How does light water prevent criticality of natural uranium even as it is outside the zirconium cladding?

Question

The main reason heavy water is used to allow nuclear reactors to operate with nuclear reactor is that they do not absorb neutrons, allowing an increase in neutron temperature that lets the few U-238 to reach criticality.
However, from the design point of a PWR reactor, this does not make sense to me. The uranium is contained inside ceramic pellets, with the water acting as a coolant and carrying the heat towards the turbine. So even if light water does absorb neutron, wouldn't the uranium within the pellets still reach levels of criticality needed for energy production?

Alwin · Answer

When the fuel is surrounded by water, whether pellets or rods, the neutrons emitted from 1 pellet or 1 rod travel through the water and can hit another pellet or another rod. Light water absorbs neutrons, but more importantly, when neutrons collide with hydrogen, it slows the neutrons down. For nuclear physics reasons, slower neutrons are better at causing chain reactions. When the chain reactions are too fast, the water expands, which increases gaps for neutrons to escape without colliding with the water, so neutrons travel faster again, and the chain reaction rate is lowered. This helps control the rate of reactions.
See https://en.wikipedia.org/wiki/Pressurized_water_reactor#Moderator
which is about the role of water in PWR, and
https://en.wikipedia.org/wiki/Neutron_cross_section which shows that slower neutrons with lower energy have a higher cross section and probability of causing a chain reaction.
tl;dr neutrons travel through the water between the fuel, and those neutrons are causing chain reactions.

How does light water prevent criticality of natural uranium even as it is outside the zirconium cladding?

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