Study of fracture toughness of Zr-2.5Nb alloy used in pressure tube of Indian PHWR having more than 200 wppm hydrogen

Abstract

Due to chemical reaction between heavy water and Zr-2.5 alloy pressure tube (PT) material, hydrogen is released which get absorbed by PT. The hydrogen in excess of terminal solid solubility precipitate out in the form of brittle Zirconium hydride. The hydrides are of platelet shape and may get oriented in normal direction to the applied hoop stress above threshold stress value. Stress reoriented hydrides in the radial axial plane are called as radial hydrides and can cause severe embrittlement in PT. This study was done to evaluate the fracture toughness of CWSR Zr-2.5Nb pressure tube containing more than 200wppm hydrogen oriented in radial direction. The charging of a pressure tube spool was done by gaseous hydrogen charging in constant volume. After hydrogen is charged into the pressure tube spool, circumferential hydrides are formed. The Circumferential hydrides are reoriented into radial hydride using a setup to pressurize the tube from internally at 350°C , so that the hoop stress developed into the spool will be greater than threshold stress value. The cooling from 300°C to 150°C at constant pressure above threshold value will reorient the hydride in radial direction. Curved compact specimen of 17 mm width was machined using wire electric discharge machining from a CWSR Zr-2.5Nb pressure tube spool of Indian pressurized heavy water reactor. Metallographic examination of each sample shows 100% radial hydrides. The specimen where pre-cracked and then tested between the temperature range from 25°C to 300°C. The fracture toughness of sample was calculated as per standard ASTM E1820-13. The fracture toughness versus temperature graph shows clear brittle to ductile transition. Fracture toughness reaches 207 KJ/m² at 300°C, where as at room temperature the fracture toughness is 3 KJ/m².