The exposure of reactor vessel materials to a fast neutron flux causes a number of mechanical property changes. For instance, the brittle-to-ductile transition temperature increases and the fracture toughness is reduced. Pressure water reactors and boiling water reactors contain a series of pressure vessel steel surveillance capsules as the principal means of monitoring radiation effects on the pressure vessel.
To date, twelve capsules have been removed and analyzed from seven of the nine Spanish reactors in operation. These capsules typically contain neutron dosimeters, temperature monitors, and metallurgical samples from the vessel.
The surveillance data of six Spanish nuclear power reactor vessels have been used for determining their reference temperatures and upper shelf energies at any time. The results have shown that the six vessels are in excellent condition to operate safely with regard to brittle fracture beyond their design life. The worst case permits extension of the vessel life for an additional 22 effective full power years without the necessity of an annealing treatment to recover the original properties of the materials.