By Jim Ungrin and Chris Saunders
A significant number of industrial applications for high-power electron beams at an energy of around 10 MeV began to look very promising in the early 1980s. (The limitation of energy to 10 MeV for industrial applications is driven mainly by the fact that there are virtually no neutrons generated by electron beams striking most targets at this energy and therefore the residual radiation fields near the accelerator are essentially zero and access to the accelerator and irradiated products is immediate.)
The Electron Test Accelerator (ETA) and medical accelerator programs within the Accelerator Physics Branch (APB) during the 1970s (See website page on on Applied Accelerators) proved that the technology existed to produce a 10 MeV radiofrequency-driven (rf) linear accelerator (linac) with a beam power in the 50-200 kW range. In 1984 Joe McKeown, who had been leading the ETA program, proposed to further stretch the limits and lobbied for the construction of a 100% duty-factor, 10 MeV industrial linac with a power of 500 kW. The name proposed for this accelerator was RAPELA (Radiation Processing Electron Linear Accelerator). A preliminary market survey of the industries where a powerful electron beam could be used failed to find a potential customer and immediate application for such a high power.
The market survey indicated however, that a significant opportunity existed in a number of industries for a 10 MeV accelerator with a power in the 50-100 kW range and a design was initiated for an Industrial Materials Processing Electron Linear Accelerator (IMPELA) operating at a frequency of 1.3 GHz and adjustable in power from 50 to 250 kW by simply increasing the duty factor and the power of the rf generator. The first unit would be designed to have a power of 50 kW and was designated I-10/50 (Industrial, 10 MeV, 50 kW).
The ETA accelerator operated in the continuous-wave (cw) or 100% duty-factor mode while the medical accelerators operated in a short pulse (6 microsecond) mode with a duty factor of ~0.1%. At 50 kW, IMPELA would operate in the long-pulse (200 microseconds) mode at a duty factor of 5%, corresponding to a pulse repetition rate of 250 Hz. Higher or lower beam powers could be achieved by simply increasing the repetition rate or the pulse length, or both. To operate in the long-pulse, a modulated-anode klystron would be used as the rf power source.
Primer on Accelerator Frequency Choices
The higher the operating frequency, the smaller and more compact the accelerating structure. On the other hand, the smaller the structure, the smaller the central beam aperture and the greater the chances of beam losses to the structure causing high radiation fields and mechanical damage or overheating. Finally, the frequency choice depends also on the commercial availability of suitable rf generators at the frequency desired. For IMPLELA a frequency of 1.3 GHz, higher than that for ETA (805 MHz) and lower than that used for the medical accelerator (3 GHz) development program at CRNL was chosen. American and European military and commercial applications had used this frequency for communications and had developed klystrons which were commercially available with output powers as high as 1 MW.
An increase in the momentum of the AECL programs on accelerator-based radiation sources for industrial applications began to occur in 1985. A corporate evaluation of the potential market for high-power radiation sources was sufficiently positive to lead to the formation of the Accelerator Business Unit (ABU), with offices at the Radiochemical Company building complex on March Road in Kanata. Andrew Stirling, formerly the Division Head of the Reactor Control Division in Chalk River, became the General Manager of this Unit while Joe McKeown became Director, Science and Technology, and Peter Brown who had been involved in the medical accelerator program at the Radiochemical Company became the Commercial Manager. At the same time, AECL also established a Radiation Applications Research Branch (RARB) at its Whiteshell laboratory under the leadership of Stuart Iverson to investigate new industrial applications for electron beams and to interact with industrial customers. Details of the work of this Branch follow at the end of this section.
The ABU received funding to direct and control programs at Chalk River to develop a prototype accelerator and to fund programs in the RARB to study, develop and expand industrial applications of 10 MeV electron beams. Since RARB did not have easy access to an appropriate electron accelerator, a second program was started – the development of an industrial-grade, 1 kW accelerator called I-10/1 (Industrial, 10 MeV, 1kW). Peter Brown had been experimenting for some time at Kanata, before the formation of the ABU, with raising the power of a standard 3 GHz medical accelerator to the 1 kW range by using a higher power, magnetron-based, rf source.
The need for a well-engineered approach to the design of industrial accelerators to ensure robustness and high reliability was understood to be essential. These requirements were very similar to those of the CANDU reactor systems, which at the time of the launch of the ABU, were setting world records for performance factors. A team of accelerator physicists from APB, bringing accelerator expertise, and engineers from the Reactor Control Branch, bringing control and engineering expertise and discipline, was therefore assembled for the design and construction of the I-10/50 and I-10/1.
Several options were considered for the location of the I-10/50 prototype before the decision was taken to disassemble the ETA accelerator in Bldg. 610 at CRNL, modify the shielded enclosure there and significantly increase the concrete shielding thickness required to accommodate the higher energy and higher power machine. Some members of the I-10/50 team also worked on the engineering upgrades, control system, beam scanner and product conveyor needed to make the upgraded medical linac in Kanata a reliable and suitable device for producing 10 MeV beams for researchers in the RARB at Whiteshell. This work was carried out mainly in a lab space in Kanata.
Work on both the I-10/1 and I-10/50 continued over the next two years and by 1988 the I-10/1 was shipped from Kanata to Whiteshell for commissioning in a newly-constructed shielded facility. Following commissioning, the unit was quickly put to use by RARB researchers for such diverse applications as validation of sterilization of medical products, materials enhancement (for example, by crosslinking of plastics), cellulose processing for the pulp and paper industry and studies of biological control of pathogens in cosmetic, hygiene, medical and food products. (1)
At Chalk River the first beam was extracted from the I-10/50 prototype on 14 April 1989 and the design power of 50 kW was demonstrated eight months later on 08 November 1989. Further improvements both of the scanned-beam quality and of the reliability of components continued over the following months and by mid-1990 “industrial-standard” operation with excellent beam quality was achieved over a full five-day period with an availability of greater than 97%. In September of 1990 IMPELA was recognized as “One of the 100 Most-Significant New Products of the Year” by the R&D Magazine. The award was presented to Terry Rummery, President of AECL Research, at a ceremony in Chicago on 26 September.
In July 1991 AECL Accelerators (formerly known as the ABU) announced the signing of an agreement to install the first commercial I-10/50 unit at the E-Beam Services Inc. irradiation service center in Cranbury, New Jersey. The majority of the effort for this unit would come from a team assembled in Kanata, most of it composed of former Chalk River employees who had worked on the I-10/50 prototype. This unit was installed by June 1992 and successfully commissioned and in service in August 1992.
A second commercial 50 kW IMPELA was shipped in August 1993 to Iotron Industries Canada Inc. in Port Coquitlam BC. This unit, which was built to service a wide range of irradiation applications, was brought into full operation by the end of 1993. (2)
Unfortunately, the promise of a large increase in the number of industrial applications for electron irradiation failed to develop in spite of the stellar performance of the prototype and commercial units and over the next several years no additional IMPELA units were sold. As a result, in 1998, AECL, while continuing to retain the rights to the IMPELA technology, decided to abandon the electron accelerator market and the activities of AECL Accelerators were terminated.
Three years later, on 03 August 2001, AECL announced the sale of its IMPELA Electron Accelerator technology to Iotron Industries Canada Inc.
Radiation Applications Research at Whiteshell Laboratories
The Radiation Applications Research Branch (RARB), under the leadership of Stu Iverson, was formed in 1985 to carry out research on industrial applications of ionizing radiation. Work at Chalk River Laboratories had led to the development of a high energy, high power industrial accelerator called IMPELA or the I-10/50. The RARB worked with industry to understand and solve the practical problems of using accelerators to treat commercial products. A major focus was irradiation to decrease the incidence of bacteria including salmonella, listeria and E. coli in common foods like chicken, hamburger and fish. A second thrust was to extend the range of products polymerized, crosslinked, grafted or broken down by irradiation. Early studies on polychlorinated biphenyls (PCB) and sewage sludge demonstrated technical feasibility. Other key technical advances included electron beam curing of composite products, the production of viscose and the breakdown of heavy oil products.
The centre piece of the RARB was the I-10/1 linear accelerator. A conveyor system moved products from the receiving area, through a maze, past the electron beam at a controlled rate and finally to the shipping area. Other necessary capabilities, such as gamma and electron dosimetry and a microbiology laboratory, were part of the facility.
The RARB was a hub of research activity for over 10 years at Whiteshell. The Food and Biomass Sections, led by Joseph Borsa, advanced our understanding of how radiation interacts with bacteria and degrades organic materials. This research also supported the efforts of many to secure regulatory approval for food irradiation in Canada. Ajit Singh led the group that developed electron beam processing as a manufacturing and repair technology for composite products. Key accomplishments included large international development programs to build new aerospace products, repair composite aircraft parts, and study the radiation stability of space components. His group also led the efforts to treat wastes with radiation to make them less toxic.
Another key focus for the RARB was education, both for the public and for industrial leaders considering irradiation for their businesses. The scientists and engineers of the RARB published extensively and travelled the world promoting the industrial benefits of irradiation. The RARB also hosted many researchers and visiting dignitaries to show them first-hand how accelerators worked, the benefits for their specific products, and demonstrated how an accelerator facility could be operated effectively and safely.
Federal budget cuts in the mid-1990s led to AECL deciding to shut down and decommission Whiteshell Laboratories. The RARB programs were also scheduled to end. However, the success of the RARB led directly to the formation of Acsion Industries in 1998. Acsion was formed by nine RARB staff, led by Chris Saunders, as a private corporation to commercialize AECL research on industrial applications for radiation. Acsion’s first major accomplishment was the installation and commissioning of a new accelerator in 2001. Acsion has been engaged in several lines of business, including the production of medical isotopes, radiation treatment of products in the healthcare, agricultural and consumer products sectors, decommissioning of radioactive sites in Canada, and aircraft repair services at Acetek Composites. Acetek was formed in 2001 as a joint venture with Air Canada.