How was that particular valley unaffected by radiation/fallout?

So many people don't understand what radiation and contamination actually are. Radiation is energy, the main types being alpha particles, beta particles or gamma rays (all are types of radioactive decay). This radiation is emitted by radioactive materials as they decay. Decay is the process whereby radioactive materials try to attain a more stable and generally non-radioactive state. A half-life is the time it takes for a particular material to decay 1/2 of it's mass. Contamination is the pollution of an object or substance with radioactive material.

Alpha particles are large and cannot penetrate much, they cannot penetrate dead skin or paper for example. However if you were to inhale alpha emitting materials then the alpha particles would destroy your lung tissue quite quickly.

Beta particles are smaller and can penetrate a little further than Alpha, most living tissue for example but aluminium foil is sufficient to stop Beta particles. It can cause cancer and mutation if living tissue is exposed, especially if beta emitting contamination is ingested.

Gamma rays are highly penetrative and a large mass is required to shield/stop them, the denser the material the better as it takes up less space. Gamma rays cause massive tissue damage.

All of these types of radiation have similar health hazards. Radiation does not travel great distances and diminishes in intensity as an inverse square of the distance .. ie. Intensity = 1/D^2 so at 2m it is 1/4 as intense as at 1m and at 3m it is 1/9th as intense.

Contamination is the big problem, contamination consists of radioactive materials as particles and that is what can easily spread vast distances by air (as smoke or dust), water or surface contamination depending on the size of the particles. Fallout is the spread of any contaminated materials.

Irradiated is not the same as radioactive or contaminated, Irradiated means to have been exposed to radiation, you get irradiated when you go for an x-ray or take a walk in sunlight for example. The clouds do not produce radiation, clouds are water vapour that may contain radioactive particles, this water vapour comes from surface water evaporating. In this scenario, scientifically speaking, pretty much any rain in the valley would be contaminated unless the valley had a closed weather system, which it really could not.

Scientifically the situation in the movie with the valley is just not possible and there is no use trying to figure it out. It is irrelevant to the story, the uncontaminated valley is merely a plot device to set the scene for the love triangle.