Funny isn't it, that early Takumar lenses are still renowned as stirling performers with a bit of thorium thrown in. Nothing to get all hot and sweaty about.
Something very similar happens in a photographic emulsion. A light photon knocks a silver or halide ion out of its place in a silver halide grain. This makes what is called an activation site which is more easily reduced by a developing agent than other portions of the grain.
I think too much is made of any color change caused by these lenses. Certainly BW films will not be effected. For color films there may be a slight warming of the colors. It is well known that humans tolerate any change toward a warmer hue much better than a change to a cooler one. Thus color labs prefer that slides and prints tend toward warm tones rather than cool ones and adjust their processing accordingly. We don't like it if Aunt Nellie is blue or green.
Ive got 3 of the Nikkor 35mm 1.4 radioactive ones. One I've cleared as best I can, 3 weeks in the bright UV rich Hawaiian Sun, and one I've left in its natural yellow state, and one that is somewhat in the middle. The yellow one looks interesting in color, but looks quite nice in B&W. I use the cleared one all the time.]