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Nuclear Reactor Simulator

dalton-nrs.manchester.ac.uk

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Re: Nuclear Reactor Simulator

#81

Post by niekze »

gosub100 wrote: post #75
niekze wrote: post #41

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 intere…

I have a question about the control rods: are they normally removed completely when you want the reactor to run? If they are partially inserted, that would seem to mean that the reactor fuel would burn unevenly, with the pellets at the bottom used up sooner than the top. Is that true, and is it a problem that has mitigations?

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.

Re: Nuclear Reactor Simulator

#82

Post by philipkglass »

gosub100 wrote: post #75
niekze wrote: post #41

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 intere…

I have a question about the control rods: are they normally removed completely when you want the reactor to run? If they are partially inserted, that would seem to mean that the reactor fuel would burn unevenly, with the pellets at the bottom used up sooner than the top. Is that true, and is it a problem that has mitigations?

Regardless of control rod use, there is non-uniform burnup. Fuel manufacturers use different enrichment levels in fuel pellets throughout the length of the rod to partially compensate for the non-uniformity.

The majority of PWR fuel assemblies have similar axial-burnup shapes – relatively flat in the axial mid-section (with peak burnup from 1.1 to 1.2 times the assembly average burnup) and significantly under-burned fuel at the ends (with burnup of 50 to 60% of the assembly average). Figure 1 shows a representative PWR axial burnup distribution. As is typical, the burnup is slightly higher at the bottom of the assembly than at the top. This variation is due to a difference in the moderator density. The cooler (higher density) water at the assembly inlet results in higher reactivity (which subsequently results in higher burnup) than the warmer moderator at the assembly outlet.

Quoted from "ORNL/TM-1999/246: Review of Axial Burnup Distribution Considerations for Burnup Credit Calculations"

https://www.osti.gov/servlets/purl/763169

Re: Nuclear Reactor Simulator

#83

Post by vectoral »

gosub100 wrote: post #75
niekze wrote: post #41

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 intere…

I have a question about the control rods: are they normally removed completely when you want the reactor to run? If they are partially inserted, that would seem to mean that the reactor fuel would burn unevenly, with the pellets at the bottom used up sooner than the top. Is that true, and is it a problem that has mitigations?

Former submarine nuke with a masters in NucE here (it's fun to see us come out of the woodwork for this).

Rods are always in the core. To start a reactor that is shut down (with the rods are all the way on the bottom), you withdraw them slowly until the reactor is self-sustaining. From there, you increase power by increasing steam demand (as described in the parent comment above) and continue raising rods to increase or maintain temperature.

When the reactor is operating at power, the control rods are used primarily to 1) control steady state coolant temperature and 2) provide a safe and reliable way to shut the reactor down quickly (by dropping them to the bottom of the core -- this is called a reactor scram). If you have a short-duration power transient for any reason, you can "shim" the rods in to prevent a power spike that might cause a protective action to occur (you shouldn't really ever have to do this except for during emergency drills).

If the rods were drawn outside of the fuel region at power, they wouldn't be able to absorb any neutrons and wouldn't give you any way to control temperature or power. During some specific maintenance when the reactor is shut down, you sometimes might pull one rod further out for testing.

Your question on uneven burning of fuel is insightful. That can happen, and it's caused by an uneven neutron flux (# of neutrons traveling through a unit surface area per unit time) distribution. The core designers take rod positioning into account when determining how to distribute fuel throughout the core in order to maintain a "flat" flux profile.

Re: Nuclear Reactor Simulator

#84

Post by yason »

The only destined purpose of a nuclear reactor simulator is to gradually let a massive meltdown happen, with gauges slowly increasing in tandem with thrill in anticipation of the upcoming drama and fireworks until you discover a bit too late that... it's too late.

Re: Nuclear Reactor Simulator

#85

Post by jtriangle »

dekhn wrote: post #49
cratermoon wrote: post #13

The very first nuclear plant simulator I ever played with was a primitive text-based program, running on a DEC 11/780. To win the game you had to average a certain number of MWh over each turn (days). It seemed easy, just turn it on and let it run, but there was a catch. The game simulated normal fatigue, meaning that the longer it ran, the more likely something would go wrong. Also, the higher power you ran, the fas…

I didn't understand how nuclear reactors work until I was in grad school and even then I was aghast. You'd think if we could split an atom, we'd have a better way to make electricity than to make steam to turn a turbine. Of course, there are RTGs, and https://en.wikipedia.org/wiki/Betavoltaic_device

It's an incentives problem. Nearly all of the world's power is generated by spinning a turbine with steam, so, we have a direct, actionable, incentive to make that process as efficient as possible, and we have indeed done this and continue to do this. Modern steam turbines are wildly efficient.

That doesn't mean that they are the only thing that can be efficient, just that they're the current best option, and, any other option is going to have to show immense promise in order to get the funding to catch up to where we are now with steam.

The only thing on my radar that bypasses steam entirely is what Helion Energy is doing up in washington with their experimental fusion devices. The basic idea being that you're using a pulsed fusion reaction and intentionally not containing it, instead using the energy produced to push back on the magnetic containment and generate power. No idea if it'll work out to be viable, but it at least makes sense on paper.

Re: Nuclear Reactor Simulator

#86

Post by cratermoon »

jtriangle wrote: post #85
dekhn wrote: post #49

Earlier quoted context omitted.

I didn't understand how nuclear reactors work until I was in grad school and even then I was aghast. You'd think if we could split an atom, we'd have a better way to make electricity than to make steam to turn a turbine. Of course, there are RTGs, and https://en.wikipedia.org/wiki/Betavoltaic_device

It's an incentives problem. Nearly all of the world's power is generated by spinning a turbine with steam, so, we have a direct, actionable, incentive to make that process as efficient as possible, and we have indeed done this and continue to do this. Modern steam turbines are wildly efficient. That doesn't mean that they are the only thing that can be efficient, just that they're the current best option, and, any ot…

You're correct. The design of nuclear reactors was intended from the beginning to replace steam generation, with the goal of not needed to redesign the entire generating plant and take advantage of the known tech as much as was reasonable. There were ideas around for just replacing existing coal/gas/whatever with reactors, to get the most value out of the capital expended on the plant. There are still studies around retrofitting retired coal generation with nuclear reactors.

Re: Nuclear Reactor Simulator

#87

Post by Obscurity4340 »

Obscurity4340 wrote: post #20
staplung wrote: post #15

Seems impossible to make it meltdown and explode. Shutting off the coolant just makes it automatically SCRAM. No incoming tsunamis threaten to swamp your diesel generators, no xenon pits to slowly climb out of. Booooring. ;-)

My biggest beef with nuclear (and I've always said this) is its just so unsexy. No mushroom shapes, no passionate explosions followed by restful oblivion at the end. Its just reliable and faithful and keeps on pluggin' away. I like to live a little dangerously, and you should too!

The foreplay also takes like 10-15 years, c'mon?!

Re: Nuclear Reactor Simulator

#88

Post by invalidator »

gosub100 wrote: post #75
niekze wrote: post #41

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 intere…

I have a question about the control rods: are they normally removed completely when you want the reactor to run? If they are partially inserted, that would seem to mean that the reactor fuel would burn unevenly, with the pellets at the bottom used up sooner than the top. Is that true, and is it a problem that has mitigations?

You don't pull all of the control rods at the same time. You can fully withdraw some number of them, then control the reaction with a few more rods.

There are several strategies to provide additional control so it's not all at the bottom. For instance, in a PWR, reactivity decreases with temperature, so additional coolant can be injected where additional reactivity is needed. In the RBMK, a small number of rods were inserted from the bottom to provide more axial control.

Re: Nuclear Reactor Simulator

#89

Post by niekze »

paulnpace wrote: post #58
niekze wrote: post #41

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 intere…

> If the water is colder, it is denser and the more likely neutrons will bounce back into the fuel. My memory is that the denser water thermalizes the neutrons in a shorter period of time and this is why reactivity is increased.

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!

Re: Nuclear Reactor Simulator

#90

Post by gosub100 »

vectoral wrote: post #83
gosub100 wrote: post #75

Earlier quoted context omitted.

I have a question about the control rods: are they normally removed completely when you want the reactor to run? If they are partially inserted, that would seem to mean that the reactor fuel would burn unevenly, with the pellets at the bottom used up sooner than the top. Is that true, and is it a problem that has mitigations?

Former submarine nuke with a masters in NucE here (it's fun to see us come out of the woodwork for this). Rods are always in the core. To start a reactor that is shut down (with the rods are all the way on the bottom), you withdraw them slowly until the reactor is self-sustaining. From there, you increase power by increasing steam demand (as described in the parent comment above) and continue raising rods to increase…

thank you so much for the response. one more q if you dont mind: on the "how its made" show they show how the fuel comes from ore, to yellow cake, to pellets in zircon rods, to collections of rods in an assembly.

This is completely safe (compared to spent fuel), but how do you get the reaction started? do you have to "light" it with a neutron source when you're ready to use the fuel for the first time? or do you "light" it with radioactivity from existing fuel? or a neutron reflector?

In How-it's-made they didn't say anything like "the fuel assemblies are shipped to power plants with graphite moderators to prevent unwanted reactions during transit", so obviously there's no danger of an unwanted reaction outside of a reactor. So what kicks it off?

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