Nuclear power plants

[SarcasticDwarf]

Originally posted by: marvdmartian

Originally posted by: BrownTown
No, it wouldn't, PWR control rods are inserted from the top, and will fall in due to gravity, BWR control rods are inserted from the bottom since the steam equipment has to been on the top of the pressure vessel. However the BWR has some other failsafe mechanism so that the control rods will be inserted, but I don't remember what it is. Also, inserting the control rods only knocks down the power about 95%, there is still a substantial amount of power being generated by the spontaneous decay of short lived actinides produced by the reaction. So, you still have to maintain cooling or bad stuff can happen. So, its not a good thing if you just shutdown a reactor and then leave (it aint gonna blow up or anything though). The Gen 3+ reactors have additional passive safety measures in order to ensure cooling for ~3 days with not outside intervention.

EDIT: looked it up for BWRs, and they use a pressurized water system to insert the control rods on a loss of power.

Civilian power plants are different enough from navy power plants that I won't even pretend to be an expert on them, but a buddy of mine worked for a while at a civilian plant. The way he explained it to me, the control rods take you down below sustainable reactivity, but to completely shut down the reactor, you have to inject (if I remember right) boron into the coolant, which acts as a massive neutron "sponge", and pretty much kills the reaction.
Navy power plants, on the other hand, will pretty much kill all but background reactivity with the insertion of the control rods on a SCRAM action, and don't require the injection of boron into the coolant (which is a good thing, since I imagine flushing that crud back out is somewhat of a chore). Also, the production of fission daughters tends to act as neutron sponges after a reactor shutdown, making an immediate restart harder than the initial startup of the reactor (this effect is much more exagerrated toward the end of the reactor core life). Add to that the negative reactivity mentioned above, caused by the rise in coolant water temperature, and a scrammed navy reactor stays shutdown, unless a restart is initiated by the reactor operator.

Geez, I almost sound like I still know what I'm talking about, don't I?? The funny thing is, I can't hardly tell you what I had for dinner 2 nights ago, but I can still remember crap like this, that I learned over 25 years ago!!! :roll: :laugh:

Hopefully that answers your questions, SarcasticDwarf?? Oh yeah, here's something else I remember......navy nuclear power class number 8107. Dang, that was a loooooong time ago!!

Yes, this definitely helps.

So by my understanding, the reaction at a PWR can be fully stopped by a scram or 95% stopped by a gravity release of the control rods. With a BWR the reaction can be SCRAMed to 100% but the control rods have to be forced up (using a spring for instance).

 

[FoBoT]

i don't know civilian design, but i would be surprised if they rely 100% on gravity for control rod insertion, it was learned pretty early (from some accidents) that control rods might need springs to ensure they are seated during some accident scenarios

but maybe that is only in naval reactors

 

[BrownTown]

I think you might be putting a little more emphasis on the difference between a PWR and a BWR, they are really not very different at all. The only difference here is where the control rods come from, they both do the exact same thing to the reaction which is essentially cut it down to around 5% of the rated power. However, this power is NOT coming from the same source as the power that makes up the other 95%, it is coming from very short lived byproducts fissioning due to their instability.

 

[FoBoT]

this is an interesting article about Chernobyl

http://www.npp.hu/tortenelem/balesetek3-e.htm