An apparatus and method for inter-unit transfer of spent nuclear fuel. In one aspect, the invention is a method of transferring high level radioactive waste comprising: a) loading high level radioactive waste into a water-filled cavity of a canister body having an open top end at a first location; b) coupling a lid to the canister body to enclose the open top end; c) removing a volume of water from the cavity so that a water level of the water within the cavity is above a top end of the high level radioactive waste and a space exists between the water level and a bottom surface of the lid; d) hermetically sealing the cavity; and e) transferring the canister to a second location, the water level remaining above the top end of the high level radioactive waste during the transfer.