Induction Heating Makes Nuclear Fuel Rods Safer

In a pressurized water reactor, thousands of gallons of water are heated by being pumped past long fuel rods filled with nuclear pellets. The water, under intense pressure, flows to a heat exchanger where it heats water from another source, producing steam. The steam powers a turbine, generating electricity. Processes similar to this account for over 10% of the electric power in America and a much higher percentage throughout the world.

The designers of pressurized reactors take great pains to keep the water heated by the nuclear pellets from coming into direct contact with the nuclear material. The pellets are contained in specially-constructed, treated fuel rods. In turn, heated water is kept separate from the water that becomes steam and powers the turbine. By separating the stages from each other, the chances of nuclear contamination are minimized and problems can be more easily contained.

Detecting a Risky Situation
When engineers in the Commercial Nuclear Fuel Division (CNFD) at Westinghouse Electric Corporation received reports that, in rare cases, fuel rod cladding could be worn away by abrasive debris in the rushing water, they treated the problem very seriously. CNFD, one of the world's largest manufacturers of nuclear fuel rods, operates under a zero defects policy. Even the slightest risk must be eliminated.

Westinghouse engineers investigated and found that debris damage occurred only when the rods were new. To explain this, they theorized that the zirconium rods were oxidizing in the hot water surrounding them, developing a wear-resistant exterior coating. This coating, formed gradually during operation, protected the rod from debris. In the period before the coating developed, however, the rods were vulnerable. To eliminate any chance of debris damage, CNFD needed to find a way to achieve the effects of oxidation protection before the rods were placed into reactors.

Westinghouse put together a team led by Mark Goldenfield, the Project Manager, David Lambert, an electrical engineer, and Steve King, a metallurgical design engineer, to develop a solution. Deciding to enlist the assistance of a heating expert, they formulated their needs and sent out RFPs (request for proposal) to the leading induction heating firms.

After reviewing the proposals that came back, and meeting with representatives of the respective companies, Westinghouse chose Lepel Corporation, based in Englewood, NY. "We were convinced that we needed an induction solution," Lambert explained. "It's the only viable heating method that offers the reliable temperature control and consistent, accurate heating that fit our design criteria."

The decision to go with Lepel was heavily influenced by the team's perceptions of the company's technicians. "We were impressed with Lepel's expertise," Lambert continued. "No one else knew as much about the possibilities of induction heating, or heating in general." King, the metallurgist, was also impressed with Lepel's experience in heat treating various materials to specific tolerances in manufacturing. Westinghouse's comfort level with Lepel's technical knowledge was critical in a project like this.

From Testing a Theory to Designing a Solution
The first step was to create a prototype environment and analyze the proposed solution. Lepel designed a sample induction process to grow a thin, hard, oxide coating on sample fuel rods and found that, as the Westingouse engineers had predicted, it protected the rods against debris damage.

With the success of these tests, Lepel began an aggressive schedule to design an efficient induction heating solution. They needed to design a system that could heat as many as 33 fuel rods to the required specifications found in the testing stage. Because Westinghouse manufactures a variety of fuel rods, the system also had to be able to recognize and treat different types of rods in different ways.

To deal with the large quantity of rods, Lepel designed a long, oval load coil with 15 parallel-mounted susceptors on 2" centers. The system was engineered to utilize three of these load coils, each heating 11 rods. The manufacturing tolerances involved were so critical that the copper coil winding and the metal tubes had to be positioned within 0.005" for required accuracy.

The heating coil, a water-cooled wound coil that surrounds the piece being heated, has to provide extraordinarily even heating. Since it would be inefficient to heat the fuel rods directly, due to the poor thermal conductivity of zirconium compared to steel, Lepel engineers designed an alternative process. They placed a steel tube, or susceptor, between the heating coil and the part to be heated. The heating coil heats the tube, and the tube heats the fuel rod, giving efficient even heating for a consistent oxide coating.

Lepel's coils use an electromagnetic, non-contact, induction heating process to bring the rods to a specified temperature. Because of the precise nature of induction heating, the coils are able to keep the specific rod sections at a tightly-controlled temperature for a specified time. Since the induction process produces heat uniformly, there is little chance of distortion. The process is so tightly controlled that no post-process inspections are required.

According to Don Blau, Lepel's lead Sales Engineer, the most challenging part of the job was to manufacture the three heating coils to perform to Westinghouse's demanding heating specifications. "We had to keep the temperature within a very thin range," Blau said, "from tube to tube, batch to batch, and coil to coil. This is the kind of advanced application that is made possible by induction technology."

Designing the Rod-Handling System
While Lepel dealt with the heating aspects, another company worked on a system to prepare the rods for heating and to deliver them to the heating apparatus. Lepel had contacted William Salvesen of Datum Industries, Inc., a custom machine builder, to design the rod-handling system.

Datum designed a system that utilizes various stations to perform a series of necessary pre-induction functions. A conveyor powered by an Emerson servo motor carries the 33 properly-spaced rods into the Lepel heating coils. Another conveyor positions the three load coils so that each surrounds a batch of 11 rods.

Throughout the process, proximity sensors ensure that the rods remain properly positioned, and a computer interface controls rod processing through the various stages, while maintaining a real-time graph of the heating process. With multiple video bar code readers and the computer, the system tracks the progress of every rod to verify correct processing and provide a record for later analysis.

Westinghouse has been pleased with the system's performance. "We keep it running around the clock, five to seven days a week," Lambert noted, "and after a year of operation, we haven't had any problems." Lepel Corporation, MAN - April 1996