; or (B) protect persons against the risk of death or serious bodily injury, upon written notification to the Director by the Attorney General or, at the Attorney General's direction, by the head of a Federal law enforcement agency, or the head of an element of the intelligence community (as specified or designated under section 3(4) of the National Security Act of 1947 ( 50 U.S.C. 3003(4) )), that there is a substantial risk that the communications of an inmate or contacts by the inmate with other persons could result in death or serious bodily injury to persons, or substantial damage to property that would entail the risk of death or serious bodily injury to persons, as described by section 501.3 of title 28, Code of Federal Regulations, or any successor thereto. (15) Special housing unit The term special housing unit means a housing unit in an institution of the Bureau of Prisons in which inmates are securely separated from the general inmate population for disciplinary or administrative reasons, as described in section 541.21 of title 28, Code of Federal Regulations, or any successor thereto. (16) Special management unit The term special management unit means a nonpunitive housing program with multiple, step-down phases for inmates whose history, behavior, or situation requires enhanced management approaches in order to ensure the safety of other inmates, the staff, and the public. (17) Substantial and immediate threat The term substantial and immediate threat means any set of temporary and unforeseen circumstances that require immediate action in order to combat a threat to the safety of an inmate, other inmates, staff, or the public. (b) Use of solitary confinement (1) In general The placement of a Federal inmate in solitary confinement within the Bureau of Prisons or any facility that contracts with the Bureau of Prisons to provide housing for inmates in Federal custody shall be limited to situations in which such confinement— (A) is limited to the briefest term and the least restrictive conditions