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emergency cooling using decay heat

emergency cooling using decay heat

In an emergency shutdown. a nuclear plant requires on-site generators to drive the emergency cooling pumps. I am wondering why the decay heat is not used. Of course that is a simple way of describing a complex system, if one is possible. I am not a nuclear engineer, but I guess it is not wrong to speak of decay heat.

This would not be important if the shutdown lasts only a few days, maybe up to a month. The diesel fuel might last that long. But if most or all of the grid goes down for a long time, because of cyberwar or a solar storm or an EMP, most if not all nuclear plants would run out of diesel fuel long before the grid comes back. A cooling system using the decay heat might be all that stands between life and death. What other source of power could there be?

I know the decay heat declines. Could we still use it for long-term emergency cooling?

RE: emergency cooling using decay heat

"...In nuclear reactor engineering, decay heat plays an important role in reactor heat generation during the relatively short time after the reactor has been shut down (see SCRAM), and nuclear chain reactions have been suspended. The decay of the short-lived radioisotopes created in fission continues at high power, for a time after shut down..."

RE: emergency cooling using decay heat

In an planned, orderly shutdown that decay heat probably is used to generate electricity for a while. In an emergency shutdown something has gone wrong with the reactor so completely independent systems are required to provide reliability.

RE: emergency cooling using decay heat

In the case of a failure of the grid, nothing has gone wrong with the reactor. After it is shut down, the core is still hot and it is producing more heat by 'decay' of the fuel. To prevent the disaster of a meltdown, the core must be cooled for some considerable time after shutdown.

I am asking whether it would be possible to use this heat, which would otherwise be wasted, to produce power to drive the emergency cooling system. Eventually the temperature of the core would drop to a point where it would not be practical to produce power. Maybe it would never be practical.

I believe the industry and the NRC are finally taking this cooling problem seriously because of the growing threat of cyberwar and EMP, which could knock out the electrical grid for a long time. Aside from these man made threats, the sun could take down all of the electrical grids in the world at any time, and we might not be able to bring them back on line for months or years. In the meantime every nuclear plant in the world would melt down and every spent fuel pile would catch fire and spread deadly radiation everywhere. Unless there is a way to keep cooling them over the long term.

I hope someone with the necessary knowledge and expertise will take time to comment on this question. There is a lot of heat generated in a shutdown nuclear station. It must be removed. Can we use it to operate the emergency cooling systems instead of relying on diesel or combustion turbine generators?

Those emergency generators require fuel, and fuel may become unavailable in a relatively short time if the grid failure is widespread. Production and delivery of diesel fuel and natural gas depends on electrical power at the least. And there would be many other problems with fuel delivery caused by a widespread, long term failure of the electrical grid.

RE: emergency cooling using decay heat

However logical it is to harvest the energy from a dying system, there is likely to be some institutional discomfort with the idea - it was an scheme a little bit like that (albeit one to avoid having DGs running on standby, rather than avoiding needing them at all) that was being tried out at Chernobyl when it all went wrong.


RE: emergency cooling using decay heat


“…In September, the commission’s top officials warned lawmakers that the threat of an EMP attack from a rogue nation ‘becomes one of the few ways that such a country could inflict devastating damage to the U.S...’”

“…John Norden, director of operations at ISO New England Inc., which manages a grid serving six states, said the industry is unprepared for a full-scale electromagnetic attack. The power industry doesn’t really have any standards or tools to handle ‘black sky events’ such as an extreme cyber or EMP attack, or even conventional war, Norden said at a recent conference…”

“…But the best experience utilities have had in preparing for an EMP is tied to a natural phenomenon: solar flares..”

“…I don’t think we have an illusion we will prevent it,” Bryson said in an interview. “That’s really the government’s job….”

“…Managing that kind of threat right now—no one really has the resources to do that,” Mroz said…”

RE: emergency cooling using decay heat

There are basically only a couple of ways of making use of the heat:

> thermopile or thermoelectric generators -- grossly inefficient, and would probably not survive the environment
> generate steam to run turbine -- this is where we started, so that's a non-starter

TTFN (ta ta for now)
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RE: emergency cooling using decay heat

IRstuff, there is another way. Compress air isothermally, heat it with the waste heat and expand it in a turbine to produce power. This is a modified Brayton cycle. The compressor of the Brayton turbine is replaced by the isothermal compressor. With a recuperator, this cycle is the most efficient of all, because isothermal compression is the most efficient process.

It is also the simplest plant. No steam, no need for water treatment and makeup, no condenser, no feed pump, none of the equipment required for a Rankine cycle. All of the equipment required for a modified, recuperated Brayton cycle is available today. There is only one problem to solve. What is the best way to transfer the waste heat to the compressed air?

I am not claiming that such an arrangement would be practical because I don't know enough about the quality and quantity of the waste heat. I hope someone with the required knowledge and expertise will comment.

RE: emergency cooling using decay heat

The energy in decay heat can help cool a core. If a heat exchanger is placed above the elevation of the core, and connected across the reactor, e.g., with one side connected to the core inlet and the other side connected to the core outlet, with isolation valves appropriately located, when the valves open, a thermosiphon results. The reactor and heat exchanger temperature differences provide the driving head. Something still needs to pull heat from the heat exchanger, though, and a large enough temperature difference must exist to keep the flow passing through the core.

"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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RE: emergency cooling using decay heat

IRstuff, good question. The power developed by the modified Brayton cycle [MBC] must drive both the isothermal compressor and the core cooling water pump. This is no different in principle from the operation of a standard Brayton cycle engine, which drives its own adiabatic air compressor while producing additional, useful power. The main question is whether the MBC can capture enough waste heat to drive the cooling pump. We must know the quantity and quality of the waste heat.

If there is a spent fuel pool it must also be cooled, for as long as ten years from what I read. Does spent fuel generate enough heat to operate its own MBC cooling system, at least for a few months? But one thing at a time.

xnuke, do you know whether a thermosiphon like that has ever been installed in a nuclear plant? As you say, the heat must be removed from the heat exchanger. Isn't that the flaw in this plan? Natural convection would probably not be enough. It will require power to remove the heat, and the premise is that we don't have it. The grid is down, the diesel fuel is used up and we can't get any more, the natural gas for a CT has probably stopped flowing because the grid is down. I know that NG compressors can run on the NG they are pumping, but with the grid down there are a lot of things that can happen to cut off the flow of NG.

RE: emergency cooling using decay heat

Thermosiphons (aka natural circulation) have been installed in nuclear plants for emergency cooling, and even for power operation for some naval vessels. As a matter of fact, the Westinghouse AP1000 design has this type of passive emergency cooling system known as Passive Residual Heat Removal.

I wrote "heat exchanger," but it could really be any heat sink. It could be water-to-air with convective heat transfer, water-to-water to a large water volume, or the flow could go to a cooling tower. See the IAEA report for discussions of passive decay heat removal methods, some of which also supply injection water, some of which do not.

"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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RE: emergency cooling using decay heat

How long are the HPCI and RCIC turbines in BWRs supposed to run?
what was the test that caused the Cherynobil accident supposed to find out. That was a main generator test on residual heat?[/indent]

RE: emergency cooling using decay heat

xnuke, thanks for the link to the IAEA report. It covers a lot of what I have been asking about. I didn't know that the nuclear industry has been conducting a study of passive emergency cooling, i.e. systems which operate solely on decay heat.

A brief look at the report indicates that these systems are intended for future plants. Current plants require emergency power for cooling when the grid is down.

Except for the AP1000, as you mentioned. The design is complete but I don't know whether any have been built. Its passive emergency cooling is good for seventy-two hours. Is that long enough? If so, I should not have been concerned about emergency cooling in current plants during a grid failure. Their emergency generators must be able to operate continuously for more than three days. Isn't the rule two weeks?

In FIG. 1 of the report, "Pre-pressurized core flooding tank (accumulator)", the gas at the top is in direct contact with the bromated water, all under high pressure. I wonder whether the engineers have accounted for the fact that the cold water will absorb a considerable quantity of the gas while the system is on standby. When the system is activated, the pressure on the water will drop and the temperature will rise when the water flows into the hot core. This will cause the dissolved gas to come out of solution, possibly a violent process. This would complicate the calculations.

RE: emergency cooling using decay heat

Two weeks is certainly better than nothing, but it seems like an accident in one unit of a multi-reactor site produces enough contamination to prevent access the to the remaining units for an extended duration. At Fukushima, a small amount of radiation prevented workers from operating a value in unit 1. When the upper part of building 1 exploded few hours later, the hydrogen explosion spread radiation to units 2 & 3. This contamination prevented workers from providing emergency cooling to units 2 & 3, leading to explosions in those units as well.

RE: emergency cooling using decay heat

You need also to realize that the decay heat generated by a reactor reduces significantly(!) over time. Further, the decay heat "available" at the start-of-accidental-loss-of-power depends on the previous reactor power history (36-48 hours for a full analysis, 6-12 hours for a order-of-magnitude analysis).

Thus, you CANNOT "depend" on decay heat being available when you need it for emergency recovery. Granted, if the reactor has been running at full power for months, you can predict what decay heat is available for a "analysis-worst-case" calculation. Further, you can assume that when the reactor has been shutdown or running at low power for a while there will be less decay heat to remove. But, decay heat builds up (as in the Fuki. fuel storage ponds) and needs to be removed.

So, what do you do when the reactor has been shutdown for maintenance for three weeks (or for three hours) and loses power for an extended amount of time? Could be as simple as refilling the fuel (being emptied by evaporation/boiling) with fire trucks.

RE: emergency cooling using decay heat

Most light water plants have 2 emergency feed water pumps, one powered by diesel the other by residual heat from the reactor in the form of steam. This true for both PWR and BWR. There is also an electric auxiliary feed water pump which is used during normal shutdowns.
At Fukushima the diesel driven pump was flooded and the control power for the turbine driven one was lost with the flooding of the emergency generators which run everything else in the plant as needed for control. In my understanding of the actions taken, the turbine driven pump was shut down on unit 1 because it was cooling the core too fast which can cause cracking. The turbine driven pump on unit 2 was left running which is why it did not over heat right away.
keep in mind, BWR the reactor is at 1000 PSI and PWR are at 2500 psi, in order to cool the reactors, pumps have to overcome this pressure, if you vent this pressure too fast the water boils away and again it overheats due to lack of water around the fuel.
When fuel gets too hot the zircaloy cladding takes the oxygen from the water to convert to rust releasing the hydrogen gas which later accumulates to cause the hydrogen explosions as seen at all the major events. zircaloy is used due to neutron cross section properties.

The newest reactor designs use more passive cooling methods such as storing the borated water inside containment that is used during refueling. They also use a natural air circulation move the heat from the reactor to a heat sink such at the refueling water storage tank. The newer designs are also better equipped to handle the rapid cool down effects

Once it was known at Fukushima that these units would never run again using any means available became the option such as using sea water through a fire truck. Had the operators made this decision when the emergency power when out, (with no hope of restoration) the consequences would have been confined to the plant perimeter, with 3 dead power plants. But this is armchair quarterbacking.


RE: emergency cooling using decay heat

Quote (BJC)

what was the test that caused the Cherynobil accident supposed to find out. That was a main generator test on residual heat?

Not on residual heat- they were testing the ability of the turbine to run the cooling pumps after shutting off steam flow.

Basically, attempting to determine if the turbine rotor assembly contained enough KE to drive the cooling pumps until the diesels could be started and run up to a high enough speed to generate enough electrical power on their own.

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