ML083330159
| ML083330159 | |
| Person / Time | |
|---|---|
| Issue date: | 11/26/2008 |
| From: | Brett Rini NRC/NRR/DIRS/IOEB |
| To: | Marion A Nuclear Energy Institute |
| Brett Rini, NRR/DIRS/IOEB, 301-415-3931 | |
| Shared Package | |
| ML083330227 | List: |
| References | |
| Download: ML083330159 (3) | |
Text
Background Information on Control Rod Cracking Found at Swedish Boiling Water Reactors (BWRs)
Control rod cracks at Oskarshamn-3 prompt inspections at other BWRs (Nucleonics Week, October 23, 2008, pgs 9-10)
Forsmark-3, Oskarshamn-3 face outages for control rod cracking (Nucleonics Week, October 30, 2008, pgs 14-15)
Chronology of Information Received on the Control Rod Cracking Found at Oskarshamn and Forsmark (Note: this information has been obtained from the utilities, the Swedish regulators, or media outlets and much of it has been derived from translations of the Swedish text. Subsequent information may contradict what was initially reported.)
10/20/08 The outage for Oskarshamn Unit 3 (O3) has been extended. This is a result of the control rod drive (CRD) problem that was detected during the annual refueling and maintenance outage.
On October 17, OKG (the utility) decided to extend the year's outage on O3. This is because during the annual inspection of CRDs, OKG found that they were not intact, but had a fracture on a CRD shaft. OKG decided then to do further samples in order to see if the error was on more CRDs. The samples showed that some additional CRDs had similar problems.
OKG views this occurrence seriously and has therefore decided to do further inspections and analyses of the established deficiencies before O3 can complete its annual outage. The first results of the analyses and inspections are estimated to be completed by October 24.
Oskarshamn-1 and -2 (O1 and O2) do not have the same CRD structure as Unit 3. Surveys have nevertheless been done on O1 and O2 to ascertain that the drives are clear.
10/20/08 The Swedish Radiation Safety Authority, SSM, has been informed by OKG about a fracture and cracks on full length control rod clusters in O3. With a similar structure, Kraftgrupp Ltd, FKA, decided to close Forsmark 3 in order to see if a similar error can to be in the plant.
During the ongoing outage in Oskarshamn 3, one fracture on the full length control rod clusters was detected and subsequent inspections found cracks in additional full length control rod clusters. SSM was informed during the previous week about the deficiency and will take part in OKG's root cause investigation. SSM has also started its own investigation into the event in order to assess OKG's conclusions.
SSM has decided to request accounts of bases, analyses, and considerations concerning the problems with CRD extensions that were reported from O3. SSM is expecting to receive the information on October 23, 2008.
In order to ensure that similar damage has not occurred in the other plants CRDs, SSM has instructed all BWRs to show that their CRD extensions do not have the same problem as O3.
10/24/08 For O1, O2, Forsmark 1, and Forsmark 2, SSM has received basis and analyses that the authority requested last Monday (October 20) in connection with the indications that have been detected in O3. After the authority checked the basis, there is no reason to question that the reactors comply with the authority's safety requirements.
As bases for its assessment, OKG and Forsmark have checked these CRDs without finding damage. Moreover, the reactors at O1, O2, and Forsmark 1 & 2 use another type of CRD that, according to their own assessment, is not vulnerable to the same problem. In the report that was sent to SSM, it is assessed that this type of cracking will not arise in these reactors because the temperature differences in their CRDs are lower than in the reactor at O3.
Preliminary analyses show that the CRD damage at O3 has arisen as a result of thermal fatigue.
10/27/08 Forsmark 3 was instructed last Tuesday (October 21) to implement an inspection of CRDs in the reactor. The reason was that O3, which is of the same structure as Forsmark 3, found a fracture on one of its CRDs. Moreover, subsequent inspections have found cracks on additional CRDs at O3.
Forsmark 3 initiated inspections of CRDs on Sunday, October 26. One can now establish that the problems that were identified at O3 also exist at Forsmark 3. So far, fractures on one CRD and cracks on additional CRDs were found at Forsmark 3. Inspections continue and the plant is expected to return to operation at the earliest, November 28. Also Forsmark 1 and Forsmark 2 CRDs have been checked, but there were no deficiencies found. CRDs in these plants are of a different design/structure than that of the Oskarshamn 3 and Forsmark 3 CRDs.
10/28/08 The annual maintenance outage at O3 was extended, and its new restart date is December 3.
This new restart date depends upon whether OKG finds additional fractures on full length control rod clusters. The company chose last Friday (October 24) to extend the O3 outage in order to do more detailed inspections on full length control rod clusters. This decision was made in the light of additional defects on some of the 169 full length control rod clusters. These analyses and inspections will help to establish the extent of the problem and the reason for it.
So far, 15 full length control rod clusters been checked in O3 using a camera. Of these, six have shown the beginnings of similar problems. Now almost 100 full length control rod clusters will be checked with the aid of different (complementary) testing methods. The utility expects to take almost a week to prepare for this inspection.
Cameras have been used to check 20 full length control rod clusters on O1 & O2 with no issues found. SSM received the report from OKG and has cleared O1 & O2 to continue operating.
These plants have been returned to operation. Analyses and inspections on O3 are being reported continuously to SSM, which must approve the conclusions that OKG submits before the plant can restart.
10/29/08 An inspection of CRDs is taking place at Forsmark Unit 3.
The fuel around the CRDs that will be checked has to be removed from the reactor. A machine retrieves the CRDs from the core and places them in a pool where they will be studied with
qualified visual technology. The examination concentrates on the area at the CRD extension, approximately 20 cm from the blade, where indications may be found. Also, ultrasonic inspections will be used in order to detect possible cracks.
11/04/08 Thus far, just over half of Forsmark 3s 169 CRDs have been checked. Cracks or indication of cracking have been found on approximately 30 percent, but no more fractures have been found.
Every CRD will be checked. The results will then be analyzed in order to determine what damage has occurred. The utility estimates that Forsmark 3 can to be returned to operation, at the earliest, at the end of November.
11/11/08 Restart of Forsmark 3 has been postponed.
Forsmark 3 has now checked all CRDs in the reactor and other CRDs in the fuel pools, totaling 197 CRDs. The result of this inspection is that 25 percent of the CRDs have been found with crevices or relevant cracks.
The technology that has been used is qualified visual technology, (i.e., mound resolution television technology). In connection with these results, a decision has been made that every CRD that has been approved with visual technology will be re-inspected with qualified eddy current measurement. Inspection is expected to take place over approximately two weeks. In connection with the detected CRD cracks, special inspections are scheduled for other components in the reactor thought to have similar concerns.
The root cause investigation and its analysis is taking place concurrently and is expected to be complete the last week of November. Restart of the plant is estimated for the end of the week of 15 December.
11/13/08 The current schedule of continued inspections, analyses, measurements and documentation indicates an estimated start up time on Saturday, December 20. Startup of the unit must be approved by the (regulatory) Authority.