A fracture mechanics based calculation procedure is provided in the CSA Standard N285.8 to define pressure-temperature limits for fracture protection of CANDU Zr-Nb pressure tubes. The calculated pressure-temperature limits are used to construct a plant-specific operating envelope for pressure tubes under reactor heat-up and cool-down conditions. The current calculation procedure to define pressure-temperature limits for fracture protection is deterministic, and makes use of conservative inputs, including an axial through-wall crack of 20 mm in length, lower-bound fracture toughness, and a safety factor of 1.3 on the calculated critical internal pressure. The deterministic procedure is straightforward to use, and has a long history of successful applications of protecting pressure tubes from rupture. However, the deterministic procedure will potentially impose challenging operational constraints on pressure tubes at late life conditions, due to the predicted low fracture toughness of pressure tubes with high levels of hydrogen equivalent concentration.
As an alternative, the CSA Standard N285.8 also permits probabilistic evaluation of fracture protection, which implies the acceptability of using risk-informed pressure-temperature limits for pressure tubes under reactor heat-up and cool-down conditions. The feasibility of developing a risk-informed procedure to define pressure-temperature limits for fracture protection of pressure tubes under heat-up and cool-down conditions is described in this paper. The intent is to use the risk-informed methodology to develop alternate pressure-temperature limits that allow more operational flexibility and still satisfy safety goals. The proposed risk-informed approach is consistent with risk-informed approaches that have been used in the U.S. nuclear industry.