Integrity assessment of Doel I and II reactor pressure vessels

Doel 1 and 2 are two-loops pressurized water reactors of 390 MW electrical output, which were commissionned in 1974 and 1795 respectively. As in many other units of this vintage, the weld assembling the two core shells (forged rings) displays a relatively high copper content, detrimental to irradiation embrittlement sensitivity. The surveillance program results are rather atypical, and the surveillance specimens might not be fully representative of the vessel welds, which makes reliable predictions of the embrittlement difficult. This raises the issue of reactor vessel integrity in case of pressurized thermal shock, which is further complicated in these units by a relatively unusual safety injection system, with direct injection in the downcomer along the vessel wall. The present paper describes the approach used to demonstrate the integrity of the reactor pressure vessels. The first part consists of the assessment of the material degradation under service exposure, which involves: - Evaluation of surveillance program results; - Charpy and mini-tensile specimens reconstitution, in particular to optimize the use of surveillance material; - Chemical analyses; - Microstructural examinations; - Damage modeling (copper precipitation vs. other embrittlement factors) and micromechanics (effect on fracture toughness). The second part is the thermal-hydraulic and fracture mechanics analysis and the definition of mitigating measures (flux reduction, safety injection preheating, hardware modifications). The approach used and the corrective actions that have been implemented (or will be implemented in the near future), have made it possible to demonstrate successfully the integrity of the pressure vessels in case of pressurized thermal shock.