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The Babcock & Wilcox Mark-C Fuel Assembly: A Nuclear Rarity

A few days ago I visited a high school to talk to a nuclear science class. I was pretty impressed that there was such a class but even more impressed to find that the school has a fuel assembly mockup. I have to admit that I was puzzled by it, though. It was unlike anything I had seen before. Since the fuel rods were in a 17 by 17 array, I assumed that it was an early Westinghouse design. After the visit, a little digging through old reports proved that assumption was incorrect. The mockup is a Babcock & Wilcox Mark-C, and it is a nuclear rarity.

All of the Babcock & Wilcox reactors currently operating in the U.S. use fuel assemblies from the Mark-B series, and all fuel assemblies in the Mark-B series have a 15 by 15 array of rods. Therefore, for devotees of nuclear fuel, "B&W" is almost synonymous with "15 by 15".

Mark-C fuel was designed for a later model of Babcock & Wilcox reactor. These reactors are often called "205 plants" because the design called for 205 fuel assemblies in the core. (The operating B&W plants in the U.S. are all "177 plants.") Four "205 plants" were ordered: Bellefonte 1 and 2 plus Washington Nuclear 1 in the U.S and Muelheim-Kaerlich in Germany. None of the U.S. plants was ever completed. Mark-C fuel was built for at least one of the Bellefonte reactors but, of course, not irradiated. The fuel was eventually returned to the manufacturer for dismantling and recycling. Nevertheless, four Mark-C fuel assemblies were irradiated in the U.S. The irradiation was in the Oconee 2 reactor, which was built for Mark-B fuel.

How can you irradiate Mark-C fuel in a Mark-B plant? The Mark-C was cleverly designed with the same height and width as the Mark-B series, so Mark-B and Mark-C fuel assemblies were interchangeable in terms of size. But their control rods were quite different. A Mark-B control rod assembly has 16 rods, and a Mark-C control rod assembly has 24. And the control rod positions don't line up. Reactor buffs will know that even if the Mark-C fuel assemblies were shipped with their own control rod assemblies, those control rod assemblies would be incompatible with some of the hardware (known as "brazements") inside the reactor. Fuel assemblies have empty guide tubes to accept the individual control rods in a control rod assembly, and when the control rod assembly is withdrawn from the fuel assembly, the brazements serve as extensions of the guide tubes, supporting the withdrawn parts. The brazements at Oconee 2 were designed for Mark-B fuel, not Mark-C. Therefore, it would seem to be impossible to use Mark-C fuel in Oconee 2, right? Wrong, as the Oconee 2 irradiation showed.

The explanation is that pressurized water reactors are designed so that fewer than half of the fuel assemblies are equipped with control rod assemblies. The remaining assemblies are fitted with components that fit into the guide tubes but don't get withdrawn during operation. These components do not have to be compatible with the brazements. So the explanation is simple: The Mark-C assemblies irradiated at Oconee 2 were placed in locations without control rod assemblies.

The more I think about it, the more I am amazed that this fuel assembly mockup, which may be the only Mark-C mockup in the world, is languishing in an obscure classroom. Finding it was like puttering around in an attic and discovering something of great historical value. I don't imagine that there's much market value in an old fuel assembly mockup, but it seems that there should be a museum that would collect and display such things. Some of them have stories to tell.

For more on the nuclear fuel production process, click here. For more on Babcock and Wilcox's current nuclear business, click here.

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