Recent comments in /f/askscience

PHATsakk43 t1_jc33vnn wrote

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

You’re answering your own question.

The low reactivity of the RBMK is why the moderator tipped control rods existed. The xenon-precluded startup is inherent in any reactor, regardless of enrichment as the xenon is from fission of U-235 and is a function of time at power (equilibrium xenon) or for a startup, time after shutdown (peak shutdown xenon.)

Repetitive startup/shutdowns that were being performed at Chernobyl would create the same xenon problems with any reactor, at any point in operating cycle if done excessively.

8

sir_jamez t1_jc33tex wrote

Reply to Is there a type of precipitation that exists on other planets but not ours? Or theoretical precipitation that doesn’t happen here? by ButIHateTheTaste

Any conditions with extremes and compounds where cells butt against each other and form self-contained weather loops suspended vertically.

E.g. A heavy hot gas layer with a lighter "water" layer above it (or whatever the precipitation medium is); so the precipitation collects, falls, hits the hotter bottom layer, and evaporates up to condense and collect again without ever reaching the surface of the planet. From a surface observer's POV (within or below the hotter gas layer), this would be a continuous rain event that never reaches them and just loops itself in the sky.

29

PHATsakk43 t1_jc32sb1 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

While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.

Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)

So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.

This is a gross simplification and there are other things that can affect this, but it’s part of the equation.

1

PHATsakk43 t1_jc31lte 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

While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.

Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)

So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.

This is a gross simplification and there are other things that can affect this, but it’s part of the equation.

2

aphilsphan t1_jc2znif wrote

Reply to comment by Zondartul in Is there a type of precipitation that exists on other planets but not ours? Or theoretical precipitation that doesn’t happen here? by ButIHateTheTaste

CO2 deposition would occur at the Martian poles. The dry ice sublimes when the temperature gets high enough and deposits again when it gets colder. This drives a lot of the changes in atmospheric pressure.

I’d like to see a planet with enough pressure and the right temperature range for CO2 to be a liquid. I’m sure there are ammonia dominated planets.

12

axloo7 t1_jc2z7z5 wrote

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

It's not much of an answer but: because that's how it was designed.

Reactors are very complex machines with so very many things that effect the way they operate.

Running a reactor is like balancing a stick by its tip. There are things that make it more reactive and things that make it less reactive. One of those things are the control rods (but they are not the only mechanism) The people who designed the RBMK reactor decide to design the reactor to have more "powerful" control rods. The graphite ends are just a way to assert more control over the reactor.

This design decision made it possible to put the reactor in to an unstable state.

There probably exist other reactors where Nuance of their design allows the same thing but we don't hear about them because those operating regimes are never used because they are not safe. I would asume most have safeguards (like the rbmk has) to prevent it.

It's worth mentioning that just because it's posible to break a thing by using it incorrectly doesn't mean that the thing is poorly designed.

7

Zondartul t1_jc2urj2 wrote

Reply to comment by NomenNescio13 in Is there a type of precipitation that exists on other planets but not ours? Or theoretical precipitation that doesn’t happen here? by ButIHateTheTaste

We distinguish rain, snow, slush and hail by the phas-of-matter and composition of the falling "water"... as alternatives to "falling" water, we have fog, dew, jack frost, icycles... different ways for water to arrive on something. We also have different end results: puddle, mud, snow, black ice.

Differentiating precipitation by the chemical that falls is easy (we have water, acid, ash, and fish here on earth) but I find the different physical processes of precipitation also interesting. For example, frozen CO2 (aka dry ice) sublimates into gas, and the opposite process (deposition?) would be interesting to see.

32