DEFECT CHARACTERISATION ON ZIRCONIUM ALLOYS EMPLOYING MULTI-NDE

ABSTRACT:

 Non-destructive Evaluation(NDE) is a tool widely used in research purpose and industry for evaluating the properties of material, component or system’s fitness for use, material soundness, and in-service inspection. The primary advantage of NDE is material evaluation &testing without causing damage. Non- destructive evaluation is one part of the function of quality control and is complementary to other long established methods. By definition NDT is the testing of materials, for surface or internal flaws or metallurgical conditions, without interfering in any way with the integrity of the material or its suitability for service. This project was aimed to characterize the defect present in Zirconium alloy components employing multi-NDE tools for ascertaining their root cause.

Keywords: .Zirconium Alloy. NDE

1.  INTRODUCTION:

1.1. Zirconium Alloy:

 Zirconium is a commercially available refractory metal with excellent corrosion resistance, good mechanical properties, very low thermal neutron cross section, and can be manufactured using standard fabrication techniques. Zirconium alloys have superior thermal properties compared to other traditional materials in consideration for spent nuclear fuel containers. Zirconium alloys have a thermal conductivity more than 30% higher than stainless steel alloys. The linear coefficient of thermal expansion for Zirconium alloys is nearly one-third the value for stainless steel giving zirconium alloys superior dimensional stability at elevated temperatures. This is an advantage in nuclear waste containers where temperatures could exceed 200ºC for hundreds of years.

 1.2. Zirconium Alloy Properties:

  Zirconium resists corrosive attack in most organic and mineral acids, strong alkalis, and some molten salts. Solutions of nitric acid (HNO3 ), sulfuric acid (H2 SO4 ), and hydrochloric acid (HCl) with impurities of ferric, cupric and nitrate ions generally result in corrosion rates of less than 0.13 mm/a (5 mpy) even at temperatures well above the boiling point curve. A tightly adherent and protective oxide film protects the metal-oxide interface to provide corrosion resistance. An additional benefit for zirconium alloys in long-term geological disposal options is the inert nature of zirconium oxide^[1]. Application of zirconium alloys alleviates the concern of nickel and chromium contamination in the ground water in severely corroded spent fuel containers^[2]. Zirconium alloys produced by ATI are available in a wide variety of sizes and shapes including plate, strip, sheet, foil, tubular products, rod, and wire. Wrought products are typically supplied in an annealed and conditioned form.

Table no: 1: THERMAL NEUTRON CROSS SECTIONS (BARNS)

Non destructive testing:

 NDE stands for non-destructive evaluation. In other words it is a way of testing without destroying. NDE can be used to ensure the quality right from raw material stage through fabrication and processing to pre-service and in-service inspection.

 In today’s world where new materials are being developed, older materials and bonding methods are being subjected to higher pressures and loads^[4], NDE ensures that materials can continue to operate to their highest capacity with the assurance that they will not fail within predetermined time limits.

 Apart from ensuring the structural integrity, quality and reliability of components and plants, today NDE finds extensive applications for condition monitoring, residual life assessment, energy audit, etc.

We have conducted NDE techniques mainly referring to these characteristics

• Material Types
• Defect Type
• Defect Size
• Defect Location (Refer to TABLE NO:2)

TABLE NO:2 COMMON NDE METHODS:

2.Characterization of  zirconium alloy:

In our work we carried out tests on 4 types of specimen  INGOT, BILLET, BAR, WELD PART

2.1. Characterization Of Zirconium Alloy  Ingot:

 Triple melted ingots having a diameter of 350mm are machined to around 337mm. Visual examination is carried out on the machined ingot surface for defects such as porosities, blowholes, cavities, etc and the ingots, which are free from all these defects, are accepted for further processing. Before further processing these materials are examined by Ultrasonic Testing for ensuring soundness. During regular processing, one ingot shown significant indication in Ultrasonic Testing causing rejection. This ingot was sectioned and analyzed by NDE methods

Ultrasonic testing parameters:

Material: Zirconium alloy

Unit: Olympus

Probe frequency: 2 MHz, + 0.5″Ø

       Result:-

On conducting ultrasonic testing for an ingot, we have observed a surface echo, defect echo and a back wall echo.

(refer to fig 1 (II.A))

Fig.1  Defect echo in ingot

2.2.Characterization Of Zirconium Alloy Billet:

 Hot extruded and machined Zirconium alloy billet stocks are being processed further for fabrication of precision tube and components. Before further processing these materials are examined by Ultrasonic Testing for ensuring soundness. During regular processing, one billet shown significant indication in Ultrasonic Testing causing rejection. This billet was sectioned and analyzed further employing Ultrasonic Testing, Radiography and Metallography.

 Ultrasonic Testing:

       Parameters:-