It is proposed to store spent nuclear fuel elements in welded-shut steel containers, which are filled with argon at about 2 atmospheres pressure. It is desirable to be able periodically to check the containers for leaks. The invention proposes to do this by providing apparatus for sensing the pressure within the container from outside the container. To achieve this, a pressure responsive bellows is mounted inside the container, such that changes in the container pressure move a metal plate attached to the bellows towards or away from a portion of the inside wall of the container. Then. if the container starts to develop a leak, the pressure will begin to drop, and the gap between the metal plate and the wall will change. This gap can be sensed from outside the container, by an ultrasonic pulse echo technique. Preferably. the bellows and plate are mounted within the head of the combined valve and lifting pintle by which the container is sealed.