The article shows the solution to the problem of defining the critical section of the hardware with inconstant volume operating conditions. The sample for the solution is nuclear reactor BN-350 assemblages. It is established, that the assembly of the BN-350 fast-neutrons nuclear reactor is characterized by complex operating conditions when each elementary volume of a product is influenced by variable temperature, pressure and density of a neutron flow. The author offers to use the resulted parameter to analyze reliability of the equipment. This parameter has revealed the critical section of an assembly cover which is located below the center of the active zone and where the cover is likely to be destroyed.