a member of both the European Union and of the European Atomic Energy Community. Prior to the commencement day, nuclear safeguards in the United Kingdom, were regulated by the EURATOM treaty, as amended by the Treaty of Lisbon, which was signed on 13th December 2007, and by Commission Regulation (EURATOM) 302/2005 (OJ L 54, 28.2.2005 p1-71). With effect from commencement day, the United Kingdom’s domestic provisions on nuclear safeguards will be set out in the Energy Act 2013 (c. 32), as amended by the Nuclear Safeguards Act 2018 (c. 15), these Regulations and the Nuclear Safeguards (Fissionable Material and Relevant International Agreements) (EU Exit) Regulations 2019 (S.I. 2019/195).
Link: The Nuclear Safeguards (EU Exit) Regulations 2019
Source: Legislation .gov.uk