 INES 2 |
Inoperability of 22 out of 61 control rods at Kozloduy NPP unit 5 |
On 01.03.2006, 06:08 AM, one main circulation pump (MCP) of Kozloduy NPP unit 5 trips. As designed, the reactor automatic power reduction system PZ-1 (PZ-1) actuates and the reactor power is automatically reduced to 67% of the nominal power as required by the Technical Specifications (TS).
Following the power reduction, the control room personnel identifies that 3 control rods of group No. IX remain in upper end position (are not moving). According to the TS, the reactor power is reduced to hot-standby state following the normal operating procedures. With the reactor in hot standby, all control rod drives were tested and as a result it was identified that 19 more control rods are not movable. Finally, the reactor is cooled to cold standby state.
As a result of the event, a program was established to investigate event direct and root causes with involvement of the operating organization and the general designer OKB Gidropress. The program included rods testing, dismantling and other kind of tests. The tests showed that the direct cause is sticking of the contact surfaces of the fixating electromagnet of the control rod drives moving system. The analyses of the root causes are continuing. In the preliminary event report sent to the NRA, the Kozloduy NPP rated the event as INES Level 1.
Following the legislative requirements, the NRA has been timely informed about event beginning on the same day and as a result we took actions to obtain additional information through our resident inspectors at the Kozloduy NPP site. To clarify the case and to obtain detailed information, NRA sent immediately to Kozloduy NPP two of the most experienced experts. NRA prescribed the following:
1. The reactor shall be put in cold shutdown state.
2. The causes shall be investigated and the necessary short-term compensatory and corrective actions shall be determined with consideration of the standpoint of the general designer.
3. Unit restart after the event shall be done only after authorization by the NRA Chairman, based on the report on the implementation of the activities, specified above.
Based on review and assessment of the NPP reports, the reports from the inspection team on-site and after consultations with the Institute of Metal Science, on 9 March 2006 NRA authorized the start-up of the unit. By the authorization granted NRA have prescribed a schedule for movement tests of the control rods – every day for the first week after start-up and once a week till the end of the fuel campaign. Also NRA set additional limitations on the operation of the unit at power – namely in case of inoperability of any control rod the unit shall be put in cold shutdown state. Also NRA laid down a condition that till the end of the annual outage of the unit the root causes shall be identified and corrective actions for preventing recurrence of such events shall be put in place. NRA resident inspectors report daily to the Deputy Chairman about the conditions of the control rods.
Independently from the NPP investigation team, NRA formed an event analysis commission within the Agency to deal with event evaluation and also contracted an independent external expertise to a technical support organization to perform thermal-hydraulic and neutron-physical analyses for the case with failure of some of the control rods.
The safety analyses included in the Safety Analyses Report of the plant and the results from the independent expertise demonstrate that during the event the reactor protection system was capable to safely shut down the reactor by itself and that the function “Control of Reactivity” has been more than adequate to cope with the expected and possible design basis accidents. Based on those analyses and results and applying the IAEA/OECD INES User’s Manual, the NRA commission decided a basic rating for the event as INES Level 1. The analysis of the additional factors carried out by the NRA showed that in the particular case there are reasons to take into consideration the additional factors: “Procedural Inadequacies” and “Common cause failures”.
Taking into consideration the work done by the investigation teams, the performed expert studies and analyses, the influence of the additional factors and the international experience of using the INES rating procedure, the NRA defines the final rating of the Unit 5 event of 1 March 2006 as Level 2 – Incident according to the INES scale.
Event date: Wed, 01-03-2006
