San Onofre's MOST fatal flaw is this... and this... and this...

8/10/2012

Dear Readers,

If they thought about it at all, most SoCal residents used to think of San Onofre Nuclear Waste Generating Station as reliable, although this view was false all along.

The false impression was accomplished in part because when one reactor would go down, the other would usually stay up, so the unreliability was masked from public scrutiny. Refueling outages were planned accordingly.

However, recent events have revealed the truth. A widespread blackout less than a year ago brought SanO down for about a week -- both reactors -- as will just about every type of major disaster conceivable, whether it's SanO's fault or not. San Onofre has to shut down if it loses offsite power, which is nearly inevitable in most major natural disasters.

But even if SanO were more reliable, it's two operating units have several basic design flaws. For example, each of the two reactors has only two steam generators. That is a basic design flaw that cannot be fixed.

Most Pressurized Water Reactors have three or four steam generators. Having ONLY a single backup steam generator if one of the two fails has become a serious issue because of SanO's attempts to restart Unit 2 at reduced power due to design flaws inside their new steam generators.

All four steam generators were replaced in 2010 and 2011 with a new design that was supposed to last longer and wear better. But on January 31st, 2012 one of Unit 3's two new steam generators sprung a leak, and a closer inspection of Unit 2's steam generator tubing revealed excessive wear there as well. Technically, Unit 3 is "trashed" as one expert put it recently, but Unit 2 might still be operated at lower power -- if you don't mind risking losing Southern California to do it.

However, so far no one can figure out a "workable" power level. As of this writing, Southern California Edison hasn't applied for permission to restart either reactor.

On the intakes to the steam generators they sometimes put devices called "restrictors". The restrictors narrow the inlets to physically limit the maximum flow of water into the steam generator. If they want to run at reduced power, they probably will need to put properly-sized restrictors on, because it would not only be against regulations to run at a higher flow rate, but it would also potentially be very dangerous.

However, this presents a problem if there's an emergency. If one steam generator fails, they would have to cool the reactor with the other steam generator running at significantly reduced maximum flow! That might not be possible.

Why not just set the "maximum flow rate" to the amount that might be needed in an emergency, and run at less than that?

The answer is that they have to limit the flow because a catastrophic accident might occur if the flow rates are too high. So in essence, they will already be operating at their maximum permissible flow rates. That's why simply telling the operators: "Don't turn that dial past 50%!" won't be sufficient. The entire system is designed to push water through those steam generators as fast as practical, and normally that's supposed to keep happening unless they insert the control rods to shut down the reactor and it starts to cool.

But in San Onofre's case, increased flow in the primary loop can cause overheating of the water in the secondary loop, causing there to be more steam than water in the upper area of the steam generator, which can cause tube-to-tube collisions and wear, because steam doesn't dampen vibration as well as water does. This is what caused the radiation leak that occurred in January. So even in an emergency, you can't have excess flow.

So that's one problem: Only one backup steam generator. If the potential failure point of concern was somewhere other than a steam generator, that would be one thing, and maybe two steam generators per reactor is enough. But with the potential point of failure being the steam generators themselves, this becomes a "fatal flaw".

Another "fatal flaw" at San Onofre is that the problem with the steam generator tubes banging into each other and wearing out too fast can't be fixed by going back to the old design.

The old design was made with an alloy (Inconel 600) that had a 10% better heat transfer rate. But that alloy had flow induced vibration problems (i.e., it was too wobbly) AND premature wearing and aging problems (i.e., the "super" alloy wasn't so super after all). Those problems were only partially relieved by the industry's newest alloy (Inconel 690) that the new tubes were made with. There are nearly 10,000 of these tubes inside each steam generator. If one fails, it could cause the whole bundle to fail in quick succession (known as a "cascading" failure).

The new tubes had to be thinner to fit in the same space and have the same overall total heat transfer rate as the old tube bundle had. It's possible the designers purposely tried to make the heat transfer rate even higher so they could produce more steam. In any event, they succeeded way too much.

They took out a "stay cylinder" in the center of the steam generators in order to fit more heat transfer tubes, which may have added to their vibration problems.

In fact, they thought they had lots of better ideas for various other parts of the steam generators -- dozens of areas were redesigned along the way. The whole process took about 10 years. There were hearings, cost estimates, attempts by citizens (including this one) to stop it... same as what's happening today, except the reactors were running at the time.

The NRC was assured by SCE that the new steam generators were "like-for-like" and were going to be "plug compatible" after they tore a hole in the containment domes to cram the new steam generators in and take the old ones out. The assurance of "like-for-like" replacement avoided a layer of public scrutiny that might have stopped the project -- or found the problem.

At least one Unit 3 steam generator registered a "1.3 g" deviation on all three of its accelerometers somewhere along its journey from Mitsubishi Heavy Industries in Kobe, Japan to SanO. That bump may have contributed to its excessive wear: 1,300,000 pounds (the weight of over 16 fully-loaded tractor-trailer trucks) doesn't take kindly to being knocked around.

Before they left the factory, some of the steam generators were rotated many extra times during repairs of cracks found during inspections.

Steam generator problems aside, an even bigger problem is all the used reactor cores that are stored on site at San Onofre.

Used reactor cores are called "spent fuel" even though they actually are entire reactor core assembly modules, each weighing thousands of pounds. They ARE the "reactor." When they are removed from the reactor pressure vessel, they are thermally and radioactively "hot", and are chock full of "fission products" (i.e., radioactive decay products that are, themselves, radioactive).

These fuel bundle assemblies are yet a third fatal flaw at San Onofre -- and at every reactor. Every 18 to 22 months, a third of the reactor core is removed from the reactor and set aside -- forever. For the first five years the reactor cores must be stored deep under water, they are so hot. After that they are precariously loaded into "dry casks" which are recipes for disaster as well.

The older the fuel, the cooler it is both thermally and radioactively. Therefore, the safer and safer it becomes -- slowly.

San Onofre has many more "fatal flaws" than just these. For example, it's not nearly strong enough to survive large earthquakes that might occur in its vicinity. Its tsunami wall isn't nearly high enough -- just like Fukushima's wasn't. Its backup diesel generators have been left uninspected, have been mis-wired, have had their backup batteries fail, have had vibration detectors set improperly, have failed to start during tests, and their fuel is located below grade and would have to be pumped up through a flotilla of tsunami debris. The sump pump for its emergency core cooling system could become clogged with debris. Workers are intimidated or even fired for reporting safety violations.

Any of these problems -- and many more -- might be "fatal flaws" for San Onofre.

The solution is to keep it shut down forever, and turn to renewables.

The amount of energy from San Onofre (about 2,200 megawatts) has been "replaced" many times over since SanO was built -- sometimes with clean energy, sometimes with cheap natural gas. SanO should immediately be decommissioned and alternative energy options utilized instead.

Sincerely,

Ace Hoffman
Carlsbad, CA

The author is an educational software developer. He has illustrated nuclear reactors, mechanical pumps, the human heart, and many other processes. He has studied nuclear issues for over 40 years and has interviewed hundreds of scientists in related fields.



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** Ace Hoffman, Owner & Chief Programmer, The Animated Software Co.
** POB 1936, Carlsbad CA 92018
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