After a nuclear reactor is shut down, the top of a nuclear fuel assembly in the core of a reactor pressure vessel is covered with a cap through which an air supply line, a warm water supply line and a sampling line extend. Air is supplied through the air supply line to form an air layer in the cap to stop the passage of a current of cooling water through the fuel assembly. A portion of the cooling water in the reactor pressure vessel is pumped up and heated to prepare warm water which is supplied through the warm water supply line to the fuel assembly so as to replace the cooling water in the fuel assembly by the warm water. After the fuel assembly is filled with the warm water, the fuel assembly is allowed to stand as it is for a predetermined period of time, and then a portion of the warm water in the fuel assembly is sampled. The radioactivity of the sample of the warm water is measured thereby to detect whether any of the nuclear fuel elements constituting the fuel assembly under test is a faulty element.