Major design problems and their solution concerning an underwater machine for insertion of nuclear irradiation samples into the NASA test reactor at Sandusky, Ohio are described. The machine, entirely remote controlled, contains extensive interlocks and inserts a radiation experiment capsule into the reactor against 165-psi primary coolant water pressure with precise positioning control. Capsule loading and machine operation is under a 20-ft head of water. The machine was designed for machine base rigidity, prolonged immersion in high purity water, environmental housing of “off-the-shelf” components, and force, torque, and carriage position sensing systems. Fabrication of machine components was limited to a handful of structural materials exposed to the reactor shielding water. The machine bed is similar to a planer bed with heat-treated stainless-steel ways. The capsule carriage is built in two sections with the insertion capsule clamp section spring mounted to the driven portion for differential motion and load sensing. The carriage rides on a unique roller system bearing against the two heat treated ways of the bed. The drive system consists of an electric motor, a “T” gearbox, a brake drum coupling, and a 60:1 right-angle gearbox driving a Saginaw ball screw with the ball nut attached to the carriage for rotary-to-linear-motion conversion. A flex shaft from the T gearbox stub provides manual drive capability at operator level plus carriage position indication. Excessive thrust and clutch slip monitoring is provided by a solid state control system. Experience to date by Westinghouse and the current user, NASA Lewis Research, indicates a successful approach to and execution of a difficult design problem.