A spent fuel assembly handling tube closure valve is provided which assures minimal or zero leakage of trapped liquid metal coolant from the handling tube. One sealing surface of the closure valve is spherical while the other is conical. The spherical-conical combination enables an adequate seal even when the valve cap is misaligned. A further safeguard against leakage is made possible by a sacrificial inert gas system utilizing the diving bell principle. A metered gas supply system supplies additional sacrificial gas should a leak develop. A high-low level mutual inductance probe operates as a leak detector allowing the timely addition of greater quantities of sacrificial gas.