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Transformer Explosion Closes Nuclear Plant Unit North of NYC
Reuters reports that a transformer failure and related fire have forced the closure of a generating unit of the Indian Point nuclear plant, about 40 miles north of New York City; another generator at the same facility was unaffected. Witnesses reported seeing an explosion, as well as (according to NBC News) a "huge ball of black smoke" when the transformer exploded, which led to the shut-down of the site's Unit 3. The Reuters article says the plant "has long been controversial because of its proximity to the United States' largest city.
Indian Point is one of 99 nuclear power plants licensed to operate in the United States and which generate about 20 percent of U.S. electricity use, according to the U.S. Nuclear Regulatory Commission website.
A nuclear power power plant may have a dozen or more [back-up generators] in their generator building. Even replacing just one is not some sort of couple day task.
Quite right. But the reason there are so many is to provide redundancy - they are not all needed at once - and by having a "dozen or more" they are not all going to fail at the same time because of a transformer explosion. The power stations I am familiar with (I am a nuclear engineer in the UK) do not put them all in the same generator building either. Nor are they sited in locations prone to tsunamis and it does not look like Indian Point is either.
generators which are only rarely tested
On the power stations I deal with they are tested frequently. It is hard to judge the size of the generators in your linked picture because it is obviously taken with a very wide-angle lens. The ones I deal with are the same type as used in railway locomotives, and there are mobile trailers available with such generators.
A transformer blew, they do do that.
It is not uncommon for a large transformer to blow. I am a power station engineer and know of two events over 10 years at UK nuclear power stations. It is not a big safety deal apart from the possiblility of injuring people within say 50 yards, and I have been within sight of one (yet someone was worried about NYC 40 miles away!). These transformers tend to be in bays shielded from each other by thick masonery walls.
One unfortunate problem with nuke plants is that IIRC you have to have a continuous connection to the grid. If that connection fails, the plant has to scram to avoid damage to the generators (overspeed).
BS
Power stations (even non-nuclear) always have back-up generators that kick in on loss of grid to allow control of the plant to be maintained and for cooling pumps to take the heat out of the system in a controlled way. Generators will not overspeed if cut off the grid - their speeds are controlled by sophisticated control systems, and if they even fail then an old-fashioned back-up mechanical governor will cause the main steam supply valve to slam shut.
The plant would not be "scrammed" on loss of grid. Scramming means hitting a big red panic button. The plant would be kept spinning at first, obviously with the reactor power reduced to near zero, with residual heat being dumped through purposed heat exchangers and possibly releasing steam to atmosphere (unless it is a BWR - Indian Point is not), while the cause of the loss-of-grid was investigated - like getting the grid company on the phone. Many losses-of-grid are quite brief, but if it looked like it was going to be a while then the plant would be shut down in a controlled way, not by a scram button.