[paulnpace]

> If the water is colder, it is denser and the more likely neutrons will bounce back into the fuel.

My memory is that the denser water thermalizes the neutrons in a shorter period of time and this is why reactivity is increased.

[niekze]

It's kind of the same idea, right? The more stuff there is to bounce off of, the faster it will slow down, the smaller the net distance it will travel, the increased chance it will thermalize, and less likely it is to escape. I could be missing a lot of nuance there as it's been almost 20 years since I went to naval nuclear power school. I'm definitely not the one to ask about the specifics!