While additive manufacturing is often lauded for its ability to create rapid prototypes to speed up the design process, or produce small batches of end user products, it has another compelling application: the manufacture of obsolete parts that would be too difficult or costly to create using traditional fabrication methods. Still, it’s one thing to print a replica of a knob or other non-critical part, it’s another to create parts that meet the most stringent industrial safety regulations.
The Krško nuclear power plant in Slovenia became operational in 1981 and began producing commercial power in January of 1983. The plant, a two-loop pressurized light water reactor developed in cooperation with Westinghouse, has a capacity of 696 MW and provides electricity for 25 percent of Slovenia and 15 percent of Croatia (the countries share responsibility for waste disposal). The reactor, which is the only nuclear power plant in Slovenia, was due to be decommissioned in 2023, but an application was approved in 2015 by the Slovenian regulatory body to extend this date by 20 years to 2043. As could be expected at the operations level, however, parts of the plant were beginning to wear out.
Recently, the plant found it necessary to find a replacement for a metallic, 108-mm diameter impeller for a fire protection pump. The original part was created in 1981 out of cast metal, and its original manufacturer had long since gone out of business. Despite its small size, the part played a critical role: it’s in constant rotating operation and provides pressure for the plant’s fire protection system. Building a new part using traditional fabrication methods was out of the question, Ales Presern, Head of Power Generation Services at Siemens Slovenia, told Design News.
“It would have been necessary to obtain original drawings from the 1970s and repeat the casting and machining process, which is typically not suitable for small-series production,” he said.
Instead, a team of experts from Siemens Slovenia reverse-engineered the part using x-ray tomography to obtain a three-dimensional model of the impeller. Once it was rendered into digital files, workers at the company’s additive manufacturing facility in Finspång, Sweden created a perfect copy of the original using an EOS M290 3D printer.
The original obsolete part on the left, the 3D-printed prototype in the center and the final 3-D printed part on the right. (Source: Siemens)
“Without 3D printing the whole pump would have needed to be exchanged,” Presern told Design News. “The replacement of the part itself was very short. The certification of the 3D printing process by the required entities in the nuclear power industry took the most time. Now, since we have the certification of the overall process, delivery of new 3D-printed impellers could be very quick -- within a few weeks.”
The project to create the new part, code-named “Perun” after the Slavic god of lightning and metallurgy, is particularly noteworthy because of the stringent safety regulations that cover nuclear power operations. The success is being hailed as a validation and a vote of confidence for additive manufacturing technology in the nuclear power industry. Extensive testing of the new part by CT scan and other quality assurance technologies took months, but the part not only met the steep requirements for safety, it exceeded the original part’s material properties as well as its creators’ expectations.
“The original material was replaced by a new, superior material in terms of mechanical properties, but those had to be proven by extensive testing by an independent institution,” said Presern, noting that unlike the original part, the replacement showed nearly zero vibration behavior.
According to Presern, the part created for the Krško nuclear power plant was one of Siemens’ first obsolete part replacement applications in a commercial sense, but that the company is looking to expand in this area. A recent breakthrough by the company in the 3D printing of gas turbine blades holds promise for updating other aging power generation infrastructures that requires obsolete parts to remain operational. The first 3D-printed burner component for a Siemens heavy-duty gas turbine has been in successful commercial operation in a power plant in Brno in the Czech Republic since June 2016. Successes at Siemens’ additive manufacturing facility in Finspång, Sweden has led to a 50 percent reduction in lead time and a 75 percent reduction in development time for gas turbine parts.
Siemens and personnel at the Krško installation have stated that they plan to continue joint research and development for the nuclear power plant and are looking to advance the design of parts that would be difficult, if not impossible, to produce using traditional manufacturing techniques, such as lightweight structures with improved cooling pattern. Ultimately, additive manufacturing technology may allow them to produce a variety of parts that exceed those they replaced in performance and safety.