The tests performed on a full-scale prototype of the closure designed for the Pathfinder reactor vessel are described. This head was bolted to another head designed to simulate the reactor vessel. The design pressure of the facility is 700 psig at 500 deg F. Strain gages and brittle coatings were used to determine the stresses in the closure. Dial indicator gages were used to measure flange rotation after the head was secured and while it was subjected to hydrostatic pressure. Tests were performed at reactor operating temperature and pressure to evaluate the closure and gasketing. The vessel was thermal cycled, and temperature gradients in the head measured to establish allowable heating and cooling rates. Hydraulically operated bolt tensioners were used to seal and open the closure. Special tools and procedures were developed for underwater operation. (auth)