As a retired meteorologist, I would like to point out that even if the United States and North Korea tried to confine a nuclear exchange to the Korean peninsula, they would not succeed, due to prevailing winds.
These winds mean that the nuclear fallout unleashed into the atmosphere could be carried to Japan and possibly even to the US, depending on the scale of such a war.
Rain would also wash this radiation into the soil, rivers, reservoirs and seas.
Radiation-free food and potable water would likely be hard to find in the affected areas for a very long time.
In the Fukushima disaster, radioactive seawater was carried by oceanic currents to the western shores of the US.
No one can control the wind and rain.
Chang Wen Lam