Viewing a single comment thread. View all comments

galqbar t1_jc4lgm7 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

What a superb and informative comment. I thought I knew a fair bit about RBMK reactors and the accident but there was a lot of interesting new information here.

Boiling coolant inside of the vessel seems like a Bad Idea when one of the effects is to vary the amount of moderation at different depths in the core.

1

Hiddencamper t1_jc4rz0u wrote

We do that in a boiling water reactor. But in that case the water is the coolant and the moderator, so as you boil, you get less moderation, which protects the upper portion of the core. We also “shape” the flux profile by adjusting enrichment and gadolinium content (burnable poisons) in the fuel.

Normally in a BWR, power leaks in the bottom 1/4 of the core, and as you deplete the fuel in the bottom later in the cycle, the water is able to “climb” further up the core before boiling, which improves the moderator in the upper portion of the core. By the end of core life, the power peak is in the top 1/3rd of the core.

So it can work when designed right.

But yeah in nearly all other cases, you want to keep your coolant and moderator in a single phase (for the most part)

2