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The LaSalle report April 6, 2006

Posted by dr. gonzo in Nuclear Power.
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*Note, I have linked as many of the technical terms and abberviations as possible to the definitions maintained in the NRC online reading room glossary.

The Nuclear Regulatory Commission Special Inspection Report on LaSalle County Station Unit 1 was sent to Exelon on March 23, 2006, 31 days after the ‘site area emergency’ at the LaSalle nuclear plant. The reactor came back online March 20, 2006.

The report is broken up into several sections. There is a summary, details and a chronology of events on Feb. 20, 2006. The inspection also looked at the liscensee’s (plants) own investigation and troubleshooting, as well as the Control Rod Scram Insertion Failure, Human Factors and Procedural Issues, Site Area Emergency Declaration Issues, and the meetings that covered the incident.

The meat of the report

The report is careful to back up the statement in Mark A. Satorius, Director
Division of Reactor Projects, Region III’s letter to Exelon’s President and Chief Nuclear Officer Christopher Crane, “Based on the results of this inspection, no findings of significance or violations of NRC requirements were identified,” his letter stated.

From the report:

“A. Inspector-Identified and Self-Revealing Findings

No findings of significance were identified.

B. Licensee-Identified Violations

No violations of significance were identified.”

But, this doesn’t mean that the NRC didn’t find some problems at LaSalle Unit 1.

Back to the Letter:

However, several issues remain outstanding regarding control rod performance and control rod position indication. First, the indicated positions for several control rods following the scram, as well as the operation of the rod worth minimizer (RWM) in scram mode, have not yet been fully resolved. Additionally, based on a review of a source range monitor (SRM) nuclear instrumentation count history, it appears that control rod 38-43 did not fully insert into the core in response to the initial scram signal for a significant period of time. Given the potential safety significance of these issues, the NRC is treating these questions as Unresolved Items, pending review and inspection of your completed root cause analyses of these matters and any other relevant documents.

Issues indeed.

Electro-Hydraulic Control (EHC)

First, from the report’s background section:

“For as yet unknown reasons, the EHC system reactor set pressure experienced a temporarystep change that caused all five main turbine bypass valves to go fully open. The resultingreactor water level and pressure transients with the reactor in the run mode at approximately 6 percent power and in the process of being shut down for a scheduled refueling outage resulted in a reactor scram and Group 1 (Main Steam Isolation Valves) containment isolation. For several hours following the scram, plant operators were unable to verify that all control rods had inserted into the core as designed and declared a Site Area Emergency in accordance with the station’s emergency plan.”

Basically, it looks like the whole incident was chalked up to various mechanical failures. Including the Electro-Hydraulic Control (EHC), which investigators thought was due to an output diode on the PMG power supply. But after further testing it was determined that this failure was “insufficient to explain all the EHC anomalies encountered from the February 20, 2006, Unit 1 scram.”

The team from LaSalle has yet to complete their analysis of the EHC failures but did provide the NRC with several preliminary conclusions and solutions in the short-term. The report notes that the “licensee [plans to do a] wholesale replacement of the main turbine EHC systemwith a new digital EHC system during the next refueling outage on each unit.”

So that, problem will, hopefully, be solved permanently.

Control Rod Scram Insertion Failure

First the inspection found several significant pieces of data:

-Each control rod was, in fact, fully inserted beyond position 02.
-The control rod drift alarms were functional.
-The control rods required an inordinate amount of drive water pressure, in some cases the maximum allowed by the normal operating procedure for the system, in order to be moved.
-Control rod settle time was excessively long. In some cases, no control rod settle was observed.

As with the EHC problem, the full scope of the failures surrounding the control rod insertion failure is not yet known. For this reason and the safety significance of the problem, the NRC is treating this as an Unresolved Item (Def: An unresolved item is a question that requires more information to reach a conclusion. Licensee response and NRC evaluation are required).

There were two other items that caught my attention from the report. First, this was not the first time control rod insertion had been a problem at the LaSalle station. In 1994, Unit 2 was the culprit.

From the report:

“On October 19, 1994, LaSalle County Station, Unit 2, scrammed from full power due to an EHC system malfunction. Nine control rods failed to indicate fully inserted for approximately two minutes following the scram. In the aftermath of the event, the licensee determined that the nine control rods had been inserted slightly beyond the full in reed switch position, and did not indicate fully inserted until each had settled back out to the full in reed switch position.”

The second, and final thing that I am going to mention in this piece is that on the morning of Feb. 20, 2006 the LaSalle plant not only declared a site-area emergency but also an Anticipated Transient Without Scram (ATWS). Click on it, the definition is pretty scary. Things could have ended up a lot worse that morning, glad they didn’t.

I will keep my eyes open for additional information on this incident, as the NRC is required to follow up on the Unresolved Item. Some useful links below.

Related Links:

Feb. 23 NRC release on incident
Feb. 20 Exelon release on incident
FTG’s U.S. nuclear emergencies database
NRC Special Inspection Report on Feb. 20 incident. (24 pages)

Related Posts:

Notes on the LaSalle nuclear plant
Trouble in Fissionville
Other FTG posts on nuclear power

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