I have a question about constant coolant temperature program

[jbs87]

I am trying to figure out exactly how the feedback works in this process. What I know so far
when more power is needed

•This greater loading causes more heat to be extracted from the heat exchanger, and for a short period of time, the heat capacity of the heat exchanger and coolant can normally supply the additional load.
• However, this extra energy extracted from the system requires that the temperature of the coolant into the reactor must drop.
The dropping of the inlet temperature causes a drop in the average temperature, and consequently the reactor will posses more reactivity.
• If the reactor was initially in a critical state, it now temporarily becomes supercritical.

• The output temperature of the coolant rises, and more energy is then available from the reactor.
• Finally in the steady state the reactor returns to its critical condition with the average coolant temperature the same as it was initially.

Now if you include fuel temperature feedback, won't the increase in reactivity result in an increase in fuel temperature and due to negative feedback cause a decrease in reactivity. Won't this decrease cancel out the increase in reactivity due to decrease in inlet coolant temperature. So how does this process produce power? Any help would be much appreciated

 

[Astronuc]

Usually a large LWR is based loaded so there really isn't a change in demand. Small changes can be handled by the balance of plant (BOP) usually involving the turbine control valve. BWRs are typically more flexible with flow control, but PWR can load follow with the use of grey control rods. In load-follow operation the core can go up and down in power with fairly good swings. In a PWR, grey rods are more responsive than the boron dilution system.

There is also frequency control, but I'm not too familiar with that. The French utility EdF uses load-follow and frequency control quite a lot. It's not really done in the US.

In BWRs, the increased flow collapses the void in the core which quickly adds reactivity, and blades could be moved as needed, but usually flow control suffices.

In PWRs, a set of grey rods is partially withdrawn, which adds reactivity. And increase in reactivity means the fuel and coolant will get hotter. As the moderator heats up, it becomes less dense, so it doesn't moderate neutrons as well. If there is more nucleate boiling in the upper spans, this also reduces moderation in the PWR. As fuel heats up, Doppler reactivity becomes more negative. After power reaches a certain level, the grey rods can be maneuvered back in position, and boron concentration adjusted as needed.

At higher core power, the fuel is hotter. To balance Doppler coefficient and reduced moderator coefficient, flow control can be used in a BWR for fine control, and in a PWR, the soluble boron concentration can be reduced to compensate for any negative reactivity from hotter fuel or reduced moderator density. Small increases in reactivity are generally offset by decreases due to Doppler and moderator density.

The core exit temperature (and delta-T and average temperature) must naturally be greater for an increase in reactor power, if the core inlet temperature remains the same. Also, keep in mind that in response to an increase in demand, the flow rate in the BOP side of the SG also increases. It is a matter of keeping primary and secondary side in balance.