Stress and creep damage analysis for HTR heat exchangers at very high temperatures

Abstract The Helium-Cooled High Temperature Reactor (HTR) can supply process heat for methane reforming at temperatures of nearly 1000°C. As is the usual requirement for equipment used in a nuclear plant, the process heat exchangers must operate with a high reliability. This implies both that the exchanger tubes must be made from alloys which are very resistant to high temperatures and that the operational pressures are adjusted so that primary stresses in the tube walls are small. However, secondary stresses—an order of magnitude larger than primary stresses—are produced by the temperature profile through the wall. With consideration of creep, the time-dependent stress-strain distributions in the tube walls were calculated using the Finite Element Programme ASKA. The creep damage was estimated using Robinson's rule. The results showed that operation was possible for 100 000 h. Under accident conditions, load-controlled stresses in the tube walls may be an order of magnitude larger than under normal operation conditions. Here the stress distribution near the tube closure was investigated and it was found that lower stresses occurred in the transition and closure region than in the undisturbed tube area. In this way it could be shown that the closure is not seriously put at risk.