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PostPosted: Feb 18, 2015 6:31 pm 
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Say a BWR like Fukushima was shutdown properly, has cooling, but 4 weeks after shutdown it looses all power, and cooling stops.
Is the reactor safe ? Could it meltdown still ? Could manual procedures save the reactor from damage, assuming no cooling.
Somebody is trying to say that even after 4 weeks a reactor is still in a fragile state if it looses power. I think not.
At this point it's at 0.1% decay heat prior to shutdown. Sounds like something the reactor might naturally convect and radiate.
I'm assuming that at a very low power level the reactor thermal margins increase, meaning perhaps that coolant temps could go above the normal top of the range without meaning a meltdown, increasing convection and radiation.

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PostPosted: Feb 18, 2015 8:59 pm 
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By 4 weeks after shutdown the reactor has probably been defueled if it was a planned shutdown.


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PostPosted: Feb 19, 2015 2:58 am 
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Four weeks after shutdown the decay heat is still quite high.
Even in a spent fuel pool, the water supply and circulation must be maintained reliably.

https://dl.dropboxusercontent.com/u/116 ... y_mod1.JPG


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PostPosted: Feb 19, 2015 2:59 am 
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macpacheco wrote:
Say a BWR like Fukushima was shutdown properly, has cooling, but 4 weeks after shutdown it looses all power, and cooling stops.
Is the reactor safe ? Could it meltdown still ? Could manual procedures save the reactor from damage, assuming no cooling.
Somebody is trying to say that even after 4 weeks a reactor is still in a fragile state if it looses power. I think not.
At this point it's at 0.1% decay heat prior to shutdown. Sounds like something the reactor might naturally convect and radiate.
I'm assuming that at a very low power level the reactor thermal margins increase, meaning perhaps that coolant temps could go above the normal top of the range without meaning a meltdown, increasing convection and radiation.


Internal recirculation natural convection cooling works at some 50% of normal rated power for BWRs without fuel failure. This isn't the problem. The problem is nuclear reactors are closed systems. A 3000 MWt BWR making 0.1% decay heat is still 3000 kWt. That's a big heater if you leave it on all the time and sit back to read the newspaper.

Very simple operations would suffice for emergency cooling if all else fails. During shutdown the pressure is low and the cold water inventory in the vessel is large.

There are different shutdown states. If the reactor is just shut down it is operating at pressure and temperature. Decay heat is the highest. So this could be called the most limiting operating mode, but actually cooling systems are available such as the reactor core isolation cooling system (RCIC) or isolation condenser system (IC). The next operating mode is cold shutdown, low temp and low pressure, but with the pressure vessel closed. This is better than the previous mode because you have the option of fire water injection (low pressure). And you could still let things heat up and start the RCIC or IC again if fire water injection fails. The next operating mode is when the vessel has been removed. This means no possibility of going to RCIC or IC operation. On the other hand it is now easy to flood with the water in the upper pools or add fire water. During this operating mode the water is at the top of the vessel. That's a lot of water you could just boil away. Quick calc for knowing the grace time. Say a 15 meter column of 7 meters diameter, this is 577 m3 of water. To dump 3000 kWt you'd be evaporating some 1.2 liters/sec including heat to 100C. This is some 133 hours of boiloff. This is the time you have to add water. In reality you have some more time, a few hours, before the fuel heats up to failure. If you cannot add water it gets bad, you can get a big release of radionuclides and the containment and pressure vessel are both open and we have to assume the HVAC and filters are unavailable to mitigate the release.

Most of the shutdown time however, if you're considering weeks post shutdown with somehow (as Ed points out won't be the case) the fuel is still not unloaded, would be with the reactor well flooded too. This is the area above the pressure vessel. There's a seal skirt around the upper part of the drywell, that is used to seal off the containment at the upper pressure vessel, this allows flooding of this area. This gives a lot more water available, like multiples.

None of the reactors in Japan that were shut down sustained core damage. That's a simple fact.

I would not call a 133+ hour grace time for a simple water pumping operation a "fragile state". In my opinion 133+ hours to put water in a pool is perfectly doable. That's a simple opinion.


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PostPosted: Feb 19, 2015 3:06 am 
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jaro wrote:
Four weeks after shutdown the decay heat is still quite high.
Even in a spent fuel pool, the water supply and circulation must be maintained reliably.

https://dl.dropboxusercontent.com/u/116 ... y_mod1.JPG


The situation in a spent fuel pool is quite different, depending on the spent fuel age. This is because spent fuel pools are not closed systems, they are open to a big building which has ventilation and cooling (or could just open some doors if there is no power). Anti nukes have used this as a critique of spent fuel pools but as usual reality is more complicated. Air cooling works quite well, depending on the arrangement of the spent fuel and age, to prevent fuel failures. The NRC has done extensive modelling recently using NRC approved codes such as MELCOR, that showed that, depending on configuration, beyond a few months of age, the spent fuel is no longer at risk of self overheating. In fact the analysis showed that old spent fuel can be checker-boarded alongside new spent fuel in order to reduce the heat load. The older fuel assemblies become effective heat sinks upon a postulated draindown event.

This is not really surprising. The wisdom with nuclear energy risk is, there is potential risk with freshly discharged spent fuel or fission products in some other form, and no risk at all with old spent fuel.


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PostPosted: Feb 19, 2015 1:11 pm 
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Could I bug you guys a little more and explain a little about CANDU ?
I'm assuming on CANDU, it's possible to remove a fuel rod (simultaneously replacing with a new one) with the reactor at full or a very high power setting.
So the fuel rod would come out with decay heat = 6.5% of full power (or close to that).
That suggests there is some time until the rod melts, otherwise they would be risking it melt before reaching the spent fuel pool. Perhaps 15 minutes at least.

Is there another type of reactor that is designed for faster refueling (still undergoes shutdown, but doesn't quite need to wait a month) ?

This knowledge is helping me combat a long time anti nuclear moron, that can't accept simple nuclear engineering facts. On slashdot.


The main issue about refueling in typical reactors is that it must be depressurized before refueling, so the reactor must be cold enough before you can mess with fuel rods, is that the critical aspect (coolant below 100C after depressurization) ? And CANDU has this special design that allows horizontal refuelling, and the fuel rods are under no pressurization.

Thanks for the info. this shall help me put some anti nuclear types to shame in discussions, with facts.

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PostPosted: Feb 19, 2015 1:20 pm 
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Cyril R wrote:
macpacheco wrote:
Say a BWR like Fukushima was shutdown properly, has cooling, but 4 weeks after shutdown it looses all power, and cooling stops.
Is the reactor safe ? Could it meltdown still ? Could manual procedures save the reactor from damage, assuming no cooling.
Somebody is trying to say that even after 4 weeks a reactor is still in a fragile state if it looses power. I think not.
At this point it's at 0.1% decay heat prior to shutdown. Sounds like something the reactor might naturally convect and radiate.
I'm assuming that at a very low power level the reactor thermal margins increase, meaning perhaps that coolant temps could go above the normal top of the range without meaning a meltdown, increasing convection and radiation.


Internal recirculation natural convection cooling works at some 50% of normal rated power for BWRs without fuel failure. This isn't the problem. The problem is nuclear reactors are closed systems. A 3000 MWt BWR making 0.1% decay heat is still 3000 kWt. That's a big heater if you leave it on all the time and sit back to read the newspaper.

Very simple operations would suffice for emergency cooling if all else fails. During shutdown the pressure is low and the cold water inventory in the vessel is large.

There are different shutdown states. If the reactor is just shut down it is operating at pressure and temperature. Decay heat is the highest. So this could be called the most limiting operating mode, but actually cooling systems are available such as the reactor core isolation cooling system (RCIC) or isolation condenser system (IC). The next operating mode is cold shutdown, low temp and low pressure, but with the pressure vessel closed. This is better than the previous mode because you have the option of fire water injection (low pressure). And you could still let things heat up and start the RCIC or IC again if fire water injection fails. The next operating mode is when the vessel has been removed. This means no possibility of going to RCIC or IC operation. On the other hand it is now easy to flood with the water in the upper pools or add fire water. During this operating mode the water is at the top of the vessel. That's a lot of water you could just boil away. Quick calc for knowing the grace time. Say a 15 meter column of 7 meters diameter, this is 577 m3 of water. To dump 3000 kWt you'd be evaporating some 1.2 liters/sec including heat to 100C. This is some 133 hours of boiloff. This is the time you have to add water. In reality you have some more time, a few hours, before the fuel heats up to failure. If you cannot add water it gets bad, you can get a big release of radionuclides and the containment and pressure vessel are both open and we have to assume the HVAC and filters are unavailable to mitigate the release.

Most of the shutdown time however, if you're considering weeks post shutdown with somehow (as Ed points out won't be the case) the fuel is still not unloaded, would be with the reactor well flooded too. This is the area above the pressure vessel. There's a seal skirt around the upper part of the drywell, that is used to seal off the containment at the upper pressure vessel, this allows flooding of this area. This gives a lot more water available, like multiples.

None of the reactors in Japan that were shut down sustained core damage. That's a simple fact.

I would not call a 133+ hour grace time for a simple water pumping operation a "fragile state". In my opinion 133+ hours to put water in a pool is perfectly doable. That's a simple opinion.


Thanks a lot. This matches my on the spot analysis (as far as conclusions goes). But it's excellent to have confirmations, hard data, technical terms to silence those that replace analysis with paranoid conclusions. Thanks again.

When you say boil away, it would first go inside secondary containment, or does it boil directly outside ? I'm assuming it goes into secondary containment. No matter how much you explain people that as long as the reactor doesn't meltdown, the water itself is hardly radioactive, the problem is radiation sources (radionuclides or mostly the fuel).

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PostPosted: Feb 19, 2015 5:18 pm 
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macpacheco wrote:
Could I bug you guys a little more and explain a little about CANDU ?
I'm assuming on CANDU, it's possible to remove a fuel rod (simultaneously replacing with a new one) with the reactor at full or a very high power setting.
So the fuel rod would come out with decay heat = 6.5% of full power (or close to that).
That suggests there is some time until the rod melts, otherwise they would be risking it melt before reaching the spent fuel pool. Perhaps 15 minutes at least.

CANDU refuelling machines are probably the most complex equipment in the plant.
They include water cooling of course, for the spent fuel bundles.


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PostPosted: Feb 19, 2015 6:09 pm 
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The boiled water isn't radioactive, the core hasn't been damaged. It goes to secondary containment, but it hardly suffices as a containment. Its just a work area, with a high ventilation rate. If there is power there may or may not be a filterering system. But if there is power then a 133 hour period with no water makeup is not a plausible scenario, in my opinion. So we'd be looking at some settling of any radionuclides on the service building/roof part. But not much more than that. Main thing is it is just water that we're boiling away. No detectable emissions at the plant boundary.


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PostPosted: Feb 19, 2015 6:15 pm 
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jaro wrote:
macpacheco wrote:
Could I bug you guys a little more and explain a little about CANDU ?
I'm assuming on CANDU, it's possible to remove a fuel rod (simultaneously replacing with a new one) with the reactor at full or a very high power setting.
So the fuel rod would come out with decay heat = 6.5% of full power (or close to that).
That suggests there is some time until the rod melts, otherwise they would be risking it melt before reaching the spent fuel pool. Perhaps 15 minutes at least.

CANDU refuelling machines are probably the most complex equipment in the plant.
They include water cooling of course, for the spent fuel bundles.


Jaro,

Thanks for the imgs. Very interesting.

Over the years I've learned that complex is a relative term. My mother thinks her home boiler is terribly complicated. To the maintenance man that comes around on a yearly contract that is rather a good joke. He repairs these kettles for a living.

As it turns out, not only beauty but also complexity, is in the eye of the beholder.


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PostPosted: Feb 19, 2015 9:08 pm 
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Cyril R wrote:
jaro wrote:
macpacheco wrote:
Could I bug you guys a little more and explain a little about CANDU ?
I'm assuming on CANDU, it's possible to remove a fuel rod (simultaneously replacing with a new one) with the reactor at full or a very high power setting.
So the fuel rod would come out with decay heat = 6.5% of full power (or close to that).
That suggests there is some time until the rod melts, otherwise they would be risking it melt before reaching the spent fuel pool. Perhaps 15 minutes at least.

CANDU refuelling machines are probably the most complex equipment in the plant.
They include water cooling of course, for the spent fuel bundles.


Jaro,

Thanks for the imgs. Very interesting.

Over the years I've learned that complex is a relative term. My mother thinks her home boiler is terribly complicated. To the maintenance man that comes around on a yearly contract that is rather a good joke. He repairs these kettles for a living.

As it turns out, not only beauty but also complexity, is in the eye of the beholder.


And I thought a BWR / PWR were complex things.
The important aspect is the refueling parafernalia isn't a core safety feature (I would assume), so they aren't quite as expensive to develop and build. Not subject to so much regulatory burden.
Hopefully we'll see lots of CANDUs migrate over to Pu-Th fuel and U233-Th fuels (I think I read it would be a U233 breeder).

One last question, secondary containment with a reactor online is a no go zone for humans, correct ?

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PostPosted: Feb 19, 2015 9:27 pm 
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macpacheco wrote:
Hopefully we'll see lots of CANDUs migrate over to Pu-Th fuel and U233-Th fuels (I think I read it would be a U233 breeder).

Yes, with Thorium-HEU the Candu could indeed be a breeder.
The fissile would have to be at least 60% U235, thus HEU. That simply won't fly politically (ditto for the Shippingport experiment many decades ago).

Several other modifications would be required, including buffering of irradiated fuel in the SNF pool (to allow Pa233 to decay to U233), as well as a reverse path for the irradiated fuel back into the reactor (not feasible in existing Candu plants).

Heavy water quality would also have to be improved a bit, and it would help if there were less zirconium and other parasitic neutron absorbers in the core.

A more direct approach would be to switch to liquid fuel, with on-line processing (and no fuelling machines!)


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PostPosted: Feb 20, 2015 6:08 pm 
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jaro wrote:
macpacheco wrote:
Hopefully we'll see lots of CANDUs migrate over to Pu-Th fuel and U233-Th fuels (I think I read it would be a U233 breeder).

Yes, with Thorium-HEU the Candu could indeed be a breeder.
The fissile would have to be at least 60% U235, thus HEU. That simply won't fly politically (ditto for the Shippingport experiment many decades ago).

Several other modifications would be required, including buffering of irradiated fuel in the SNF pool (to allow Pa233 to decay to U233), as well as a reverse path for the irradiated fuel back into the reactor (not feasible in existing Candu plants).

Heavy water quality would also have to be improved a bit, and it would help if there were less zirconium and other parasitic neutron absorbers in the core.

A more direct approach would be to switch to liquid fuel, with on-line processing (and no fuelling machines!)


I'm not defending construction of new CANDUs instead of MSRs. I want MSRs.
But we have to separate hopes and dreams from the possible in the short term. The earliest we might have enough MSRs worldwide to match a large hydro dam is 2025, best case. Like 50GWt total of MSRs running. And we don't expect existing PWR/BWR will be trashed, specially all Gen III/III+ already running.

Even a reactor able to run on Thorium and make 95% of the fissile it consumes, with unlimited recyling of all fissile and fertile left in the fuel, seems the best way to use Thor Energy's work with CANDU. PWR/BWR will probably convert 80-90%.
The thought is SNF undergoes some reprocessing that only removes fission products from fertile/fissile. A little Pu is added to makeup fissile levels, and enough Thorium to complete total inventory. An almost sustainable cycle. We kill the arguments of those that make Plutonium and other transuranics as a big evil.

There is a lot of Plutonium in existing SNF stockpiles to start this cycle. This might also be an option for ThorCon since they are going with Thorium based fuels. In the meantime lots of U233 is being made. Best case for MSRs might even be if nuclear operators opt for Pu+Th fuel once through. Interesting U233 mine to startup MSRs latter.

But in order for reprocessing to be interesting the most important criteria is burnup, and apparently CANDU + Thorium fuels could help there too. If burnup is doubled, reprocessing cost / kWh produced halfs. Of course is MSRs much better, due to online fueling/batching for reprocessing, but we should make short, medium and long term plans.

Thor Energy is hoping to put the first full length Pu+Th rods in a commercial reactor 2020. Enough time for multiple burnup cycles until 2025. Enough time to migrate dozens of water cooled nukes too.

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