Dave -
Ouch! Drill 'n' retap is an unhappy process. If it happens again, would you consider using a screw extractor or an EZ-OUT, or even a drill with a left-hand thread? With one of these devices, you can sometimes drill only partway through the stuck bolt (or in this case, the anode rod plug). If you can do that, you avoid introducing metal chips into the tank. Regardless, I would always start with Dick's oil and tap-tap-tap, then put the extractor device to work. Try turning it with a hand-turned tap wrench at first. If you really can't get it, an impact wrench as Wavery pointed out might be the way to go. But avoid over-slam-torquing the extractor. If it breaks off in the hole, you are in really deep yogurt because it is hardened steel, and you can't drill into it without special bits.
Just as thought, I have heard that if you press a piece of dry ice against the broken stub, the thermal shock may break it loose. Or at least it may shrink the diameter of the stub enough to loosen the grip. Never tried it, though. Dry ice is available in supermarkets around here..
Incidentally, I followed the old mechanic's rule. You only need an extractor if you don't have one. So buy a set of extractors and put them on the shelf. Sure enough, I bought a set of extractors 20 years ago, and I've never had to use them.
I don't envy you that process.
Bill
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