In the context of a three phase high voltage power network, a "réactif" is a detrimental power component generated by inductive loads on the network and which must be absorbed by the client (often by using a condenser) to prevent it from being passed back to the network. The direct translation of "réactif" in this context is "reactive power component", but that's a bit clumsy, "reactive power" conveys the meaning.

There are two types of electric power in a three phase grid, "actif" (translated "real", since that's what you get billed for), and "réactif".

For a brief explanation of how it works, see the links. (Page 2 in th epdf)

Although this is a chemical plant, I don't htink this use of the word "réactifs" has anything to do with chemicals in this case, but the plant's electric power substation.

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Note added at 6 hrs (2009-08-12 02:50:05 GMT)
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I already added a note but it didn't appear, so I hope this isn't a duplicate. The second URL above is way too long to work here, so try this one instead: http://tinyurl.com/qpwje9

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Note added at 6 hrs (2009-08-12 02:55:37 GMT)
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To clarify further, "component" in this context is not a physical object, its a part of the electricity waveform.