Category I Nuclear Material meaning

Category I nuclear material describes, in practice, the highest‑risk nuclear material (for example, specified quantities and forms of plutonium and highly enriched uranium) that attracts the most stringent physical protection and transport security controls. The term is an international classification set out by the International Atomic Energy Agency in its Nuclear Security Recommendations on Physical Protection of Nuclear Material and Nuclear Facilities (Nuclear Security Series No. 13, INFCIRC/225/Rev.5), Annex I, superseding earlier Rev.4 references. It is not defined in UK or Irish primary legislation, but UK regulators (including the Office for Nuclear Regulation under the Nuclear Industries Security Regulations 2003) and Irish competent authorities apply the iaea categorisation in regulatory guidance, security assessments, approvals and in site or transport security plans. In legal practice, identifying material as Category I triggers the highest level of security planning, guarding, access control, material accountancy and transport arrangements, and informs licence conditions, contract drafting (allocation of security responsibilities) and insurance requirements. Usage and effect are broadly consistent across England & Wales, Scotland, Northern Ireland and Ireland, reflecting the IAEA framework. Practitioners should consult the current IAEA thresholds to determine whether particular holdings meet Category I, as the categorisation depends on material type, enrichment and quantity.