Prototype radionuclide traps were tested in sodium loops containing irradiated sources (Source Term Control Loops 2 and 3) at 604 and 538°C (1120 and 1000°F). Prototype traps were 70 to 87% efficient in removing 54Mn from the sodium, and also effective for 60Co. Extensive screening tests showed that pure nickel is the most effective getter material, working best above 450°C (842°F) with increasing effectiveness at higher temperatures. Of the several possible trap sites considered for reactor use, a location within the top of the fuel assembly was chosen as the most convenient and effective. This position would facilitate trap handling by making trap insertion and removal an implicit part of the normal fuel handling procedure. A cost/benefit analysis shows that the radionuclide trap will be economically attractive. One radionuclide trap has completed a year of testing in an Experimental Breeder Reactor II driver fuel subassembly with good results, and a second trap is being tested in the same reactor.