[candiodari]

Or you could go with a substance that both stops radiation AND extracts the energy, used in every nuclear reactor: water. It's not quite as good as lead but pretty good, and it will heat up while stopping the radiation, then conveniently through convection can transport itself to a point where the energy can get extracted.

[s_y_n_t_a_x]

Water extracts thermal radiation at the infrared wavelength, not the higher energy wavelengths that these fungi eat. (X-ray, and Gamma).

Funny enough, these little critters are actually found in reactor pools.