Radiation embrittlement reactor pressure vessels

Popp, K., Brauer, G., and Leonhardt, W.-D. et al., (1989) in Radiation Embrittlement of Nuclear Reactor Pressure Vessel Steels An International Review, ASTMSTP1011, ed. By L.E. Steele (American Society fro Testing and Materials, Philadelphia)... 

Key words pressurized water reactor (PWR), reactor pressure vessel (RPV), surveillance database, radiation embrittlement. 

Abstract This chapter describes the embrittlement processes in WWER reactor pressure vessel (RPV) materials during operation - radiation embrittlement and thermal ageing. Current trend curves for both types of WWER RPV materials are given and explained. Surveillance specimen programmes are shown, as their results are used for RPV integrity and lifetime evaluation. Finally, anneahng of the RPV is proposed as the most efficient mitigation measure. 

Key words WWER reactor pressure vessel, radiation embrittlement,... 

It is widely known that radiation embrittlement behaviour of reactor pressure vessel (RPV) steels depends on various parameters such as material composition, neutron flux and irradiation temperature. Sound understanding and modelling of embrittlement mechanisms require systematic knowledge of effects of individual parameters and their synthesis on microstructural development and then mechanical properties. Most such knowledge has been obtained from single-parameter experiments using test reactor irradiation. This is because test reactor irradiation allows researchers to obtain mechanical property data together with microstructural data on materials with well-controlled chemical compositions under well-controlled irradiation conditions such as flux and temperature. Surveillance data in commercial power reactors are non-systematic in this context and relevant microstructural data are very scarce. 

A. Munier, R. Schaller, O. Mercier and W.B. Waeber, Interactions of defects with dislocations in reactor pressure vessel steels . Radiation Embrittlement of Reactor Pressure Vessel Steels An International Review (Fourth Volume), ASTM STP1170, L.E. Steele, ed., American Society for Testing and Materials, Philadelphia, PA, 1993,269-282. 

H.S. Palme, Radiation Embrittlement Sensitivity of Reactor Pressure Vessel Steels, BAW-10056 Topical Report, Babcock and Wilcox, Lynchburg, VA, March 1973. 

C. EngUsh, S. Ortner, G. Gage, W.L. Server and S.T. Rosinski, Review of phosphorus segregation and intergranular embrittlement in reactor pressure vessel steels . Effects of Radiation on Materials 20th International Symposium,... 

K. Dohi, S. Ishino, K. Nishida, A. Nomoto and N. Soneda, Embrittlement correlation method for the Japanese reactor pressure vessel materials . Effects of Radiation on Nuclear Materials and the Nuclear Fuel Cycle, 24th Volume, J.T. Busby and B. Hanson, eds, ASTM International, West Conshohocken, PA, 2010, 64-93. 

KTA, Monitoring the Radiation Embrittlement of Materials of the Reactors Pressure Vessel of Light Water Reactors, KTA 3203 (3/84), Nnclear Standards... 

It has been recognised that decommissioning sites can be a useful source of information that is relevant to those stations that are still operational. For example, one of the key issues relevant to the safety cases for the continued operation of the steel reactor pressure vessel Magnox stations is the effects of neutron irradiation induced embrittlement. The reactor vessels at Trawsfynydd have been identified as a source of representative and highly irradiated material that can be sampled and subjected to detailed analysis to determine the actual rather than just the predicted effects of embrittlement. This has required special remotely operated equipment to be developed that is capable of reaching the base of one of the reactor vessels, which is not readily accessible and is in a high radiation area, and that can then remove and retrieve steel samples for laboratory analysis. This equipment is now being deployed successfully. 

Reactor pressure vessel supports are subject to neutron irradiation at low temperature during plant operation. The neutron flux is lower than that at RPV, but the low irradiation temperature could result in higher embrittlement rate. Some reactor pressure vessel supports were fabricated without special requirements on fracture and radiation resistance. Steel surveillance specimens that had been irradiated in an environment believed to be similar to that in operating reactor cavities exhibited a greater than expected shift (increase) in the nil-ductility-transition temperature. This indicated that there was a potential for excessive embrittlement of reactor vessel supports. Moreover, the RPV supports are in many cases difficult to access or inaccessible for in-service inspection. 

NUREG/CR-5644, Consequence evaluation of radiation embrittlement of Trojan reactor pressure vessel supports, Oak Ridge National Laboratory, October, 1990. 

One of the main tasks of nuclear-reactor safety research is assessing the integrity of the reactor pressure vessel (RPV). The properties of RPV steels and the influences of thermal and neutron treatments on them are routinely investigated by macroscopic methods such as Charpy V-notch and tensile tests. It turns out that the embrittlement of steel is a very complex process that depends on many factors (thermal and radiation treatment, chemical compositions, conditions during preparation, ageing, etc.). A number of semi-empirical laws based on macroscopic data have been established, but unfortunately these laws are never completely consistent with all data and do not yield the required accuracy. Therefore, many additional test methods are needed to unravel the complex microscopic mechanisms responsible for RPV steel embrittlement. Our study is based on experimental data obtained when positron annihilation spectroscopy (PAS) and Mdssbauer spectroscopy (MS) were applied to different RPV steel specimens, which are supported by results from transmission electron microscopy (TEM) and appropriate computer simulations. 

Low pressure in the primary circuit enables to reduce the thickness of reactor vessel walls and to use for its fabricating less strong austenite steel being resistant to radiation embrittlement under operation conditions and eliminate the possibility of vessel brittle damage. This enhances safety and eliminates the restrictions on temperature change rate on conditions of thermocycling strength and radiation life time of reactor vessel. 

R B Jones and C J Bolton, Neutron radiation embrittlement studies in support of continued operation and validation by sampling of Magnox reactor steel pressure vessels and components, Proc. 24 Water Reactor Safety Information Meeting, NUREG/CP-0157, US Nuclear Regulatory Commission, Washington, DC, 1997, 2,25-48. 

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