ThermalÐstructural calculations are performed and the results are used to predict the lifetime of a possible ITER EPP blanket. During the power plant life various in-service loading events can inflict damage to the structure and it is important to identify the main structurally damaging loading events and their operational limits. The calculations use the first wall configuration and coolant flow parameters based on the high temperature, helium cooled, martensitic steel, European blanket design [1,2] proposed for DEMO, but assume loading conditions anticipated in the ITER EPP. The lifetime estimation accounts for the cumulative damage from both creep (thermal and irradiation) and fatigue, and includes loading due to normal power on, over/under power transients, disruptions, and powerÐoff or scheduled maintenance periods.

The calculations and analysis of the results suggest that the zeroÐfatigue lifetime (creep damage only) for low coolant temperature is 6.2 years, and for high coolant temperature is 3.08 years. When nominal contributions from all the anticipated loading conditions are accounted for, the calculated lifetime for low coolant temperature (315.5 C) is 3.24 years, and for high coolant temperature (375.0 C) is 2 years. In the case of normal load operation only, the respective lifetimes for the low and high coolant temperature values are 4.55 years and 2.61 years.

All cumulative damage mechanisms examined in this study generally exhibit a dampened behaviour as the operational temperature decreases. A breakdown of the relative contributions of creep and fatigue to component cumulative damage, shows that ~60% is due to creep at normal load, and ~40% is due to fatigue, with ~15% due to power up/down and ~25% due to overpower transients. The effect of the helium to displacement damage ratio on the design creep curves is not accounted for in this analysis. More experimental data and analysis of the microstructural behaviour of martensitic steels are needed in order to make accurate judgement of the effects on lifetime. Creep, both thermal and irradiation, is the primary limiting mechanism for good component lifetime; the bulk of the cumulative damage is due to the powerÐon period. Lifetime improvement is possible if the time to failure at the full powerÐon stress level can be extended. This can be achieved through geometrical shape optimisation for minimising the stress level at the critical points of the component.

L. Giancarli (Ed.), Report DMT/ 94/538, CEA, France, 1994.
M. Dalle Donne (Ed.), Report KfK 5429, Germany, November 1994.

This work was jointly funded by the UK Department of Trade and Industry and Euratom.