Why are reactors attractive near shadowed lunar craters?

Answer: They can power ice-rich dark regions

They can power ice-rich dark regions ✓ — Correct! Permanently shadowed craters are interesting because they may hold water ice, but they receive little or no sunlight. A reactor can keep drills, processors, heaters, and communications running there without depending on the Sun.

Shadow makes uranium safer — Not quite. Uranium does not become safer just because a crater is dark. The value of nuclear power there is that it can provide steady electricity where sunlight is unreliable or absent.

Crater walls block all radiation — Not quite. Terrain can help with local shielding geometry, but crater walls do not block all harmful radiation and that is not the main reason reactors are discussed there. The big reason is reliable power for dark, resource-rich zones.