Determination of the Threshold Conditions for Delayed Hydride Cracking in Zircaloy-4 CANDU Fuel Cladding – AECL’s Contribution to IAEA Coordinated Research Programme

Abstract

Delayed Hydride Cracking (DHC) is one of the cracking mechanisms for Zircaloy-4 fuel cladding and is a concern for spent fuel storage. To determine the threshold conditions of the material, the International Atomic Energy Agency (IAEA) initiated a Coordinated Research Program (CRP); AECL, representing Canada, participated in the CRP. A flaw in a cladding may extend by DHC due to hydrogen pick up by the material. The hydrogen diffuses up the tensile stress gradient at the flaw tip; if sufficient hydrogen is accumulated to form hydrides and if the stress is large enough, the hydride cracks, the flaw extends, and finally, may lead to a through-wall failure. From this description of the phenomenon, threshold conditions for DHC can be related to:

• Sufficient hydrogen must be present for forming hydride at the flaw tip;
• The local tensile stress must be large enough to crack the hydride. A crack will not extend if a threshold in stress intensity factor, called KIH, is not exceeded;
• A high temperature limit exists when the yield stress at the flaw tip becomes too low to crack the hydride.

This presentation will describe measurements of KIH and crack growth rate, VDHC, in un-irradiated Zircaloy-4 CANDU fuel cladding in cold-worked, stress-relieved condition. The cladding contains about 130 ppm hydrogen. Two methods were used to evaluate KIH:

• testing several specimens at various values of KI and finding the value below which no cracking is detected, and
• gradually increasing the load until cracking starts.

The test specimen and fixture was the Pin Loading Tension configuration, and the test temperature ranged from 250 to 296°C. In the CANDU material the mean value of KIH below 280°C slightly increased with temperature; it ranged from 7 to 8 MPa√m at 250°C and was 10 MPa√m at 280°C. At higher test temperatures, KIH rose sharply to 16.7 MPa√m while the crack growth rate declined towards zero. This behaviour suggests that fuel cladding is immune from DHC above about 300°C. The implications for spent fuel storage will be discussed.