Effects of Neutron Irradiation on Mechanical Properties of Heat Trested Zr-2.5Nb Pressure Tube in Prototype Advanced Thermal Reactor FUGEN

Abstract

The heat-treated Zr-2.5Nb is a pressure tube alloy utilized in prototype advanced thermal reactor Fugen. Fugen surveillance specimens as well as three kinds of modified HT Zr-2.5Nb, i.e. as received. 200 ppm hydrogen enriched and radially hydride reorientated specimens, were prepared and irradiated in the special fuel assemblies of Fugen up to neutron exposure of 7 X 10^25 n/m2 (E > l MeV) at around 560 K. A series of post irradiation examination including tensile test, bending fracture test, compact tension test and metallographies have been extensively conducted to investigate the effects of neutron irradiation on mechanical property changes of pressure tube. Tensile strength of pressure tube increases within the initial stage of 1 X 10^25 n/m:, and thereafter takes saturation around 1000 MPa and 800 MPa at room temperature and at 573 K respectively. The fracture toughness of the modified HT Zr-2.5Nb pressure tube is improved at both unirradiated and irradiated conditions. The fracture toughness is very sensitive to the orientation relationship between hydride platelets and crack extension axis. The radially reorientated hydride specimens degrades in fracture toughness but satisfy the Fugen design value. Based on present results the integrity of the Fugen pressure tube was appropriately demonstrated for 30 years operation.