Vessel material embrittlement due to neutron irradiation

This is one of the major safety concerns for the pressure vessel of PWR reactors and, therefore, for the safety of the reactors themselves. For boiling water reactors this problem is smaller because of the lower neutron flux on the vessel walls, which is a typical characteristic of this type of plant, principally due to dimensional factors (the design pressure of BWRs is roughly one half than that of PWRs and 

It is known that the neutron irradiation causes, in the carbon steels used for vessels, an increase of the transition temperature between the brittle and the ductile behaviour of the steel (RTNDT, Reference Temperature for Non-Ductility Transition). This temperature is typically — 10°C at start-of-life and, with increasing irradiation, may increase by many tens of degrees in the course of years. 

It is obvious that below the transition temperature a crack which reaches a super-critical size may propagate and cause the brittle and catastrophic rupture of the vessel and, moreover, the stresses for which a crack becomes critical are lower. 

As the vessel must not break, the importance of the control of the embrittlement of the material during the plant life can be easily understood. One of the fundamental safety assumptions of water reactors, in fact, is that the break of the vessel is made impossible by design, construction and operation provisions. 

It must be remembered that an explosion of the vessel might break all the four barriers against the external releases of radioactive products at the same time (see Chapter 9). 

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