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Thursday, February 2, 2012

San Onofre - Primary to Secondary leak

Unit 3 at California's San Onofre Nuclear Generating Station has experienced a primary to secondary leak in one of its steam generators. This kind of failure cannot be described as everyday or commonplace to the point of normalcy, but this kind of failure does happen frequently enough that there are widely known and standardized procedures to deal with it. This type of failure usually results in plugging the U-tube or tubes if multiple which are leaking in the steam generator. Since each generator has many thousands of tubes, plugging several or even dozens does nothing to the heat transfer from the primary to secondary and in no way makes the plant any more dangerous than it was before the leak occurred. Many plants have operated with dozens or hundreds of plugged steam generator tubes for many years.

Here is the text from the official NRC report:

MANUAL TRIP DUE TO A PRIMARY TO SECONDARY LEAK GREATER THAN 30 GAL/HR

"At 1505 PST, Unit 3 entered Abnormal Operation Instruction S023-13-14 'Reactor Coolant Leak' for a steam generator leak exceeding 5 gallons per day.

"At 1549 PST, the leak rate was determined to be 82 gallons per day. At 1610 PST, a leak rate greater than 75 gallons per day with an increasing rate of leakage exceeding 30 gallons per hour was established and entry into S023-13-28 'Rapid Power Reduction' was performed.

"At 1630 PST, commenced rapid power reduction per S023-13-28 'Rapid Power Reduction'. At 1731 PST, with reactor power at 35% the Unit was manually tripped. At 1738 PST, Unit 3 entered Emergency Operation Instruction S023-12-4 'Steam Generator Tube Rupture'.

"At 1800 PST the affected steam generator was isolated."

All control rods fully inserted on the trip. Decay heat is being removed thru the main steam bypass valves into the main condenser. Main feedwater is maintaining steam generator level. No relief valves lifted during the manual trip. The plant is in normal shutdown electrical lineup.

Unit 2 is presently in a refueling outage and was not affected by this event.

The licensee has notified the NRC Resident Inspector. The licensee has issued a press release.

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San Onofre 3 is a Combustion Engineering pressurized water reactor. The containment style at this plant is dry, ambient.

The steam generators on San Onofre 2 and 3 are however not original to the plants. These steam generators were manufactured by Mitsubishi Heavy Industries in Japan, and were installed as replacements for both plants' original CE steam generators.

There are now reports circulating fairly widely that NRC officials have noted excessive wear on a number of the steam generator U-tubes in San Onofre Unit 2, which is presently shut down for refueling. Clearly, with a tube rupture or leak having occurred in identical equipment while at power on the same site, a much deeper investigation will take place.

There are reports circulating that "a radiation leak has shut down the San Onofre nuclear plant." The cause of the shutdown was a primary to secondary leak, not a "radiation leak," whatever that is. There are reports that a small amount of radioactive material in gaseous form may have been released to the atmosphere. I am continuing to look into this - but NO threat to the public exists.

The plant is shut down, and the affected loop is isolated from the reactor.

Here is a view of a typical Combustion Engineering pressurized water commercial nuclear plant primary system - called typically the NSSS or Nuclear Steam Supply System. In the large steam generators seen at the sides, heat is transferred from the primary coolant (circulated through the reactor by large pumps, clearly visible) into the secondary water as the primary water passes through many thousands of small diameter tubes. On the outside of these tubes is the secondary water, which boils, turns to steam, and then powers the turbine generator and other plant equipment. If a primary to secondary leak occurs, then, it is possible for some radioactive material to enter the steam plant.

Having said this, ALL steam plants at nuclear stations are designed with this eventuality in mind and primary-secondary leaks through small holes, or even tube ruptures, have occurred many times before. When controlled properly (as at San Onofre) there is no risk to the reactor, or its cooling or monitoring.

Finally for now here is Edison International's press release:

SAN ONOFRE LEAK PRESS RELEASE - FEB 1

4:50 PM Eastern Thusday February 2, 2012
ATOMIC POWER REVIEW

2 comments:

  1. If there was a "radiation leak", it probably would have been out the main condenser air ejectors. Some sort of vacuum pump or steam jet ejectors keep a high vacuum in the condenser. The noncondensables are released to the atmosphere. A steam generator tube leak would put primary coolant into the main steam, which eventually ends up in the condenser, so any noncondensable gases (such as xenon?), which are also radioactive, would cause a small release of radiation. The actual amount of curies should be low, depending on the activity in the primary coolant, and the impact to the public should be extremely low to nonexistent.

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  2. @T: Of course. I was merely attempting to indicate my displeasure at the press who use terminology like that while having absolutely no clue what they're talking about. "Radiation leak" is not synonymous with "escape of radioactive material from a controlled area."

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