ZIRCALOY WELDING IN OPAL REACTOR REFLECTOR VESSEL

Transcription

1 ZIRCALOY WELDING IN OPAL REACTOR REFLECTOR VESSEL Ortiz L. and Martínez R. INVAP SE, S. C. de Bariloche, Río Negro, Argentina Abstract This paper describes the development of the Zircaloy 4 welding processes applied in the fabrication of the Reflector Vessel for the OPAL reactor. As an introduction, a brief description is made of the geometric design of the Reflector Vessel, which is very complex, as it has many irradiation positions, beams tubes, channels, outer connections, and reinforcements. The complexity of this component demanded different welded joints and a careful manufacturing sequence, which comprised welding, machining, heat treatments, and quality control operations. Design and fabrication of different ad-hoc devices was also required, some of which are described in the present paper. For the purposes of issuing reliable welding procedures, the joints were previously set up through tests using samples and mock-ups. Procedures and welders were qualified in accordance with the ASME Code, Section IX. The welding methods used in the fabrication were GTAW (Gas Tungsten Arc Welding) and PAW (Plasma Arc Welding), processes deemed adequate to weld these alloys, as they do not contaminate the material, for they are carried out within an inert gas atmosphere. Contrary to other welding methods commonly used for other materials, no welding fluxes or chemically-active gases are applied in these two processes. The advantages of both methods used are discussed. The welds were carried out in manual, orbital, or mechanized mode depending on the type of joint. 1. Introduction 1.1 Reflector Vessel description The Replacement Research Reactor known as OPAL (Open Pool Australian Light Water Reactor) [1] features an open-pool design with a core immersed in a pool filled with demineralized water. Surrounding the core, the Reflector Vessel (RV) contains heavy water for the purpose of reflecting neutrons to maintain the nuclear reaction. The RV (see Figure 1) basically comprises a cylindrical shell plate known as middle shell plate welded to a top plate and a bottom plate. A central channel of square section, runs through the middle of the vessel, joining both, top and bottom plates. This channel is denominated chimney, and it has finned walls in order to increase its stiffness. The reactor core is placed inside the chimney, and comprises 16 low-enrichment uranium silicide fuel elements. The irradiation positions are vertically arranged inside the vessel, welded to the top plate and, in most cases, also welded to the bottom plate. Two annular flanges are welded to the vessel top plate to bind the outer housings for a cold and a hot neutron sources. There is also a central flange with squared shape to connect the chimney to the primary cooling circuit. Five beam tubes are perimetrically arranged, mounted on the middle shell plate by means of welded flanges. The heavy water is distributed and collected inside the RV by means of two circular channels internally mounted on the middle shell plate. Apart from the middle shell plate, the vessel has a lower shell plate that is part of the irradiation positions cooling circuit plenum, and an upper shell or skirt welded to the top plate. The stiffness of the middle shell plate is increased by means of a welded structure made up of rectangular-section bars. Many welded anchors are distributed on the top cover to bind different accessories and instruments.

2 There are several lateral connections on the middle and bottom shell plates for the heavy and demineralised water circuit pipelines. Top plate Chimney Flanges Anchor Irradiation positions Upper skirt Circular channel Middle Shell Stiffener Beam tube Lower shell Bottom plate 1.2 Materials Figure 1- Reflector Vessel- Cross sectional view In light of its low neutron cross section, Zircaloy 4 was the material selected to construct most of the parts of the RV, namely: middle shell plate, top and bottom plates, chimney, irradiation position housings, and beam tubes. 2. Reflector Vessel welding The selected welding processes for the RV fabrication were Gas Tungsten Arc Welding (GTAW) and Plasma Arc Welding (PAW), deemed appropriate to weld zirconium alloys as they are implemented within an inert gas protection atmosphere, and thus prevent the contamination of the material [3]. Zirconium alloys have high affinity with oxygen, hydrogen, nitrogen, and carbon at the temperatures produced during the welding operation. These contaminants deeply affect ductility and corrosion resistance. At a high temperature, the zirconium absorbs large amounts of hydrogen (up to some 50% at temperatures above 780 K). This solubility drops rapidly with temperature, while the surplus hydrogen precipitates as hydride phase, and may lead to serious embrittlement at room temperature. There are two key factors to avoid weld contamination: ensuring the cleanliness of the parts prior and during welding, and preventing the weld bead and adjacent areas from taking contact with the air during and after welding while these areas are at high temperature (T> 280 C). In the fabrication of the RV, before any welding operation the parts were thoroughly cleaned following written specifications in facilities conditioned for such purpose. Once clean, the parts remained packed in polyethylene bags until welded, and were always handled using cotton gloves.

3 The welding operations took place within a restricted access working area, under strict cleanliness conditions, and without the presence of welding-contaminant elements. The staff inside the working area wore clean garments and shoe covers. To prevent contact with air, a shielding flow of argon was supplied on the material surface while it was at high temperature. This was accomplished by using torches provided with large cups, and gas lens, see Figure 2. Figure 2 Torch with large cup and trailing shield Furthermore, the welding process involved the application of additional shielding gas on the root area, and on the adjacent area behind the torch by means of a trailing shield device, see Figure 2 and Figure 3. Diffuser plates were used to prevent turbulence effects on the shielding gas blanket. These diffuser plates were incorporated to the backing and trailing shields. Torch Trailing Shield Weld Bead Argon Lines Part Diffuser Backing Shield Figure 3 Typical gas shielding arrangement Inert gases with certified quality was used in all cases. Argon was used in most cases as shielding gas except for the plasma torch, where helium was used to yield higher weld penetration effect. To achieve the required quality levels welders and welding procedures were qualified in accordance with the ASME Code, Section IX [2]. For this purpose, welded specimens were produced and further submitted to tensile and bending tests. In light of the large number of welded joints present in the RV, it became necessary to carry out many prior experiences to set up the welding devices and equipments. Moreover, it was necessary to use gas shielding and heat dissipation devices specifically designed for each assembly configuration. Parameters were defined and written welding procedure specifications were issued for each joint. Post weld stress relieving heat treatments were applied in many cases at different temperatures and times depending on the thickness of the parts being treated.

4 The GTAW process was applied in manual or mechanized mode, the torch movement in the latter mode being at constant speed by means of a motorized mechanism. The PAW process was applied in mechanized mode, also incorporating a trailing shield. In the next paragraphs we give some fabrication examples where GTAW and PAW processes were applied to join Zircaloy 4 parts. 2.1 GTAW welding of chimney fins in manual mode a) b) Figure 4 a) Fin fixturing, b) Finished wall Manual welding was applied on most of the RV joints, as it is the most versatile mode and may be applied where the geometrical design or the accessibility to the joints make it impossible or rather unprofitable to set up mechanized or automated welding methods. In this case, the fins of 3 mm thickness were welded to the external side of the chimney walls, applying the GTAW welding process in manual mode (See Figure 4). The joint design applied was that of butt weld without bevel, welded on both sides. Intermittent weld beads were applied to reduce wall distortion. A welding fixture as shown in Figure 4a, was used to hold the fins into place with respect to the wall. This welding fixture has been outfitted with a purge-gas backup bar and a watercooling circuit. Additional shielding gas was supplied to the weld bead by means of a trailing shield comprising a gas lens placed behind the torch during welding. 2.2 Vessel closure welding in mechanized mode with the use of a manipulator This system was generally applied in those cases where a considerable amount of filler metal deposition was required in several passes, either on straight or circular seams. The advantages of mechanized welding over the manual mode are well known and include the production of seams of better weld appearance, high quality consistency, and with less working time requirement. The longitudinal seams closing the RV s middle shell plate of 9 mm thickness were achieved through this method. The circular welds joining the middle shell plate to the top and bottom plates were also carried out using the manipulator. The torch was bound to the head of a manipulator of 3 metres height, which allowed to position and apply the forward movement of the torch at constant speed with respect to the part, see Figure 5. A water-cooled torch for machine was used. The trailing shield was fixed to the torch by means of an elastic clamp. The manipulator head had the necessary mechanisms for the torch oscillation movements and filler wire feeding. Root shielding was achieved by means of a copper bar with groove, into which inert gas was fed through a diffuser plate to obtain a laminar flow of back shielding gas. The bar was water-cooled to dissipate the heat during the process.

5 a) b) Figure 5 a) Welding manipulator, b) Manipulator welding head 2.3 Welding of the irradiation position housings to the plates by the orbital GTAW method The typical geometry of an irradiation position to plate weld is shown in Figure 6a. The main problem of these joints was given by the large mass difference between the parts to weld. The tubes are 2,5 mm thick, while the plate is 25-mm thick. It was necessary to select a welding method that allowed accurate control of the heat input, as an excess of energy could pierce the tubes, while insufficient energy would yield incomplete fusion at the weld root. a) b) Figure 6 a) Tube to plate typical weld joint, b) Orbital welding head The process selected was the GTAW process in orbital mode, a variation of the mechanized method. In this case, the weld torch is bound to a head (see Figure 6b) that turns around the axis of the joint to complete the joint while the parts remain stationary. The weld head movement is achieved by means of an electronically-controlled mechanism. The orbital equipment used has a programmable power source controlled by means of a microprocessor that allowed the setting and accurate regulation of all the electric and movement parameters during the process. Through the incorporated function known as AVC (Automatic Voltage Control), the distance between the electrode and the part is controlled and constantly corrected during the process. This variable is of key significance to obtain a weld bead of even penetration and aspect. On the other hand, the electric arc was applied in pulsed mode, through which it was possible to obtain the weld penetration required, with minimum heat input. 2.4 Chimney Welding through the PAW process in keyhole mode The PAW process in keyhole mode was chosen for the 9-mm-thick longitudinal joints between the chimney walls, as it features greater energy concentration than in the case of the GTAW process. This characteristic of the PAW process allows welding at higher travel speeds than in GTAW, producing narrower beads, which result in welded parts with less distortion. Another advantage of this technique is that it ensures complete penetration without the need of using a bevelled joint design.

6 In the PAW keyhole technique, the plasma jet passes through the entire thickness of the joint. The molten metal is rejected towards the rear, where it solidifies and forms the weld bead. The chimney design required the observance of stringent fabrication geometric tolerances, which proved to be difficult to achieve when applying the GTAW process. In weld tests held on mock-ups, it was noticed that the distortions of the parts inherent to the GTAW process were incompatible with the required tolerances. By using this process (see Figure 7) it was possible to achieve the welds, within the required tolerance limits, through the application of a single pass. The joint design used was that of corner joint with square groove. No filler metal was applied. During the process, the walls were firmly clamped to a rigid fixturing device. a) b) Figure 7 a) Welding fixture b) Plasma welding head 3. Conclusions The experience gathered from the zirconium alloys welding for the construction of the RV yields the following conclusions: High quality Zircaloy 4 welds were produced in the RV fabrication. To accomplish this, the following is considered essential: a) To apply a proper cleaning procedure to the parts prior welding; b) To perform a careful design, construction, and set-up of the gas shielding devices to be used during the welding process. In every case, the gas shielding systems designed proved to be efficient in preventing weld contamination. Manual GTAW welding was applied in those cases where the application in mechanized mode was not viable. Welds were carried out by experienced welders that had been qualified in accordance with the fabrication code. Hence, all quality standards were observed. Mechanized GTAW welding was applied wherever possible, yielding even weld beads of excellent appearance and quality that are typical of this method. The welding of plate to tubes with different thicknesses was solved through the orbital welding method, yielding joints of repetitive quality. The PAW method with the keyhole technique was successfully applied to weld chimney walls without excessive distortion. 4. References [1] Miller R., Abbate P. M., Australia s New High Performance Research Reactor, IGORR 9 Proceedings, 9th Meeting of the International Group on Research Reactors March 2003, SYDNEY, AUSTRALIA [2] ASME Boiler and Pressure Vessel Code, Section IX, Welding and Brazing Qualifications [3] AWS, Welding Handbook, 8 Th Ed., Vol. 4. pp Miami, Fla.: American Welding Society, 1998