Heat transfer

A method of transferring excess heat from coolant in a primary vessel of a nuclear reactor through a space between the wall of the primary vessel and the wall of a secondary vessel containing the primary vessel, comprises disposing in the space pieces of a heat transfer medium which is solid at temperatures at and below the normal operating temperature of the coolant but which melts if the coolant temperature becomes excessive, the medium thereby making good heat transferring contact with both of the walls. The medium may be, for example, particles of aluminium. (UK)