The steam generators in light water pressurized-water reactors (PWR's) present a paradox to the corrosion engineer. They consist of shell-and-tube heat exchangers, either straight tube or U-bends, that are similar in construction (and materials) to those used successfully in the chemical industry for many years. On the other hand, their performance in light-water reactors has been less than desired. They are called upon to generate tremendous quantities of steam at high steaming rates and at high heat flux. As a result, a part per million impurity in a recirculating steam generator can be concentrated several fold in the bulk water in a short period of time if there is insufficient provision to "blow-down" a fraction of the boiler water to remove the contaminant introduced with makeup water. In once-through units, the solids remain in the steam generator since only volatiles pass over with the steam.
