# Nuclear



## Ariel (Jul 7, 2015)

I'm interested in everything to do with nuclear reactors and their meltdowns.  I've already watched documentaries on the Chernobyl incident and I've read up on that.  The lovely Schrody has also sent me some information that she has found.  Does anyone else know anything about nuclear reactors?


----------



## musichal (Jul 7, 2015)

Had an uncle who was a welder who did his part to help build one.  Welding applicants for critical areas had to pass an exam in which their welds were x-rayed  to check for weaknesses and tiny cracks.  He was a perfectionist, and they told him his were the best welds they'd seen - he wound up working for NASA, who also required perfect welds.  If that tidbit helps offer any insight...


----------



## InstituteMan (Jul 7, 2015)

I'm at least mildly knowledgeable about nuclear issues. What do you need to know? It's a big area. 

My first recommendation to someone wanting to learn about fission rates is to watch (or, better yet, do) one of the demonstrations with mousetraps with ping pong balls in aquariums. If the density of mousetraps is too low, the reaction peters out, but if the density is too high the thing goes crazy (bad in a reactor). In other words, the density off fissile nuclei makes all the difference in controlling a reaction.


----------



## Ariel (Jul 7, 2015)

I get that much, Insty. I was wondering about the causes of meltdowns. I know Three Mile Island's partial meltdown was human error and a faulty valve in the coolant system.  Chernobyl was a comedy of errors and human error.

I'm curious about the control rods and the thermal output.  I know from the Chernobyl accident that control rods play a crucial part in keeping the temperature at a safe level. I'm not sure if I understand whether this part right--the more control rods the less fission can occurs and thus the temperature lowers?  Control rods hinder the fission because they are made of non reactive materials like boron that absorb neutrons. 

I was wondering if a meltdown was possible with the all of the control rods inserted.  Also, what would a meltdown be like in space--more particularly the space between the heliosphere of two solar systems?


----------



## InstituteMan (Jul 7, 2015)

Assuming a reactor is well designed, inserting all the control rods all the way ought to stop the fission entirely pretty fast. That doesn't mean that all designs are good ones or that all control rods are in working order, though.

I would assume a meltdown in space would have nasty blobs of radioactive goo floating about. The force of gravity is so weak at the scale of a nucleus that the physics of the fission shouldn't change any, but the mess would disperse differently than on Earth.


----------



## Ariel (Jul 8, 2015)

Thank you, Insty!  And thank you Musichal!  And I'm sorry you had to interpret my garbled mess up there.


----------



## 3blake7 (Jul 9, 2015)

Not all nuclear reactors can have a melt down. Chernobyl was a RBMK and the one in Japan was a LWR sub-type BWR. Most at this point are LWR sub-type PWR, which is a lot less likely to have a meltdown. There are other types as well, such as the lead-cooled fast-reactor, which uses molten lead and convection or the PBR pebble bed reactor where a meltdown is pretty much impossible. Most likely to meltdown is RBMK followed by BWR, I think, lol.


----------



## DoubleFoxtrot (Aug 6, 2015)

In conventional fission reactors, once the control rods are fully inserted, the fission chain reaction will come to stop. By absorbing the electrons, they act like the breaks on a car freewheeling down a hill - you can use just a little to control the speed, but after a certain point, they will start decreasing the speed an eventually cause it to stop.

For low-lever power requirements (like most satellites nowadays), the nuclear reactor used is an RTG reactor that does not rely on fission chain-reaction but on the simple heat caused by the decay of the atoms. This cannot melt down.

Speculatively, if a fission reactor would go into uncontrolled fission reaction, the hazard would be much worse than on ground. It would not "melt down" in the absence of gravity, but start heating up uncontrollably and radiating both heat and radiation around. As cooling is pretty hard in space, the heat would kill humans and equipment on a space vehicle far before the radiation. Combine this with the lack of power for the cooling systems, the disaster would probably be pretty quick. Ejecting the reactor from a vessel would be very effective, as vacuum would shield from the heat and some radiation very well. In addition to this, spacecrafts' hulls are anyways shielded against many types of radiation.


----------



## kaminoshiyo (Sep 3, 2016)

They make for lively divorces...


----------

