Ultrasonic testing procedures for volumetric and surface inspection of CANDU pressure tubes

Fuel channel pressure tubes are critical components at the heart of the CANDU reactor core which require periodic inspection to demonstrate continued fitness for service. A CANDU reactor consists of a large tank, the calandria, which is penetrated by several hundred horizontally mounted fuel channels containing natural uranium fuel bundles. The fuel channel consists of a 104 mm inside diameter, 4.3 mm thick Zirconium Niobium pressure tube, inserted into slightly bigger calandria tube, and two stainless steel end fittings at the ends of the fuel channel. Ontario Power Generation; Inspection & Maintenance Services Division operates a variety of inspection systems to perform volumetric and surface nondestructive examinations of the pressure tubes. At the core of those systems is an ultrasonic probe module which contains several ultrasonic probes interrogating inspected tube from several angles and with various wave modes at the same time. The ultrasonic inspection system utilizes high frequency ultrasound (10 MHz and 20 MHz) in order to achieve high degree of detectability of small flaws and high accuracy of flaw sizing. The inspection is performed in three phases: Inspection system calibration ultrasonic flaw detection sizing of detected flaws Detection of the flaws is performed using several criteria, including signal amplitude criterion. Sizing is performed using a variety of B-scans acquired with various probes in both pulse echo and pitch catch configurations. The flaw depth sizing accuracies achieved are in 20 to 40 microns range for certain flaws. The pressure tube inspection systems in use by Ontario Power Generation and previously by Ontario Hydro have successfully inspected hundreds of pressure tubes in Canadian and foreign CANDU reactors over the last 20 years. 1. Brief description of CANDU reactor Although the purpose of this paper is not to present CANDU design, basic knowledge of the reactor is required to understand issues related to structural integrity and inspection of the pressure tubes. A CANDU reactor (Figure 1) consists of a large tank, the calandria, which is penetrated by several hundred horizontally mounted fuel channels containing natural uranium fuel bundles. The fuel channel consists of a 104 mm diameter (ID), 4.3 mm thick Zirconium Niobium pressure tube, inserted into slightly bigger calandria tube, and two stainless steel end fittings at the ends of the fuel channel. The tubes are 6.3 m long. Garter Spring spacers separate the two tubes preventing contact between the hot pressure tube and relatively cooler calandria tube. Heavy water coolant flows through the pressure tubes, removing heat from fuel bundles and transferring it to steam generators, where secondary circuit light water is being heated and converted into steam to run the turbine. During reactor operation, pressure tube material is subject to high pressure (up to 11.3 MPa), high temperature (up to 310 °C) and very high gamma and neutron radiation fields. The calandria is filled with non pressurized heavy water acting as moderator. Unlike single vessel PWR or BWR reactors, a CANDU reactor consists of several hundred smaller pressure vessels – called pressure tubes. CANDU reactors are currently operated by several Canadian and foreign utilities. In addition, a number of similarly constructed PHWR reactors of the same basic design are under operation in India and Pakistan Figure 1 Simplified schematic of CANDU reactor core IV Conferencia Panamericana de END Buenos Aires – Octubre 2007 2 2. Inspection requirements for NDE of pressure tubes Inspection requirements for nuclear reactors in Canada are determined by two inspection programs: Periodic Inspection Program In-Service Inspection Programs Periodic Inspection programs are mandated by Canadian Standard CAN/CSA-N285.494. The standard has been recently revised and issued as CAN/CSA-N285.4-05. It is currently being implemented for use. The vast majority of the pressure tube inspections are being conducted under In-Service Inspection Programs. A number of degradation mechanisms have been identified during the many years of the CANDU program. Various fuel channel life management programs have been developed in order to gain better understanding of the pressure tube degradation phenomena and to maintain monitoring of pressure tube integrity at an acceptable level. The number of inspections performed under In-Service Inspection Programs vastly exceeds the minimum level required by the standard. 3. Inspection Systems Currently Operated by OPG for inspection of Pressure Tubes Inspection & Maintenance Services of OPG operates a variety of pressure tube inspection systems for the benefit of OPG and other utilities in Canada. The focus of this paper is on full length ultrasonic inspection of pressure tubes. Such inspections are accomplished using either of the two systems: ANDE (Advanced NDE) CIGAR (Channel Inspection and Gauging Apparatus for Reactors) There are many differences in the delivery mechanisms, electronic and computer hardware and productivity rates but basic ultrasonic inspection procedures remain the same for both systems. Basic capabilities of the inspection systems provide the following: a) Ultrasonic volumetric inspection of pressure tube material b) Flaw characterization by ultrasound c) Flaw characterization by flaw replication d) Garter Spring location by Eddy Current and UT techniques e) Channel gauging (internal diameter and tube thickness measurements) by ultrasonics f) Pressure tube sag profile g) Visual inspection of pressure tube ID IV Conferencia Panamericana de END Buenos Aires – Octubre 2007 3 This paper will focus on ultrasonic inspection capabilities of the Pressure Tube inspection systems. 4. Ultrasonic testing methods and procedures The ultrasonic inspection of pressure tubes is performed in three phases: In channel system calibration Detection scan Sizing and characterizations scans Detection scans are run in helical pattern. The helix pitch is adjustable by the operator. Currently used pitch for general flaw detection scans is 1 mm through the full length of the pressure tube. The sizing and characterization scans may be done as helical or raster scans – depending on the system. 4.1 Ultrasonic system calibration The flaw detection procedure is based on CSA N285.4-94 requirements. The requirement of the standard is to calibrate the inspection system on a variety of EDM notches. The calibration is achieved by setting the signals at a predetermined (100% FSH) amplitude level. The shear wave probes are calibrated on 0.15 mm deep, 6 mm long and 0.15 mm wide notches. There are a total of 4 notches: two ID notches – one axial and one circumferential two OD notches – one axial and one circumferential The Normal Beam (material) probe is calibrated on three Flat Bottom slots, machined from the OD of the tube. The slots are 1.5 mm long, 0.75 mm wide and are machined to 15%, 50% and 85% of the tube thickness. 4.2 Ultrasonic flaw detection Once calibrated, the reportability and investigation threshold is established at 50% of the calibrated levels. Certain areas of the pressure tube may have this threshold lowered even further, to 20% due to non uniform stress field in the area. To put the calibration notch size into perspective, it should be recognized that one paper sheet (copy paper) is approximately 0.11 mm thick. For the general flaw detection scan, both pressure tube inspection systems currently employ four focused, immersion probes, 10 MHz nominal frequency and one or two normal beam probes. The shear wave probes are configured in such a way that the individual beams refract in the pressure tube material at 45 degree angle (refracted shear wave). The probes are grouped into a probe cluster (Figure 2). Two probes constitute an axial pair and two constitute a circumferential pair. All four beams meet at the same point on the pressure tube ID (full skip). Thus, any flaw is being looked at from four IV Conferencia Panamericana de END Buenos Aires – Octubre 2007 4 different directions by shear wave beams. Depending on the system there might be one or two more normal beam probes. One of them is calibrated on OD slots in pulse echo mode. This probe interrogates the material and pressure tube OD. The second normal beam probe is calibrated using back wall echo and is used for flaw detection based on amplitude drop. Figure 2 Probe cluster – axial probe pair During the detection scans the data is acquired in a number of data acquisition gates. The gates are configured in the following way: