S>Progress is reported on the research and development program being performed in connection with the design of the Pathfinder Atomic Power Plant. The topics covered include: fuel material cladding, bonding, and irradiation testing; heat transfer and fluid flow; fuel element manufacturing research and development; low-enrichment superheater fuel element design; mechanical studies of control rods, guide tubes, and control rod drives; reactor physics; and reactor and system dynamics. (C.H.)