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Originally Posted by benzfan
The trick is, of course, creating and maintaining the proper conditions for the protective oxide layer to not only form in the first place, but to preserve it. The primary and secondary sides in a reactor/steam generator/turbine system all require different chemistry to maximize system life because of different materials predominating in the different systems.
I remember the protective oxide layer on the body and structure of my old Alfa Spider just kept getting thicker...  |
Not different chemistry for the systems at my plant, a Boiling water reactor (BWR). We make the steam in the vessel, which then goes through a steam separator, then a steam drier, then out 4 main steam lines at 99.8 % quality to our high pressure turbine. We don't have a secondary loop with pressurizors and steam generators and all. We have had some problems with errosion / corrosion and 'tiger striping' in our BOP lines... when identified our usual course of action is to replace the carbon steel piping with stainless.
Yeah in PWRs (Pressurized Water Reactors for those that don't know), the chemistry is REALLY different in the reactor core. They finely control reactivity by adjusting the concentration of boron ions in the water (boron absorbs neutrons). That boron is added by adding boric acid. Using boric acid has some serious risks. I refer to the Davis Besse event.
In the BWR we finely control reactivity by adjusting the rate of water recirculation. The slower the recirculation, the more voids (bubbles) are formed on the fuel surfaces. Water is a good moderator for neutrons (slows them down into the thermal range, and thus propigates the reaction), while air is not. So the more air bubbles, the reactor power goes down. Increase the recirculation, the bubbles get swept away, and power increases.
Of course our chemistry is really strange these days. We recently added a hydrogen water chemistry system, injecting hydrogen gas into the water to bind up any free oxygen and thus try and prevent corrosion, such as intergranular stress corrosion cracking (IGSCC) in the vessel internals. During our upcoming refueling outage we are going to be injecting nobel metals in the water as well.