Does nuclear radiation go away?

No, it doesn't.

Wrong, it does. There are 2 types of nuclear radiation: prompt & decay.

• Prompt nuclear radiation occurs for a period of time while the reaction that generates it is happening. Examples are the flash of neutrons, light, x-rays, etc. when a nuclear bomb explodes as well as the sustained neutron flux as a nuclear reactor is in operation. When the reaction stops, prompt nuclear radiation goes away.
• Decay nuclear radiation occurs as radioactive isotopes decay to different isotopes. As the decay happens (which is a probabilistic process) the radioactive isotope is consumed. This follows an exponential function with one half of the current amount of the radioactive isotope consumed in each period of time called a halflife. While there will always be a tiny residue of the original radioactive isotope, for practical purposes it is considered to be negligible after 5 halflives have passed. When 5 halflives of the radioactive isotope decaying have passed, decay nuclear radiation is considered to have gone away for practical purposes.