niekze · HackerTrans

3 jaar geledendiscuss

It's kind of the same idea, right? The more stuff there is to bounce off of, the faster it will slow down, the smaller the net distance it will travel, the increased chance it will thermalize, and less likely it is to escape. I could be missing a lot of nuance there as it's been almost 20 years since I went to naval nuclear power school. I'm definitely not the one to ask about the specifics!

niekze

3 jaar geledendiscuss

The rods are not completely removed. Control rods ravenously gobble up free neutrons. As they're pulled up, more neutrons get to the uranium. You are correct in that fuel at the bottom is used up sooner. As more fuel is used, the rods will have to be pulled up higher than they were before for the same effect. The design takes this into account.

niekze

3 jaar geledendiscuss

The fuel in US Naval nuclear reactors is enriched to a much higher percentage than civilian reactors due to size and longevity considerations. It has to fit the ship and refuels take months/years. A ship undergoing a refuel isn't a ship you can use.

In a civilian plant, you can have multiple reactors and refuel them on a rotating schedule to avoid downtime, having a larger reactor vessel isn't a problem, and all of that is also going to be less expensive - which is a huge factor.

niekze

3 jaar geledendiscuss

I gotta be both the reactor operator and the throttleman?!?

niekze

3 jaar geledendiscuss

Former US Navy submarine nuclear reactor operator here.

Adjusting the steam output was kind of strange. On a submarine, the steam used to propel the submarine dwarfs all the other steam loads. As a result, there's a throttleman who controls that.

Even though this simulation is simplified, it's not too bad. It does hide some of the really interesting aspects of a water cooled/moderated nuclear reactor. The most interesting thing is that water makes the reactor self-regulating because of its negative temperature coefficient of reactivity. I'll explain.

When a uranium-235 atom absorbs a stray neutron, it becomes unstable and splits. This releases more neutrons. Very few of these neutrons will be absorbed by surrounding uranium-235 atoms. This is a good thing. Most will escape the fuel, and some will bounce around in the surrounding water. This slows the neutrons down, and some of them will bounce back into the fuel to be absorbed for more fission reactions.

Let's say 1,000 fission reactions occur. If the result is that 800 neutrons from those fission reactions are absorbed by other uranium-235 atoms, you'll have 800 more fission reactions. The reactor is sub-critical as the reaction will not be self-sustaining.

If 1,000 fissions cause 1,200 neutrons to be absorbed and react, you'll have 1,200 resulting fission reactions. The reactor is super-critical as the number of fissions will increase.

If 1,000 fissions occur and the result is that 1,000 neutrons are absorbed and cause 1,000 more fission reactions, the reactor is critical. "The reactor is critical" means the number of fission reactions is self-sustaining and neither increasing nor decreasing.

How can we affect how many neutrons bounce back into the fuel? We can change the density of the water. It makes sense if you thing about it. The denser the water, the more likely neutrons will hit a water molecule and head back into the fuel.

How can we change the density of the water? We change the temperature of the water. If the water is colder, it is denser and the more likely neutrons will bounce back into the fuel.

How do we change the temperature of the water? We pull more/less heat of out it by using more/less steam.

Putting this all together, as steam demand goes up, more heat is pulled out of the water. This causes colder water to enter the reactor. Colder water will reflect more neutrons. More neutrons means more fission. More fission means more heat. More heat means warmer water and this will attenuate the increase in fission until an equilibrium is reached.

If you're creating too much power, the coolant temperature will increase and the power output will lower. If you're creating too little power, the coolant temperature will decrease and the power output will rise. That's why water is a great coolant/moderator: its negative temperature coefficient of reactivity.

niekze

3 jaar geledendiscuss

If you had 50 invocations every second and your execution time was 2 seconds, would you set it to 100? If that's the case, you would multiply the invocations per second by the process time.

niekze

3 jaar geledendiscuss

"You should set your PC value equal to your peak TPS divided by the execution time. For example, if you have a peak of 50 TPS and your execution time is half a second, the most effective PC value would be 25."

This confused me. Wouldn't 50 / 0.5 be 100?

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