At the Combustion Engineering 840MW PWR located in Michigan, USA, Entergy’s outage workers replace all 36 in-core instrumentation tubes every other outage. They feed them into a semi-automatic hydraulic shear that cuts the 3/8in (1 cm)-diameter tubes into six-inch segments that fall into a disposal bin. A single tube takes about 15 minutes to shear.

When extracted from the Palisades reactor, the 32ft (9.8m) long flexible instrumentation tubes sit in only 24ft of water, so it is the job of two or three workers to guide the tubes to the cutter without breaking the surface of the reactor well. Although the whole tube is radioactive, only the bottom 12ft or so, which sit next to the fuel in the reactor, are intensely so, said Palisades’ senior project manager Mark Coddington.

Because the plant’s original instrumentation cutter was becoming awkward to repair, requiring extensive servicing after every use, Coddington’s team approached vendor Master Lee Engineering Services about a replacement unit. As it happened, the company had developed an improved version for Constellation Energy’s Calvert Cliffs plant located in Maryland, USA.

But there was a problem. As the first Combustion Engineering reactor (online in 1971), the Palisades reactor design is unusual, and in particular, does not have a deep end, a deeper area in the reactor well for storing highly-irradiated components for extra protection, as Calvert Cliffs does. So the original disposal canister for the tube segments, shaped like a fuel assembly (8 inches square and 12ft deep) and which was in fact removed from the reactor in the same way, by using the fuel transfer tube, would not work for Palisades.

Instead, Coddington’s team decided to split the canister into three four-foot high units that stack together. Now, the reactor’s refuelling crane feeds the cutter with a new canister; a tool on the bottom of the cutter grabs or disengages a canister. Two canisters can take all of the 36 instrumentation tube cuttings, Coddington said. The team keeps the additional canister in the reactor well for any loose debris.

A more important innovation happened above, on the reactor refuelling floor. Although relatively compact, the cutter itself requires five connections: a line in (and out) of compressed air (nitrogen) for the instrumentation grapple, electrical power (480V) for on-board lights, a camera and other systems, and a line in and out for the pressurized hydraulic oil from a pump at the top that powers the shears.

Previously each system was loaded into the reactor building from outside one at a time; first the disposal canisters came into containment through the equipment hatch, and then made their way into the refuelling canal, then the in-core shearing machine itself, and then the control unit.

Coddington’s team simplified the mobilization-demobilization operations. He describes how it happened as ‘dumb luck’: the team received all of the components of the new cutter packed in a shipping box similar to a standard jobsite tool chest (such as a Knaack piano box).

“We thought, wouldn’t it be nice if there were power in here, then we wouldn’t have to unpack it. Wouldn’t it be nice to have the monitor here, too. So we put in power and air, put in transport brackets and wheels to make it easy to move around.”

Now, the Entergy team needed to simply connect a power line, and lower the cutter down the reactor well to the head with a long pole. They also manually insert new disposal canisters into the reactor well, instead of through the refuelling machine.

Coddington said that the reorganized system reduces the number of reactor portal crane moves from 32 to 2, a crucial time savings in an outage. And, with time saving comes savings in dose. With initial dose savings of 1.714 Rem/outage, over the life of the Palisades plant that equates to 21.66 Rem.

Palisades and Master Lee worked with Westinghouse to develop the design. Initial concept work started in 2007, design and delivery in 2008-2009, and final implementation was in October 2010. Although the cutter hasn’t been used in its spring 2012 outage, which was underway at the time of writing, outage teams have again used the modular box idea to organize general and specialist tools brought into containment during the outage for other projects.

The Palisades containment is particularly awkward to work in during outages, Coddington said, for two reasons. First, equipment must first go through the auxiliary building to reach containment. Second, the Palisades containment is almost half the size (116ft long) as other reactor containments, so there is less lay-down space for tooling and job planning.

So the Entergy team consolidated many outage tools into four specially-designed containers, 8ft high, 7ft wide and 12ft long, with working platforms on top to compensate for the floor area that they take up. They are, first, a tool crib, complete with lights, lockers and a workbench; second, a container for rolling tool chests like the one containing the in-core shearing equipment, which are dispersed through the building; third, one for the reactor head tensioning equipment, and fourth, a container exclusively for outage scaffolding.

“We took a series of semi-complex things and created one plug-and-play box. Now, you pull the box up to the space and you plug in to get going. It’s simplified and effective for our outage operation,” Coddington said.

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This article was first published in the June 2012 issue of Nuclear Engineering International