It is indeed a technical term, referring to exposed (and therefore uncooled) tubes:

Two separate accident scenarios are addressed. In the first scenario, it is assumed that the non-safety grade startup feedwater system is not available to provide feedwater to the steam generators. In this event, the water level in the steam generators drops, resulting in tube uncovery and there is flashing of a portion of the primary coolant assumed to be leaking into the secondary side of the steam generators. Also, the period of steaming is terminated at 1.5 hours when the capacity of the passive residual heat removal system exceeds the decay heat generation rate.

http://www.nrc.gov/reactors/new-reactors/design-cert/ap1000/dcd/Tier%202/Chapter%2015/15-3_r15.pdf

Indeed, it is a good question. I checked several dictionaries for the word 'uncovery' but could not find it.

TheFreeDictionary has an entry for this but it does not clearly state anything on the page.

I think it is not widely used (COCA returns one result to the search 'uncovery'); MS word shows 'error' in the spelling and as I said, dictionaries including MM, OALD, Cambridge, WordWeb don't have such entry.

This seems to be a technical term or jargon in some context, not necessarily available on dictionaries. However, I had come across somewhere something like: "...leaving out an uncovery space on a glass.'

In addition, to make the verb 'to uncover' a noun, it would be 'uncovering' i.e. a gerund word. Uncovering is found in dictionaries.