[niekze]

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.

[Ericson2314]

I know land vs sea is different, but after decades of nuclear submarines working beautifully it's just so sad to me we don't have abundant SMRs by now.

[shagie]

I am of the understanding that part of this is because there are different profit margins in mind with a civilian reactor generating power to be sold and a military reactor powering a vessel.

When that is combined with deregulation (or an anti-regulatory mindset) where things like insulation on water intake is deferred or ignored because it impacts the economics of the power plant, then building one becomes difficult.

[exabrial]

Agree... although I do believe part of the problem is that a lot of US Naval reactors run on weapons grade uranium. Someone here probably knows more about this.

[niekze]

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.

[Kimm0n0]

I think the difference i >90% for military use and 3-5% for civilian use.

Some new SMRs are planning on using >5%.

[Ericson2314]

With that grade fuel, can a nuclear submarine denote like a bomb?!

[preisschild]

Large nuclear reactors have better economics (per energy produced) than small reactors due to economy of scale benefits.

But SMRs can still be useful for small towns, remote communities, district heating, process heat for nearby industry and so on.

[gosub100]

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?

[vectoral]

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.

[gosub100]

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?

[sbierwagen]

Fresh fuel pellets are “safe” in that they’re not going to kill you immediately, but they’re still fairly radioactive, not just from alpha decay, but from spontaneous fissioning, which produces neutrons. Pile em up and they’ll start a chain reaction all on their own. There’s even geological evidence of natural chain reactions in some uranium ore seams: https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

[vectoral]

Sorry for the slow reply -- didn't realize there weren't notifications on HN.

U-235 (the fuel used in naval reactors) does undergo spontaneous fission, but not at a rate high enough so reach criticality. Like one of the other posters mentioned, you can make it easier to achieve and maintain criticality by changing the shape of the core (so that fewer neutrons leak out) but in general you do need a neutron source inside the core that is just always spitting out enough neutrons to help the reactor achieve criticality as the control rods are withdrawn.

Once the core has operated at power for long enough, some core materials become "active" (from irradiation) and may help contribute to the neutron source.

Straight U-235 is fairly safe (iirc), but even then I don't think you'd want to ship the fuel assemblies with any moderator as moderated neutrons are what make fission more likely.

[OkayPhysicist]

Criticality is a result of geometry. If you modify the geometry (by placing the rods in a reactor, by removing control rods, etc), you can vary the system from subcritical, to critical, to super-critical. No external neutron source necessary.

[philipkglass]

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

[invalidator]

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.

[niekze]

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.

[tiffanyg]

Thank you so much for sharing that! Beautiful / elegant and simple!

It's been decades since I looked at any of the details involved in any of the various types of reactors that have been designed. When I did, in the past, I hadn't even encountered concepts like "control theory" or spent any time with the subject matter of "systems engineering" or even "chemical engineering". I.e., areas where you start thinking about how to combine all of the different simple "laws"* and properties and such of energy and matter to create "robust" (ideally) or even just practical "systems".

Although I had read about the Chernobyl disaster, and "run-away" that occurred - the massive volumes of water being pumped in, partly as a result of such levels, at near boiling ... the steam voids, etc. I'm not entirely sure whether I really encountered the point about temperature and density, but, certainly, it didn't 'click' quite the way it did now when I read your description.

I love this kind of stuff - the "how it all fits together" from what can otherwise be these seemingly dry / 'dead' "laws" and such that can seem too simple / narrow / etc. to do much of use with - even if your teachers spend as much time as possible giving you homework questions etc. that certainly seem practice-oriented - but who gives a rat's-keister about whether comparing the weight of a duck to a putative witch might establish flammability and hence witchcraft when they're 15, right? ;)

* Simplified models describing various types of matter and physical processes - models that are valid (for some definition of ... as the mathematicians &/ Humpty-Dumpty [Alice in Wonderland / Lewis Carroll] might say) given certain assumptions / pre-conditions (on scale, frame of reference, etc.)

[ok_dad]

The Chernobyl disaster was partly because the design was graphite moderated, which does not have the safety that water does since it’s not self regulating due to the GPs explanation about that above. When the reactor started to go supercritical, it was reinforced by the moderator working better to create more neutrons, the opposite of what you’d want.

[cyberax]

> supercritical

All reactors are technically supercritical. Chernobyl reactor became _prompt_ _critical_.

Normally, a small amount (just around 0.2%) of fission neutrons are emitted within a 1-3 seconds after a fission event. They are called "delayed neutrons", and this small percent of delayed neutrons is what pushes a reactor over the criticality threshold.

Since these neutrons are delayed, it gives enough time for control systems and natural feedback mechanisms to keep the reaction rate steady.

If you push your reactor past the delayed neutrons so that there are enough of prompt neutrons to sustain the criticality, you're screwed. The reaction rate can double within microseconds, far too fast for anything macroscopic to react. So within less than a millisecond your reactor can overheat, until the nuclear fuel becomes too hot to fission because its atoms move too fast (usually somewhere around 1000C).

And then it'll be followed by some extreme thermodynamics and chemistry: steam explosion, water-zirconium reaction, graphite moderator fire, etc.

Here's a research reactor that does prompt criticality excursions in controlled conditions: https://www.youtube.com/watch?v=pa0Fmcv83nw (with a countdown!)

[cyberax]

It's a tad more complicated. Light water is both a good neutron moderator _and_ a good neutron absorber.

If you vaporize the water, thus reducing its density, it reduces both the neutron absorption, and it reduces the moderation efficiency. But crucially, the moderating efficiency matters much more in regular reactors, so the overall reactor power will drop.

In a graphite-moderated reactor, water's moderating efficiency might not matter much. So if you vaporize the water, there's going to be less neutron absorption, but there's still going to be plenty of graphite moderator to help neutrons to slow down. So the reactor power will _increase_ unless compensated by other means, and this can result in a self-reinforcing loop (see: Chernobyl).

BTW, the neutron absorption is the reason it's very hard (though not impossible) to make water-cooled breeder reactors that produce more nuclear fuel than they consume.

After the Chernobyl disaster, several remaining RBMK reactors were made safer by enlarging the cooling channels. This increased the amount of water present in the core, thus increasing the dependency on water's moderating effect, greatly reducing the positive void coefficient. It couldn't be completely eliminated, but it was reduced to a level where it can't result in prompt criticality anymore.

[tiffanyg]

Excellent point - you are 100% correct, parent comment etc. were specifically about water-moderated types.

I was too grabbed by some of the later description and just connected it somewhat haphazardly to not very organized or accurate info rattling around in my head from years ago.

Thanks for pointing that out!

[paulnpace]

Additionally, they were ordered to disable all safety mechanisms and run the reactor in a known unsafe condition, causing a massive long-term disaster in... the Ukraine.

The way I attempt to explain the difference between the negative and positive coefficient of reactivity is it's like one car accelerates by pressing the gas pedal and the other car has an engine running WFO and all you ever do is press the brake pedal. It isn't a perfect analogy, but I think it gets the general concept across.

[dgroshev]

As an aside, "the Ukraine" has mildly offensive connotations. https://en.wikipedia.org/wiki/Name_of_Ukraine#English_defini...

[paulnpace]

> 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.

[niekze]

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!

[hansthehorse]

After a civilian nuclear reactor is refueled mtc is positive. It makes for a fun few weeks.

[39]

Dope explanation!