Qualification of NDT Sensors and Sensor Module used in In-Service Inspection of Coolant Channels of 220 MWe Indian Pressurized Heavy Water Reactors

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Alexander Copeland
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1 National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014) Vol.20 No.6 (June 2015) - The e-journal of Nondestructive Testing - ISSN Qualification of NDT Sensors and Sensor Module used in In-Service Inspection of Coolant Channels of 220 MWe Indian Pressurized Heavy Water Reactors H. M. Bapat, Gurpartap Singh, A. M. Kadu, Vijay Tike, A. K. Haruray, Manojit Bandyopadhyay, R. K. Puri and D. N. Badodkar Division of Remote Handling and Robotics, Bhabha Atomic Research Centre, Trombay, Mumbai , INDIA hbapat@barc.gov.in Abstract. In-Service Inspection (ISI) of coolant channels of Indian Pressurized Heavy Water Reactors (PHWRs) is carried out periodically to monitor important parameters related to health of channel. ISI is carried out using indigenously developed BARC Channel Inspection System (BARCIS) with reactor in shutdown condition and shutdown pumps running. During shutdown condition flow of coolant is maintained and temperature is around 35 to 50 degree centigrade. Also, there is differential pressure of around 5Kgf/cm 2 and estimated maximum radiation dose rate of 1MRads/hr inside the channel. Both ultrasonic and eddy current based sensors are used to monitor different parameters (Flaw detection, dimensional measurement, detection of spacer ring etc) of channel. The NDT sensors are mounted in probe holder with its mechanical and electrical signal cable arrangements and this is called BARCIS Inspection head. These NDT sensors and Inspection head need to be qualified to survive in harsh environment of channel and work satisfactorily. Both ultrasonic and eddy current sensors are potted with epoxy resin to make them leak proof. The epoxy resin used for potting should be able to withstand high radiation and pressure. Since scanning is carried out at differential pressure of 5Kgf/cm 2, the leak tight joint in the basic extension tube of Inspection head is very important as its failure will lead to loss of coolant from reactor and spread of activity in vault area. So the testing of sensors, resin potting, leak tight electrical joint and complete mechanical assembly is very important in successful completion of ISI campaign. This paper describes various tests like pressure test, radiation resistance test, dimensional test, electrical continuity test and functionality test that each and every sensor has to pass before it can be deployed in the Inspection head. In addition to this, the stringent test procedure for the complete assembly of Inspection head is also described. Once the Inspection head qualifies the prescribed tests then only it becomes ready for deployment in reactor for actual In-service inspection. Keywords: Pressure tube, In-service inspection, NDT Sensor, Radiation Introduction Pressure tubes of 220M We Indian Pressurized Heavy Water Reactors (PHWRs) is made of Zr- 2.5% Nb material. The pressure tube is placed horizontally and concentrically inside the calandria tube and it is joined at the ends with end fittings by rolled joints. There are four number of tight fit spacer rings called Garter spring between pressure tube and calandria tube. This complete assembly

2 is called coolant channel and it is around 9 meters in length. There are 306 coolant channels in a typical 220MWe Indian PHWR. A typical coolant channel assembly is shown in Figure 1. The pressure tubes houses nuclear fuel in the form of cylindrical bundles. Heavy water as coolant flows from one end of tube to other and extracts heat generated in fuel bundles. Under reactor operating conditions, these channels are subjected to extreme pressure and temperature along with very high neutron flux. Over a period of time, some degradation in the structural condition of these channels is expected. It is important to keep a track of the structural condition of these channels and for this Inservice inspection is carried out. Coolant Inlet End Shield Calandria Pressure Coolant Outlet Cables End Fitting Rolled Joint Garter Spring Inspection Head Drive Liner Figure 1. Inspection head inside pressure tube In-Service Inspection In-Service Inspection [3] of coolant channels is carried out from one end of channel with reactor in shutdown condition and shutdown pumps running. The channel to be inspected is defuelled and fuel bundles are temporarily stored in the fuelling machine at the other end of channel. The inspection is carried out using indigenously developed system known as BARCIS [1] (BARC Channel inspection system). During reactor shutdown condition there is maximum temperature of around 50 degree centigrade, differential pressure between inlet and outlet end of channel is around 5Kgf/cm 2 and radiation dose rate is around 1MRads/hr. All the components and NDT sensors used in sensor module must be able to withstand this high level of radiation, pressure and temperature for about 120 hours for inspection of at least 10 coolant channels without any degradation of sensitivity and resolution of the sensors. BARCIS system provides linear and rotary movement to sensor module for scanning the channel. Sensors Used in In-service Inspection During In-service inspection of coolant channels of Indian PHWR s different parameters are monitored to assess the condition of channel. These parameters help Regulating Authorities to take appropriate decision to grant permission for further operation of reactor. The parameters which are observed during ISI are as follows

Detection of location of Garter spring spacer ring using eddy current technique Detection of annular gap between pressure tube and calandria tube using eddy current technique Detection of flaws in

3 Detection of location of Garter spring spacer ring using eddy current technique Detection of annular gap between pressure tube and calandria tube using eddy current technique Detection of flaws in pressure tube using ultrasonic technique in both axial and circumferential directions. Detection of wall thickness of pressure tube using ultrasonic technique Measurement of internal diameter of pressure tube using ultrasonic technique The ultrasonic and eddy current sensors are fitted in one sensor module with its mechanical and electrical signal cable arrangements and this is called BARCIS Inspection Head [2]. In one BARCIS Inspection head there are total 6 ultrasonic probes and two eddy current probes. Complete assembly of BARCIS Inspection head is shown in figure 2. A brief specification of Ultrasonic and Eddy current sensors is given in table 1 & 2 respectively Figure 2 : BARCIS Inspection head Table 1: Specifications of Ultrasonic Sensors Sr No Description UT Probe For flaw detection UT Probe For Wall thickness measurement UT Probe For Diameter measurement 1 Probe Type Line Focussed for immersion testing Spot focused for immersion testing Spot focused for immersion testing 2 Crystal Diameter 10mm 6 mm 10 mm 3 Peak Frequency 10 MHz ±10% 10 MHz ±10% 10 MHz ±10% 4 Focal Length 30mm±10% 30 mm±10% 30 mm±10% 5 Operating Environment Temperature Pressure

4 6 Mechanical strength 7 Radiation resistance Drop test from a height of 24" along with mounting fixture Must be able to withstand a cumulative dose of 10 8 Rads at dose rate of 10 6 Rads/hr Drop test from a height of 24" along with mounting fixture Must be able to withstand a cumulative dose of 10 8 Rads at dose rate of 10 6 Rads/hr Drop test from a height of 24" along with mounting fixture Must be able to withstand a cumulative dose of 10 8 Rads at dose rate of 10 6 Rads/hr Table 2: Specification of Eddy Current Sensors Sr No Description PT-CT Gap Measuring Probe Garter Spring Detection Probe 1 Probe Send and Differencial Receive Differential Probe Pancake Probe 2 Operating 2.5 KHz 9 KHz Frequency 3 Probe type Surface probe Bobbin probe 4 Probe Diameter 40 mm 80 mm 5 Operating Environment Temperature Pressure 6 Mechanical strength 7 Radiation resistance Drop test from a height of 24" along with mounting fixture Must be able to withstand a cumulative dose of 10 8 Rads at dose rate of 10 6 Rads/hr Drop test from a height of 24" along with mounting fixture Must be able to withstand a cumulative dose of 10 8 Rads at dose rate of 10 6 Rads/hr Qualification of Sensors All the NDT sensors used for In-Service Inspection have to pass different tests like pressure test, radiation resistance test, dimensional test, electrical continuity test and functionality test. All these tests qualify the sensors and inspection head for use in in-service inspection.

i. Dimensional test: Inspection head assembly along with special sealing plug is loaded in to fuelling machine magazine. The fuelling machine loads this assembly in the channel.

5 i. Dimensional test: Inspection head assembly along with special sealing plug is loaded in to fuelling machine magazine. The fuelling machine loads this assembly in the channel. Since the size of fuelling machine magazine is fixed and there are very little tolerances, the length of complete inspection head assembly should be strictly within the prescribed limits. The desired performance of the inspection head assembly depends on the accurate location and fixing of the sensors in the assembly. This is achieved by maintaining tight tolerances on the mechanical dimensions of all the sensors. The Inspection head assembly is designed as modular in construction and in these modules the housing of the all the sensors are accurately machined. In dimensional test, dimension of each and every mechanical component and complete assembly is measured and those components which do not meet the criteria were rejected and not used further. ii. Pressure test: Inside coolant channel, the coolant flows from one end to other and there is differential pressure of around 5Kgf/cm 2. NDT sensors used for In-service inspection must be able to withstand this pressure. All the ultrasonic and eddy current sensors are potted with special epoxy resin to make them leak tight and withstand high pressure. In order to qualify the sensors and sensor module to work in high pressure, the complete inspection head assembly is tested for pressure in pressure test setup. The complete inspection head assembly is kept in pressure test setup for 2-3hrs and mechanical failure of the sensors and the watertight potting is checked. After this pressure test, the working of all the sensors used are checked for desired sensitivity and resolution. This pressure test of BARCIS Inspection head is carried out before and after final calibration of NDT sensors to ensure its satisfactory and successful performance during actual ISI. The Pressure test setup is shown in figure 3. Figure 3: Pressure test setup iii. Radiation Resistance Test: In-service inspection is carried out in shutdown condition but even in shutdown condition the radiation level is very high. Radiation level varies inside reactor and maximum radiation is observed at centre. The dose rate inside channel can go up to level of 1MRads/hr. At such high radiation level many components fail due to radiation therefore NDT sensors used for ISI must be able to withstand high level of radiation. In order to qualify the NDT sensors to work in high level of radiation, all the sensors and components used in

Figure 4 shows gamma irradiation facility having volume of 5000 CC.

6 inspection head assembly has to qualify for radiation. A dedicated high energy gamma irradiation facility of Radiation technology development division, BARC was used for testing different NDT sensors and components used in assembly of inspection head. Figure 4 shows gamma irradiation facility having volume of 5000 CC. The components of inspection head like resin potting material, ndt sensors, nylon material, wires and connectors used were kept in chamber at dose rate of 1MRads/hr for 100 hrs and after irradiation their performance was checked. Only those materials which qualify radiation resistance test were used in inspection head. Lot of tests were carried out using different materials to findout suitable material which can with stand high level of radiation. Figure 4: High Energy Gamma Irradiation Test Facility iv. Electrical Continuity test : All the NDT sensors are assembled and fitted in inspection head. Since inspection head operates at high pressure, it is necessary to make leak tight electrical joint. To achieve this, a potted joint is made between wires coming from sensors and wires coming from connector. At each and every stage of fabrication of potted joint electrical continuity is tested because any break in electrical continuity leads to failure of complete assembly. v. Functionality test : All the NDT sensors fitted in inspection head were tested for their proper functioning at different stages of fabrication of Inspection Head. Eddy current sensors are tested just after assembly, then after potting and then after assembly in inspection head and finally after pressure test. Similarly ultrasonic sensors are tested individually and after assembly in inspection head and finally after pressure test. Before dispatch of sensor head to plant site all the sensors are calibrated. Ultrasonic sensors used for flaw detection are calibrated by picking up notches of known size with full length of co-axial cable (Approximately 100m long). Ultrasonic sensors used for diameter measurement and wall thickness measurement are calibrated by measuring diameter and wall thickness at known points in tube. Eddy current based garter spring detection probe is calibrated by picking up the signal of garter spring and similarly gap measuring probe is calibrated by using fixture which has arrangement for variation of gap between pressure tube and calandria tube. Thus all the NDT Probes are calibrated after performing all the required tests like pressure test, continuity test, dimensional test etc. Again

7 after final calibration, pressure test(hydro test) is conducted at a miminum pressure of 5Kgf/cm 2 before putting into reactor channel for actual In-Service Inspection. Conclusion The operating condition inside reactor pressure tube is very severe and it was big challenge to develop sensor head which can work satisfactorily. The state of art design of Inspection Head has been carried out at Division of Remote Handling and Robotics, Bhabha Atomic Research Centre. The system has successfully worked at all the reactor units in the country and till now more than 1500 channels were inspected using this type of inspection heads. Acknowledgement The authors are thankful to Shri Y. K. Bhardwaj and Shri Sunil Shejwal, Radiation Technology development Division, BARC for their support in carrying out radiation testing of sensors and other components of inspection head. References [1] R. K. Puri, Manjit Singh, "BARCIS (BARC Channel Inspection System)", International Conference on Peaceful Uses of Atomic Energy-2009 held at New Delhi, (2009). [2] Amit Kumar Haruray, R.D. Veerapur,R.K. Puri, Manjit Singh, "Inspection Head Design for In- Service Inspection of Fuel Channels of PHWRs", International Congress On Advances in Nuclear Power Plants ICAPP06 at Reno, Nevada, USA (2006). [3] R. K. Puri, Manjit Singh, "In-service Inspection System for Coolant Channesl of Indian PHWRs Evolution & Experience", 7th International CANDU Maintenance Conference, Toronto, Canada (2005)

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