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Innovative NDT techniques for life assessment of coolant channels of Indian PHWRs

B.B. Rupani, Kundan Kumar and J.N. Kayal

Bhabha Atomic Research Centre, Trombay, Mumbai-400 085 e-mail: rupani@barc.gov.in,kundan@barc.gov.in,jnkayal@barc.gov.in

ABSTRACT Coolant Channel is considered as a heart of primary heat transport system of pressure tube type reactors. A Coolant Channel Assembly, consisting of pressure tube, end fittings, calandria tube and associated supporting hardware, is subjected to severe operating conditions i.e high temperature, high pressure and high radiation environment. The most effected component of a coolant channel assembly is a pressure tube, made of zirconium alloy. These conditions lead to a number of degradation mechanisms in a pressure tube. The degradation mechanisms, such as irradiation creep and growth, corrosion & hydriding, and irradiation hardening cause changes of physical dimensions, hydrogen pick and change in the material properties of a pressure tube. To some extent, margins for accommodating dimensional changes are provided during design stage. Operating experience has shown that effective monitoring of the degradation mechanisms of major reactor components is essential for plant safety and also for plant life. Therefore, ageing of the reactor components must be effectively managed to ensure the availability of design functions throughout the plant service life. Timely monitoring and management of these degradations in the components are requirement for safe operation of any plant. But the monitoring health level of such components may not be always easy as it may require development of methodology suitable to them. These methodologies preferably should be of NDT type and remotely operable and should have features of in-situ application. NDT techniques are required to preserve its residual service life without affecting the integrity of the component.

In order to monitor the changes taking place in the pressure tubes, a number of innovative NDT techniques have been developed at Reactor Engineering Division, BARC. These techniques are in-situ and remotely operable and have been used at various reactor sites for the assessment of safety of the pressure tubes and calandria tubes. The techniques have been used for visual inspection, dimensional inspection, hydrogen estimation, and measurement of mechanical properties of the pressure tubes.

The experience of development of innovative tools for PHWR coolant channels is being extended for development of NDT tools for coolant channels of Advanced Heavy Water Reactor (AHWR). These tools will be used for life management and pre-service inspection of coolant channels of AHWR. This paper gives schematic detail of evolution of all these techniques and their qualification in PHWRs.

Keywords:

NDT, AHWR, PHWR, coolant channel, pressure tube, calandria tube, garter spring, sliver sampling, visual inspection, degradation mechanism, ageing management

1.0 INTRODUCTION Coolant channel assemblies are main components of PHWRs. Each coolant channel primarily consists of a pressure tube (PT), a calandria tube (CT) and garter spring (GS) spacers. Fig. 1 shows a typical coolant channel of Indian PHWR. Pressure tube en-houses fuel bundles and guides the coolant flow through it to carry forward the heat generated due to fission reaction. The coolant temperature, at inlet to the pressure tube, is about 250ºC and at outlet it is about 300ºC. Due to operational requirement, the moderator temperature has to be maintained at 65ºC. The calandria tube helps in keeping the colder moderator separate from the hotter pressure tube. Garter spring spacers provide supports between pressure tube and calandria tube for maintaining the radial gap between the two. A proper location of garter spring spacer plays a vital role in keeping pressure tube separate from calandria tube.

Due to severe operating conditions of high temperature, high pressure & high radiation field, coolant channels assemblies, particularly pressure tubes, are subjected to the following degradation mechanisms:

Fig. 1 : Schematic arrangement of Coolant Channel of PHWR

• Wear and fretting phenomena, loss of surface integrity • Displacement of loose Garter Spring from its design location leading to change in PT-CT gap • Corrosion and Hydriding, i.e. picking up of hydrogen by pressure tube material • Irradiation creep & growth cause pressure tubes to elongate, diametrically expand and lateral sag. • Irradiation effect leads to the change in material properties

The design and development of NDT tools & techniques for monitoring the above degradation mechanisms and its implementation at reactor sites have progressed hand-in-hand with the operation and inspection experience. The solutions have been delivered in time to enable taking well-considered and conservative measures in time, rather than adhoc actions. The evolution process of NDT tools and techniques, have been described in brief further, individually for each degradation mechanism. Above-mentioned techniques are remotely operable and in-situ non-destructive techniques.

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2.0 WEAR AND FRETTING PHENOMENA The inner diameter of pressure tubes are subjected to wear and fretting phenomenon due to fuel loading, unloading and in presence of some debris. Dry Channel Visual Inspection System (DRYVIS) was developed to visually inspect and grab visual images of inner diameters of pressure tube and calandria tube. The system gives the surface condition of the pressure tube and calandria tube one inner diameter side. In addition it can give details of garter spring marks, contact spot, presence of muck or any foreign material and other abnormality within the tubes. The DRYVIS system, Fig. 2, comprises a radiation resistance video camera, a separate illumination head, a grating for sizing of indications and a walker. The walker is pneumatically operated array of cylinders, which can be remotely positioned at a desired location, within the pressure tube or calandria tube. DRYVIS was used to carry out visual inspection within drained and de-fuelled pressure tubes of Rajasthan-2 and calandria tube of Narora-1. The visual

inspection of pressure tube and calandria tube would help in detection of surface flaws which can cause a site for crack initiation and cold contact spots in case of displacement of garter springs.

Fig. 2: DRYVIS Inspection Head

3.0 DISPLACEMENT OF LOOSE GARTER SPRING FROM ITS DESIGN LOCATION 3.1 Garter Spring Detection System Early generation of Indian PHWRs were having only two loosely fit garter spring spacers in each coolant channel. Garter Spring are located in the annulus of PT and CT, as shown in Fig. 3. A typical Garter spring is shown in Fig. 4. These loose fit garter spring spacers had tendency to get displaced from their installed locations, during various stages of commissioning of the reactor, causing a pressure tube-

Garter Spring Calandria TubePressure Tube

Fig. 3 : Schematic Arrangement of Garter spring in the PT-CT annulus

Fig. 4 : Garter spring

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calandria tube contact. The contact location provide site for various degradation mechanisms to initiate and finally cause failure of pressure tube, if left unattended. Therefore techniques were required to identify such problems and to do preventive maintenance to avoid such phenomena to occur. One such technique was developed to detect and move the garter spring within the coolant channel . This technique helped to ensure the safety of the coolant channel by avoiding pressure tube and caldaria tube contact. The Garter Spring detection system, Fig. 5, was developed for detecting the axial location of the garter spring, in-situ, within a coolant channel. The principle of the detection system is based on eddy current technique. The system consists of a Garter Spring Detection Probe (GSDP), Fig. 6, signal-conditioning unit, and data acquisition system and carrier mechanism. The probe consists of two sets of primary and secondary circumferential coils embedded in a nylon former. The garter spring detection system is having good repeatability, operable from 100 metres cable distance, Dynamic detection capability, working cablity at 100ºC, Software based surge/ noise removal capability etc It has Capability to detect and distinguish between garter springs in presence of common irregularities viz. tube wall thickness variation, circumferential fuel marks etc., which give synonymous signal to garter spring signals

MOTOR

CARRIER MECHANISM

ENCODER DAS ADD

-ON CARDS

INDUSTRIAL COMPUTER

SIGNAL CONDITIONING

UNIT

GSDP SIGNAL ON PC

MONITOR

VARAIBLE FREQUENCY

POWER SUPPLY

GSDP

DISTANCE

AM

P

TOOL EXTENSION TUBE

Fig. 5: Schematic arrangement of the Garter Spring Detection System

GARTER SPRING

COOLANT CHANNEL ASSEMBLY

Fig. 6 : Garter Spring Detection Probe

3.2 PRESSURE TUBE-CALANDRIA TUBE CONCENTRICITY DETECTION SYSTEM

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Amount of the gap between PT and CT is required for estimation of creep contact time. Concentricity Detection System was developed to estimate the eccentricity between pressure tube and calandria tube, which is based on eddy current phase mode detection principle. The Concentricity Detection Probe Fig. 7,

consist of two eddy current sensors embedded in the former. The Concentricity Detection Probe consists of two probes, which are located at 3’O clock and 12’O clock positions on the phenolic load bearing. The sensor at 3’O clock position is used as reference sensor. The sensor in 12’O clock position is used as the measurement sensor. These sensors are configured in differential mode for compensation of the thickness variation of pressure tube and temperature variation of the coils. The Concentricity Detection Probe is designed so that they can be embedded in 10mm thick phenolic load bearing and it consists of C-Shaped mu-metal former wound with primary and secondary copper windings. The high permeability mu-metal former based Concentricity Detection Probe, which efficiently couples flux linkages to calandria tube provides high sensitivity. The schematic arrangement of the Concentricity Detection Probe is shown in Fig. 8.

Fig. 9: Data Acquisition System

SecondaryWinding

Mumetal formerL32xW8xT1.5mmSize

Primary Winding

OUTER METALICTUBEPhenolic

bearing

SENSOR AT 12’O’ CLOCKPOSITION

INNERMETALLICTUBE

REFERENCESENSOR AT 3’ O’CLOCKPOSITION

METALLICTOOL

Fig. 8 Schematic Arrangement of the transducer

Fig. 7: Concentricity Detection Probe

The output signal from the transducer, after processing through the signal conditioning unit and PC based data acquisition system, directly gives the pressure tube-calandria tube eccentricity value on computer screen, Fig. 9.

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4.0 CORROSION AND HYDRIDING 4.1 Sliver Sample Scraping Technique Hydrogen pick-up, is another significant phenomenon, which is caused due to corrosion. Hydrogen, normally present in the pressure tube as impurity, is not appreciably soluble below 100ºC and precipitates as Zirconium hydrides, in platelet form. These hydrides reduce the ductility of the pressure tube. Due to zirconium and heavy water reaction, additional deuterium is generated. Part of the generated deuterium is picked up in the metal and hydrogen concentration goes on increasing to the extent that pressure tube may become brittle in nature. For safe reactor operation, information of hydrogen concentration in pressure tube is necessary.

A

R1

R2

Pressure Tube Dia

Oxide Scrape

DETAIL ’A’

Pressure Tube

Effective Oxide Width

Oxide Layer

Oxide Width

Metal Width

Taper 10° (Exaggerated) on both sideLength

Total Depth of Scrape

Oxide Thickness

Fig. 10 : Scraped region contour in transverse & longitudinal directions

Sliver sample scraping technique was developed for obtaining a very small amount of metal sample from the bore of a pressure tube and retrieve it securely. In the process, a sliver sample of 0.1mm thickness and 90 mg average weight is obtained in such a way that the scraped region, Fig. 12, i.e. contour created inside the bore of the pressure tube, does not lead to loss of remaining service life of the pressure tube. 4.1.1 AXIAL SCRAPING TECHNIQUE Fig. 11 shows the present version of axial scraping tool, which is being used extensively in Indian PHWRs for obtaining sliver samples from pressure tubes. The tool consists of two tool-bit holders housed within a common cartridge. For scraping operation the cartridge is pushed forward to engage the tool-bits with the

bore of the pressure tube. The scraping operation is done in two tandem steps. In first step, outside hydrogen enriched oxide layer of the pressure tube, called oxide sample, is removed. In the second step, inner layer, called metal sample is removed for hydrogen measurement Fig. 14 shows oxide and metal

ge

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Fig. 11 : Pressure Tube Scraping Tool

Tool-bit Cartrid

Fig. 12 : Oxide and Metal Samples

samples obtained from a piece of pressure tube, obtained during mock-up trials. The scraped regions were qualified on the basis of various non destructive testing on the specimens 4.1.2 CIRCUMFERENTIAL SCRAPING TECHNIQUE

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he circumferential scraping tool complements the earlier discussed scraping tool as it can obtain sample

.2 Hydrogen Equivalent Assessment Technique (HEAT):

A new technique known as “Hydrogen Equivalent Assessment Technique (HEAT)” is being developed for

The principle of assessment of hydrogen is based on measurement of resistivity of material of pressure

Circumferential Scraping Tool, Fig. 13, has been developed for obtaining samples in the circumferential direction. It consists of a rotating cutting head, bearings and locking arrangement. During scraping operation the cutting head rotates by 360 and in the process scrape samples are obtained in the tool cavities.

Fig. 13: Circumferential Scraping Tool

Cutting Head

Tfrom the extreme ends of the pressure tube, which is not possible by the later. Further, it can obtain samples from a precise axial location of the tube, nearly 7mm, whereas the other scraping tool obtains sample over 50mm length. Thus it can give hydrogen value at a precise axial location. The cross-section of scraped region is crescent shape. 4

in-situ estimation of hydrogen concentration of an operating pressure tubes. In this technique there is no need for removal of physical samples from the pressure tubes and hydrogen content in the pressure tube can be obtained by scanning method.

tube. Two factors contributing to the conductivity of the pressure tube are inherent resistivity property of the Zirconium alloy and the resistivity property exhibited by the hydrogen present in the alloy when it is in solid phase, which is function of the temperature. The maximum hydrogen in ppm can exist in the solution phase in Zirconium alloy at particular temperature is described by the Terminal Solubility Curve. The pattern of hydrogen in solid phase changing to liquid phase as the temperature increases can be exploited for the estimation of the hydrogen concentration in ppm present in the Zirconium alloy. The technique involves a Tool and various sub-systems. The schematic arrangement of the tool is shown in Fig. 14.

Fig. 14 Schematic of Hydrogen Equivalent Assessment Tool (HEAT)

5.0 IRRADIATION CREEP & GROWTH Irradiation creep & growth along with thermal creep cause increase in diameter and length of pressure tubes. Increase in ID causes coolant bypass and overheating of fuel bundles, with consequent localized boiling of coolant. This situation is undesirable and may call for de-rating of the reactor. Hence, inside diameter of pressure tube needs to be monitored periodically during its service life. A hydraulically operated three legged micrometer type inside diameter measuring instrument, named HYRIM, was developed for this purpose. It has a measurement accuracy of 0.05 mm and an inspection range is 82 to 86 mm.

Fig. 15: HYRIM inspection head partially inserted in pressure tube

Inspection Head Pressure Tube

HYRIM system consists of an inspection head, Fig. 15, and a control unit, Fig. 16, connected by a hydraulic hose. The inspection head goes into the pressure tube for measurement. The inspection head has three radial legs. These legs are made to move over conical piston with the help of hydraulic pressure. The control unit is actuated by a stepper motor fitted with an incremental encoder. Piston displacement and operating pressure are monitored during measurement. Important features of the system are:

• Operational principle similar to conventional three leg micrometer motion • Movement of radial legs is created by a conical piston instead of tapered screw for ease of

remotisation • Contact point is established from the inflexion point of the graph • In-situ calibration facility

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Fig. 16: Control Unit

Stepper Motor Controller

Pressure Conditioner

Motor with Encoder

Prototype version of HYRIM was designed for manual operation. Performance trial of this system was carried out successfully at coolant channel mockup facility at Kakrapar reactor site. First version was utilized at Rajasthan-2 for measuring the ID of 16 pressure tubes. Measurement accuracy and compatibility for operation in water-filled channel was established and an improved version of the system, having fully automated operational features, is being developed. 6.0 CHANGES IN MECHANICAL PROPERTIES Monitoring the change in mechanical properties of pressure tubes is very important for ensuring the safety of the reactor. Presently, for surveillance, pressure tube is removed from the reactor to measure its mechanical properties. The present version of IProMS is for use with Zr 2.5 wt% Nb pressure tubes of 220 MWe PHWRs. In order to avoid the difficulties and economic penalty involved in removal of pressure tubes, an In situ Property Measurement System (IProMS) based on ball-indentation is being designed and developed for monitoring the mechanical properties.

Measurement of mechanical properties is based on cyclic ball indentation technique. Ball indentation is a non-destructive technique, used for estimation of mechanical properties of structural materials. It is based on multiple indentation cycles at the same penetration location on a metallic surface by a spherical indenter. A minimum of eight cycles is used and each cycle consists of indentation, partial unload and reload sequences. An analysis of the load and indentation depth data recorded during the test gives an estimate of the yield strength, ultimate tensile strength (UTS), strain hardening exponent and Brinell hardness of the material. The schematic arrangement of the system is shown in Fig. 17.

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100 meters Cable & hose

IPROM Tool head with LVDT & Indenter

Feeding Mechanism

Pressure Tube

Fig. 17 Schematic arrangement IProMS for Pressure Tube 7.0 CONCLUSION The NDT techniques developed are custom designs, based on inspection requirements. The visual inspection system, garter spring detection & repositioning system and sliver sample scraping technique have been developed as a part of life management activity of coolant channels of Indian PHWRs. These techniques have been successfully deployed at various reactor sites. The sliver sample scraping technique is currently being used as a standard sampling tool during every annual shut down of plant. All these techniques can be extended for NDT examination of other components, such as pressure tubes of AHWR, as per the requirement and available margin, by slightly modifying the design of the system. ACKNOWLEDGMENTS

The authors are thankful to Shri R.K. Sinha, Director, Reactor Design & Development Group and Shri D. Saha, Head, RED for their valuable guidance. The authors are also thankful to their colleagues in Reactor Engineering Division for their support in developing and implementing the techniques in mock up and at reactor sites. The authors are also thankful to personnel from NPCIL head office and from various reactor sites for their active support in implementing the Scraping Tool at their sites.

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