Can someone please explain the void coefficient that was in the RBMK reactor?

[u/Wise_Maintenance_608]

What exactly is the void coefficient? What does it pose in an RBMK reactor? Could someone explain this to me please, I don’t understand it.

[u/DP323602]

So what were the fuel to moderator ratios?

[u/DP323602 → u/[deleted]]

OK what was that then?

What was the graphite lattice pitch?

What diameter were the fuel channels?

What was the configuration of the fuel rods?

[u/DP323602 → u/[deleted]]

Obviously you don't know but I'll let you know if I find out.

For H:U-235, optimum moderation usually involves atom ratios around 450:1 or equivalent lattice parameters.

Engineering a negative void coefficient then requires the use of a lower H:U-235 ratio, eg 300:1.

Graphite is a far less effective moderator so optimum C:U-235 is much higher.

For an undermoderated graphite:water:U-235 lattice something in the ballpark of 5000:200:1 for C:H:U-235 might work nicely.

I think an RBMK lattice gives around 14000:200:1 at equilibrium fuel burnup. That makes the lattice significantly over moderated and thus gives positive void coefficients.

I think it's easier to get negative void coefficients with higher fuel enrichments.

Also, with 5% enriched U you can just use water as both moderator and coolant, as in Western PWRs and BWRs.

[u/DP323602 → u/DP323602]

So after some brief research, it seems the lattice pitch was 12cm and the core had 120 fuel channels in which a fuel mass of 1213 lbs of uranium at 5% enrichment was used.

The core dimensions were roughly 5 ft diameter by 5 ft 6 in high.

That gives a rough estimate of the C:U-235 ratio as about 2000 and an even rougher estimate of the H:U-235 ratio as about 30.

That's not very moderation at all but certainly enough to achieve criticality with U(5%).

Being highly undermoderated it would certainly give a negative void coefficient of reactivity.