TMI Steam Generators: Unexpected Flaw

by Scott Portzline

The new steam generators at Three Mile Island have an unexpected flaw which is yet to be understood by Areva, the twice financially rescued French company (last year by US tax dollars) which built them, or by the engineers in the US nuclear industry. Fortunately, the engineers at TMI correctly determined that abnormal wear was occurring at unexpected locations on 257 steam generator tubes. Their observation caused Arkansas Nuclear to re-evaluate data which it had glossed-over indicating that the same thing was occurring at their reactor site.

These new "enhanced" steam generators were supposed to save money by decreasing maintenance and by increasing the amount of electrical power generated. The Nuclear Regulatory Commission (NRC) was so confident in the new metal alloys used in the steam generators that they would automatically extend inspection cycles to any licensee which requested it. But, now the NRC should mandate that the inspection of these new steam generators be more thorough and occur during each refueling outage. TMI will do this voluntarily.

The problem will probably be blamed on a design flaw which allowed steam tubes to bang against each other. Either the proximity of the tubes to each other, or inadequate stiffness can be blamed for the unexpected wear. The high temperatures of the pressurized water running through (and around) these tubes cause the tubes to expand and to bow sideways.

There are concerns that under abnormal conditions, when the temperatures can double or triple, additional tubes will bow even farther and then additional tubes may be compromised. Add to that the vibrations and shock waves caused by steam voids (which happened at TMI in 1979) and you have a large break loss of coolant accident on your hands as numerous tubes fail.

At the moment, there is still enough safety margin at TMI during normal conditions. Exelon claims that a "large break loss of coolant accident evaluation also demonstrates significant margin" as far as the steam generator tubes are concerned. Their report allows for tripled pressure differentials to support their analysis. However, there is no discussion of the temperature differences which allow the tubes to bow. During the 1979 emergency, temperatures in the reactor rose ten-fold. Steam generator "A" tore its guts apart.

The NRC's Advisory Committee on Reactor Safeguards, which conducted a review in 2001 stated that, "The NRC staff does not currently have a technically defensible analysis of how steam generator tubes, which may be flawed, will behave under severe accident conditions in which the reactor coolant system remains pressurized."

Additionally, there is a special manner in which tube failure can cause what is known at a guillotine rupture accident. If a high temperature jet rushes from a failed tube and cuts through an adjacent tube, the process can repeat itself in a rapidly cascading failure. A report on experiments by the Rockwell International states "[Damage] occurred on the surrounding tubes due to the high temperature reaction."

Knowing this, and despite the chances of its occurrence being very low, the NRC should address this as a Safety Issue. A spokesman for TMI stated that the newly found flaws are not considered a safety issue.

The problem reminds me of a fatal flaw that existed during the Apollo Moon Program. The original Saturn rockets had a vibration problem called "pogo." Liquid fuel slammed against the bottom of the fuel tank and then slammed against the top. The forces threatened to tear apart the rocket. Designers solved the problem by installing baffles to prevent the sloshing of the fuel.

Now back to nuclear thinking: For years the industry compared itself to "rocket science" and that ordinary citizens were unable to understand the complexities involved. Therefore citizens who worried about nuclear safety were viewed as ignorant, over-emotional pests. As the years went on and citizens became very sophisticated with their knowledge and familiarity of the regulatory process, citizens were accused of wanting to over-regulate the industry. New cries went up from the industry and even from the NRC's chairman Nils Diaz, "This is not rocket science."

The steam generator concerns are on the level of rocket science. These are new materials, higher pressures and temperatures may be involved, and new operational conditions and data; just like rocket science. No one should forget that experiments are still being thrust on the public in this nuclear realm.