nuke reactor circulation pumps

[johansen]

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.

Well for one you dont have to pump water, just let it fall down the shaft

[Njk]

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.

As an example, one unit at a modern NPP generates 1.2 GW of electric power, 7.5% of which is required for its own needs. So each unit requires 90 MW, from internal or external source. Typically, there are several units at NPP.

[Nominal Animal]

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.

Cooling is an interesting question, because thermal-induced vertical flows in water are pretty strong and effective, and cool water is a lot heavier than hot water.  You'd probably use large vertical tubes to surface, which would also mean quite high water pressure at the reactor, unless intermediate pumping stations and water reservoirs would be used –– which in turn could be used for kinetic and thermal energy storage from solar and wind power too.

As to maintenance: The same way you maintain a mine, or service anything related to large city infrastructure.  At least here, they tend to be underground already.  (In central Helsinki, about 100m underneath the Esplanadi park, there is a 8m wide, 40m tall, 80m long artificial "lake" with 25,000 cubic meters of water solely for district cooling.)
There is some extra cost involved in transporting stuff, as you use long spiraling ramps, not elevators, so it's comparable to having it say 30 miles outside your major city.

It really is much less crazy than might appear at first, especially for experienced civil engineers.

[sparkydog]

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.

Well for one you dont have to pump water, just let it fall down the shaft

Great idea! Now your plant is flooded with hot water. (Although, in theory, the pressure of the incoming water should make it easier to pump the water back out, and the outgoing water being hot might mean you get some assistance from convection. But you'd still need to pump the water, which means pipes, and pipes to carry that much water are expensive.)

[5U4GB]

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

Fukushima is a boiling water reactor not a pressurised water reactor so things work differently there, the control rods are inserted from underneath because the top half is used for steam separation and dryers, so there's no gravity-assisted way to insert them, you need power. Also since the control rod mechanisms are at the lowest part of the reactor building, if it floods...

[5U4GB]

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.

It's also a major problem at Sellafield in the UK, google "First Generation Magnox Storage Pond" for details.

[5U4GB]

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.

If you're referring to the high pressure coolant injection system (HPCI) which was never activated, that's an interesting one.  What that would do is inject cold water onto a very hot containment vessel that's gone through decades of radiation embrittlement (the impact of neutrons creates voids that make the reactor vessel more and more brittle over time), the TMI operators actually shut it down after a few minutes out of concern that the thermal shock would cause a breach of the primary containment.  Nuclear physicists are divided 50:50 on whether this was a good thing or a bad thing.  I certainly wouldn't want to be anywhere near a reactor where such an experiment was performed.

[johansen]

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.

If you're referring to the high pressure coolant injection system (HPCI) which was never activated, that's an interesting one.  What that would do is inject cold water onto a very hot containment vessel that's gone through decades of radiation embrittlement (the impact of neutrons creates voids that make the reactor vessel more and more brittle over time), the TMI operators actually shut it down after a few minutes out of concern that the thermal shock would cause a breach of the primary containment.  Nuclear physicists are divided 50:50 on whether this was a good thing or a bad thing.  I certainly wouldn't want to be anywhere near a reactor where such an experiment was performed.

There is a lengthy youtube vid on this. They didnt know what valves were open or closed for various reasons.  They should have been able to do it, would have made a cubic mile sized steam cloud, but they assumed it was working because of a small steam cloud.

Sorry trying to simplify it..

[jmelson]

[...] 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.

The only plant I have been inside was the Calloway County plant in central Missouri.  It is a pressurized water reactor.  The circulation pumps are inside the containment, along with the steam generators.  This keeps the primary coolant loop very small, for safety reasons.  There are 4 pumps and 4 steam generators, each pump has a 1000 Hp motor.  If a steam generator develops a leak, it can be valved off and they can run  on only 3.
Jon

[WatchfulEye]

Fukushima is a boiling water reactor not a pressurised water reactor so things work differently there, the control rods are inserted from underneath because the top half is used for steam separation and dryers, so there's no gravity-assisted way to insert them, you need power. Also since the control rod mechanisms are at the lowest part of the reactor building, if it floods...

However, this can be done hydraulically. Hydraulic pressure can be stored in accumulator pressure vessels. Therefore all that is needed is to open the valve from accumulator to hydraulic rams and rod insertion will proceed.

You don't even need an oil hydraulic system - you can use the reactor coolant itself as hydraulic fluid. This means that force on the rams is proportional to reactor vessel pressure - with the correct sized hydraulic pistons and mechanical advantage, it is possible to guarantee that the insertion force will exceed hydrostatic pressure in the vessel.

Add a normally open hydraulic valve, and if power is lost, or the control system output is lost, the valve opens and the rods are inserted with no additional energy or control input.

[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.

The only plant I have been inside was the Calloway County plant in central Missouri.  It is a pressurized water reactor.  The circulation pumps are inside the containment, along with the steam generators.  This keeps the primary coolant loop very small, for safety reasons.  There are 4 pumps and 4 steam generators, each pump has a 1000 Hp motor.  If a steam generator develops a leak, it can be valved off and they can run  on only 3.
Jon

I'm trying to get some idea of the "Residual Heat Removal (RHR)" pumps' required power.  There's a few different cooling loops, it's complicated. Reactor cooling is not in the steam (generation) loop so the high feed pressure is not a requirement I believe.
But still 1,000's GPH flow. RHR seems to run for 4 hours min. from 350°F and 425 psig but then it says "The design heat load is based on the decay heat fraction that exists at 20 hours following reactor shutdown from extended operations at full power." That's all in the diesel generator league, not battery/UPS or hamsters running in a wheel.

"The RHR system transfers heat from the reactor coolant system to the component cooling water system. During shut down plant operations, the RHR system is used to remove the decay heat from the core and reduce the temperature of the reactor coolant to the cold shutdown temperature (< 200°F). The cooldown performed by the RHR system (from 350°F to < 200°F) is referred to as the second phase of cooldown. The first phase of cooldown is accomplished by the auxiliary feedwater (AFW) system (Chapter 5.8, steam dump control system (Chapter 11.2), and the steam generators.

Westinghouse Technology Systems Manual, Section 5.1, Residual Heat Removal (RHR) System
Westinghouse Technology Systems Manual, Section 5.2, Emergency Core Cooling Systems

Antique "Fukushima Daiichi reactors are GE boiling water reactors (BWR) of an early (1960s) design supplied by GE, Toshiba and Hitachi, with what is known as a Mark I containment." has process diagram.

[Terry Bites]

Where's the power going to come from when the reactor is shut down, from the grid.
Off on a bit of a tangent. I did some work at big old coal fired station.
They had an enormous battery room. A beautiful tiled floor covered in glass sided cells over a metre high. Fab, I should have taken a photo.

[jmelson]

You don't even need an oil hydraulic system - you can use the reactor coolant itself as hydraulic fluid. This means that force on the rams is proportional to reactor vessel pressure - with the correct sized hydraulic pistons and mechanical advantage, it is possible to guarantee that the insertion force will exceed hydrostatic pressure in the vessel.

Add a normally open hydraulic valve, and if power is lost, or the control system output is lost, the valve opens and the rods are inserted with no additional energy or control input.

As far as I know, all (water-cooled) reactors use WATER hydraulic systems to operate the control rods.  Advantages are they have lots of high-pressure water pumps already there, and if there's a leak, it doesn't inject oil into the reactor coolant.
Jon

[Ground_Loop]

Primary loop pumps are used to heat the reactor prior to startup, so an external power source is required.  Once the reactor is on line, local steam gen sets can be paralleled with the outside grid and switched in to power the pumps.  Further, upon shutdown, decay heat needs to be removed for weeks to months afterwards which requires loop circulation.  I was a reactor operator on an aircraft carrier.