ECCS activation will stop the temperature rise and start to cool the core by injection from the bottom of the core in a PWR and from the top of the core in a BWR (Strasser et al, 2010b). The ‘cooling’ process as shown in Fig. 5.9 is relatively slow until the emergency coolant contacts the fuel that has been at the PCT. At that point, in the range of 400-800°C and identified as ‘quenching’ in Fig. 5.9 , the water from the ECCS will reduce the cladding temperature at a rapid rate (1-5°C/s) by re-wetting the cladding heat trans­fer surface. The process will collapse the vapour film on the cladding OD and cooling will be by nucleate boiling. Thermal shock due to the sudden change in heat transfer conditions can fracture the cladding at this stage and the ability of the cladding to withstand the thermal stresses will depend on the extent of oxidation and degree of cladding embrittlement that occurred during the LOCA transient.

The oxidation embrittlement process and final structure of the cladding after completion of the LOCA cycle is as follows (Strasser et al., 2010b):

• First, the increasing water and steam temperatures during heat-up increase the reaction rates with the cladding and increase the conver­sion of the cladding surface into thicker ZrO2 films.

• As the LOCA temperature passes the levels where a ^ в transforma­tions start and finish, the resulting structure consists of:

— The growing ZrO2 layer.

— A zirconium alloy layer with a very high oxygen content which sta­bilizes the a phase.

— The bulk cladding which is now in the в phase.

• The ECCS initiated quenching phase cools the cladding back down through the в^а transformation temperature and the bulk cladding is now re-transformed from the в into the a phase and referred to as the ‘prior or former в phase.’

Oxygen and hydrogen affect the formation of the structure as follows dur­ing the oxidation (Strasser et al, 2010b):

• Oxygen diffuses from the ZrO2 to the bulk cladding which is in the в phase at the high temperature (HT); however, the в phase has a low sol­ubility for oxygen.

• Increased hydrogen levels from the oxidation reactions prior to and dur­ing the LOCA increase the diffusion rate and solubility of oxygen in the в phase >1000°C.

• Wherever the solubility limit of oxygen in the в phase is exceeded, the excess oxygen stabilizes the a phase.

• The oxygen stabilized a phase forms next to the ZrO2 layer and grows, as does the ZrO2 layer, at the expense of the bulk cladding in the a phase and as a result after quenching in the ‘prior в phase.’

The final integrity of the cladding is based on the properties of the prior в phase, since the ZrO2 and oxygen stabilized a zones are too brittle to sus­tain a load (Strasser et al, 2010b). ‘Oxygen is the major source of cladding embrittlement as noted above and hydrogen is less likely to contribute to the embrittlement except to the extent that its presence increases the oxy­gen solubility’ (Strasser et al., 2010b ).