Ageing mechanisms

This section describes the ageing mechanisms that can affect BWR RPVIs and evaluates the potential significance of the effects of these mechanisms on the continued performance of safety functions of the RPVIs throughout the plant service life. 

Ageing mechanisms are specific processes that gradually change characteristics of a component with time and use. Ageing degradation are those cumulative changes that can impair the ability of a component to function within acceptance criteria. Service conditions outside prescribed limits can accelerate the rate of degradation. 

Evaluation of age related degradation mechanisms is based on BWR service experience, pertinent laboratory data, and relevant experience from other industries. The following mechanisms will be reviewed and assessed for relevance to RPVIs  

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Aging. When a gel is maintained in its pore Hquid, the stmcture and properties continue to change long after the gel point. This process is called aging. Four aging mechanisms can occur, singly or simultaneously polycondensation, syneresis, coarsening, and phase transformation (9,21). 

Polycondensation, 10 189-190, 191 as an aging mechanism, 23 64 of polyamide plastics, 19 781-782 polyester formation by, 20 390-391 silicone polymerization via, 22 556-558 in the sol-gel process, 23 61-62 Polycrystalline alloys, 13 523 Polycrystalline diamond films, deposition of, 24 744-745... 

Accelerated tests usually require the application of a higher electrical stress (voltage) coupled sometimes with an increase in temperature as a second accelerating factor. As in all accelerated testing, care must be taken to ensure that this does not introduce ageing mechanisms different to those that occur in service. The most common relation used to predict insulation life in the presence of partial discharges is the power law ... 

On the other hand, it is necessary to be aware of the ageing, mechanical resistance and thermomechanical behaviour, which are different from those of metals. The recycling of plastics presents some difficulties that are not generally solved in a satisfactory way. 

Bergamini, E., Cavallini, G., Donati, A., and Gori, Z. (2007). The role of autophagy in aging. Its essential part in the anti-aging mechanism of caloric restriction. Ann. N. Y. Acad. Sci. 1114, 69-78. 

Fig. 13. C6 aging mechanism ring isomerization site deactivation.

Unusual features of vitamin E substances as observed by various investigators include (1) the vitamin may be involved in aging mechanisms by protecting unsaturated fatty adds and membranes against free radicals ... 

Finally, there are no systematic differences in the aging mechanisms of linear and network polymers. What is different is essentially (i) the degree of difficulty of analytical studies, especially determinations of the crosslink density and (ii) the lack of theories (in thermosets) linking the network structure to properties, especially ultimate mechanical properties. There is a need for basic research in this area to reach a full understanding of aging effects on use (especially fracture) properties. 

Myers BL, Badia P. Changes in circadian rhythms and sleep quality with aging mechanisms and interventions. Neurosci Biobehav Rev 1995 19 553-571. 

Lane MA, Baer DJ, Rumpler WV, et al. Dietary restriction lowers body temperature in rhesus monkeys consistent with a postulated anti-aging mechanism in rodents. Proc Nat Acad Sci USA 1996 93 4159 1164. 

Knights S, Colbow K, St-Pierre J, Wilkinson D, (2004). Aging mechanisms and lifetime of PEFC andDMFC. Journal of Power Sources, 127 127-134. 

RHEOLOGY OF PYC PLASTISOL - X. VISCOSITY AGING, MECHANISMS AND RESIN TYPES... 

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