THE PEBBLE BED REACTOR PROGRAM. Current Fuel Element Developments and Their Effects on the Pebble Bed Reactor Development Program

Recent fuel element irradiations demonstrated the structural integrity of spherical uranium-graphite fuel elements at burn-ups in excess of the design requirements of a 125 Mw(e) Pebble Bed Reactor (PBR) power plant. Other irradiations indicated the successful development of a coated fuel particle which permits fabrication of fuel elements meeting the PBR design objectives of a fission-product release rate (R/B) ? /sup 10-6/. beta plus gamma system activity in a 125 Mw(e) PBR is 490 c, assuming complete release of /sup lO-6/ of all fission products volatile at or below 2500 deg F. The low R/B being obtained from PBR fuel elements indicates that decay during diffusion of the short-lived volatile precursors of nonvolatile daughter products will result in further reduction of this system activity, and an increase in the average half life of the fission products remaining in the system will increase the efficiency of a bypass clean-up system. The method of fabricating coated particles by the hydrolysis of metallic chlorides to produce oxides or pyrolysis of hydrocarbons to produce carbon on a suitable substrate, is described, and preirradiation test results are given. An accelerated coated-fuel-particle program is discussed as well as development work on the Pebble Bed REactor concept as a whole. aut