Recent comments in /f/askscience

UnamedStreamNumber9 t1_jc2o0o9 wrote

Reply to comment by saluksic in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

It can, but by enriching the 235 relative to the 238, you reduce fraction of the fuel that can be jumped from U 238 up to Pu 239 vs the fraction of U 235 that breaks down into barium, krypton and 3 neutrons. There’s still U238 in the fuel rods but with enriched uranium, there’s less of it available to be transmuted

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Hiddencamper t1_jc2nwl9 wrote

Reply to comment by saluksic in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

All uranium based reactors produce plutonium.

It’s a feature! We use U-238 as the filler material in the fuel, knowing we will get some breeding and use that plutonium to extend the fuel cycle.

When you pull fuel out of a LWR after three cycles, it’s running on about as much Pu-239 as it is U-235.

We have to account for that in fuel cycle analysis, hot excess reactivity / shutdown margin, and the Beta factor (fast/thermal fission ratio). It also can impact moderator temperature coefficient and cause it to shift to zero or even slightly positive.

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Hiddencamper t1_jc2nqpi wrote

Reply to comment by radioactive_dude in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

That’s a great picture.

And just so people are thinking about this the right way. From a safety perspective We don’t care about the graphite as long as it is in the fuel region or below the fuel region, because during a scram they go down which means graphite will be exiting the fuel region and control rods will be coming in.

It’s only a problem when those followers are all the way up and partially out. They will raise power below them as they drive down, right into the power peak.

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Chemomechanics t1_jc2nqn9 wrote

Reply to comment by mesouschrist in If the temperature of a system depends on its average kinetic energy, does it mean the "de facto" temperature depends on the speed of the observer? by Dryu_nya

> Your answer seems to imply that if the system was spinning, you would call it higher temperature

That would be a misreading, because the context of the answer is a question about translational motion. More generally, the bulk motion is typically subtracted before we do thermodynamics. If you don’t see that stated in definitions of temperature, it’s because it’s already been implicitly assumed.

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Hiddencamper t1_jc2ms2l wrote

Reply to comment by PHATsakk43 in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

That’s true. They wouldn’t have attempted to do what they did if they weren’t flooded with xenon.

To be fair though, all light water reactors can overcome xenon except for the very end of the operating cycle. So you avoid issues related to xenon in most reactors out there which eliminates risk of potential power spikes. And the CANDU design simply doesn’t have enough reactivity to pull through a xenon peak (which is why their reactor protection systems will try to stabilize the reactor at 60% or 2% power when it is safe to do so, to allow the operators an opportunity to keep the unit running).

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Hiddencamper t1_jc2mhcl wrote

Reply to comment by BrightCharlie in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

Exactly.

Today if you say the words “reactor safety limit”, that’s an inviolable parameter. If a reactor safety limit is exceeded the plant cannot restart without approval (10cfr50.36). And if there is a potential to exceed one, you are in a reportable event (for example is a safety system was found degraded such that it would actuate too late to protect the safety limit).

As reactor operators we are required to know them from memory.

The same level of deliberate caution around those limits likely did not exist with the USSR and the RBMK design, as evidenced by them withdrawing rods as much as they did. When rods are they far out in the RBMK, you not only get a positive reactivity spike on a scram, but you also magnify your positive void coefficient.

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hunguu t1_jc2lsux wrote

Reply to Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama

This is a very simple question to answer actually. Reactor power is all about controlling the number of neutrons. Control rods absorb neutrons to help control and lower power. But if you pull out a rod to raise power, it doesn't work well if water fills in that space because water is great at absorbing neutrons too. So the designers had a very bad idea of adding 4.5 meters (over 14 feet) of graphite to the bottom of the control rod because graphite adds positive reactivity compared to water. When the rod is pulled out, graphite takes its place. I find saying the "tip" is graphite causes confusion because it's actually a long rod of graphite nearly the height of the core entire.

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Doc_Lewis t1_jc2ju74 wrote

Reply to comment by sejanus21 in As they still have a neutral charge, can antineutrons replace neutrons in a regular atom? by Oheligud

For a real world application, see PET scans. Positron emission tomography, a common imaging technique in healthcare, relies upon certain radioactive isotopes that undergo beta decay. That is to say, an up quark in a proton flips to down, and turns the proton into a neutron, and ejects a positron (antimatter electron). When the positron meets an electron, they annihilate and release gamma rays, which are detected.

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