Laser Cleaning for Nuclear Decontamination & Decommissioning

A viable nuclear decommissioning technology should be able to minimize the waste volume (thus reducing the overall waste disposal/management costs) and remain safe and reliable to operate. Fig.2. A typical nuclear reactor decommissioning procedure. High-power lasers have been recognized as noncontact, remotely controllable precision tools for cutting, welding, and surface cleaning. They are widely used in modern industries. Whether these existing techniques can be applied directly to nuclear decommissioning is still a subject of investigation. There are several competing conventional techniques that can be readily available for use in nuclear decommissioning. These include chemical decontamination, dry ice blasting for surface cleaning, diamond saw cutting (for concrete), plasma arc cutting and flame torch cutting (for steels). These processes can deal with complex surfaces, large areas and thick section structural materials. It is necessary to understand the special characteristics and limitations of laser technology as compared to alternative methods, so that its suitability for specific applications in nuclear decommissioning can be identified. 

Laser Cleaning for Nuclear Decontamination and Decommissioning  

In a nuclear power plant, radioactive contamination can be found on the inner surfaces of piping, valves, pumps, heat exchangers as well as on the surfaces of large machinery, tanks, vessels, reservoirs, handling equipment, instruments and painted ¦ Removal of spent fuel § System Decontamination / Waste Separation ¨ Temporary Storage (5-10 years) © Dismantling ª Waste Disposal « Site / Material Re-use / Recycling ¬ Waste Management walls/floors. The materials that the low / medium level radioactive contamination is normally associated with can generally be grouped into three categories: a) Oxides / rust on a metal (mainly stainless steel and carbon steel) substrate. The depth of contamination is normally within the oxide layer that is less than 100 μm. b) Multiple layer paint (e.g. chlorinated rubber and epoxy paint) on concrete/plaster and metal substrates. Most contamination is within the paint layer which is normally less than 1 mm in thickness. c) Contaminated concrete. Most of the radioactivity remains within 5 mm from the surface [1,28] Therefore nuclear decontamination involves the removal of these contaminated layers from various substrates. Damage to the substrate is not a factor to be seriously concerned. The required decontamination factor (DF, the ratio of radioactivity before and after decontamination) needs to be above 100 to meet the work-atmosphere dose – equivalent rate [4]. The decontaminated surface should have radioactivity below 5000 dpm/100 cm2 for release [5]. Furthermore, secondary waste generation needs to be minimized in the decontamination process. A minimum of 2 m2 / hour decontamination rate would be desirable to enable the technique practically feasible. Various decontamination techniques have been investigated to meet the above targets. These include the use of lasers, chemical solutions as well as abrasive blasting techniques. These are discussed and compared in the following 

Oxide layer removal from metal substrates large area metal surface decontamination (removal of contaminated metal oxides) can be achieved by immersing the contaminated components in concentrated acids (e.g., oxalic acid, chloric acid, and nitric acid) as well as diluted acid solutions (e.g., dilute chloric acid reduction agent of inhibitor-laced chloric acid and vanadium chloride mixtures [4]). Decommissioning factor of 100 can be achieved. The chemical method has an advantage of high efficiency for decontaminating complex geometry components and difficult to access areas such as inner walls of pipes and small holes. The method is, however, not suitable for decontaminating large objects such as tanks/flasks. In addition, despite up to 70% chemical solution recycling rate [4], considerable secondary waste is generated. Furthermore, metallic surfaces can be activated by the chemical process, thus making them vulnerable to recontamination. Another non-laser-based technique that has been investigated is the use of abrasive jet (e.g., water + zirconia powder) to remove metal oxides [4]. Decontamination factor of 100 can be achieved. It can be applied effectively to the exterior of objects and hot spots. Again, this process generates considerable secondary waste, and the collection of the waste is difficult due to the high-pressure abrasive jets. Laser decontamination techniques involve the use of a short-pulsed (typically with a few nanosecond pulse widths) and high peak powered (up to several MW) laser to remove the oxide layer by thermal ablation (vaporization) and the associated thermal shock effects. Since the thermal penetration depth is proportional to the square root of beam-material interaction time, the use of short-pulsed lasers enables minimum energy loss through thermal conduction to the substrate material. Multiple pulses or multiple scans are normally needed to provide high DF. Since vapour/plasma plume generated during laser ablation travels in the direction normal to the surface, to avoid plume interference with the laser beam, laser beam can be fired to the surface at a lower angle (i.e., < 90°). A low-pressure gas jet (a few l/min) coaxial to the laser beam can be used to protect the laser optics and to disperse the laser-generated plume. Helium gas has been found to improve removal efficiency by 50% over other gases when a 90° beam incidence is used [6] since helium gas has higher ionization potential (24.6 eV) than Ar (15.6 eV), N2 (15.6 eV) and O2 (12.1eV). Uniform, rectangular beam geometry, rather than a Gaussian beam with a circular spot, is desirable to achieve uniform ablation and to maximize coverage rate. Therefore, a cylindrical lens, rather than a spherical lens, is preferred in laser ablation cleaning. The removed particles can be collected via high efficiency particulate air (HEPA) filtration. Up to 95% collection rate has been reported [7] and a decontamination factor up to 257 has been demonstrated [7]. Re-deposition of the removed particles can sometimes occur. Multiple passes and the use of a cylindrical lens can minimum this effect. For example, in a case study, with a cylindrical lens, re-deposition rate was < 0.2% compared with 5% with a spherical lens [6]. A typical processing arrangement for laser ablation cleaning is shown in Fig.3. 

Paint Stripping in Nuclear Processing Plants 

Paints are used to form a sacrificial layer on the surfaces of buildings and equipment in low-level radioactive environment. Repeated paint coating is used to trap down and fix the particulate contaminants. During decommissioning, these paints must be removed and disposed of in a controlled manner. A common practice is the use of chemical solutions, such as methylene chloride, phenolics, alkaline and acid activators [16-17]. The use of chemicals not only increases the waste volume but also likely creates extra disposal difficulties and further contamination of the substrates. Some of the other physical paint stripping methods such as plastic media blasting (PMB) [18-19], sodium bicarbonate media blasting [20], water jet systems [21-22] and ice particle blasting [23], currently used for aircraft and industrial equipment all result in increased waste volumes. Alternative physical paint stripping methods, which do not result in waste volume increase, include dry-ice (solid CO2) blasting [24] and light based (laser and flash lamp) devices. The dry ice system presents difficulties for waste collection due to local high pressure and particle scattering. Also, the excessive CO2 gas involved in the process makes it mainly suitable for outdoor paint stripping. Light – based systems are considered to be the only true non-contact methods which have the advantages of controllability, flexibility, convenience for waste collection and minimum opportunity for recontamination. Laser paint stripping has been successfully demonstrated on helicopters and military aircraft to facilitate necessary periodic metallurgical inspection. The lasers used are pulsed TEA CO2 lasers, Q-switched Nd-YAG lasers and excimer lasers. Some recent studies also revealed the possibility of using continuous wave diode lasers for paint stripping. The characteristics of various laser paint stripping techniques are described in the following sections. 

By passing a laser beam across the surface of concrete, the top layer can be made to eject violently, without melting or vaporizing. This is known as laser scabbling or stress fracture pioneered by the author of this paper [36,37]. This effect is believed to be caused by the rapid dehydration and evaporation of the moisture in the concrete. The process is more effective using large beams, especially when the beam size is larger than that of aggregates (10 –20 mm diameter). Single pass removal depth up to 20 mm has been achieved was found optimum for the process. There is no significant difference in removal characteristics between the two types of lasers. Typical removal rate is 1000 cm3 / hr.kW [38]. The process was most effective for the first layer concrete removal [1]. This process has the highest material removal rate compared with other laser-based methods as described in the following. 

SUMMARY  

A review has been presented on the use of high-power lasers in nuclear decommissioning. The work shows that, laser processing can offer significant advantages in terms of much reduced secondary waste, scalability and remote controllability. There are also limitations to the laser processing techniques for large area and thick section applications. For contaminated metal oxide removal, short pulsed (typically nano-second pulse width), high peak power (108 -109 W/cm2 ) lasers are preferred. Shorter wavelength lasers down to UV range can provide higher removal efficiency. The potential of using fiber beam delivery for excimer laser beams and diode pumped solid state laser beams would provide practical means for hot spot decontamination. 2-6 m2 / hr. removal rate can be achieved. Combined UV and IR beam ablation could increase the removal rate further. For paint stripping, although CO2 lasers would have the highest removal efficiency (e.g., 17 m2 /hr at 500 W power), high-power diode lasers can play a significant role when combined with an O2 gas jet. Since an HPDL beam can be delivered through optical fibres and much higher power diode laser systems can be made available in the future, HPDL applications in large area paint stripping in nuclear decommissioning could become practical. For paint stripping from metal substrates, complete removal of paint requires much higher laser energy density. Spraying a small amount of dimethyl formamide on the paint before laser paint stripping has been found to enable complete removal of paint on metal substrate. Concrete decontamination can be carried out by vaporization, glazing, and scabbling. While vaporization and glazing delamination are more controllable, scabbling is the most efficient method. Typical material removal rate for laser scabbling of concrete is 1000 cm3 /(hr.kW) at a laser power density of 100-300 W/cm2 . The scabbling process is also insensitive to laser wavelengths. Although CO2 and Nd:YAG lasers have been used, the potential use of an HPDL for concrete scabbling cannot be ruled out. Contamination tie-down on concrete can be best carried out using an HPDL. A coverage of 1.94 m2 /hr. with a 2.5 kW diode laser has been demonstrated. 

Due to its effectiveness and quick, clean operation, laser ablation or laser cleaning for nuclear decontamination and decommissioning is definitely the preferred method for thorough nuclear decontamination. As an environmentally friendly option that keeps your personnel safe and free from exposure to secondary pollution, LaserClean makes it a priority to exceed your expectations.