To lessen the threat of terrorists stealing cobalt-60 to fashion a radiological weapon, the Department of Energy’s National Nuclear Security Administration (NNSA) says it wants to encourage alternative technologies for sterilizing disposable or single-use medical devices such as gloves, masks, and stents.
Currently about half of US medical products are sterilized with gamma rays; most of the rest are sterilized chemically. But chemical treatment is only suitable for products with permeable packaging, and it requires a post-treatment aeration period to dissipate the toxic ethylene oxide gas. The agency is sponsoring a new study to detail the economic and regulatory obstacles to converting the sterilization industry from gamma irradiation to accelerator-generated x rays.
“Our electron accelerators could perform equivalent sterilization to cobalt-60 without a radioactive source,” says Timothy Meyer, chief operating officer of Fermilab, whose Illinois Accelerator Research Center is performing the NNSA study. Robert Kephart, the center director, says the lab is engaging a major irradiation company, an accelerator manufacturer, and a medical-device manufacturer to help with the market assessment. “As a national lab, our goal is to provide unbiased science-based advice to NNSA,” he says.
About 50 commercial irradiators in the US use 60Co, according to the Nuclear Regulatory Commission. The International Atomic Energy Agency estimates that about 200 gamma facilities are in operation worldwide. All use 60Co. Nordion, the Canadian company that supplies most of the world’s 60Co sources, says there are about 100 gamma irradiators that provide services on a contract basis in 30 countries. Other irradiators are operated in-house by large medical-device manufacturers.
The facilities are known as panoramic irradiators because the 60Co sources bathe everything in sight with gamma rays. Products pass through the radiation-shielded room on conveyor belts.
This diagram shows a facility driven by an electron-beam accelerator that sterilizes equipment such as single-use medical treatment devices. The packaged devices move along the conveyor belt so that they are exposed to the electron beam. The accelerator can alternately be used to produce x-ray irradiation.
This diagram shows a facility driven by an electron-beam accelerator that sterilizes equipment such as single-use medical treatment devices. The packaged devices move along the conveyor belt so that they are exposed to the electron beam. The accelerator can alternately be used to produce x-ray irradiation.
Although gamma rays can sterilize entire pallets of stacked products at once, electron beams can penetrate only a single box at a time, says Philippe Dethier, marketing and business development director of IBA, an accelerator manufacturer based in Belgium. Electron-beam accelerators of 10 MeV or less have been used for decades to sterilize some low-density medical devices. But simply by inserting a tantalum target in the beam path, companies can cheaply and easily modify such accelerators to produce x rays, which penetrate as well as gamma rays. IBA built a US x-ray sterilization facility for the US Postal Service; that accelerator treats mail destined for government offices. Other x-ray capable systems are located in California and Hawaii.
An accelerator, whether it produces electron beams or x rays, is a “nonnegligible investment,” Dethier acknowledges, and 60Co will continue to be the more economical solution for small-volume operations. But x rays will be cheaper in the long run for larger facilities.
Another significant barrier to x-ray treatment is the requirement that a manufacturer demonstrate to the Food and Drug Administration that the process is effective for each kind of product to be treated. Since regulatory standards are currently gamma based, getting a new irradiation method approved will cost producers hundreds of thousands of dollars, says Kephart. Although that may not be a major cost for the big medical manufacturers, there is no incentive for them to bother, Dethier notes.
In 2015 the radioactivity of all the world’s 60Co sources totaled 440 million curies, half of it in the US, according to Dethier. A doubling of that quantity is expected in the next 15 years to accommodate the projected growth in disposable medical devices such as syringes, masks, and clothing. The isotope, which has a half-life of 5.3 years, must be periodically replenished. It is produced in reactors by neutron activation of 59Co targets. Nordion, which has produced 60Co in Canada’s National Research Universal reactor, last year signed a long-term agreement to produce the material in the Bruce Power nuclear-generating station’s CANDU reactors.
Nordion’s 60Co sources emit 9000–11 000 curies. The sources measure 450 mm in length and 11 mm in diameter and weigh about 250 grams. Dozens may be used in an irradiator, with the exact number depending on the size of the facility. A sterilization provider that wants to expand need only to increase the number of sources.
Although the NNSA is tasked with curtailing the commercial use of radioactive materials suitable for a dirty bomb (see Physics Today, March 2013, page 30), the agency sometimes acts at cross-purposes. “What is really funny is that NNSA has the mission to secure the gamma facilities,” says Dethier. “So they go to the sites and they finance the upgrades to improve safety and security on the site. That is basically a subsidy to gamma. Someone who wants to build an x-ray site will not get anything from the government.”
The NNSA declined requests for an interview. In a statement, a spokesperson said the agency’s Office of Global Material Security supports the voluntary transition to nonradioactive sterilization approaches. According to the statement, the NNSA “has also worked with industry partners to identify security solutions for panoramic sterilization irradiators.”
Dethier says that IBA has sold accelerator systems in Europe, Japan, and China. All are used for sterilization with electron beams, although a few can be modified to produce x rays. In some cases, electron-beam sterilization is efficient enough to be cheaper than gamma-ray sterilization, even in the short term. “If your products allow for it, e-beam is a no-brainer,” he says. That’s not the case for the US market. The Canadian company Mevex also produces electron-beam sterilization plants.
For now, accelerators are effectively excluded from the US. Two facilities would need to be built for the industry to consider x-ray sterilization, since manufacturers typically require a backup plant within a reasonable distance, Dethier says. For that to happen, he notes, government intervention may be required. PT
