Sentence:  Furthermore , the exchange would not be expected to be sensitive to trace amounts of impurities because it would not be apt to be a chain reaction since the <WSD>activation</WSD> energy for abstraction of chlorine by a chlorine atom would be expected to be too high ; also it would be expected that * * f would compete very effectively with any impurities as a scavenger for * * f radicals . 
Meaning of the word marked with <WSD> tags:  the energy that an atomic system must acquire before a process (such as an emission or reaction) can occur