Yes, I think that is the where the debate is over the exact nature of things. Fission happens spontaneously, even the bismuth in Pepto Bismol has a bit of a fission going on, slowly decaying into thallium.

²⁰⁹Bi decay to ²⁰⁵Tl is alpha decay — the bismuth nucleus splits into an alpha particle (helium) and a thallium nucleus. I think you’d be hard pressed to find a nuclear physicist or engineer who calls that “fission”.

Here’s the Wikipedia article on fission. It’s a rather different process:

https://en.m.wikipedia.org/wiki/Nuclear_fission

Can you link to what you are reading? I am a nuclear engineer, I suspect they are using a very loose definition of critical, one that would imply the entire universe itself is critical.

Maybe the fault is with me for using the word "reactor" a bit loosely.

I don't think anyone knows for sure what is going on down there.

My theory is that it's both critical and quiescent, with a violent dynamic at play. Gravity will concentrate the heaviest and least stable elements as you'd expect. As that happens pockets of dense matter in close neutron proximity will approach critical and heat billions of tons of matter which then convects outwards, dispersing the mixture. Think of a lava lamp.

Geochemists say uranium concentrates in the crust, for chemical reasons -- which is, in fact, where we find it.