PSI - Issue 14

Altaf Ali et al. / Procedia Structural Integrity 14 (2019) 273–281

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Author name / Structural Integrity Procedia 00 (2018) 000–000

1. Introduction For second stage nuclear power program, Prototype Fast Breeder Reactor (PFBR) is India’s flagship nuclear reactor that is coming up at Kalpakkam, Tamilnadu. PFBR is a sodium cooled pool type fast spectrum nuclear breeder reactor based on two enrichments of mixed oxide (MOX) fuel.

Nomenclature SNR

Signal to Noise Ratio Contrast to Noise Ratio Image Quality Indicator

CNR ሺ ሻ ୔Ǥ ሺο†ሻ ୔Ǥ ሺ ሻ ୵Ǥ ሺο†ሻ ୵Ǥ IQI

Signal to Noise Ratio in penetrameter zone Thickness of smallest visible IQI wire Signal to Noise Ratio in weld zone

Material loss depth along X-ray path in weld zone L up or (LOP%) unpenetrated height as percentage of tube wall thickness

PFBR core consists of various types of subassemblies (SAs) and radial Blanket SA (BSA) is one of them. BSA provides a radial blanket of Deeply Depleted UO 2 (DDUO 2 ) to the fuel region and produce more fissile material from fertile material. A BSA consists of 61 blanket pins arranged in a triangular pitch. Blanket pins (see Fig. 1 (a)) are manufactured by encapsulating DDUO 2 sintered pellets along with other minor components in a thin walled (0.6 mm) 20% cold worked D9 clad tube and is then TIG welded in helium environment with SS 316LN end plugs. It has been observed that the profile of the weld is not repeating i.e. there is considerable variation in weld fusion zone. Penetration and weld spread depend on many parameters like relative position of plug-clad interface and electrode tip along the tube axis, gap between end plug-clad interface and electrode, variation of other process parameters like current and voltage. As per the experience gained in quality control, a combination of clad and plug having loose fit generally leads to root pocket kind of defect. So, usually a tight fit end plug and clad combination is ensured for the weld. Axial location of the electrode precisely over the clad and end plug interface is also very important as it may otherwise lead to weld shift which in turn causes a lack of penetration (LOP) kind of defect (see Fig. 1 (d)). Cleanliness of the joint before weld also plays a big role as it may otherwise cause the greasy substances to vaporise and get trapped which in turn leads to porosity as the defect. Inclusion is also a probable defect that occurs. It can be of two types: (i) Tungsten inclusion that occurs due to blunting of the electrode tip, it gets melted and transferred to weld pool. Its chances are high specially in the helium environment because of its low conductivity and (ii) DDUO 2 powder particles left at the junction after pellet loading that gets included in weld pool. Some of the defects with good weld are shown in Fig. 1 below. BSAs will operate in a very harsh environment of fast reactor core having very high operating temperatures of about 550 0 C, high neutron flux and steep thermal gradients, Raj et al. (2015). It is well known that exposure to neutron flux of this order causes materials to swell, thereby reducing their lifetime in nuclear reactors and the irradiation process produces radioactive by products and fission gases. However, subassemblies are designed to normal operating conditions and other envisaged accidental conditions for smooth operation of the plant, Prasad et al. (1992). Since blanket pin forms the first barrier that prevent the release of radioactive products and gases that would have otherwise contaminated PFBR coolant circuit. So the integrity of the blanket pin end plug welds plays an important role for the reliable and economic operation of the reactor. Presently, 100% radiography of production lot end plug welds is carried out using double wall single image full compensating block tangential radiography technique (RT). As per the experience gained in PFBR first core, 70% of total rejections were due to LOP and remaining rejections were due to various other defects mentioned above. And LOP can reflect ultrasonic waves efficiently because of its planar nature, Choi and Yang (1991). Thus, ultrasonic testing (UT) poses to be a good option for it and can detect LOP with a good sensitivity. So, this study has been carried out to explore UT as an alternative/supporting technique for the present RT. Many studies have been carried out earlier also to replace RT with UT because of the main disadvantages of radiography towards potential hazard to health associated with the ionizing radiations which are the basis of the method, Goujon (2005). In U.S. nuclear regulatory