Irradiation embrittlement mitigation

Neutron irradiation affects the material properties, and hence the structural integrity, of reactor pressure vessels in nuclear power plants. Mitigation of irradiation damage is one of the major issues within nuclear plant life management. An overview is given of proposed and utilized mitigation methods. Low-leakage loading schemes are commonly used in (PWRs) to mitigate a consequence of reactor pressure vessel (RPV) embrittlement. Dummy assemblies have been installed in WER 440-type and in some old western power plants where exceptionally fast embrittlement has been encountered. Shielding techniques for pressure vessels have been developed, but these are not in common use. Prestressing of the pressure vessel has been proposed as a potential method for preventing RPV failures due to pressurized thermal shock (PTS) loading, but applicability of the method to nuclear pressure vessels has not yet been demonstrated. The large number of successful annealing treatments performed in WER 440-type reactors, and the intensive research and development work done on the methods and benefits of annealing treatments during the recent years, suggest that more applications can be expected in the near future also in western PWRs. The emergency core cooling systems have been modified in WER 440- type reactors in connection with other mitigation measures, and in some western PWRs. Efforts to extend the service life of reactor pressure vessels further emphasize the role of plant specific surveillance programmes. The embrittlement management of PWRs should include evaluation of all realistic mitigation alternatives, and generation of additional material data whenever possible, before final decisions on life extension measures are made. The use of decision tools are recommended in dealing with the complex PTS issue.