[en] Further to work on the first prototype unit which has been described in D.P. Report 118, experimental work has now been carried out on the Stage II Prototype Unit. The Stage II Prototype which was manufactured by Plessey Nucleonics Ltd., is a full scale test model for the valve unit which will finally be installed in the actual Reactor System and has been built to assess the degree of reliability and performance which might be expected from the final unit. It had been hoped that the unit would run for 2,000 operations in pure helium without deterioration of the sealing qualities of the valves or failure of the actuator mechanism, Neither of these objectives has been fully achieved but much useful information wes obtained which influenced the design of the final unit. For example, due to early failure of three individual valve units, experiments were made with valves of alternative design which appeared to be more satisfactory.