Two innovations in the design of the NPD primary system were to use carbon steel for piping and to fill and test with heavy water only. Resulting savings were $400 000 and $100 000, respectively. Specifications ensured satisfactory surface and fluid states following commissioning. Operation of the carbon steel system has been very satisfactory except for a fueling-machine failure when foreign materials were introduced into the system. The pD of the system has been maintained between 10 – 11, the dissolved O2 concentration at < 0.01 parts/106 and the crud level during steady-state operation at < 0.01 parts/106 with peak crud concentrations of < 0.1 parts/106. During pressure testing, 496 lbs of reactor grade D2O were lost at a cost of $12 400 at today's prices; nevertheless this loss is acceptably low. Significantly, the corrosion rate, the radiolytic damage, and the gas formation were never as great as had been expected. Carbon steel is therefore recommended for other pressurized water systems. The experience gained in operating NPD for two years showed the economic feasibility of pressurized D2O as a reactor coolant and pointed out the improvements required. Consequently, the target figure of 10 lb/day loss of D2O for a 200-MWe reactor now appears feasible.