Nuclear reactor geometry

[spidey07]

There's lots of smart folks here. So can anybody explain the specific geometry I've seen in nuclear reactors? It seems to be specific hexagonal positions of the control rods and fuel rods. Is this based on distance? Surface area? Velocity of neutrons?

 

[rcpratt]

The fuel geometry? Or what exactly?

 

[Bignate603]

If we told you we'd have to kill you.

Actually, I don't know. I'd be interested in hearing too.

 

[spidey07]

The fuel geometry? Or what exactly?

The top down view of the rods, be they fuel or control.

 

[TridenT]

How about you look for it online from multiple sources instead of taking someone else's possibly made up information?

They put them in a specific order only because then the fairies can be sucked dry of their life force quicker that way. It allows them to maintain a good surface area to depletion rate ratio.

 

[spidey07]

How about you look for it online from multiple sources instead of taking someone else's possibly made up information?

They put them in a specific order only because then the fairies can be sucked dry of their life force quicker that way. It allows them to maintain a good surface area to depletion rate ratio.

In all seriousness, it's always a honeycomb structure and I've yet to find a reason why on the intarweb. Maybe the bees had it all figured out.

I understand the significance of the honeycomb, but can't draw the connection as it pertains to this and seek knowledge of the "hive". Get it? Hive.

 

[rcpratt]

The overall shape is not really that important. It's as circular as it can be because that's the shape of the pressure vessel, which is that shape largely because it's more area/volume efficient than a square. And because it can handle higher stresses than a square, I suppose.

ESBWR core layout. Rods are always arranged at the center of a four-assembly square. There are that many because more would be unnecessary from a neutronics standpoint. When control rods inserted, it is easy to see that neutrons would be heavily shielded from traveling outside their little four-assembly box, which sufficiently stops fission.

There are several things you don't see. First of all, each assembly has hundreds of fuel pins in it. These pins are filled with fuel, water (for moderation), or burnable neutron poisons. The other thing you don't see is the varying enrichment of each assembly, due to both intentional enrichment loading differences and enrichment loss from being burned. A quarter to a third of the fuel is replaced each cycle (18-24 months), meaning that that are varying degrees of "freshness" and thus enrichment in the fuel. The arrangement is carefully planned for flux shaping purposes, both axially and radially. Varying degrees of insertion of different control rods also allows for flux shaping.

I have no idea if I answered any of your question.

 

[Turin39789]

Bees build the reactors. They are radiation proof and prefer the honeycomb. Reminds them of food.

Too bad they can't carry water.

 

[spidey07]

The overall shape is not really that important. It's as circular as it can be because that's the shape of the pressure vessel, which is that shape largely because it's more area/volume efficient than a square. And because it can handle higher stresses than a square, I suppose.

ESBWR core layout. Rods are always arranged at the center of a four-assembly square. There are that many because more would be unnecessary from a neutronics standpoint. When control rods inserted, it is easy to see that neutrons would be heavily shielded from traveling outside their little four-assembly box, which sufficiently stops fission.

There are several things you don't see. First of all, each assembly has hundreds of fuel pins in it. These pins are filled with fuel, water (for moderation), or burnable neutron poisons. The other thing you don't see is the varying enrichment of each assembly, due to both intentional enrichment loading differences and enrichment loss from being burned. A quarter to a third of the fuel is replaced each cycle (18-24 months), meaning that that are varying degrees of "freshness" and thus enrichment in the fuel. The arrangement is carefully planned for flux shaping purposes, both axially and radially. Varying degrees of insertion of different control rods also allows for flux shaping.

I have no idea if I answered any of your question.

Believe it or not, I can follow that and it makes sense. Barely.

Now you're touching on the why I was looking for. The flux shaping properties must be related to radial distance.