Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

- 6 -

1 INTRODUCTION

In reactor pressure vessel steel, several factors such as irradiation, cyclical, thermal and static stress as well as the hydrogen absorbed in the steel cause the properties of the steel to deteriorate. The reactor pressure vessel steel comes into contact with the reactor water locally if the pressure vessel's welded stainless-steel cladding is penetrated by defects such as stress corrosion or corrosion fatigue cracks. The underclad cracks generated in the pressure vessel steel during manufacturing are also associated with the elevated hydrogen content of steel.

This study looks mainly into hydrogen and irradiation embrittlement as well as the effects of the stainless steel used as cladding material under the PWR conditions.

2 HYDROGEN CONTENT OF PRESSURE VESSEL STEEL

Pressure vessel steel absorbs hydrogen:

- when the hydrogen in the reactor water is dissociated on the surface of the steel; - from the corrosion reactions on the surface of the steel; - from the radiolysis of water; - and from the transmutations caused by irradiation. Hydrogen absorbed during steel corrosion is by far the most important of these because the hydrogen generated in radiolysis, for instance, does not increase the hydrogen content of steel (Harries and Broomfield, 1963; Rossin, 1967). Consequently the steel's maximum hydrogen content is obtained when it is assumed that all the hydrogen developed in the corrosion is absorbed by the steel. At equilibrium, the same amount of hydrogen is diffused from the external wall of the pressure vessel as is absorbed by the steel from the internal wall. Tomlinson (1981) has shown that in oxygen-free high temperature water, more than 90% of the hydrogen generated in the corrosion is absorbed by the steel because the evolution reaction of the hydrogen takes place on the metal/magnetite boundary and the diffusion of hydrogen in magnetite is slow.