In the second decade of design and construction of water-cooled and watermoderated power reactors in the Federal Republic of Germany (FRG), the neutron irradiation effect on reactor pressure vessels (RPV) was reduced in a twofold manner: 1. Steel optimization for high toughness at the end of design life. 2. Fluence reduction by adaptation of core geometry and RPV diameter.

The methods of fluence evaluation and results for different core configurations are described. They provide a satisfactory fluence surveillance of the RPV. If necessary, operational measures are possible to reduce the end of life (EoL) fluence of the RPV. The provident fluence reduction measures taken in two older commercial power plants are described. These measures reduced the EoL fluence by more than a factor of two without loss of power.

The new generation RPV provides a larger water gap between core edge and RPV wall and therefore much lower EoL fluences. The design EoL fluence of all 1300-MWe and the modern 1000-MWe PWR is 5 × 10 ¹⁸ cm ^-2 and 3.3 × 10 ¹⁸ cm ^-2, respectively.

Fine-grain forged steels 22 NiMoCr 3 7 and 20 MnMoNi 5 5 and corresponding weldments are used for the RPV in KWU-built nuclear power plants. Extensive experience has been gained on the irradiation behavior of these steels. By specifying high purity, especially for the elements copper (Cu) and phosphorus (P), and by keeping Nickel (Ni) at medium levels, which improves preirradiation toughness but supports irradiation effect, high postirradiation toughness is maintained.

Therefore, in combination with the low EoL neutron fluences, the nil ductility transition temperature shift Δ T ⁴¹ was reduced by nearly 90% by comparison with the first generation plants (high fluence, high copper).