I always thought that it was somewhat ironic that a big nuclear power plant must rely on diesel backup generators for emergency shutdowns in during a black out--but such are the necessities of technology. It is interesting that pressurized water reactors--and most nuclear reactors in general--must preheat the coolant water generally using electricity to bring the coolant up to a minimal operating temperature--before the reactor core is even allowed to 'go critical' and begin generating heat.

The pressurizer in PWR's uses a bank of resistance heaters--kind of a gigantic version of a house's hot water heater--to heat some of the water to the point that it boils and creates some trapped steam in the head space of a confined vessel. Pressure is transmitted from the bottom of this vessel to the main coolant inlet lines going to the core (or is it the hot return lines--I don't remember!) Anyways, the point is, that a 'cold start' of a nuclear reactor is kind of a misnomer--and most plants when they start are initially large "loads" on the electrical grid before they come up to operating power. These diesel backup generators do not generate enough power to start a reactor (in a cold start condition where the coolant has been allowed to cool to ambient temperature--such as after a refueling operation or such.) Only by connecting a nuclear powerplant to an energized grid will the plant recieve enough power to actually commence operation--before it can generate any.

How many Megawatts does it take to "Cold Start" a nuclear power plant?

Googlenaut,

Commercial boiling water reactors can actually perform a cold start, i.e., they can go critical at below 200 F and use reactor heat to raise RCS temperature and pressure to normal operating levels.

Submarine pressurized water reactors can also perform a cold start and use reactor heat to raise RCS temperature and pressure to normal operating levels.

The reason why commercial pressurized water reactors cannot do this is because they use boric acid as a means of controlling reactivity. When RCS temperature is below 200 F, the temperature co-efficient of reactivity in a commercial PWR tends to be slightly positive due to the neutronic effects of boron - cold water, higher density, more boron atoms per unit volume - thus, when temperature rises, distance between boron atoms increases and the chances that a neutron will escape capture by boron increases, thus making more neutrons available for fission, thus introducing a positive reactivity effect. This does not occur at normal operating temperatures and pressures because the macroscopic cross-section for neutron scattering from the light hydrogen in water ends up 'outweighing' the macroscopic cross-section for neutron absorption in boron. That inverse relationship causes a net negative temperature co-efficient of reactivity.

Submarine PWRs do not use chemical shim of core reactivity. Commercial BWRs use two methods of controlling core reactivity: control rod movement and variation in reactor water recirculation flowrate. Both submarine PWRs and commercial BWRs tend to have very strong negative temperature co-efficients of reactivity, even at low temperatures and pressures. Because boric acid reactivity shim is used in commercial PWRs, this is not the case and PWR startup below normal operating temperatures and pressures is forbidden.

I would be very curious to learn from Jaro what is permitted for Candu reactors which are quite dissimilar from submarine PWRs, commercial PWRs and commercial BWRs.

Regards,

Paul

Thanks for the clarification, Paul. I forgot about Boiling Water Reactors! And yes, it stands to reason that a naval PWR ought to be able to undergo a cold start--it sure wouldn't make much sense to put a power source onboard a ship that was incapable of being started by onboard diesel auxiliary power! The Navy generally doesn't roll out 10,000 mile long extension cords to connect a nuclear vessel to shore power! ;)

I think the Navy tends to shy away from 'chemical shims' because of corrosion concerns. Corrosion inside the cooling system of a nuclear sub could be real bad news--I wonder how the Russians have dealt with this problem in their Naval reactors...?

Anyway, that's a bit off topic. It's interesting to understand that there is a certain minimum level of power needed to start a power plant. I remember taking a tour of our PG&E Humboldt Bay Power Station, which has two fossil fuel (natural gas primary, oil secondary) fired boilers, Units 1 and 2, while unit 3 is an old General Electric Boiling Water Reactor of about 65MW. Unit 3 has been shut down since 1976, and was one of the very first commercial nuclear reactors to be decommisioned in the United States. Any how, I remember seeing a large bank of lead acid batteries in a room predictably called the "Battery Room." Now what on Earth would a power plant need batteries for? The Engineer who was giving the tour smiled and said basically that incase of a major power disruption--something that required the plant to completely shut down--which has never happened yet--then the only way to 'cold start' the plant was to provide enough initial electricity to run certain pumps, control circuits, and ingiters to get the initial fires started again. Once those boilers came up, in something less than an hour the steam turbines would begin to spin again. Although he said the batteries were something of a 'throwback' they still kept them because '...you never know when they might be needed..." Still, even a modest diesel backup generator supplying 100 kW could easily displace the batteries for any contigency. I can only imagine how big the backup generators are at a large commercial nuke plant is--probably several megawatts just to run the emergency backup coolant pumps.

-- Edited by GoogleNaut at 09:39, 2006-02-06