Qualification of In-service Inspections of NPP Primary Circuit Components

Transcription

1 Qualification of In-service Inspections of NPP Primary Circuit Components ABSTRACT Matija Vavrouš, Marko Budimir INETEC Institute for nuclear technology Dolenica 28, Zagreb, Croatia Maintenance and inspection programs for primary circuit components in nuclear power plants are necessary for safe plant operation. Primary circuit components present reactor coolant boundary and integrity of these components is essential for safe and reliable operation of nuclear power plants, and consequently, environmental safety and reliable electrical power production. Non-destruction examinations are commonly applied in nuclear industry for purpose of primary circuit component in-service inspections. Each in-service inspection activity is determined by regulatory requirements and standards for each specific component, through which methods, scopes, frequency and other parameters of in-service inspection are defined. In order to determine selected inspection processes effectiveness, qualification requirements were introduced into regulatory requirements and standards to asses that capability of selected inspection process meets criteria defined by applicable standards. Increasing experience in nuclear power plant operation provides more information about primary circuit components, which results in improvements of regulatory requirements and standards. Improved regulatory requirements and standards include more demanding and more reliable inspection programs and methods. Due to these changes and necessity of safe plant operation, significant emphasis is placed on issues of inspection qualifications and reliability, to ensure that inspection requirements are met. This article will focus on recent status of most common qualification requirements, and will present specific experience gained from qualifications performed by INETEC. 1 INTRODUCTION Nuclear power plants are plants in which electrical power is generated through energy transformation from thermal energy that is created inside the core of the nuclear reactor. The common design of nuclear power plant includes separate flow circuits, one through which only radioactive coolant is passing, and one or two circuits through which non-radioactive coolant flows. No direct contact exists between primary and secondary coolant and only the heat generated in core of nuclear reactor is transferred from primary to secondary coolant through steam generator heat exchangers. As radioactive coolant is restricted to only to primary circuit components, it is absolute necessity to ensure integrity of primary circuit components. Primary circuit components represent primary circuit boundary and ensuring its integrity is critical for safe operation of entire nuclear power plant and environmental safety. Additionally, loss of primary circuit 509.1

2 509.2 integrity would result in undesired outages and loss of electrical power production capability. Monitoring of condition of primary components is done through in-service inspections. In-service inspection is maintenance activity that is performed with purpose of obtaining information about the condition of the component subjected to the inspection. Based on acquired condition information further conclusions about integrity and possibility of safe operation of the component can be done. In-service inspection in nuclear power plants are always done by application of nondestructive testing. This way inspected components remain in same condition as prior to inspection but information about component condition is gained. As in-service inspection is done to ensure integrity of nuclear power plant components, capabilities of in-service inspection process are related to safe operation. To evaluate capabilities of in-service inspection, process of inspection qualification is required and performed. 2 QUALIFICATION CONCEPT Qualification of in-service inspection is a process in which assessment of inspection capabilities is performed to verify if they meet requirements determined by applicable standards. Qualification is always applied to three main elements of in-service inspection: equipment, with which the inspection is performed, personnel that are performing the inspection and written procedure according to which the personnel use the equipment and perform the in-service inspection. Figure 1 presents the typical stages for the qualification process. Technical Specification Input Document NDT procedure & TJ Qualification Procedure Qualified Procedure & Equipment Practical Blind Trials Personnel Qualified Personnel Figure 1: Typical qualification process stages In-service equipment and procedure are always assessed through open trial evaluation, where capabilities of in-service inspection are done on open sample. Sample in inservice qualification refers to material sample with artificially made degradation, like cracks and notches. When sample is considered open, all information about existing degradation or flaws on it are available, and when true information about degradation is compared with information obtained through non-destructive testing, assessment can be made about capability of inspection equipment and procedure. On the other hand, blind trials are applied for qualification of personnel. Blind samples include degradation like cracks and notches but no information is available about number, sizes, distribution and locations. Personnel applied for qualification are required to detect and characterize flaws, and if specified criteria are met, they are considered qualified. Representative samples are closely related to success of qualification. Material and design of sample piece should be similar to the actual component and flaws made on samples should be similar to flaws expected on actual component. Figure 2 presents typical samples used for qualification.

3 509.3 a) b) Figure 2 Flawed specimens used for in-service inspection qualifications; a) VVER Reactor pressure vessel shell welds specimen; b) VVER reactor pressure vessel nozzle specimen 2.1 In-service inspection qualification standards The requirements for performance of qualification in-service inspection are defined in various standards depending on the field of application and country. In United States, for PWR power plants standard that defines the qualification requirements is ASME Boiler and Pressure Vessel code, Section XI [1]. This is standard that defines all parameters for inservice inspection, including qualifications. To ensure that all qualification are performed in accordance with [1], independent institution Electrical Research Power Institute (EPRI) performs qualification body duties for all qualifications done in US, and also for plants located outside US but following US regulations regarding nuclear power plant operation. In Europe, there is no unique qualification body or unique standard, and qualification is done through guidelines created by European Network for Inspection Qualification (ENIQ). ENIQ qualification guidelines [2] do not present strict standard but guidelines how the qualification should be implemented. For number of European countries their regulatory body creates its own qualification standards and procedure. Technical aspects in most of such standards are based on ASME Boiler and Pressure Vessel Code, Section 11 requirements, modified depending on circumstances while legal proceedings are based on ENIQ recommendations. Based on ENIQ proceeding International Atomic Energy Agency made its own qualification requirements [3] which are similar to ENIQ requirements with slight difference in roles of qualification body, and other participating organizations. Specific aspect of ENIQ approach to qualifications is introduction of technical justification (TJ) document. Technical justification is document that assembles all evidence about capability of selected non-destructive method for component examination. Evidence can include reasoning for selection of certain technique, modelling, simulation and open trail results. Benefit of technical justification is that provides feedback information to organization preparing qualification input information resulting in more optimized and efficient inspection as a result. 3 RECENT QUALIFICATION EXPERIENCE In recent time number of different qualifications of in-service inspection was performed by INETEC. With increasing demands defined by latest in-service inspection standards and

4 509.4 increasing demands of quality systems, most of the plants or organizations that require inservice inspection activities, require also the verification of inspection capabilities. 3.1 PWR Reactor pressure vessel inspection qualification For purpose of 2010 inspection of reactor pressure vessel in Krško NPP, qualification of ultrasonic inspection method of RPV shell welds was required. As applicable standard was [1], Appendix 8 and its applicable Supplements that define all requirements for qualification, entire qualification was performed in EPRI facilities in Charlotte, USA. Number of open specimen, with all information about existing flaws available were scanned and examined by group of ultrasonic experts. After the obtained results and flaw measurements were compared and found to be inside defined criteria, equipment and procedure were considered qualified. Personnel qualification was done through examination of blind specimens, and their ability to adequately detect and size existing flaws was assessed by independent experts from EPRI. Each candidate independently evaluated the data. Criteria for detection and measuring of flaw dimensions are presented in Table 2. Table 1: EPRI PWR RPV qualification requirements Flaw Detection Flaw Sizing No flaw is undersized for depth by more than Depends on number of 5.08 mm flaws i.e. Flaw lengths shall be 10 flaws/10 detected the true length 6 / flaws/14 detected mm 20 flaws, 18 detected The mean deviation of flaw depth is less than 6.35 mm 3.2 ENIQ qualification for VVER Reactor pressure vessel For purpose of 2010 and 2012 inspections of reactor pressure vessel in Loviisa NPP Units 2 and 1 respectively, qualification of in-service inspection was required prior to performance of actual inspection. After detailed input was received from Loviisa NPP representatives, appropriate in-service inspection methods were selected. For surface of RPV material eddy current non-destructive method was applied while for volumetric examination ultrasonic method was selected. After methods of inspection were determined, number of examination and testing were done on open test samples provided by Loviisa NPP, and its owner, FORTUM. Through examination performed on these samples, tuning of equipment and procedure was done, inspection parameters defined more precise and carefully, which resulted in precise flaw measurements and sizing capabilities.

5 509.5 a) b) Figure 3 VVER RPV qualification scanning; a) nozzle specimen scanning; b) vessel shell specimen scanning Entire assessment of qualifications was done by Finnish Qualification body, at INETEC facilities. In presence of Qualification body open trials were scanned and examined and results presented to members of QB. After the QB evaluated the obtained results and measurements and compared to true, manufacturer flaw values it was determined that all results met the defined criteria. Personnel qualification was done on additional blind specimens. These specimens were shipped to INETEC facility under surveillance of members of Qualification body. Specimens were unsealed prior to scanning and resealed immediately after the scanning. Data obtained was independently evaluated by number of eddy current and ultrasound experts. Their results were assessed by QB and compared to true values of flaws, and compared to defined criteria. Criteria for acceptance of qualification regarding detection and sizing capabilities for this qualification were defined Finland Regulatory Guides YVL 3.8 [3] and YVL1.3 [4] that are developed based on ENIQ guidelines. 3.3 VVER Control Rod Drive Protection Pipe inspection qualification Similar to RPV inspection qualification, for VVER Control Rod Drive Protection Pipe inspection in Loviisa, qualification was required prior to performance of actual inspections. Based on input specification defined by Loviisa NPP personnel, new inspection manipulator tool was developed and inspections methods selected. Surface was to be examined by eddy current while volumetric examination was to be done through application of ultrasonic examinations. Number of open specimen with all flaw information available were scanned and examined. This way equipment was tested on open samples and parameters of non-destructive testing were optimized for best actual inspection performance. Figure 4 presents scanning of qualification samples during CRDPP qualifications. As VVER CRDPP has two different dimensions of welds, two different types of samples had to be made by FORTUM and Loviisa NPP to have representative qualification test pieces.

6 509.6 a) b) Figure 4 VVER CRDPP qualification scanning; a) CRDPP weld no.1 specimen scanning, b) CRDPP weld no.3 Actual qualifications were done in presence of Finnish Qualification Body. First open specimens were examined and results presented to QB and compared to true results. After it was determined that measured results meet defined criteria, blind specimens were scanned under the surveillance of Qualification Body members. Acquired data was evaluated independently by a number of eddy current and ultrasonic experts and their results compared to defined qualification criteria. 4 CONCLUSION In-service inspections as a part of maintenance activities are of great importance in ensuring the integrity of nuclear power plant primary circuit components. Integrity of primary components is directly related to safe operation of nuclear power plants and environmental safety as well as reliable operation of plants and continuous electrical power production. To make sure that performed in-service inspections have capabilities to address potential degradation and deterioration of primary circuit components, assessment of inservice inspection capabilities is done through qualification process. Qualifications ensure that inspection activities are done in accordance with applicable standard requirements. Detailed qualification process can result in more optimized inspection methods. This way actual inspection on-site are performed in more efficient manner while maintain high level of reliability. REFERENCES [1] Rules for Inservice Inspection of Nuclear Power Plant Components, ASME Boiler and Pressure Vessel Code, Section XI, ASME, New York, [2] European Methodology for Qualification of Non-Destructive Testing (third issue), ENIQ, EUR EN, Luxembourg, [3] Nuclear power plant pressure equipment, STUK Regulatory Guide YVL3.8, Helsinki, Finland, 22 September 2003 [4] Mechanical components and structures of nuclear facilities, STUK Regulatory Guide YVL1.3, Helsinki, Finland, 17 March 2003