Refuelling error at Dampierre unit 4 02/04/2001 Lessons learned. EDF th and 14 th 2003 Expert Meeting KTG - error on Dampierre unit 4

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1 efuelling error at Dampierre unit 4 02/04/2001 Lessons learned

2 eminder of the facts Dampierre 4 was running a forced outage at mid cycle (SG leakage) Error in the running of the sequences in the Spent fuel pool building undetected. (Sequence 25 missed out) Shuffling of the assemblies (wrong positioning) in the reactor up to sequence 138, constitution of a more reactive mass than initially calculated. Nevertheless, No flux alarm annunciator was actuated. (113 assemblies moved) Detection of the anomaly at the time of sequence 139 by the reactor building crane operator. Causes essentially of human origin

3 Normal plan for Dampierre unit 4 P N M L K J H G F E D C B A CNS n G G SA NI NI GI NI GI CNS n 2 N de séquence Nom du groupe de grappes assemblage de premier cycle assemblage de second cycle assemblage de troisième cycle assemblage de quatrième cycle

4 Wrong plan obtained for Dampierre unit 4 CNS n G G SA G G CNS n 2 N deséquence Nomdugroupedegrappes erreursdeséquence assemblagedepremier cycle assemblagedesecondcycle assemblagedetroisièmecycle assemblagedequatrième cycle

5 DAMPIEE UNIT 4 FLUX COUNTING CHANNEL EVOLUTION AET FOTUIT TANCHE 4/ COMPTAGE (c/s) CNS1 CNS2 T4/ N DE SEQUENC E

6 Actual consequences Preservation of margins regarding the risk of criticality. (Keff 0.96 as against 0.95 required in the Final Safety Analysis eport FSA. In case of completion of the 157 sequences, the final Keff should not have been increased) - Boron concentration was above the operating technical specifications (STE) (2,345 ppm against 2,000 required) but realistic (required boron concentration in efuelling Water Storage Tank: 2,425 ppm) - Burn-up of 2,000MWj/tonnes

7 «Dampierre unit 4 Scenario" Findings of the corporate study With the current operating tech. specs. (STE), the risk of criticality cannot be excluded on the sequencing modes for CP0 and CPY 900 MW plants. Margins available in 1,300MW and N4 plants are sufficient. Transmission to the regulatory authority of a document at the end of 2002 isk closely linked to: (See Slides 2 ). Type of core map (position of new assemblies). Sequencing (coiled and diagonal pattern) Source range channels do not automatically detect criticality approach before it occurs (at this point, it should be detected) classified as INES level 2 Limited consequences of criticality event in the specific case of Dampierre unit 4

8 efuelling not in compliance with the core map: issue Wrong positioning of several assemblies. Combined effect too complex for engineering the safety demonstration. Errors with adverse effects (c.f. Dampierre unit 4) The lack of the risk of criticality cannot be demonstrated only by engineering studies equirements of article 45 of the PGE (General regulatory requirements). Basis adopted to keep in hands the risk of criticality. Drawing up of a criticality reference document (transmitted to the regulatory authority at the end of 2002 beginning of 2003)

9 PGE Decree of the 31/12/99 (Article 45) A criticality accident can in no way be the result of a single anomaly, such as the failure of a component or a function, human error (for example, noncompliance with a procedure) or accident situation (forexample,fire) If a criticality accident can be the result of the two anomalies occurring at the same time, then the following must be proven: - The two anomalies must be completely independent; - The probability of each of the two anomalies happening is quite low; - Each anomaly can be highlighted using appropriate and reliable surveillance methods within an acceptable period of time for corrective actions.

10 efuelling not in compliance with te plan Single anomaly Wrong positioning of one assembly Non-criticality demonstrated by studies Choice (fuel building) and/or position (reactor building) Multiple anomalies Wrong positioning of several assemblies Non-criticality cannot be demonstrated by studies Criticality accident

11 Studies (criticality reference) Demonstration of unique refuelling errors. Transmission to the regulatory authority in accordance with a schedule presented in October 2002 Continuation of engineering on other risks. aising of reactor vessel head with the rods undeconnected (end of 2003). Presence of air in fuel handling tools (2003). Dilution during handling activities, capacity of the source range channels to detect this situation (2003)

12 Trends - Operations Short term (2001): corporate prescription = resolving errors. Independent verification of the assembly, mandatory communication between reactor building and fuel building coordinators, management of handling interruptions, checking of CCAs position consistency, counting rate monitoring and positioning in the reactor vessel other location than final one, even temporary, is prohibited (because of positionning difficulties due to bowing issues) In 2002: corporate precription (CPY CP0) = preventive actions. esult of studies conducted: boron concentration + loading in a coiled pattern, request for OEF and formal inspection of positioning within the reactor vessel. Diagonal pattern is no more allowed Sustainable application of the reference base (PC) and amendment to the operating tech. Specs. for boron concentration in the refuelling cavity Studies to ensure increased safety of fuel handling activities. Assessment of fuel handling process reliability. Feasibility of reading the FA number in the reactor building on 900MW plants Coaching, training and OEF

13 Methods of monitoring and taking action Other methods for surveillance inside the reactor vessel. Feasibility studies (mechanical, neutronics in process first results in 2003, but little chance to obtain a reliable outcome) Alarm for high flux at shutdown. Standardisation of documentation (operating tech. Specs., Low power physics testing rules for start-up). eview plan of actions to be taken Definition of emergency operating procedures (review in February 2003) Longer term studies:. On-line Keff calculation depending on the configuration. Use of the source range channel signal