We show that it is possible to denature the plutonium produced in pebble bed modular reactors by doping the nuclear fuel with either 3050 ppm of 237Np or ~2100 ppm of isotopic composition of Am. A correct choice of these isotope concentrations yields denatured plutonium with isotopic ratio 238Pu/Pu 6%, for the entire fuel burnup cycle. The penalty for introducing these isotopes into the nuclear fuel is a subsequent shortening of the fuel burnup cycle, with respect to a nondoped reference fuel cycle, by ~40 and 20 full-power days, respectively, which correspond to 4.1 and 2.0 GW(d)/ton reduction in fuel discharge burnup.