4 ms·

Where does the 1500MW of heat the reactor is producing go, immediately after the generators are isolated from the grid?

by Symbiote 2y ago

csours 2y ago
Water? Same place it was going
- Scoundreller 2y ago
  I mean, at least some percentage was going down the wire. That’s why we do it, right?
  - csours 2y ago
    It would be funny if we were doing the whole song and dance just to amuse the electrons. If the turbines are 50% efficient, then the heat load on the cooling towers would double, which I have to assume is WELL within design limits. You have made me wonder about turbine overspeed and steam bypass.
rtkwe 2y ago
The reactor is turned down and rapidly stops producing that much heat. On site backups keep the pumps running to continue cooling the residual heat. There's a long slow tail as lower energy fusion chains tail off towards lead but the vast majority of the power comes down quickly.
- deleted 2y ago
  [deleted]
pstrateman 2y ago
Enthalpy of vaporization for water is very very large. The amount of heat that can be dissipated by evaporating water is kind of incredible.
- Symbiote 2y ago
  Thanks, that's what I was asking. (It's obviously not a meltdown situation!)
theideaofcoffee 2y ago
Instead of that energy being directed to the turbines to convert the thermal energy into rotational energy (and thus it to electricity), the reactor begins cooling from the primary reactions (but not completely since secondary reactions/decay still are going) since there is no reaction heat (reactor idled). All of that excess residual heat contained in the mass of the fuel and reactor water is carried away by the cooling water loop, and either out to the cooling tower where it evaporates and releases its energy, or through the next stages of the loop into the cooling supply, most likely a neighboring river or lake.
gunapologist99 2y ago
It seems like you might really be asking about a meltdown scenario. The reaction can be slowed with control rods, which stops/minimizes the heat from being generated. The previously generated heat still needs to be handled, however (by evaporation). https://en.wikipedia.org/wiki/Nuclear_reactor https://en.wikipedia.org/wiki/Nuclear_reactor A meltdown occurs when the reaction can't be slowed down through normal means because the safety systems fail. For example, Fukushima and Chernobyl: https://en.wikipedia.org/wiki/Fukushima_nuclear_accident https://en.wikipedia.org/wiki/Fukushima_nuclear_accident https://en.wikipedia.org/wiki/Chernobyl_disaster https://en.wikipedia.org/wiki/Chernobyl_disaster However, those safety systems are obviously designed expressly to prevent such a disaster. For example, control rod systems are often designed to be fall (via gravity) into the reactor in the event power fails. These failsafe systems are typically very reliable, in the absence of other external events (such as the flooding and earthquake that took place in Fukashima.)
- thijson 2y ago
  The control rods don't stop decay heat from being produced. That's what happened with Fukushima. Also, nuclear poisons (neutron absorbers) build up after the reactor is shut down. After the control rods are withdrawn it takes a few days for the poisons to be burned up and the reactor can resume power production. I think they call that poisoning out the reactor. https://en.wikipedia.org/wiki/Neutron_poison https://en.wikipedia.org/wiki/Neutron_poison
  - thijson 2y ago
    When the Northeast power grid went down down in 2003, the Bruce Nuclear Power Development was almost entirely kicked off the grid. It was able to keep ticking over at a few percent of power output though because of a feeder line that went North. This prevented the reactors from poisoning out, and allowed them to come back faster than if they had poisoned out.
- euroderf 2y ago
  > These failsafe systems are typically very reliable From the RISKS archives: "A fail-safe system fails by failing to fail safe."
bell-cot 2y ago
In general, any sort of "boil water to steam, use steam to drive turbines" system can vent the hot steam from the boilers to the atmosphere. There's a big reserve tank of cold boiler feed water, to replace the water you're no longer getting back from the steam condensers (attached to the "OUT" steam pipe on the turbine). So the boilers will keep soaking up just as much heat, while the engineers scramble to reduce the heat coming in from the burning wood, or burning coal, or burning oil, or fissioning atoms, or whatever.
quickthrowman 2y ago
I would assume the cooling loop is still running on generator and/or backup utility feed power, so the heat from the reactor would go into the water of the cooling loop, and then the heat would be rejected into the atmosphere when the cooling loop water evaporates in the cooling towers, with the leftover heat being discharged into the river/lake/cooling water source.

Where does the 1500MW of heat the reactor is producing go, immediately after the generators are isolated from the grid?

csours 2y ago

Water? Same place it was going

Scoundreller 2y ago
I mean, at least some percentage was going down the wire. That’s why we do it, right?
- csours 2y ago
  It would be funny if we were doing the whole song and dance just to amuse the electrons. If the turbines are 50% efficient, then the heat load on the cooling towers would double, which I have to assume is WELL within design limits. You have made me wonder about turbine overspeed and steam bypass.

rtkwe 2y ago

The reactor is turned down and rapidly stops producing that much heat. On site backups keep the pumps running to continue cooling the residual heat. There's a long slow tail as lower energy fusion chains tail off towards lead but the vast majority of the power comes down quickly.

deleted 2y ago
[deleted]

pstrateman 2y ago

Enthalpy of vaporization for water is very very large. The amount of heat that can be dissipated by evaporating water is kind of incredible.

Symbiote 2y ago
Thanks, that's what I was asking. (It's obviously not a meltdown situation!)

theideaofcoffee 2y ago

Instead of that energy being directed to the turbines to convert the thermal energy into rotational energy (and thus it to electricity), the reactor begins cooling from the primary reactions (but not completely since secondary reactions/decay still are going) since there is no reaction heat (reactor idled). All of that excess residual heat contained in the mass of the fuel and reactor water is carried away by the cooling water loop, and either out to the cooling tower where it evaporates and releases its energy, or through the next stages of the loop into the cooling supply, most likely a neighboring river or lake.

gunapologist99 2y ago

It seems like you might really be asking about a meltdown scenario. The reaction can be slowed with control rods, which stops/minimizes the heat from being generated. The previously generated heat still needs to be handled, however (by evaporation). https://en.wikipedia.org/wiki/Nuclear_reactor https://en.wikipedia.org/wiki/Nuclear_reactor A meltdown occurs when the reaction can't be slowed down through normal means because the safety systems fail. For example, Fukushima and Chernobyl: https://en.wikipedia.org/wiki/Fukushima_nuclear_accident https://en.wikipedia.org/wiki/Fukushima_nuclear_accident https://en.wikipedia.org/wiki/Chernobyl_disaster https://en.wikipedia.org/wiki/Chernobyl_disaster However, those safety systems are obviously designed expressly to prevent such a disaster. For example, control rod systems are often designed to be fall (via gravity) into the reactor in the event power fails. These failsafe systems are typically very reliable, in the absence of other external events (such as the flooding and earthquake that took place in Fukashima.)

thijson 2y ago
The control rods don't stop decay heat from being produced. That's what happened with Fukushima. Also, nuclear poisons (neutron absorbers) build up after the reactor is shut down. After the control rods are withdrawn it takes a few days for the poisons to be burned up and the reactor can resume power production. I think they call that poisoning out the reactor. https://en.wikipedia.org/wiki/Neutron_poison https://en.wikipedia.org/wiki/Neutron_poison
- thijson 2y ago
  When the Northeast power grid went down down in 2003, the Bruce Nuclear Power Development was almost entirely kicked off the grid. It was able to keep ticking over at a few percent of power output though because of a feeder line that went North. This prevented the reactors from poisoning out, and allowed them to come back faster than if they had poisoned out.
euroderf 2y ago
> These failsafe systems are typically very reliable From the RISKS archives: "A fail-safe system fails by failing to fail safe."

bell-cot 2y ago

In general, any sort of "boil water to steam, use steam to drive turbines" system can vent the hot steam from the boilers to the atmosphere. There's a big reserve tank of cold boiler feed water, to replace the water you're no longer getting back from the steam condensers (attached to the "OUT" steam pipe on the turbine). So the boilers will keep soaking up just as much heat, while the engineers scramble to reduce the heat coming in from the burning wood, or burning coal, or burning oil, or fissioning atoms, or whatever.

quickthrowman 2y ago

I would assume the cooling loop is still running on generator and/or backup utility feed power, so the heat from the reactor would go into the water of the cooling loop, and then the heat would be rejected into the atmosphere when the cooling loop water evaporates in the cooling towers, with the leftover heat being discharged into the river/lake/cooling water source.