[stoppingdown]

I've not used a flash for about ten years, some time before moving from Nikon to Sony. At the time it was (seldom) used for fill light in bird photography (with the Better Beamer). I still have a bunch of AA NiMH batteries that haven't been used since then.
Now I bought a small Meike Mk-320S for macro photography and AA batteries are useful again. I'm trying to guess whether it is safe to reuse the old ones, or whether there are risks of fires, chemical leaks etc...
Of course the problem is not to buy new NiMH batteries, they are cheap, but to avoid to dispose things that are a special waste.

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Ah, I forgot another possible problem: the Mk-320S is able to recharge the batteries by connecting to a power source and of course I don't want to risk to damage it, even though it's cheap.

[toni-a]

I am actually using them without any issues, of course they don't last just like new ones but they are still okay, just have another set as backup if you will need to use them extensively (fashion, weddings, etc)
I noticed also they self discharge rather quickly, so don't forget to recharge them before using them,although this can be due to Qatar very hot weather, Li-ion batteries in phones and cameras and lead acid batteries in cars are already struggling, Ni-Mh shouldn't be an exception.

[wim]

If you haven't used them and recharged them in a long time, they may have passed on to battery-heaven, though, as in, they will not work anymore.

Just try and charge them and check if they will hold at least a 1.2 to 1.3 volt charge.

[olandese volante]

With NiMH AA cells, there's no real danger of explosion or fire unless you dead-short a fully charged cell, they're not as touchy as Li cells.
Old cells might leak, don't bother with cells that have white or greenish crud on them near the seal (+ end), these should be disposed of safely.

The simple way to find out if an old NiMH cell is still good is to put it in a smart charger. Such chargers can sense and reject bad cells.
One problem that a smart charger can't detect is excessive self-discharge, which would drain the cell in a few weeks. The solution is to let your charged set sit on the shelf for a few weeks and put them in the charger again. The charger will tell you the state of charge of each cell, either directly or by the time it takes before the charger signals "charging complete". Good cells can hold their charge for months.

One problem that old cells invariably suffer is tarnished contacts. This is easily fixed by a gentle pass with a fine wire brush. It is essential that the contact be nice and shiny, even a slight tarnish will render an otherwise good cell useless.

[apricotsceiling]

Placing an old NiMH battery in a smart charger is the easiest approach to determine whether it is still functional. Bad cells can be detected and rejected by such chargers. Excessive self-discharge is one issue that a smart charger can't identify since it would quickly exhaust the cell. The fix is to put your charged set back in the charger after a few weeks of sitting on the shelf. 

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