How does radiation affect reaction rate?

What kind of radiation are you referring to? Photochemical reactions are promoted by ultraviolet radiation since it triggers the formation of free radicals. Other kinds of photons can also promote bonding electrons to antibonding orbitls breaking chemical bonds. This also happens in the UV region ofthe spectrum and sometimes even in the visible. Isomerizations can occur in this way as well. X-ray and gamma radiation may also break chemical bonds.