nuke reactor circulation pumps

[p.larner]

is there any reason the pumps need power from outide and not use the power the plant produces,apart from cost?.

[jbb]

Buckle up! This could get bumpy.

As I understand it, you need to consider cases where something has gone wrong and the turbine has ‘tripped,’ ie disconnected from the grid and stopped generating.

Alternatively, we could think about refueling stops, where there’s no generation happening but you still need to cool the reactor core. They apparently generate a lot of heat even after all the fission reactions have stopped, because the immediate products of the fission reactions are very radioactive (and hence generate lots of heat).

[CatalinaWOW]

It is my understanding that most of them do use internally generated power, with multiple back ups like grid sources and on site gas or diesel generators to deal with situations jbb mentioned and many more. 

[Andy Chee]

Using Fukushima as an example.  Immediately upon earthquake detection, all the control rods dropped into place and shutdown the nuclear reaction and power generation.

Despite shutdown, the core remains hot, much like a car engine that has just had its ignition key removed a minute ago.  Unlike a car engine though, the Fukushima reactor is unable to dissipate the heat naturally, and so requires cooling pumps.

But with the reactors shutdown, how do you power the cooling pumps?  You use either or both, grid power or onsite diesel generators.  In the case of Fukushima, the tsunami wave that arrived an hour after the earthquake, flooded and destroyed the switch gear for both.

Evidently an hour of cooling is not enough.  The shutdown reactors remained hot enough to split water into hydrogen & oxygen.  The hydrogen production could not be contained, the hydrogen ignited, and blew the tops off the buildings (though the reactor pressure vessels & concrete radiation containment remained intact).

In summary, external power for cooling systems is mandatory for a nuclear power station (either grid power or diesel generators).  You can't rely on a shutdown core to generate enough power for the cooling pump.

Incidentally, there are newer Generation IV (four) reactor designs which use passive cooling techniques.  These new reactor designs could potentially do away with external power.  The Fukushima reactors are Generation II (two) design.

[floobydust]

Power plants require grid power to startup and run, usually called "station power". Ones I worked in (coal, natural gas) had separate lower voltage feed like 25kV bus lines from another grid interconnect. They did not power themselves, the output was 240kV or 500kV. You need power for plant SCADA and control, to operate valve actuators etc.

Reference the Fukushima nuclear disaster where the earthquake scuttled grid power lines, total loss of plant power.
Leaving only the emergency diesel generators but of course they were soaked with seawater by the tsunami and could not start, as well as the switchgear flooded. I'm not sure of the role of the DC battery system.

[Andy Chee]

As a slight tangent, you might be interested to know that even regular non-nuclear power stations require a source of external power. 

For example this turbine explosion was also a result of loss of external power:

https://www.abc.net.au/news/2021-06-02/qld-cs-energy-releases-photo-of-damaged-callide-power-station/100186330

[moffy]

Another nuclear power station event, Chernobyl, there is a computer simulator you can download to reconstruct the accident:

The video describes in detail what happened, and it looks like that even a shut down reactor has to deal with many MWs of thermal energy.

[Someone]

it looks like that even a shut down reactor has to deal with many MWs of thermal energy.

Somewhere around 6% of the thermal dissipation compared to full operation:
https://en.wikipedia.org/wiki/Decay_heat

[Jeroen3]

The scale of the systems required for the generators used in power plants requires more than 24V backup.
Each panel has that, but there also is a backup bus and emergency bus.

For example, you need to operate valves, fuel pumps, louvres, fans. 24V isn't cutting it.
Also lighting and pc's for the crew to command and control what is going on.

Then there is the problem that exciting these large machines so that they begin generating voltage requires external energy.
They're not self-starting, even if you get them spinning!

[Ranayna]

Despite shutdown, the core remains hot, much like a car engine that has just had its ignition key removed a minute ago.  Unlike a car engine though, the Fukushima reactor is unable to dissipate the heat naturally, and so requires cooling pumps.

Most cars nowadays keep the fans running for quite some time after your turn off iginition.
Some are really loud doing that.

[ejeffrey]

It's not just the main reactor.  The spent fuel rods are kept in wet storage for some time (a few years?) after being removed from the reactor before being moved to dry storage.  Even here the decay heat is enogh to boil the water away and overhead if pumps are not running.  This was a secondary problem in the Fukushima site, although as I remember it takes days before that becomes an issue.

[CaptDon]

The generators were available but mounted at ground level and made useless after having been submerged by the first wave!! I have always thought emergency generating equipment should be inside concrete buildings and at least 20 to 30 feet above local ground level. This is how our equipment was housed at the hospital where I worked. The fuel tanks were at ground level but the vents and fillers were up at the second floor level and fuel was supplied from an access road on a hill behind the emergency generator facility. The generators operated without fail in the flood of 1972 and in 1975. Strangely enough, the hospital had a diesel generator in a sub-basement vault. It was 1MW capacity. I never figured out the logic behind that installation?

[mikerj]

Despite shutdown, the core remains hot, much like a car engine that has just had its ignition key removed a minute ago.  Unlike a car engine though, the Fukushima reactor is unable to dissipate the heat naturally, and so requires cooling pumps.

Most cars nowadays keep the fans running for quite some time after your turn off iginition.
Some are really loud doing that.

And some have DC power auxiliary coolant pumps that continue running as well.

[floobydust]

The generators were available but mounted at ground level and made useless after having been submerged by the first wave!! I have always thought emergency generating equipment should be inside concrete buildings and at least 20 to 30 feet above local ground level. This is how our equipment was housed at the hospital where I worked. The fuel tanks were at ground level but the vents and fillers were up at the second floor level and fuel was supplied from an access road on a hill behind the emergency generator facility. The generators operated without fail in the flood of 1972 and in 1975. Strangely enough, the hospital had a diesel generator in a sub-basement vault. It was 1MW capacity. I never figured out the logic behind that installation?

I have seen hotels, hospitals etc. have either above or below ground diesel generator rooms. They are kind of an afterthought on something very rarely used.

Fukushima design should not have been approved. It was due to Tepco being corrupt and greedy, they wanted it cheap and found a way to rationalize the emergency generators, switchgear etc. being so low on the ground and math that tsunami's that powerful never really happen 

A two-fault failure scenario is super dangerous, that is when people get killed and things go very bad.
Example: Combined flood and grid power outage. How are you running those sump pumps?
Or power plant I worked in, lost grid power due to switchgear explosion and the plant UPS also was dead, did not work, never gets tested beyond initial commissioning.
So that was instant lights out

[Neepa]

snip

snip....

Exactly THIS. Good old greed and some corruption coming in to ruin the promise of nuclear power. Did that ever happen previously......

AFAIK they also skimped quite heavily on emergency training and several procedures to ensure information flow; ESPECIALLY to emergency personell responding from outside the powerplant.

[richnormand]

If you are interested about the subject (and why the plant needs an auxiliary power source like batteries, diesel, connection to the power grid or a large flywheel to generate power as some used to on start-up) I would suggest you download Nucleares from Steam.
This game/simulation makes it pretty obvious how and why you need auxiliary power for the electronics, motors, turbines and such while starting the plant from an off-line situation.

[johansen]

AFAIK they also skimped quite heavily on emergency training and several procedures to ensure information flow; ESPECIALLY to emergency personell responding from outside the powerplant.

it is quite possible they could have saved the reactors had the operators known which pipe to put water into the reactor through, instead they just further flooded the basement iirc, or the concrete shell around the reactor.

no one at the site was alive when the last emergency cooling operation was done (only one test ever, at commissioning of the system 40+ years ago) (basically, just let the reactors blow off steam open to the atmosphere)

as a result, the small amount of steam escaping the building was assumed by many at the site, to be passive cooling of the reactors boiling off water, when in fact none of the water from the fire trucks made it into the reactor.

[Nominal Animal]

I find it rather telling in the sociological sense that whenever any major accidents occur, be they nuclear or otherwise, you always have individual workers risking (and voluntarily giving) their lives to save others.  However, whenever it comes to making a profit, at least some are always willing to risk workers' and other lives just to make a little bit more of that sweet, sweet profit.  Weird species.

[Circlotron]

I find it rather telling in the sociological sense that whenever any major accidents occur, be they nuclear or otherwise, you always have individual workers risking (and voluntarily giving) their lives to save others.  However, whenever it comes to making a profit, at least some are always willing to risk workers' and other lives just to make a little bit more of that sweet, sweet profit.  Weird species.

Sounds like every war since the beginning of time.

[CatalinaWOW]

I find it rather telling in the sociological sense that whenever any major accidents occur, be they nuclear or otherwise, you always have individual workers risking (and voluntarily giving) their lives to save others.  However, whenever it comes to making a profit, at least some are always willing to risk workers' and other lives just to make a little bit more of that sweet, sweet profit.  Weird species.

Unless you think Darwin was just totally off base, the evidence is that this behavior is at the very least not harmful to species survival, and there is a fair chance that it actually is good for the species.  Tough on individuals though.  And maybe our ability to create really large scale disasters is changing that equation, though the reality is a that even Chernobyl wasn't/isn't very large scale from a species point of view.

[floobydust]

It's human behaviour - the greed-driven ones have no feelings for others and could care less about viewing people as anything other than tools for profit and meat for the grinder.
Top notch engineering is always ruined by corner cutting, corrupt construction practices, and insufficient maintenance. People want quality and reliability in a nuke plant or airplane, and expect it but behind the scenes there are problems.

France has a huge nuclear power industry, I think 58 reactors. 22 of them are more than 40 years old. More than 12 were/are shut down due to stress corrosion on the cooling pipes. 32 are down right now. They want to make another 14 reactors.
"On 21 February 2022, S&P Global Ratings and Moody's downgraded the credit rating of EDF citing the technical issues at its nuclear power plants."

The outrageous construction costs of new nuke plants is the only thing preventing them from sprouting up like mushrooms, in the green energy collapse.
"French state-owned utility EDF has raised its cost estimate for the construction of six new nuclear reactors to 67.4 billion euros ($73.18 billion)" source uh that's a lot of money.

[jmelson]

A whole bunch of reasons.  First, the MASSIVE alternators for normal operation may become unstable at very light loads.  Either the RPM governor or the rotor exciter.  US nuclear plants have huge battery-powered UPS'es to cover the pump load while emergency Diesel generators start.  They have fast-start generators, but what if one of them FAILS to start?  Yikes, so they need battery backup to make sure the pumps don't stop.
The whole Chernobyl disaster was caused when a really bad reactor design (positive void coefficient) was planned to use rotational inertia to power their circulator pumps during a station blackout event.  They did a test of a new exciter design that could regulate at the load of the plant, but the alternator slowed down during the test, and so the PUMPS slowed down!  This was only part of the damn foolery that caused the disaster, but it was a part of it.  The whole idea was SO DAFT it should never have been attempted!

OH, many of the older GE boiling water plants of similar design to the Fukushima plants have a Tenney turbine, that is capable of providing sufficient cooling water a couple minutes after a reactor SCRAM to prevent meltdown, through a cooling tower on the roof.  This small turbine runs off the residual heat from the cooling reactor.  Fukushima Dai-ichi plant #1 did NOT have this feature.  All the newer plants did have it, and they worked fine until the batteries that kept the valves open went dead.
Jon

[Nominal Animal]

My point was that humans are weird: some capable of extremely altruistic actions, some of extreme exploitation, and surprisingly often both in the same human.

Note that even including Hiroshima and Nagasaki and all nuclear tests, accidents, and leaks, the radioactive particles released by coal power plants and the related death toll is still higher; and that does not include the chemical toxicity of coal power plant pollution at all.
(Which reminds me: depleted uranium is amazingly toxic heavy metal, more dangerous than lead or mercury fumes.  When you get enough uranium in your body to kill you, it is the chemical toxicity and not the radiation that kills you first.)

A big problem with nuclear plants is that they just want to build them way too large; simply because the return on the investment takes a couple of decades to realize, and people feel the return must be maximized by scale.  It's the age-old "I want my profit now, or at the end of this quarter at the latest" problem.
I like the way it is said Romans commissioned their bridges: half when the bridge was finished, the other half fifty years later if the bridge was still standing.

A question one can ask is why don't we build the reactors where we intend to store the radioactive waste long-term, a mile or two beneath the surface, at least half a mile below groundwater deposits.  Such repositories already have to be accounted for the total price of nuclear energy, so why not build them there in the first place.

[uer166]

My point was that humans are weird: some capable of extremely altruistic actions, some of extreme exploitation, and surprisingly often both in the same human.

Note that even including Hiroshima and Nagasaki and all nuclear tests, accidents, and leaks, the radioactive particles released by coal power plants and the related death toll is still higher; and that does not include the chemical toxicity of coal power plant pollution at all.
(Which reminds me: depleted uranium is amazingly toxic heavy metal, more dangerous than lead or mercury fumes.  When you get enough uranium in your body to kill you, it is the chemical toxicity and not the radiation that kills you first.)

A big problem with nuclear plants is that they just want to build them way too large; simply because the return on the investment takes a couple of decades to realize, and people feel the return must be maximized by scale.  It's the age-old "I want my profit now, or at the end of this quarter at the latest" problem.
I like the way it is said Romans commissioned their bridges: half when the bridge was finished, the other half fifty years later if the bridge was still standing.

A question one can ask is why don't we build the reactors where we intend to store the radioactive waste long-term, a mile or two beneath the surface, at least half a mile below groundwater deposits.  Such repositories already have to be accounted for the total price of nuclear energy, so why not build them there in the first place.

How the heck are you going to cool and service a reactor miles underground?? There's a good reason most/all reactors are next to a large body of water.

[floobydust]

[...] US nuclear plants have huge battery-powered UPS'es to cover the pump load while emergency Diesel generators start.  They have fast-start generators, but what if one of them FAILS to start?  Yikes, so they need battery backup to make sure the pumps don't stop. [...]

That's not how it is in the coal plants, the UPS doesn't have enough power for the boiler feed and cooling pumps.
The plant UPS is strictly for SCADA visibility and the computers, the Control Room needs those monitors so they can tell what's going on.

It's a good point relevant to OP is the power requirements for the pumps.
Steam loop, the boiler feed pumps are huge for the 15MPa, 100's HP as I remember. We had one one bag due to PLC software mistake.