Abstract: A tamper-proof closure 10 is inserted by means of positioning tool 20 into a nuclear fuel consolidation storage canister 30. The closure 10 includes a grooved weakened portion 44, which if it exceeds its elastic limit when the base 12 is flexed during removal attempts will indicate any attempts to remove or otherwise tamper with the thus closed canister. Locking tabs 18, on transverse legs 13 and 14 extending from base 12 are engaged in slots 40 in the wall of the opening of canister 30 when the closure 10 is in fuel storage position. Tool 20 has a portion 46 which enters a slot 48 in base 12 at 90.degree. to its working position and then twists to engage spanning portion 42's flanges 43 in slots of flanges 16 on the legs 13 and 14 to retract tabs 18. The base is prevented from flexing on insertion into the container opening by support it receives from portion 46.