How Long Would The Lights Stay On?

[mwbrooks]

@chorpler

ETA: On a more serious note, isn’t Una’s report saying that the plants would go down because when part of the grid fails it forces the plants that are still online to shut down lest they try to supply too much power and overload? So if you could isolate power plants from the grid (like Cerowyn mentioned), they should still be able to produce power as long as things in the plant are running, right?

The power grid is a sort of balancing act. There is no significant power storage capability, so power plants have to continuously adjust their output to match the demand.

As I understand it, past regional blackouts have occurred when a plant or transmission line shut down for some reason, causing the load to exceed the remaining plants’ capacities, so all those plants shut down to protect themselves. Supposedly steps are taken after each blackout to prevent this kind of thing. There’s always another blackout, but maybe the next one isn’t as big as it would have been.

Also as I understand it, a plant can’t just “disconnect itself” from the grid and keep generating, because it will also be damaged if it continues to operate without a load. So while Hoover dam could conceivably continue operating, someone or something would have to be doing a quickstep to keep its output matched to the remaining load. The smaller the remaining grid, the harder this is. This might be easier with hydroelectric than with other power sources, I don’t know.

I’ve read about steps being taken to put some storage capacity into the national electric grid, but I gather it’ll be a long time, if ever, before there’s any significant amount.

[Spoke]

@robby

Anyway, the show’s repeated emphasis on the supposed aftereffects of spent nuclear fuel somehow resulting in catastrophic meltdowns makes me think that someone involved with the show has an anti-nuclear power agenda.

I kinda got that vibe, too.

They also seemed weirdly obsessed with poisonous gases, given that even if there were a leak of such gases, the effect would be short-term and localized.

[chorpler]

@mwbrooks

Also as I understand it, a plant can’t just “disconnect itself” from the grid and keep generating, because it will also be damaged if it continues to operate without a load. So while Hoover dam could conceivably continue operating, someone or something would have to be doing a quickstep to keep its output matched to the remaining load. The smaller the remaining grid, the harder this is. This might be easier with hydroelectric than with other power sources, I don’t know.

No problem – we’ll keep a bunch of big ol’ resistors in the lake and cycle a new one in when we shut off the microwave.

[tracer]

@zev_steinhardt

I live in New York City. Suppose I were to wake up one morning and find that I was the only person in the city.

You can rest easy in that case. New York City gets plenty of its power from power plants outside the city – and you didn’t say anything about the OTHER cities being deserted!

[mwbrooks]

Post deleted by poster out of sheer embarrassment.

[si_blakely]

@robby

So anyway, I finally watched the show myself last night, and actually, they did NOT say that the nuclear power plants would melt down. Instead, their concern was with the spent fuel in holding pools.

I’m not an expert regarding spent nuclear fuel, but my initial reaction is that the show’s concern is greatly overblown. I find it hard to believe that that much residual decay heat would be produced from spent nuclear fuel. After all, the point of the Yucca Mountain storage facility is to encase spent fuel in glass containment with no cooling water at all. It’s my understanding that spent fuel repositories do not depend on active cooling (which would require sustained backup power), but instead use passive cooling (such as some type of natural recirculation).

Initially, the spent fuel generates a lot of heat. After most of the shortlived radionucleotides have decayed, the cooling requirements fall off (allowing noncooled longterm storage). And while passive cooling is sufficient at this stage, the water does get hot. And if it evaporates away in the absence of human monitoring, the spent fuel is not longer cooled or moderated, and a disaster is waiting to start.

Of course, there is no-one around to worry about the situation - this is a posthuman world, after all

Si

[robby]

@si_blakely

Initially, the spent fuel generates a lot of heat. After most of the shortlived radionucleotides have decayed, the cooling requirements fall off (allowing noncooled longterm storage). And while passive cooling is sufficient at this stage, the water does get hot. And if it evaporates away in the absence of human monitoring, the spent fuel is not longer cooled or moderated, and a disaster is waiting to start.

Of course, there is no-one around to worry about the situation - this is a posthuman world, after all

Si

Perhaps you could elaborate a bit more on this?

First off, it seems to me that the lack of moderation is a good thing, not a bad thing. After all, the point of moderation is slow down the fast neutrons (which have little tendency to cause subsequent fissions) down to thermal neutrons (that are much more likely to cause a chain reaction).

Secondly, if passive noncooled storage is sufficient for the spent fuel, what difference does it make if the fuel is surrounded by water or not?

[Nuke]

I hate to tell you this, but when the Diesel Generators run out of fuel and there’s no offsite power, the cores will melt. Also, the water in the spent fuel pools will eventually boil and evaporate, making even more of a mess. Better to keep some folks around…

[si_blakely]

@robby

First off, it seems to me that the lack of moderation is a good thing, not a bad thing. After all, the point of moderation is slow down the fast neutrons (which have little tendency to cause subsequent fissions) down to thermal neutrons (that are much more likely to cause a chain reaction).

In the context of a nuclear reactor, you are correct. However, the radioactivity from spent fuel is not fission generated neutrons - it is mostly alpha, beta and gamma radiation from radioactive decay. There may be some slow decay neutrons as well, but the surrounding water will slow them even more. And the spent fuel rods are no longer fissile - they are spent, full of neutron absorbing fission products and a bit of residual Pu and U. If all the spent fuel melted together, it would be a mess and there may be some additional fission, but probably not much. The bigger problem is the whole mass catching fire, because then you have radioactive combustion products spewing into the air.

@robby

Secondly, if passive noncooled storage is sufficient for the spent fuel, what difference does it make if the fuel is surrounded by water or not?

For the same reason cars are mostly water-cooled - water (even just from internal convection) is a better heat transfer medium than air, needed during the initial decay period when the fuel is much hotter thermally than later, when all the ‘hot’ radioactive material has decayed away and the temp has fallen away. Only then can the spent fuel be processed into passively cooled storage (and by using deep underground storage, they are using the surrounding rock as a massive heatsink). One proposed disposal scheme was to place enough waste in a massive heatproof container, drop it down a shaft in a granite slab, and let the heat melt the rock so that the container just kept on going slowly down through the mantle. Eventually, it would melt, but it would be deep enough not to matter at that point. The risk was that a volcano could force back up the descent tube, adding radioactive lava to the localities problems.

Si

[rocking_chair]

did any of the shows mention hydro power that is run off of falls? niagara, for example.

[Una_Persson]

@jimmmy

Well, I certainly don’t have the chops or inclination to say that Una is mistaken overall. Perhaps she was speaking generally or perhaps she didn’t check deeply into Hoover-as-exception or perhaps things have changed since the report was written.

I was speaking of hydro plants in general, but even then I challenge that Hoover could stay running and generating fully unattended for anywhere near 2 years. “Alarms” happen all the time at power plants, even the hydro ones I’ve worked at, and they very often require a human response to allow the plant to continue running. Unless Hoover is set up with something like sextuple-redundant intelligent systems and such, I just can’t see it. Remember that the folks programming the plant systems are not setting them up to run unattended - they’re setting them up with the expectation of running reliably, but with human oversight. While I won’t speak for Hoover in specific, IME even a worthless BS alarm that actually means nothing can shut down a plant if it’s ignored long enough, simply because someone put in the code “if this warning about gate position sensors is still going on after 2 days, and no one’s noticed, then something is seriously fucked up, so start shutting that gate.”

Hoover probably could be set up to run unattended for months or years (until it lost connection and tripped…), and that may be what they really mean to say, but I would bet that it is not currently set up to do that.

Like mwbrooks says - the whole system is a very delicate balancing act in some places, and it can fall prey to very sudden and unexpected weaknesses - witness the latest blackout in south Florida, cascading from a relatively simple issue at a single point.