Pressurized heavy-water reactor
No amount of 238U can be made "critical" since it will tend to parasitically absorb more neutrons than it releases by the fission process.
Water makes an excellent moderator; the ordinary hydrogen or protium atoms in the water molecules are very close in mass to a single neutron, and so their collisions result in a very efficient transfer of momentum, similar conceptually to the collision of two billiard balls.
And so using ordinary water as a moderator will easily absorb so many neutrons that too few are left to sustain a chain reaction with the small isolated 235U nuclei in the fuel, thus precluding criticality in natural uranium.
The degree of enrichment needed to achieve criticality with a light-water moderator depends on the exact geometry and other design parameters of the reactor.
One complication of this approach is the need for uranium enrichment facilities, which are generally expensive to build and operate.
This is not a trivial exercise by any means, but feasible enough that enrichment facilities present a significant nuclear proliferation risk.
These features mean that a PHWR can use natural uranium and other fuels, and does so more efficiently than light water reactors (LWRs).
Some CANDU reactors separate out the tritium from their heavy water inventory at regular intervals and sell it at a profit, however.
[4] While prior to India's development of nuclear weapons (see below), the ability to use natural uranium (and thus forego the need for uranium enrichment which is a dual use technology) was seen as hindering nuclear proliferation, this opinion has changed drastically in light of the ability of several countries to build atomic bombs out of plutonium, which can easily be produced in heavy water reactors.
Although this process takes place with natural uranium using other moderators such as ultra-pure graphite or beryllium, heavy water is by far the best.
The Soviet nuclear program likewise used graphite as a moderator and ultimately developed the graphite moderated RBMK as a reactor capable of producing both large amounts of electric power and weapons grade plutonium without the need for heavy water or - at least according to initial design specifications - uranium enrichment.
U fission cross section - while a nonlinear relationship is apparent, it is clear that in most cases lower neutron temperature will increase the likelihood of fission, thus explaining the need for a neutron moderator and the desirability of keeping its temperature as low as feasible.