Analysis of a delayed return to full power using a 3D simulator

For an optimal operation of power plants, it has now become necessary to install expert system in the control rooms, particularly to give a help to the operators for predicting, following and explaining the operating events. The 3D MICROLUX code, foreseen for such system, was tested on the following operating event occurring on the DOEL-3 reactor. After a scram of about 6 hours, the return to full power (PN) was limited at 15% PN because of unacceptable axial offset deviation which was believed to be caused by the Xenon buildup during the shutdown. The reactor must then be operated at reduced power during 18 hours before reaching again the nominal power. From the study, it seems that there was no danger to return directly to full power in spite of the ex-core indications. The 3D calculations and the ex-core results show that there is a need of investigating the ex-core and in-core relationship when the reactor is operated outside the calibration field. These preliminary results indicate that the axial offset concept should be analyzed on the basis of a large number of transient operating conditions with the help of three-dimensional methods which give a better description of the core behaviour during transients. The use of such methods could contribute to a safer and more economical operation of the reactor