Fuelling additive manufacturing10 October 2017
Work being carried out by Westinghouse hopes to drive innovative design, learning and acceptance of additive manufacturing forward to benefit the nuclear industry. By William Cleary and Zeses Karoutas
Complex geometries, tight tolerances and other challenges to manufacturing have long been hallmarks that reflect both the allure of nuclear fuel design and the limitations designers encounter. But what if these challenges could be alleviated? What if nuclear fuel designers could concern themselves only with improving fuel performance rather than with the restrictions traditional manufacturing methods place on what can be achieved?
Westinghouse is freeing those design constraints through the use of additive manufacturing. As early as 2018, those efforts will position the company to be able to be the first to place an additively manufactured fuel component — a thimble plugging device — in a commercial reactor.
Driven by safety, quality, manufacturing excellence and innovation, Westinghouse is incorporating the promising benefits of additive manufacturing to take fuel performance to new levels, and to do so with reduced supply chain and shorter time from concept to product fruition. The potential benefits of additively manufactured fuel and core components include:
- Lower fuel assembly pressure drop;
- Better flow mixing, greater heat transfer ability and improved seismic performance;
- Less potential for fuel to leak;
- Greater heat tolerance;
- Better fuel margins;
- Extended fuel cycles;
- Customisable fuel assemblies;
- Less dependence on supply chain;
- Fewer overall suppliers;
- Reduced time from design concept to product availability.
When Westinghouse embarked on this mission, the nuclear industry did not have any direct radiation experience with additively manufactured materials. To address this and take the primary step towards putting the first additively manufactured component in a commercial nuclear reactor, Westinghouse had mini-tensile specimens of Stainless Steel Alloy 316L and Inconel Alloy A718 additively manufactured for testing. The specimens were printed using the laser powder-bed system, which lays down thin layers of powdered metal and fuses them together. Westinghouse selected these alloys because they are the most promising commercially available alloys for crossover to nuclear applications.
Working with the Massachusetts Institute of Technology (MIT), the specimens were placed in the MIT test reactor in October 2014 and removed in May 2015. Westinghouse evaluated the specimens at its U.S. Materials Center of Excellence Hot Cell Facility in Churchill, Pennsylvania, and found the results to be very promising. Mechanical properties, such as yield strength and ultimate strength, behaved similarly to their conventionally wrought and cast counterparts. Corrosion potentials were evaluated in pressurised water reactor conditions and found to be similar as well.
With these results, Westinghouse selected Stainless Steel Alloy 316L to be irradiated in a commercial reactor. The company chose to additively manufacture the rods of a thimble plugging device, combining a traditionally cast piece — in this case the faceplate from which the rods that plug the thimble tubes extend — with additively manufactured rods. The company selected the thimble plugging device to be the first additively manufactured fuel component for testing in a commercial reactor because consequences of less-than-expected performance are minimal, posing very low risk. In addition, the thimble plugging device provided an opportunity to enhance understanding and refinement of the design-and-build process for a cast and additively manufactured component combination.
The thimble plugging device has a fairly complex design due to its size and the tight tolerances required for the diameter, spacing and alignment of the rods. Westinghouse is working with Moog, owner of a laser powder-bed system that has been demonstrated to enhance the quality of complex parts. Following successful proof-of- concept demonstration of the prototypes, the engineering teams of Westinghouse and Moog manufactured the thimble plugging device to leverage the unique capabilities of the laser powder-bed system. With it, Westinghouse reduced the total number of parts for this component from 30 pieces to three and eliminated connective bolts and process steps to fabricate the final product. The thimble plugging device faceplate was placed in the powder bed, allowing the rods to be printed layer by layer directly onto it, which saved the precise drilling and machining of 24 holes into the faceplate, as well as the need to attach each of the 24 rods to the faceplate with bolts.
The additively manufactured thimble plugging device will be placed in a region of the reactor with a comparable fluence — or exposure to the number of particles per second streaming across it — to that which occurs in the region of the bottom of the fuel assembly within the core. It will be removed during the normal refuelling cycle (18 months, 36 months and 54 months) when it will be visually inspected to confirm its reaction to the harsh reactor core environment.
Westinghouse has also designed and additively manufactured other fuel components such as a bottom nozzle and an advanced tubular grid. The designs fully leverage the advantages of additive manufacturing in which components are built in layers with fine three- dimensional printing versus traditional subtractive manufacturing where metal would be cut away. It also affects the freedom component designers have in applying the advantages of the process to think differently. This new opportunity in manufacturing has allowed them to better address flow, pressure and other challenges to fuel performance with improved designs that traditional manufacturing methods may not have been able to accommodate.
Additionally, new prototypes can be built and tested quickly since additive manufacturing is data-driven, allowing three-dimensional models to directly inform three-dimensional printing. This speed, as opposed to the time-consuming steps of sending that information to a supplier to cast molds, fabricate tooling, and perform stamping and machining, among other steps, reduces the overall time frames from several months to days.
For new designs, Westinghouse can quickly produce unique additively manufactured plastic prototypes. Each prototype can be tested in the company’s 5x5 rod array hydraulic test loop, which has the same velocity as the reactor core. The overall process allows the hydraulic performance and other characteristics of new prototypes to be determined, optimising the design for improved performance at a cost and speed not possible before.
Westinghouse recently utilised the quicker development lifecycle achieved with additive manufacturing to rapidly screen improved advanced tubular grid design prototypes. The company’s goal with this work is to develop the advanced tubular grid to be stronger and achieve better flow, mixing and seismic performance than current designs. The development effort has also provided an opportunity to progress three-dimensional computational fluid dynamic, computer- aided engineering and finite element analysis modelling. Improving these modelling techniques will allow designs to be better perfected before performing the expensive testing processes required for new components to be ready for production and commercial use. Westinghouse is also investigating the neutron radiation effects on additively manufactured zirconium alloys for light water reactors. In 2017, this project received $830,000 in funding from the US Department of Energy. Westinghouse continues to work with MIT, where three sets of additively manufactured zirconium samples were irradiated in the institute’s test reactor this year. Two sets were returned this summer for exposure to higher radiation levels while the first set is analysed for early results.
The samples are made from a zirconium-based powder that Westinghouse’s project associate, ATI Specialty Alloys & Components, developed for use with additive manufacturing. The development process demonstrated that additive manufacturing can be used to conglomerate zirconium-based alloys. Following irradiation, Westinghouse will test the additively manufactured zirconium samples at its Hot Cell Test Facility to ascertain the radiation’s effect on their mechanical and microstructural properties. Using statistical comparison of the results, researchers will determine whether or not the changes realised in the additively manufactured zirconium specimens are different than the changes in cast or wrought zirconium materials when exposed to the same levels of radiation. The results can help further understanding and acceptability of using additive manufacturing to produce zirconium-based nuclear fuel components.
Future Westinghouse plans for developing additive manufacturing include discussions concerning progress and testing with the US Nuclear Regulatory Commission.
William Cleary, Nuclear Fuel Additive Manufacturing Technical Lead, Global Supply Chain Solutions and Zeses Karoutas, Chief Engineer, Fuel Engineering & Safety Analysis Global Technology Office at Westinghouse