Structural zones, degradation process

As for linen and other natural fibres, silk is sensitive to a variety of environmentally driven degradative processes, though in most cases the actual damage is caused by hydrolysis and/or oxidation. Attack on the polymer chains is generally initiated in the amorphous zones as a consequence of their more open structure and the incidence of reactive amino-acids (specifically histidine, lysine, phenylalanine, proline, threonine, tryptophan, tyrosine and valine). 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

Based on available literature, the in-situ decay of MTBE is in principle difficult and occurs under natural conditions very slowly or not at all. A substantial reason for this is the structure of the MTBE molecule in the form of a tertiary chain and an ether bond. Only very few microorganisms are able to consume MTBE as an exclusive carbon source for minerahzation [6]. Further, in spill areas the microbial activities create an oxygen-depleted zone and under such anaerobic conditions MTBE is difficult to degrade by natural processes alone. A better chance for MTBE decay occurs in the downstream area after MTBE is separated from the other gasoline components due to its higher transport velocity. 

In the event of SCB failure, the structures comprising the boundary of the Zone 2A canyon provide a secondary confinement barrier to the release of radioactive materials to the HCF and to the environment. This is a major contributor to defense in depth in that, in abnormal or accident conditions where the SCB boundary has been degraded, an independent and redundant confinement capability exists to mitigate the release of radiological materials. The process of filtration of this airflow is similar to that for Zone 1, however only one stage of charcoal filtration is provided in the MER, based on the expectation that radiological inventories in the Zone 2A canyon will be significantly less than those in Zone 1, except under accident conditions. 

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