russ_watters said:
Can you also increase the output of the reactor by lowering the water temperature? Ie, by drawing energy off faster with those heat exchangers? Heat exchangers are designed for adverse conditions (warmer cooling water) and if the cooling water is unusually cold, you can pull more heat from a heat exhchanger than it is designed for. Similarly, I would think you can over-rev a turbine (not certain) by putting more electrical load on the generator -- for example by increasing the RPM or prop pitch of the propulsion system.
For a PWR, if you increase the steam extraction from the steam generators (either through increasing turbine load, opening a steam relief valve or PORV, or opening turbine bypass valves, for example), this will remove more steam from the generator, and effectively increases the heat removal from the primary cooling loop. This increase in effective cooling causes lower temperature water to enter the reactor. Due to negative temperature coefficients of reactivity for most operating states of a PWR, decreasing inlet temperature will increase reactor power.
There are limits to this though. Increasing the power in this way affects thermal limits. To protect the fuel, all plants have some type of thermal limit, and many plants have a "Delta-T" interlock which will trip the reactor if the hotleg and coldleg temperatures are too far apart, OR they may have a power-flow imbalance trip where if power is greater than flow allows a trip will occur.
For a BWR, if you increase effective heat removal by making the turbine draw more steam, you have the OPPOSITE effect. An increase in steam removal causes the pressure in the reactor to decrease, which in turn increases the amount of voiding in the core (remember BWRs operate at saturated steam conditions and temperature is a function of pressure). BWRs have negative void coefficients for virtually all operating modes, and as a result, an increase in voiding results in a decrease in power. This will also challenge thermal limits as well (mainly critical power ratio CPR), however BWRs typically are capable of operating with no forced cooling flow (natural flow is enough), but their power will be limited by lack of cooling flow. Additionally during high power/low flow conditions (typically less than 60% flow and greater than 40% power), you run the risk of core thermalhydraulic oscilliations, causing power in the core to oscillate with a very specific frequency (in the 1-2 second range). All BWRs have OPRM (Oscillating power range monitors) which detect these oscillations and will automatically scram the reactor if they are present, and most, if not all plants, have procedures to manually scram the reactor upon entering the region where core oscillations are known to exist.
The control systems between PWR and BWR turbines are different as well. PWR turbine control systems typically operate in the load-set mode. In this mode, you tell the turbine how much steam it is allowed to draw, and that sets the electrical power being generated. If too much steam is suddenly available the bypass valves will lift automatically to limit the pressure increase. To make power changes, you dial in a load-set targer power, and a rate of change. When you enter this in, the turbine will ramp at the specified rate to the new power level.
A BWR CAN operate in the load set mode, and typically is for turbine startup/shutdown and special operations, but normally BWRs operate in the "Pressure Control" mode. In this mode, the operators tell the turbine what reactor pressure they want to maintain, and the turbine will only draw the amount of steam required to maintain that pressure. This means, if you increase reactor power through some method (control rods or cooling flow), the turbine will automatically follow the reactor power increase and increase its power output as well. There is no need to change the load set when you change reactor power, as the turbine will automatically adjust to the new power level.
tl;dr - In a PWR, the reactor follows the turbine load. For a BWR, the turbine follows the reactor.
Other ways to change power in BWRs include subcooling and forced cooling flow. Subcooling is how cold the water entering the core is. You can make the water colder by taking feedwater heaters out of service. You have to take penalties to your core thermal limits to do so, and US plants will only do this at the end of cycle to extend the fuel cycle a couple weeks. The other way to change power in a BWR (and is used from roughly 40% power up to 100% power), is to increase forced cooling flow in the reactor. The reactor recirculation pumps drive jet pumps in the core (except BWR/2 plants, where the recirc pumps directly cool the core). As you increase the jet pump drive flow, it 'pushes' more water through the core and pushes steam out of the core faster. This effectively decreases your void coefficient and increases inlet subcooling. The core will respond by increasing thermal output, until it starts boiling the new volume of water fast enough to return it to equilibrium boiling conditions.
