Wednesday, April 9, 2014

The Truth About Nuclear Power - Part Ten

Subtitle: Nuclear plants require costly upgrades after 20 to 30 years
Steam generators inside containment structure
Purple-cutaway view.
source: NRC
One of the favorite arguments of the nuclear proponents is that, even though a nuclear plant costs more to build, it lasts for 60 years.  The second part of the statement is not true, although the first part is definitely true.  Equipment wears out, and must be replaced at significant cost.  As an example, the pressurized water reactors, PWR, have an expensive heat exchanger – the steam generator – that suffers tube degradation over time. see image.  NRC requirements cause these steam generators to be replaced when tube degradation reaches a certain level.  For some plants, the replacement works.  At California’s San Onofre plant – SONGS – however, four replacement steam generators failed recently very soon after startup.  The plant owner, Southern California Edison, SCE, elected to shut down the plant permanently rather than complete the steps required by the NRC to ensure the steam generators could be repaired and operate safely. 
Details of the SONGS steam generator troubles can be found at the NRC website: see link.  
The NRC described the tube degradation as “unexpected.”  Apparently, the type of tube wear and degradation is one that has never been witnessed before.   The tube wear was due to adjacent tubes rubbing against each other, and tubes rubbing against retainer bars.    The safety concern, unique to nuclear power plants using the PWR design, is a sudden loss of main steam header pressure.   In the words of the NRC, this is a main steam line break.   The reason this is a safety concern is that radioactive hot water under high pressure flows on the inside of the tubes in the steam generator.  At a somewhat lower pressure, water flows on the outside of the tubes.  The water on the outside of the tubes is heated, boils, and turns to steam (hence the name, steam generator).  The tube walls must retain their strength to prevent leaks of the radioactive water through the tubes and into the steam system.  The steam system's pipes run outside the containment building, into the steam turbine, and from there steam flows into the condenser.   With both systems operating normally, pressurized radioactive water on the inside of the tubes, and lower pressure water/steam on the outside, the tubes have an easier task in keeping the two water systems separate.  But, if a main steam line breaks, the pressure difference across the tube walls increases suddenly and dramatically.  Weak tubes would, of course, fail and send radioactive water and steam into the atmosphere.  This is unacceptable, but is a natural consequence of choosing to generate power using nuclear fission as the heat source.
Indeed, this is exactly what happened at SONGS when the new steam generators sprung a leak, radioactive water entered the steam system, and a small amount of radioactive steam was released into the atmosphere.  See link  As required, SCE shut down the plant to investigate. 
The sticking point in the order from NRC to SCE was this: “SCE will determine the causes of tube-to-tube interaction and implement actions to prevent recurrence of loss of integrity in the Unit 3 steam generator tubes while operating.”   That is a most reasonable requirement, find out what happened, and implement steps to make sure it does not happen again.  SCE, however, either could not, or would not take the time and expense to determine the causes.  Instead, SCE shut down both reactors in the plant.
It should be noted that minor tube wear is normal and expected.  Indeed, with the more than 9,000 individual tubes in one steam generator, a tube that is near failure due to excessive wear can be plugged to remove it from service.  The difference in this case was the rapid tube wear so very soon after the new steam generators were placed in service.   The original steam generators lasted not quite 30 years, as the SONGS reactors came online in 1983 and 1984, and the steam generators were replaced around 2010.  The radioactive steam leak occurred in January, 2012. 
There is much more to the story of the leaking tubes at SONGS.  As time permits, that story will be told.  It involves SCE trying to obtain an extension to the operating permit by claiming the replacement steam generators were sufficiently similar to the original equipment to qualify for "like-for-like" status, when the new steam generators were not "like-for-like."   A United States Senator became involved.  A re-licensing procedure would have been lengthy and the plant would be shut down for the duration of that procedure.   
In addition, the 2000 MW of electricity was lost to the grid, and had to be replaced somehow.  A part of that story is related at this link.  The good news is that at least 75 MW of the power must be from energy storage systems.  That will provide a significant boost to the grid-scale energy storage firms.  
Other nuclear plants also have been in the news due to tube wear and degradation, including the St. Lucie plant in Florida.  See link   Also, the Watts Bar plant has suffered tube wear and has ordered replacement steam generators.   Finally, the Davis-Besse nuclear plant in Ohio is replacing its steam generators, also.
It’s an exciting time.  How many more nuclear plants will go the way of SONGS, due to faulty replacement steam generators that have tubes wear on each other? 
It can be seen, then, that the nuclear power plant in California lasted a bit less than 30 years, not the 50 or 60 years as nuclear proponents claim.   Nuclear plants require costly upgrades after 20 to 30 years, but the anticipated added life does not always appear.  

Previous articles in the Truth About Nuclear Power series are found at the following links.  Additional articles will be linked as they are published. 

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

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