Study on the Step by Step Process and Performance of Laser Welding for the Spent Fuel Pool Floor

The spent fuel pool is constructed by means of welding with a steel structure module and steel liner, which is the key area for temporary spent fuel storage in the nuclear power plant. The pool is filled with borated water during operation. These steel liner welds are exposed to radiation, water pressure, and load-bearing environment for a long time. After long-term operation, they are prone to stress corrosion and pitting corrosion, which can lead to pool leakage accidents in serious cases [1].

Most of the nuclear power plants in operation or under construction in the world have only one spent fuel pool, and it is almost impossible to empty the spent fuel pool for maintenance. Laser welding has the characteristics of small heat input, narrow heat-affected zone, small welding residual stress and deformation, less welding spatter, and good weld formation [2]. Local dry underwater welding uses an inflatable draining device to create a local dry environment on the surface of the steel plate to be welded by filling high-pressure gas, which not only greatly improves the quality of underwater welding, but also saves complex welding devices. The application of underwater welding robots also improves the flexibility and engineering applicability of the devices [3]. It is a good choice to repair the steel liner of spent fuel pool by underwater local dry laser welding to renew the original appearance without affecting the storage of spent fuel grid on the floor.

A great deal of research has been done on underwater laser welding of duplex stainless steel in the world. Cui et al. [4] studied the microstructure of S32101 welded joints by scanning electron microscopy and electron backscatter diffraction, analyzed the influence of heat input on the coincidence lattice grain boundary and chromium nitride precipitation of weld metal, and tested the intergranular corrosion performance of weld metal by double cyclic polarization curve. Keskitalo et al. [5] studied the butt welding of S32101 with a 1.5 mm thin plate by using a 4 kW disk laser. It was found that increasing the heat input can soften the welded joint, and the bending performance of the welded joint can be improved by using nitrogen instead of argon as the shielding gas. The mechanical properties of YAG laser underwater welding of stainless steel were studied by Westinghouse Electric Company [6]. The results show that the tensile strength and bending properties of underwater welded joints are equivalent to those of the welds in air. Baghdadchi [7] promoted the transformation of ferrite to austenite in the duplex stainless steel weld by changing the shielding gas and adding laser reheating.

In order to develop a welding process applicable to underwater repair of the steel liner of the spent fuel pool and ensure that the welding quality and performance meet the requirements of engineering application, relevant welding process test researches step by step were carried out in this paper.

ASME SA240 S32101, the steel liner material of the spent fuel pool in the third-generation nuclear power plant, is used as the base metal to be welded in the laser welding process test. The AWS classification of the welding wire is ER2209, and the diameter is 1.2 mm. The compositions of base metal and welding wire are shown in Table 1.

The V-shaped welding groove is prefabricated on the S32101 duplex stainless steel plate. The groove size and weld bead distribution are shown in Fig. 1. Based on the three conditions of air environment, shallow water environment (laser head placed in 50 mm water depth), and simulating-repairing environment (equivalent to 15 m water depth), the laser welding process tests in flat welding position were carried out step by step. The laser welding test platform for the air and shallow water environment conditions is shown in Fig. 2. The welding test in the simulating-repairing environment is carried out in a high-pressure test chamber in Fig. 3. The laser welding actuator includes a mobile positioning mechanism, a watertight laser head, and a drainage hood, as shown in Fig. 4. The type of fiber laser for welding equipment is RFL-C6000, as shown in Fig. 5. The rated output power is 6000 W, and the wavelength is 1075∼1080 nm.

The whole set of research is based on the laser welding process test in air environment. After a large number of laser welding process tests, the weld forming quality improves under the conditions of laser power 5000 W, welding speed 480 mm/min, wire feeding speed 260 cm/min, and shielding gas flow 30 L/min, as shown in Fig. 6. Based on these process parameters, the experimental study of laser welding process in shallow water environment can be carried out.

According to the design and construction requirements of the spent fuel pool of the third generation nuclear power plant, the local dry laser welding process test in shallow water environment (laser head placed in 50 mm water depth) is carried out with reference to ASME Section IX, Section XI, and AWS D3.6 standards, as shown in Fig. 7.

In the underwater environment, increasing the flowrate of the shielding gas in the mobile gas hood can effectively improve the dryness of the welding area and thus improve the welding quality. In the process of underwater groove filling welding, due to the further increased difficulty of groove draining compared with cladding, the weld is prone to problems such as incomplete fusion and incomplete penetration. In order to improve the dryness of the surface to be welded by filling welding, a gas pipe is added at the groove position for synchronous blowing and draining, as shown in Fig. 8. After the structural optimization, the use of mobile gas hood combined with groove synchronous blowing and draining can obtain better weld forming quality in the shallow water environment, as shown in Fig. 9. The weld forming quality in underwater local dry laser wire welding with the welding parameters of laser power 5000 W, welding speed 480 mm/min and wire feeding speed 260 cm/min is good when the drainage gas flowrate is increased to 40 L/min–50 L/min.

At the same time, laser reheat treatment of laser power 2000 W and welding speed 480 mm/min is added at the position of 0–1 mm on both sides of the second bead surface shown in Fig. 1 to increase the residence time of the weld in the high temperature zone, so as to provide sufficient time for the transformation of ferrite to austenite and effectively improve the corrosion resistance of the weld [8–10].

On this basis, the welding process test can be carried out to simulate the bottom plate repairing environment of the spent fuel pool.

The underwater welding process of simulating-repairing condition was studied based on the test results of shallow-water welding process. The whole set of laser welding unit, as shown in Fig. 4, was put into the high-pressure test chamber. The laser head was placed 50 mm deep from the water surface, and the high-pressure test chamber was pressurized to 0.15 MPa (simulating 15 m water depth), thus the difficulty of hood drainage was increased.

Based on the welding parameters of laser power 5000 W, welding speed 480 mm/min and wire feeding speed 260 cm/min mentioned in Secs. 2.1 and 2.2, and further increasing the flowrate of drainage gas from 30 L/min to about 65 L/min, cooperating with synchronous groove blowing and draining(keeping blowing for more than 3 min before welding), the surface dryness of welding groove and the welding quality can be effectively improved. The final welds are shown in Fig. 10.

Nondestructive tests, including visual inspection, liquid penetrant inspection, ultrasonic inspection, and radiographic inspection of welding test pieces prepared under the above three environments met the standard requirements of AWS D1.6. Then, according to the test items and requirements listed in Table 2, specimens were sampled from the qualified welding test pieces as shown in Fig. 11. The tensile, impact, and bend test specimens are perpendicular to the welding direction of the welding test pieces, to assess the performance of the welded joints, especially the weld metal.

The corresponding performance tests are carried out, respectively. The ferrite contents of the weld and heat affected zone (1 mm from the fusion line) are tested by magnetic method according to GB/T 1954, using a magnetic instrument produced in Germany. The intergranular corrosion specimens prepared according to the standard requirements are subjected to sensitizing treatment at 675 °C for 1 h. After that, the specimens are soaked in boiling sulfuric acid-copper sulfate solution for 20 h continuously. After taken out from the test solution, the specimens are bent to 90 deg, and are examined under magnification (about 10×) to detect cracking.

The tensile strengths of the welded joints at room temperature and 130 °C in the above three environments are similar, as shown in Fig. 12. With the increase in temperature, the tensile strengths of the welded joints decrease, and the tensile strength at high temperature decreases by about 15% compared with that at room temperature.

The impact energy results of the Charpy V-notch impact test at –40 °C for the weld and heat affected zone (the centerline of the V-shaped notch of the impact specimen is 1 mm from the fusion line) are shown in Fig. 13, which meet the standard requirements. Considering that the impact energy of the impact test itself has a certain dispersion, the impact performance of the weld, and heat-affected zone under the three environments is relatively stable.

As shown in Figs. 14 and 15, the bending and intergranular corrosion specimens in the above three environments have no cracks, and the overall plasticity and intergranular corrosion performance of the welds is good.

As shown in Fig. 16, the ferrite content of the welds in the three environments is slightly higher than that in the heat-affected zones, and the results are in the range of 40%∼50%, which are relatively stable and meet the standard requirements.

The microstructures of weld metals in three environments are composed of grain boundary austenite, widmanstatten austenite, intragranular austenite, secondary austenite (γ2), and δ-ferrite, as shown in Fig. 17. Combined with the ferrite content results in Fig. 16, the underwater laser welding has achieved the two-phase structure balance of ferrite and austenite, thus improving the corrosion resistance performance of the underwater laser welds.

In order to realize the steel liner underwater repairing of the spent fuel pool in the third generation nuclear power plant, experimental research was carried out on the laser welding process of S32101 duplex stainless steel in three environments step by step: air environment, shallow water environment and simulating-repairing environment of the spent fuel pool floor. By optimizing the welding process, the obtained weld is well formed and the welding spatter is small.

The mobile hood inflation protection combined with synchronous groove blowing and draining was developed to achieve the functions of creating local air environment in underwater environment and continuous sealing during the welding process. The underwater laser reheat treatment and shielding gas ratio control have increased the residence time of the weld in the high-temperature zone and promoted austenite formation.

The microstructures of weld metals in three environments are composed of grain boundary austenite, widmanstatten austenite, intragranular austenite, γ2, and δ-ferrite. The underwater laser welding has achieved the two-phase structure balance of ferrite and austenite, thus improving the corrosion resistance performance of the underwater laser welds.

The performance test results (including tensile, impact, bending, intergranular corrosion, and ferrite content) of welding test pieces under the three environments met the standard requirements. The underwater weld performance is similar to that in the air environment, meeting the requirements of the spent fuel pool construction standards, and laying a technical foundation for the application of the spent fuel pool underwater repairing.

AWS =

American Welding Society

BPVC =

Boiler and Pressure Vessel Code