I think you could do this with Radon. Radon is radioactive and gives off alpha particles, which should be easier for a putative radiotrophic organism to handle than gamma rays. Radon is generated in the earths crust continuously. Being a gas which is generated in solid areas, it will tend to collect in pockets where it can - sort of like water collecting in favorable areas on the surface. These pockets can be shallow and so have access to nitrogen or nitrogenous compounds and other needed minerals percolating down from the surface.
You do not want your radioactive energy source to swing from nonexisting to sterilizingly strong (the latter being a problem with the spontaneous nuclear reactors). Those swings are bad for the critters. The good thing about radon in this context is that the creation of radon is fairly steady and continuous, and the half life and so breakdown (into nongas elements) is short. So these subsurface pockets of radon should have stable levels of radioactivity.