Nuclear Decontamination by Laser for Decommissioning

Nuclear energy is an important substitute for replacement of fossil fuels such as coal which contributes heavily to global warming. In the Canadian province of Ontario, where the production of electricity by coal burning power plants has been banned in the last few years, 59% of electricity is produced by nuclear power plants as compared to 23% hydroelectric, 11% natural gas and about 5% of clean energy (Solar wind). Therefore nuclear power plants play an important role in production of electricity. However, nuclear power plants do not last forever and need to be decommissioned after 30 to 40 years of operation. This is because parts get worn out and it would be unsafe to run the facility any further. Another reason is the possibility of the walls developing cracks.

During nuclear decommissioning, normally a reactor is dismantled completely and all the parts are taken to a temporary repository which will then be transferred to a permanent geological repository. Some of the parts that are to be disposed of consist of valuable metals and alloys and it would be beneficial to decontaminate them so they can be reused rather than ending up as waste. There are many precious metals such as stainless steel available in the structure of nuclear power plants and it would be economical to recycle this material after decommissioning them. Usually in the decontamination process, there are layers of oxides which contaminate the walls of a nuclear power plant that need to be removed. Figure 1, shows a picture of one of these walls:

Figure 1: The oxide layer is visible on the interior wall of the nuclear power plant. This image is taken from the site belonging to the Swiss Federal nuclear safety inspectorate ENSI.

The oxide layers normally accumulate on the concrete walls or the metallic tubing and needs to be removed before decommissioning. The concrete walls will be thrown out as waste but have to be cleaned from the radioactive residue first (Concrete scabbling). The tubes which are made of stainless steel, need to be cleaned from the oxide residue since the metal can be reused for the next generation nuclear reactor.
There are several methods of decontamination to implement concrete scabbling and oxide layer removal such as water jetting and mechanical scabbling. However some of these methods produce a lot of secondary waste (water jetting) and have the requirement for complex external control and deployment systems. There are also other wet techniques such as steam cleaning, chemical cleaning and wet abrasive cleaning. All of these techniques produce a lot of secondary waste material. Figure 2 shows the water jet cleaning of oxidized surfaces

Figure 2: Water jet cleaning. Picture is taken from ultra-high pressure water jet cleaning intelligent technology.

Another proposed method called laser decontamination which has many advantages and a few disadvantages, is uses nano-second high powered lasers of 10-100 kW power range and sends the laser beam through a fiber to reach the oxidized surface to be cleaned. The development of rugged industrial lasers have made this possible and the flexibility of fiber optics of a few meter lengths would make it possible to reach out to surfaces that need to be treated. A lens system at the end of the fiber optic delivery tube could make different shapes for the beam such as cylindrical, spherical, etc. Considering that for laser cleaning the power density can be small and the beam size could be larger (as compared to a cutting application), this is implemented successfully. The advantages of using the laser cleaning system is the low impact it has on the environment, and the lower amount of secondary waste and automation. However there is the disadvantage of requiring accurate laser beam focusing on the surface under treatment. Figure 3 below shows the laser cleaning system.

Figure 3: Laser ablation cleaning

The Fiber optic delivery of the high power laser beam is shown in figure 4.

Figure 4: Fiber optic delivery of the beam

The oxide layer on the surface of metals mainly consists of Cobalt 60 which has a half-life of seven years.

To understand the action of the laser beam on ablating the contaminated surface, one can explain that if the laser pulse energy is high enough, it can generate shock waves inside the material and will evaporate the oxide layer. This is due to mechanical instabilities that are induced within the surface structure and will cause the removal of the contaminating oxide layer. The parameters of importance for the laser beam are wavelength, pulse duration and pulse energy. The properties of the oxide layer needs to be taken into account as well to choose the correct laser parameters for efficiently ablating the surface.

For ablation of contaminated radioactive surface, the laser ablation is done layer by layer and the ejected material are captured by a suction pump, so they do not get the chance to be redeposited on the surface.

Allied Scientific Pro has introduced laser cleaning devices that can be used for nuclear decontamination purposes when a nuclear power plant is being decommissioned. The flexible 5 meter fiber with a scan head will direct the laser beam (wavelength in the near IR region, tens of nano-second pulse width and variable kHz rep rate) towards the surface. Cylindrical lenses make the beam shape in the from of a line and the contaminated surface can be removed by laser ablation effectively. Figure 5 below shows the laser cleaning system by Allied Scientific Pro.

Figure 5: LaserBlast-200 Cleaning System by Allied Scientific Pro

Laser ablation is definitely the preferred method for nuclear decontamination due to its effectiveness and clean operation which would help the environment and provides a safer operation for the personnel who get exposed to less secondary pollution.

References:
1-   Laser decontamination and cleaning of metal surfaces: modelling and experimental studies, PhD thesis by Anton Leontyev.
2-   The potential of high power laser in nuclear decommissioning, Paul Hilton and Colin Walters, TWI website.
3-   Taking nuclear power plants out of service, Swiss Federal Nuclear Safety Inspectorate ENSI website.

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Nuclear Decontamination by Laser for Decommissioning