100MW reactor must produce a 110MW

EinsteinII

Watched a scene from a Hollywood movie where the Captain of a Nuclear submarine asks his Reactor officer to give him a 110% of his reactor. That means a 100MW reactor must produce a 110MW.

How is that increased? Is that simply by pulling off control rods or is there any other way? Whats the threshold by the way?

Devils

http://en.wikipedia.org/wiki/Rocket#Net_thrust
A typical rocket engine can handle a significant fraction of its own mass in propellant each second, with the propellant leaving the nozzle at several kilometres per second. This means that the thrust-to-weight ratio of a rocket engine, and often the entire vehicle can be very high, in extreme cases over 100.

EinsteinII

Was that an answer to my question?

Devils

1. Hollywood script writers say anything.
2. Real rockets can get more than 100% thrust.

EinsteinII

Devils,

My brother is an officer on board a nuclear submarine and he says that actually is a possible thing! Can you explain me how is that not possible in a nuclear reactor's case?

Drakkith

This is about nuclear power, not rocket thrust.

Nuclear reactors can have their output varied by putting control rods in or pulling them out. Since control rods absorb neutrons, pulling them out lets those neutrons that would have been absorbed hit the fuel instead and cause another fission reaction. I assume that the reactor and powerplant of the submarine can handle going above "normal" limits if need be, but it probably leads to much faster degradation of the reactor and other components than normal. Obviously pushing the reactor too high will lead to extreme danger and likely damage the reactor and possibly cause a meltdown if you go too high or for too long.

Astronuc

Reactors may have margin to failure limits such that they can realize 110% during a short period. It is a simple matter to increase power by withdrawing control rods (if control rods are actively used in the core for reactivity control) or soluble boron in the coolant is reduced. This is not common practice.

Besides the reactor, the heat exchangers and steam turbines must be able to handle the extra power/energy.

EinsteinII

Thank you very much sir,

I was confused when Devils answered me, but again thought he might have a point there. My brother's answer was the same though ruled out a meltdown because reactor officer will warn the captain about it but faster degradation of fuel is obvious.

Thank you again Sir.

EinsteinII

Can anyone tell me whats the meltdown threshold of a reactor. Or at least a link to refer this subject.

Drakkith

That's a hugely complicated subject which depends entirely upon the specifications of each individual reactor. There's no way we could link any references other than the incidences that have already happened.

http://en.wikipedia.org/wiki/Fukushi...clear_disaster
http://en.wikipedia.org/wiki/Chernobyl
http://en.wikipedia.org/wiki/Three_Mile_Island_accident

Devils

It's the only way you're going to get over unity in this argument. Otherwise you are talking about perpetual motion.

If some arbitrary process is running "greater than 100%", say 102%, then its 102% of what?

Drakkith

There is no over unity or perpetual motion here. Such things do not exist. End of story.

Of standard operating maximum due to the design of the reactor. Anything beyond 100% is considered above normal and is likely to vastly increase the chances of something breaking. For example, my trucks engine is limited to around 6,000 RPM. I can, if I want to, remove the limiter and rev my engine even higher, however the further I get above 6,000 RPM the greater the chance of something flying apart inside it. I might reach 7,000 or even 8,000 RPM but my engine is likely to break.

Astronuc

As Drakkith iindicated, the 100% refers to a 'normal' rated thermal power (or full power).

Many reactors in the US have been uprated between 5 and 20% of their original full power ratings. It usually required modifications to the balance of plant, and some modifications to the primary system (e.g., increased flow) and core design. The core can be designed to reduce power peaking both in the axial and radial directions.

Hiddencamper

One thing to realize with nuclear plants, is that we run our reactors such that during worst case conditions, the reactor is still safe.

This means, you can run the reactor at higher power levels and tighter thermal limits, but risk unacceptable accident conditions.

So an example of this, and I'm going to use a large BWR I've worked at as an example. The critical power ratio (CPR) is required to be above 1.09 at all times during that plant's operation. This is a safety limit on the reactor. The operating limit critical power ratio in our plant is around 1.47. What we did, is we looked at every accident that affects critical power ratio, and we found the worst case one to be around .38. We then added that to the minimum critical power ratio (MCPR 1.09) and came up with a limit for normal operation. So what that means, is if I keep my reactor's critical power ratio above 1.47 at all times during NORMAL operation, then during accidents it will never go below our safety limit. On the flip side, I theoretically could operate the plant BELOW 1.47, with no problems or issues to the core, BUT if an accident happens, I risk violating my safety limit and potentially causing unacceptable damage to the fuel during an accident.

In a nuclear submarine, these risks are weighed, and there are calculations and analysis that allow for this for some period of time as well. The maximum power in a nuclear power reactor is almost always limited by thermal limits (maximum capacity of heat exchangers or heat removal from the fuel), and is almost never limited by the core itself. If you were to keep pulling rods, diluting boron, and increasing cooling flow or subcooling, you will keep pushing power up. The maximum limit is definitely over 100%. Power reactors have pre-programmed trip limits on overpower. A large commercial BWR will have a system (APRM - Average Power Range Monitors) that automatically scrams if the reactor is ever over about 118% power, and can not be overridden. Additionally if there is not enough cooling for the current power level for more than a few seconds, the reactor will automatically scram as well. Sub reactors have similar systems (which may be overridden during battle situations).

tl;dr you can easily push reactor power up by diluting boron or pulling rods (or some other reactivity insertion mechanism), but will either be pushing your thermal limits, or pushing the limits on your emergency trip/scram settings.

russ_watters

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.

Hiddencamper

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.

EinsteinII

Thank you very much Gentlemen!