It's normally far more productive to say what you mean rather than ask silly questions and say why they weren't dumped.
In general I'd agree with that, and I had my doubts about how to approach this. In Mike's case in particular I found it would feel pretentious to give a long and potentially unwanted explanation out of the blue. I would much prefer if he had just said he was on top of things.
(...)
As far as I can tell, then all of this is accurate. That is however not what I was getting at with the 'dump' comment. I am not old enough to have worked with MV rectifiers, back when they were widely used, but my mentors certainly are. According to them many industrial MV installations were actively decommissioned ASAP, as alternatives became available. This makes it even more amazing that some are still in active service.
There are plenty of details on the wikipedia entry. But basically, these have a fair amount of liquid mercury in them, stored in a brittle glass bulb. If they shatter then the result is a very nasty environmental hazard / cleanup expense. And I suppose in an explosive shatter there is probably quite a bit of vapor ejected too. Think about how freaked out people get over a single cracked mercury thermometer or tilt switch, and multiply that by a million.
Oh, but it is much better than that.
OK, here is the (overly long) explanation: Why you (probably) don't want an active MV arc rectifier in your home or museum.
The background here is that metallic Mercury at room temperature is pretty benign. It wouldn't be a good idea to ingest some, but having it sitting in for instance an open bowl would not be a major problem. The reason is that the vapor pressure of Mercury at normal room temperature and air pressure is quite low, so the rate of evaporation would be negligible. Even accidentally dropping some Mercury on the floor of your lab isn't that much of a problem. There are even standard lab tools for dealing with this situation, so you are probably fine if you pick it up in a reasonable time frame. The rate of evaporation will be miniscule due to the relatively small surface area exposed to the air.
Enter the industrial, glass encased MV arc rectifier, and its smaller sibling, the 'vacuum tube' edition with a heated cathode.
The first problem is that to make a decent live display of a large MV arc rectifier, you need to pass some *serious* power through them, along with the cooling to match. There is a video on YouTube showing Mike's substation in action, and here it powers a train, or rather a tram. To put this in perspective, then a KW level radio transmitter would just need a single pair of beer bottle sized MV rectifiers for its PSU. So you need serious power/current for your live display (or show it in complete darkness, and hope people won't be too disappointed).
The second problem is that you need flexible leads for most of the electrical connections to the rectifier. Otherwise the mechanical stresses due to thermal expansion would cause cracks in or downright failure of the vacuum vessel.
So even if gravity helps out, then in case the glass vessel breaks there is a very good chance you'd - briefly - have very conductive Mercury, lots of electricity and wildly dangling wires all over the place. This is in addition to the Mercury dropping down on the cooling fan and being splattered all over the place. Those of you, who may have seen an accident involving a car battery, may be able to guess what comes next.
"
Mercury smoke. Don't breathe this."
OK, so maybe we predicted what was about to happen, and teleported out of there just before the Mercury hit the fan. We'll just wait for the Mercury to condense out of the air and go clean up. Right?
Erm, no, this is where it gets ugly.
We just created a sizable amount of microscopic pellets of liquid Mercury, small enough to be carried short distances by the normal air currents. Genuine Mercury smoke. That stuff will go
everywhere in the building in question, snuggling into every nook and crevice in fabric, untreated wood, clay/brick, etc. Even better, the tiny droplets will create an amalgamate with many metallic surfaces, making the Mercury impossible to clean off.
And for fun, the total surface area of the combined mass of micro droplets is no longer trivial, compared to the vapor pressure. Additionally the Mercury will also stick to hot surfaces, like stove plates, room heaters, incandescent light bulbs, electronics and more. As a result you now have noticeable amounts of Mercury vapor in the local atmosphere, at least until you remove
all the Mercury smoke particles from, well,
every surface in the building. Which is physically impossible.
You now have two options:
*) Clean away any visible traces and spills from the accident, and pretend it didn't happen. This is likely to cause children spending much time in the building to grow up and be stupid. Literally. Adults are just likely to get sick, and may potentially die in due time. Mercury is like Lead, just a lot worse.
*) Condemn the building, tear it down, and store its remains and the items within as hazardous waste. This will be a popular option for a museum.
Lastly, the beautiful purple glow they emit while operating is very heavy in a wide range of UV wavelengths. I'm not sure I would want to stare at one or even stand near it for too long.
Shhh! Some people may use this as a selling point, as the museum visitors may actually want a nice tan...
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