Nuclear Power Plants Need Ultrapure Water
Tm world-wide nuclear power reactor scoreboard shapes up like this: 9 reactors running and 27 under construction or in advanced planning stages. Total gross electricity output: around 2627. Five of the reactors are in the U. S.-Shippingport being the largest. Ten more are being built and more plants will come both here and abroad. Bringing nuclear reactors to reality has been a battle with specificationswithout question, among the most rigorous ever prepared. These tough specs run the gamut from materials of construction to the quality of water used to cool or moderate nuclear power reactors. With nuclear power flexing its atomic muscles in modest fashion today, many of the problems-and solutions-in meeting nuclear specs are coming to light. This month I/EC takes a deep look at one of the lesser understood but vital phases of nuclear power-water treatment. Water for nuclear power plants is different from that used in most chemical processes. City, stream, or well water, cleaned up routinely, would lead to disastrous consequences if used in atomic power plants. Nuclear water must be ultrapure-for example :
resistivity between 15 and 20 million ohms chloride content not over 0.1 p.p.m. no free caustic dissolved oxygen not over 5 p.p.b.
There is good reason for such extreme purity. Chlorides can lead to stress cracking of the stainless steel used in reactor systems. Also, corrosion must be minimized-otherwise, metal particles accumulating in the water will pick up radioactivity while flowing through the system. And, there are processing benefits from using ultrapure waterfor instance, better heat transfer rates and shielding effects. Ion exchange via multiple bed deionization seems the best way to make the high quality water for power reactors systems. Several processes are used, and here is an example: cationic exchange operating on the hydrogen cycle decarbonization adjustment of pH to 8 with .caustic soda evaporation and the condensate passed through an anionic exchange system
Other processes have cationic exchange and carbonate removal in common, but differ in the final processing steps. For example, after decarbonization, the water goes through an anionic exchange and then a cationic-anionic exchange in series, or through an anionic exchange followed by a mixed bed treatment. The pure water is used in both the primary and secondary loops of a power reactor system, but the former is the area of most concern, for here is the greatest danger of contamination. Hence, the water must be analyzed continuously to keep it within safe limits. This task falls to appropriate process control instruments but until full confidence is placed in these devices, spot checks will be run in control labs. Also, it is necessary to check the content of additives such as cyclohexylamine or hydrazine which will be added to inhibit corrosion and water decomposition. Apparently, water treatment problems are solved but disposal remains a problem. “Hot” water cannot be simply dumped into a river or a lagoon. A solution today is to pass the radioactive water through an ion exchange system, consider the resin expendable, and dispose of it 3 miles deep in Davy Jones’ locker, or bury the resin in some remote place. VOL. 51, NO. 10
OCTOBER 1959
1253
