Reactor refrigeration without external power supply

[Gruxg]

Hi, a layman's doubt out of curiosity:
If I am correct, after a reactor is shut down, it keeps heating because of the high activity, it is necessary to extract the heat to avoid damages and this needs electricity. I think the nuclear plants have emergency diesel generators in case they get disconected from the general grid, but my doubt is:

In case of failure of the external power supply, is it possible to keep generating electricity with the reactor and operate the refrigerating systems with it? I mean, like a closed independent system? It seems extrange that a power plant needs external power (from the grid or from diesel generators) to operate.

Are there any type of reactors (used in power plants) able to refrigerate only with passive systems in case of emergency, or is it always necessary an active one?

Thanks!

 

[gmax137]

In case of failure of the external power supply, is it possible to keep generating electricity with the reactor and operate the refrigerating systems with it? I mean, like a closed independent system? It seems extrange that a power plant needs external power (from the grid or from diesel generators) to operate.

I will answer in terms of PWR (pressurized water reactor) design since this is what I am most familiar with. In a PWR, water is circulated through the reactor core by several (2 to 4) large pumps, each of which pumping ~6.3 cubic meters per second (100,000 gpm). The pump motors are about 5000 to 6000 kW each. These are normally powered from offsite (ie, the grid) because (1) they are very large loads, and (2) they will continue to operate even if the reactor is tripped. Plant recovery from a trip is much simpler if the pumps remain running. On the other hand, if the grid is lost, the pumps will coast down and the reactor will shutdown due to the low flow through the core (the details are more complicated but the result is the same).

All of this goes to show why the plant will not continue to generate electricity if offsite power becomes unavailable.

However, the idea of the reactor powering its own cooling is a good one, and in fact is implemented as follows.

A typical PWR plant design includes an emergency feedwater system which dissipates the core decay heat by boiling the water in the steam generators, with the resulting steam used to power a small turbine to pump make up feed water into the steam generators. So the decay heat provides the energy needed to maintain the make up water to the steam generators. This works whether offsite power is available or not.

There are other considerations. For instance, the pumps circulating water through the main condenser are also powered from offsite.

I hope this helps answer your questions.

 

[Gruxg]

A typical PWR plant design includes an emergency feedwater system which dissipates the core decay heat by boiling the water in the steam generators, with the resulting steam used to power a small turbine to pump make up feed water into the steam generators. So the decay heat provides the energy needed to maintain the make up water to the steam generators. This works whether offsite power is available or not.

Thanks for replying. This is interesting. The Fukushima reactors were not PWR, but is the system you describe similar to this one mentioned in wikipedia?:
"Units 2 and 3 had steam turbine-driven emergency core cooling systems that could be directly operated by steam produced by decay heat and that could inject water directly into the reactor. Some electrical power was needed to operate valves and monitoring systems"

I'm not sure if the power needed to operate the valves and monitoring systems could be provided by small batteries, or if diesel generator are necessary. Anyway, in Fukushima the power source apparently didn't work after the tsunami. My doubt is, in case the diesel generators in a plant could not be used for any reason (e.g. malfunction or fuel running out after some time), if it could be possible to get all the necessary electric power from the reactor (either by keeping it "on" or using the decay heat). I guess most plants are not designed for this, but I don't know if it could make sense.

I kept reading the wikipedia and found this:
"Unit 1 had a different, entirely passive cooling system, the Isolation Condenser (IC). [...] As the tsunami engulfed the station, the IC valves were closed and could not be reopened automatically due to the loss of electrical power, but could have been opened manually"
My understanding is that this cooling system could have worked without any electric power at all if someone had opened the valves, but for some reason nobody did it (am I correct?)

 

[Rive]

There is a contradiction you should consider. Generally, a reactor is expected to be brought (and kept) at safe, low (far below boiling point) temperature: while to extract any useful energy from it you would need far higher temperature (=> high pressure).

That's why these self-powering and passive solutions (Isolation Condenser and RCIC) are emergency measures, not for standard operation.

Isolation Condenser (IC).
[...]
My understanding is that this cooling system could have worked without any electric power at all if someone had opened the valves, but for some reason nobody did it (am I correct?)

As I recall it was a bit more complicated than that. To maintain proper cooling speed/sequence they closed those valves (that time they did not know about the tsunami).
Too fast cooling may endanger a reactor.

I think you should take a look at the passive safety systems of the ESBWR design

 

[gmax137]

There is a contradiction you should consider. Generally, a reactor is expected to be brought (and kept) at safe, low (far below boiling point) temperature: while to extract any useful energy from it you would need far higher temperature (=> high pressure).

Yes indeed. In the case of the PWR emergency feedwater system, the turbines used will operate down to about 65 psia steam supply pressure; this corresponds to a temperature of 300F. This is well below the normal operating temperature of ~550F, but is of course well above ambient temperature. The plants were therefore also provided with other systems to maintain decay heat removal at lower temperatures. Those systems do rely on motor-driven pumps and heat exchangers using cooling water from the adjacent lake, river, or ocean.

 

[gmax137]

As I recall it was a bit more complicated than that. To maintain proper cooling speed/sequence they closed those valves (that time they did not know about the tsunami).

Can anyone recommend a good book on the technicalities of the Fukushima Daiichi events? I would think that by now there would be a good comprehensive write up.

Too fast cooling may endanger a reactor.

This is a good example of a problem faced in plant design: the emergency cooling systems must be sufficient for the worst-case scenario, and they must also be redundant such that they provide sufficient cooling even with a failure in the emergency system. That means the system, if everything works, will provide at least twice as much cooling as necessary. Which can cause problems.