Sunday, August 15, 2004

Power piping

I really know how to write those titles, eh? What can I say, I'm an engineer, not a marketer. And this item is about piping in power plants, as inspired by Sean Kinsell's post here.

It seems that some steam piping failed and killed some workers in a Japanese nuclear power plant recently. Of course anything that happens at a nuclear power plant attracts media attention, and since they can't be troubled to learn anything the coverage usually is shrill. In my naivete I'll attempt to address some of this.

The plant in question was pressurized water reactor, often abbreviated as PWR. Such a reactor operates by heating water in a nuclear reactor core and then pumping it through a number of steam generators (common designs have 2, 3, or 4). These steam generators are built something like vertically oriented boilers, with tubes running vertically through them containing reactor water. The system also has a "pressurizer", which contains a bubble of steam at the top - this device serves to make sure that no steam forms within the reactor itself.

So what happens outside the steam generator tubes? This volume is filled with water at a bit lower pressure and temperature. Water comes into it at the bottom from the feedwater system. Steam forms around the hot tubes and rises to the top, where it goes through separators and driers and ultimately flows through the turbine to drive it. After passing through the turbine the steam is exhausted into the condenser, where it is cooled enough to recondense it into the condensate system.

At this point the water is at very low pressure and lukewarm. The first step is to clean it in "condensate polishers" to catch any corrosion products or other impurities in the water (initial fill is with "demineralized" water - not quite distilled, but highly purified with ion exchange resins).

After the polishers the water is slowly reheated and repressurized in stages until it is ready to return to the steam generators. By the time it returns it is at a high pressure and several hundred degrees F - if it leaks it will immediately form steam. It was piping in these stages which failed at the Japanese plant.

You might think you know a little about steam. Not under those conditions you don't. The amount of concentrated power is unlike anything you see elsewhere, and if it hits you directly it doesn't just scald you, it more or less eats you. If you inhale it live, bye-bye lungs.

Anyway, this feedwater piping is insulated heavily both to minimize personnel hazards and save the heat. In my experience this insulation usually consists of several inches of chalky calcium silicate covered with sheet metal such that none of the pipe's exterior is visible. Power plants turn heat into electric power they can sell after all - wasted heat is expensive.

(Incidentally, piping is used the same way for the same reasons with the same kind of very clean water in it at coal-burning plants. Actually the pressures and temperatures are even higher because there's no nuclear reactor to protect, which means that the personnel hazard onsite from the steam is even greater.)

You might think that if the steam is so dangerous that personnel are kept out of the area wherever pipe that might leak steam runs. In fact the area will be inhabited regularly by plant operators in their rounds checking pressures and temperatures, inspecting steam traps, and sundry other duties needed to keep the plant running. Likewise fire watch or security people or engineers like me might be around from time to time. In practice failures of such piping are uncommon, and their failure would be expensive even if the human factors were discounted, so they are designed carefully and water chemistry is maintained strictly to avoid problems.

As noted in the original article, no radiation was released. If you've been paying attention, you know why - the pipe didn't have anything radioactive in it, so it couldn't release anything. Minor steam generator leaks' results would be captured in condensate polishers further upstream, and if they can't handle it management will be taking the plant down pronto to keep the crud from spreading throughout the plant and sending maintenance costs through the roof, above and beyond any extra regulatory scrutiny and paperwork they'll suffer.

OK, but what happens to the reactor when such pipes fail? Small failures like what killed the workers are hard to pick up on instrumentation right away because nothing is really looking for them - my guess is that, depending on the design of the plant, some fire protection instrumentation might pick up on the extraordinarily high local temperature. It's possible that an HVAC (heating/ventilating/air conditioning) system would pick it up, but such a failure in the turbine building isn't the kind of thing that would alarm on an operator's console necessarily.

Should it? Definitely, if it were likely to indicate that the reactor would need attention. But small leaks are not threats to the reactor and thus are not worthy of highlighting - the risk of distracting operators can be higher than the value of the information.

Larger breaks OTOH would be unmistakeable. Temperatures would get out of hand, sumps would fill, pressures would drop and the noise would be incredible (turbine buildings are noisy but you ain't heard nothin' yet). Life in the control room would get exciting fast. They'd send operators to investigate if needed, or else they'd conclude that for safety's sake they'd have to shut down the reactor and isolate some valves.

What about the steam generators - will they overheat if their feedwater supply is cut off by a big leak or failure? Not once the reactor has been shut down, and there are numerous automatic sensors that can cause this to happen in case the control room is slow on the trigger (and if they are, they'll hear about it, from management, the US Nuclear Regulatory Commission and the industry's own Institute of Nuclear Power Operations). If feedwater is cut off entirely, there is a backup system designed to much higher QA standards to provide water to the steam generators, and similar high-QA valves would close automatically to isolate the leak from the steam generators so this auxiliary water doesn't just run back out the leak.

That's what happens if the piping *feeding* the steam generator fails (simplistically - the possible accidents are examined in much greater detail in regulatory documentation filed with the USNRC for each commercial US nuclear power plant in existence). What if the pipes coming *out* of the steam generators - the main steam pipes - fail?

That's a bit uglier, because the water escaping has even more energy in it. But this piping is even more physically remote and heavily insulated than the feedwater piping. Failures of this piping in the turbine building will immediately be isolated from the steam generators and the reactor building by the main steam isolation valves (MSIVs), of which there are two per main steam pipe, and each of those has two independent signals to actuate it. (this is for a boiling water reactor (BWR), in which the main steam system is much more significant to radiological safety than it is in a PWR, but similar controls and safeguards will be in place). That action combined with the auxiliary feedwater system mentioned earlier will keep things under control in the reactor building. Operators who are in the area when the problem occurs had better get going pronto to survive, but in fact I know of no such failures in the history of the US commercial nuclear power industry.

Alright, that's enough of a brain dump for a while. Who knows? - if the world comes to its senses and we start building nuclear power plants again, I may be able to use that background again....

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