Using Fuel Ponds to Produce Power?

[Delta Force]

Spent fuel ponds are typically kept below 50 °C (120 °F), which is below the minimum required by low temperature geothermal power systems of under 200 °C, around the mid-100s °F. However, that temperature is maintained with water constantly circulating through the fuel pond. Could spent fuel ponds be safely maintained at higher temperatures to allow them to power low temperature steam turbines? Could such a system allow the cooling pond to operate its own safety critical systems and possibly those of associated nuclear power plants?

 

[oz93666]

In theory if spent fuel rods were kept in an insulated container the temperature would continue to rise and rise . They could be made to boil water , but this water would be radioactive , and make the turbines radioactive ...

I suspect the small amount of power generated would not be worth the mess made.

 

[Delta Force]

In theory if spent fuel rods were kept in an insulated container the temperature would continue to rise and rise . They could be made to boil water , but this water would be radioactive , and make the turbines radioactive ...

I suspect the small amount of power generated would not be worth the mess made.

Boiling water reactors send radioactive isotopes to their turbines as well, although the isotopes are mostly short lived. Could a spent fuel pond power plant use another loop to avoid contaminating its turbines and end up closer in design to a pressurized water reactor?

 

[gmax137]

Spent fuel pools are open to the surrounding building to allow the plant workers to perform the associated tasks of moving assemblies in and out of the pool.

Can you estimate the amount of power that could be generated under various plant conditions? How much power does it take to run the pool cooling system? Whats the relative reliability of the current approach vs your idea?

 

[oz93666]

Boiling water reactors send radioactive isotopes to their turbines as well, although the isotopes are mostly short lived. Could a spent fuel pond power plant use another loop to avoid contaminating its turbines and end up closer in design to a pressurized water reactor?

Yes ...but more expense , more wastage of power in another loop ...I suspect the heat generated is just too small...

Have a look at big dams , there seems to be a lot of power going to waste there , room for add ons.

 

[Hiddencamper]

The heat load in a spent fuel pool is generally less than 5 MW thermal. So yes you could try to produce power, but it absolutely is not economical when you consider the need to have a new pressure vessel, a separate low power turbine, steam piping, emergency flooding systems, etc.

 

[Delta Force]

Spent fuel pools are open to the surrounding building to allow the plant workers to perform the associated tasks of moving assemblies in and out of the pool.

Can you estimate the amount of power that could be generated under various plant conditions? How much power does it take to run the pool cooling system? Whats the relative reliability of the current approach vs your idea?

There are pool type electricity producing reactors, including the sodium cooled BN-600 fast breeder reactor (I'm not sure how they manage that given sodium's volatility), so that's not unparalleled.

I'm not really sure how to determine the amount of power that could be generated or how much power is required to run the coolant pumps and associated systems (although I know how much is required for the power plants themselves). I'm from an energy policy background, not an engineering one. I'm more wondering if the fuel ponds could be used as a source of electricity because they are constantly producing heat. A fuel pond power plant would seem to be something of a hybrid between geothermal and nuclear power plants, one of which (depending on the source) is the most reliable form of power production with well over 90% capacity factor.

Also, low temperature steam systems are a relatively new technology (at least for commercial geothermal use, although my understanding is that historically they simply couldn't run on low quality steam), so it wouldn't have really been something to consider when most fuel ponds were built.

 

[Delta Force]

The heat load in a spent fuel pool is generally less than 5 MW thermal. So yes you could try to produce power, but it absolutely is not economical when you consider the need to have a new pressure vessel, a separate low power turbine, steam piping, emergency flooding systems, etc.

Couldn't it be a pool type design? Also there are already some safety systems in place due to the critical importance of keeping the fuel pond cooled.

The heat load is probably the larger issue though. If it's only 5 MWt it seems unlikely it could be used to power safety critical systems.

 

[gmax137]

The pools are filled with water; the only way to get water hot enough to make steam is to pressurize the water. So you would need to replace the open pool with some kind of vessel.

A liquid metal pool is open and at atmospheric pressure, but it can get as hot as you like (and you could pump the sodium through a heat exchanger to make steam). There are issues with using sodium coolant (keep the water out, prevent freezing, can't see through it, etc.) all of which make it far more complicated than just using a 50 hp motor to pump the pool water through a heat exchanger.

 

[Hiddencamper]

The pools are filled with water; the only way to get water hot enough to make steam is to pressurize the water. So you would need to replace the open pool with some kind of vessel.

A liquid metal pool is open and at atmospheric pressure, but it can get as hot as you like (and you could pump the sodium through a heat exchanger to make steam). There are issues with using sodium coolant (keep the water out, prevent freezing, can't see through it, etc.) all of which make it far more complicated than just using a 50 hp motor to pump the pool water through a heat exchanger.

Plus you would likely have radionuclides offgassing from a high temperature open pool. It's hard enough to keep the fuel floor deconned, especially when every few years some operator screws up and dumps fuel pool water down the ventilation...

 

[rbelli1]

some operator screws up and dumps fuel pool water down the ventilation...

Homer Simpson is only supposed to be in charge of the doughnuts. Keep him far away from the glowey bits.

BoB

 

[Astronuc]

If it's only 5 MWt it seems unlikely it could be used to power safety critical systems.

The MWt must be converted to useful MWe (electrical), and one would have to consider the Carnot efficiency. If it's ~20%, then the maximum electrical power would be about 1 MWe.

 

[rbelli1]

Maybe it could be spliced into the normal turbine water somehow. Is the water output stream of a modern turbine a lower temperature than the waste pool? Would pre-heating the make up water be useful?

BoB

 

[Hiddencamper]

Maybe it could be spliced into the normal turbine water somehow. Is the water output stream of a modern turbine a lower temperature than the waste pool? Would pre-heating the make up water be useful?

BoB

In the summer my condensate temperatures are up to 120 degF. That's warmer than the spent fuel pool.

120 degF is the max normal limit for our SFP. Above this temperature you start to challenge the ion exchange resins. By 140 degF the resin begins to break down. You need ion exchangers running for filtration most of the time, especially if you have any damaged fuel or control rod blades. Otherwise you get both dose/contamination concerns and corrosion product concerns.

So it's not really a smart idea to use it for Feedwater heating.

In the winter, my condensate temperatures are colder than the SFP. You have minimum temperature limits in the pool for materials and criticality concerns, so if you over cool the pool that's an issue too.

And as I've said previously, average SFP heat loads are 1-10 MWth generally, usually 5 or less. It's such an insignificant amount of heat that it's not really practical to use.

 

[rbelli1]

120 degF is the max normal limit for our SFP.

You need ion exchangers running for filtration most of the time,

Interesting.

When I heard 5MW of nuclear heating I just assumed it would be hotter. So where does all that heat go? How much is passive? How much active if any?

BoB

 

[Delta Force]

Interesting.

When I heard 5MW of nuclear heating I just assumed it would be hotter. So where does all that heat go? How much is passive? How much active if any?

BoB

Fuel ponds aren't that energy dense and the primary goal is to keep them cool, not produce power.

PM-3A created 9.36 MWt in a pressure vessel this large:

 

[Hiddencamper]

Interesting.

When I heard 5MW of nuclear heating I just assumed it would be hotter. So where does all that heat go? How much is passive? How much active if any?

BoB

The decay heat load is larger during and right after an outage, but typically is less than 20 MW at its peak. There is a minimum decay time before fuel is allowed to be removed from he vessel to ensure that heat removal requirements aren't exceeded, for bwrs this is typically 24 hours. For PWRs I've seen 3 days or more.

The fuel pool cooling system continuously runs to keep the pools cooled. We (BWR/6) maintain our pools between 80-100 degF, but during the summer or after a refuel it is sometimes hard to maintain and we drift up to 100-105 degF.

We will throttle flow through the heat exchanger to adjust spent fuel pool temperature. During summer and outages we run the spent fuel pool system at full cooling. We also will directly put service water through the heat exchangers sometimes as it has a lower temperature than closed cooling water and allows more heat removal. During outages we reach a point where the spent fuel pool cooling pumps can cool the spent fuel and the reactor core (with head off) on their own. Once we hit that point we will go into maintenance on our RHR pumps and heat exchangers as they are no longer necessary for safety.

If fuel pool temps are too high, it raises temperature and humidity on the refuel floor. This can be challenging as refuel teams wear protective clothing, so they have heat stress time limits, and the hotter the pools are the less fuel moves they can do before needing a break.

I don't know how much heat is lost passively. All of our calculations assume a closed system, and heat loss is only to the cooling system. This is conservative when accounting for safety. We also weekly will update our calculations on time to boil and time to top of fuel. Right now we are at 83 hours to 200 degF.

Think about that, it takes 83 hours just to reach boiling our spent fuel pool. It's not really useful for power generation.