Today, due to product improvements, pellet-clad interactions and corrosion effects are not the dominating failure causes. The main failure mechanism in PWRs and BWRs is now related to debris fretting and grid-to-rod fretting, and the industry has responded by developing improved debris filters and spacer grids (see Section 9.2). However, new failure mechanisms may emerge with increasing burn-up of the fuel. One is outside-in crack growth assisted by hydride formation with incipient cracks, which propagate to the inner surface on a power increase. This mechanism, which is associated with so-called ‘delayed hydride cracking’, has been seen in ramping of high burn-up BWR fuel and was verified experimentally by Sakamoto et al. (2010).

According to an IAEA review of fuel failures in water-cooled reactors (IAEA, 2010) covering the period 1987 to 2006, a continuous decrease of the fuel rod failure rate in PWRs was achieved. The failure rate for VVERs stayed relatively constant in that period, while BWRs and CANDUs mainly improved at the beginning of the 1990s; see Fig. 9.14.