Monday, June 30, 2014

The Truth About Nuclear Power - Part 24

Subtitle: St. Lucie Ominous Tube Wear

Tube wear in steam generators at the twin-reactor nuclear plant at St. Lucie, Florida, is the subject of this article 24.  St. Lucie in on a barrier island, a few miles north of Palm Beach.  (see photo).  There are no cooling towers, but inlets to the Atlantic Ocean are
St. Lucie Nuclear Unit 1 and 2, Florida
Atlantic Ocean on right, two containment domes in left-center
photo from Google maps - 6/29/2014
clearly visible.   This plant has the pressurized-water reactor technology, hence it has steam generators very similar to the ones that failed in 2012 at San Onofre in southern California.   The leaking tubes at St. Lucie have been in the news recently, with public concern growing over fears that they, too, may be irradiated when a tube bursts and spews radioactive steam into the skies. (see link, and link, and link for news articles.)


A steam generator is nothing more than a heat exchanger, typically with U-tubes in a vertical configuration.  Hot water from the reactor is pumped through the tubes, flowing in at the bottom, up through the tubes, around the U-bend, and back down and out again at the bottom. (this is somewhat simplified).   On the outside of the tubes, the shell-side, boiler feedwater is pumped in at the bottom.  The boiler feedwater rises in between the tubes and is heated as it rises.  At some point in the upward journey, the water begins to boil.  Steam rises to the top of the steam generator and flows out the top to the steam turbine. 

The point of concern is where the water begins to boil.  The steam bubbles exert pressure in all directions, some of it upward, some downward, and some to the sides.  Since water is incompressible (at least at these conditions), the downward pressure has essentially no effect.  The upward pressure has the most effect, because the mixture of water and steam above the boiling zone is much less dense.  Therefore, steam bubbles push water upward, and rather strongly.   However, it is the horizontal force that is of most concern.  As the water and steam mixture rises, one can imagine that the amount of water decreases while the amount of steam increases.  Therefore, water is also forced horizontally by the steam bubbles.  The tubes, which as already mentioned are vertical, resist the horizontal force and bend to some extent.  The tubes are not rigid, but have thin walls.  The tubes also are not very far apart, perhaps one-quarter inch spacing between tubes.  With violent boiling occurring, the tubes can vibrate and hit each other.  To minimize this banging, manufacturers install stability bars or stabilizer bars.  However, the tubes can also rub against the stabilizer bars. 

None of this is new and surprising, as heat exchanger designers have known this for decades.  The goal is to design and manufacture a steam generator that sustains the tube collisions and rubbing for 20 to 30 years, and continues to produce quality steam without releasing radioactivity to the atmosphere.  It is a very good thing, then, that nuclear reactors take a shutdown to refuel and inspect equipment approximately every 18 months.   Part of the inspection procedure is to pressure test the steam generator tubes, and to perform visual inspections to identify any worn spots or places that fail during the pressure test.  With approximately 9,000 tubes in a single steam generator, it is acceptable to plug a few tubes so that no water flows through those tubes.  This is not limited to nuclear plant steam generators, as pressure testing heat exchanger tubes and plugging those that leak is a common practice in many industries.  For many heat exchangers, more tubes are added in the manufacturing stage than are actually required to meet the heat transfer goal.  The over-design, or safety factor, allows some tubes to be plugged as the years go by, and the heat exchanger continues to serve satisfactorily.

Implications on Safety and Cost

With that as background, the St. Lucie plant is noteworthy due to the unusual number of steam generator tubes with wear.  NRC inspectors are reportedly aware of the steam generator tube condition and are monitoring the plant closely.    The NRC has not required the plant to shut down for safety concerns, at least not as of this writing (June 30, 2014).    The steam generators in Unit 2 were replaced in late 2007, so they have been in service for barely more than 6 years.   The original steam generators lasted 24 years, from 1983 to 2007.    An article with photos and describing the steam generator replacement process is available – see link.

The safety implications are a concern, and if the new steam generators fail prematurely, then there are cost issues also.  As Part 23 in the series showed, southern California utility customers are being asked to pay billions for faulty equipment that resulted in two shutdown reactors.   The people of Florida would also be outraged if this happens to them. 

Until a report is issued by NRC on the tube wear at St. Lucie Unit 2, which is expected late in 2014, it is perhaps best to sit and watch.  Perhaps the people of Florida will be lucky, and their St. Lucie nuclear plant will continue to run without catastrophic tube failure.  Perhaps the utility spokesperson is correct, and the tube wear is slowing down. 

This article will be updated as conditions warrant. 

Previous Articles

The Truth About Nuclear Power emphasizes the economic and safety aspects by showing that (one) modern nuclear power plants are uneconomic to operate compared to natural gas and wind energy, (two) they produce preposterous pricing if they are the sole power source for a grid, (three) they cost far too much to construct, (four) use far more water for cooling, 4 times as much, than better alternatives, (five) nuclear fuel makes them difficult to shut down and requires very costly safeguards, (six) they are built to huge scale of 1,000 to 1,600 MWe or greater to attempt to reduce costs via economy of scale, (seven) an all-nuclear grid will lose customers to self-generation, (eight) smaller and modular nuclear plants have no benefits due to reverse economy of scale, (nine) large-scale plants have very long construction schedules even without lawsuits that delay construction, (ten) nuclear plants do not reach 50 or 60 years life because they require costly upgrades after 20 to 30 years that do not always perform as designed, (eleven) France has 85 percent of its electricity produced via nuclear power but it is subsidized, is still almost twice as expensive as prices in the US, and is only viable due to exporting power at night rather than throttling back the plants during low demand, (twelve) nuclear plants cannot provide cheap power on small islands, (thirteen) US nuclear plants are heavily subsidized but still cannot compete, (fourteen), projects are cancelled due to unfavorable economics, reactor vendors are desperate for sales, nuclear advocates tout low operating costs and ignore capital costs, nuclear utilities never ask for a rate decrease when building a new nuclear plant, and high nuclear costs are buried in a large customer base, (fifteen) safety regulations are routinely relaxed to allow the plants to continue operating without spending the funds to bring them into compliance, (sixteen) many, many near-misses occur each year in nuclear power, approximately one every 3 weeks, (seventeen) safety issues with short term, and long-term, storage of spent fuel, (eighteen)  safety hazards of spent fuel reprocessing, (nineteen) health effects on people and other living things, (twenty) nuclear disaster at Chernobyl, (twenty-one) nuclear meltdown at Three Mile Island, (twenty-two)  nuclear meltdowns at Fukushima, (twenty-three) near-disaster at San Onofre, (twenty-four) the looming disaster at St. Lucie, (twenty-five)  the inherently unsafe characteristics of nuclear power plants required government shielding from liability, or subsidy, for the costs of a nuclear accident via the Price-Anderson Act, and (twenty-six) the serious public impacts of large-scale population evacuation and relocation after a major incident, or "extraordinary nuclear occurrence" in the language used by the Price-Anderson Act.  Additional articles will include (twenty-seven) the future of nuclear fusion, (twenty-eight) future of thorium reactors, (twenty-nine) future of high-temperature gas nuclear reactors, and (thirty), a concluding chapter with a world-wide economic analysis of nuclear reactors and why countries build them.  Links to each article in TANP series are included at the end of this article. 

Additional articles will be linked as they are published. 

Part Twenty Three - San Onofre Shutdown Saga
Part Twenty Four - this article

Roger E. Sowell, Esq.
Marina del Rey, California

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