[en] The shift in the transition temperature (ΔT_F) of the core region material due to neutron irradiation is one of the basic parameters when establishing safe operation of the RPV (Reactor Pressure Vessel) through the design life time. The first surveillance tests in Loviisa showed that the irradiation embrittlement was much faster than expected on the basis of the design information. As to these results, IVO was forced to take special measures to reduce the neutron flux on the vessel wall, on one hand, and, on the other hand, to soften thermal stresses on the RPV wall in a possible emergency core cooling condition. Today, investigations are concentrated on trepan testing and annealing of the RPV material. (orig./DG)