MORE POWER TO YOU: SURFACE PREPARATION OF PIPES IN NUCLEAR PLANTS

The decontamination of large-bore water pipes at nuclear power plants is a particular maintenance challenge, largely due to the production of secondary waste materials and exposure risks to plant employees. For any decontamination system to be considered viable it must minimize secondary waste and be cost effective to operate and maintain with minimal occupational radiation exposure.

Traditional mechanical decontamination includes various grit blasting techniques using either wet or dry abrasives. Yet the costs of operating time plus handling and disposal of contaminated blasting media add to the time and monetary expense associated with these methods.

Chemical decontamination techniques are often implemented that provide adequate shielding to minimize occupationsal risks; however, radioactive chemical solutions must be produced in high volumes, making disposal burdensome and expensive. Whether chemical or mechanical cleanup methods are employed, the service time plus associated costs and worker exposure to radioactivity can all present economic challenges to nuclear power facilities.

“We normally use mechanical decontamination, which can be very effective, but is also a very expensive,” says Dan Stoltz, the radiation protection supervisor at a commercial nuclear plant in the central U.S. He adds that it is quite possible for a nuclear power plant to spend many thousands of dollars on blasting oxide layers, yet not necessarily achieve the lowest radiation levels.

Dose rates are a significant consideration because of NRC (Nuclear Regulatory Commission) limits on annual millirems (units of radioactivity environmental monitoring) per worker. When dose rates are high in nuclear plant water pipes, more time and manpower – hence, higher costs  – may be required to perform pipe decontamination operations.

“We had used grit blasting for this type of operation, but were looking for a more efficient and possibly more effective mechanical method of doing the work,” Stoltz explains. “One of the ideas we discussed was honing. I had seen flexible, ball-type hones used to resurface the cylinder walls of automotive engines. So I wondered if such a hone could be made large enough for us to use in working on this 14 in (11½ in ID) pipe. Also, the hone would have to be aggressive enough to remove the tough radioactive oxide layer from the pipe, but controllable so that it would remove very little of the pipe metal.”

Limiting the amount of pipe metal removed is important because of NRC regulations for the minimum pipe wall thickness. Any significant reduction in material could require pipe replacement, which only adds to the time and money. The ideal tool would have to be controllable and flexible enough to operate effectively in pipes that, like most metal pipes, are somewhat elliptical rather than round.

Stoltz explored the Internet and found Brush Research Manufacturing (BRM; Los Angeles, CA), a supplier that offers a broad line of flexible, ball-style hones. This ball-style hone is characterized by the small, abrasive globules that are permanently mounted to flexible filaments. The flexible, relatively low-cost tool is utilized in manufacturing for specialized surfacing, including deburring, edge-blending, plateau honing and deglazing.