To ensure safety of new type fuel having no or least utilization experience for using in a reactor.

The whole of a single new type fuel element 30 for testing is installed in capsules 34a and 34b and held by a shroud tube 32. The capsules are independently contained in a compartment 14 to be a coolant flow path. A plurality of compartments are arranged around middle tie-rod 12 and its surrounding is covered with a wrapper tube 18 to be a capsule type irradiation fuel assembly 10 for testing. By loading this and irradiating with neutrons in a reactor, the use of the new type fuel elements in a commercial reactor is simulated while ensuring safety in the reactor by the capsule.