Waste not, want not18 December 2018
The UK’s ability to dispose of Sealed Radioactive Source waste has been something of a concern for many years now. But as Andrew Tunnicliffe finds out, INS has been working on an innovative, albeit challenging, solution.
IT’S A STORY THAT HAS been years in the making, quite literally. There has been a backlog of Sealed Radioactive Source (SRS) waste awaiting disposal across the UK for many years, as Martin Robb of the UK’s Nuclear Decommissioning Authority (NDA) explained in a blog last year. “We’ve been accumulating it for decades as a by-product of numerous medical, research, and industrial processes – even school chemistry experiments,” he wrote. “The old method for dealing with them became unworkable about 15 years ago and we’ve been in need of a reliable replacement ever since.”
Robb, the NDA’s national programme lead delivery manager, was commenting on his visit to Buckingham Palace to celebrate the work he and his team had done in helping identify and develop a disposal route for this type of waste. He wrote: “The material (sealed radioactive sources) was (and still is) used for entirely harmless production purposes by scores of universities, hospitals, schools and businesses. Once finished with, it’s sealed in containers around the same size as a 10-litre paint tin and needs to be disposed of carefully, in line with regulations designed to protect people’s health and the environment.
“The former method of dealing with sealed sources, which was provided by Sellafield, involved transporting the material to one of the West Cumbrian site’s specialist facilities, where it was repacked before being moved to a different store for safekeeping ahead of eventual disposal in a Geological Disposal Facility.
“But, because of increased decommissioning activities across our 17 sites, the receiving facility at Sellafield came under growing pressure. Its main function is as a transfer station for larger and more heavily radioactive quantities of waste from nuclear sites across the UK. The sealed sources were something of an extra activity and transfers were suspended at Sellafield, but continued for a while at a Harwell facility until this was decommissioned.
“There are now literally tens of thousands of ‘sealed sources’ dotted around the UK, kept in stores belonging to the users of the material and the licensed commercial collection agencies who collect them.”
“In the UK redundant SRS are collected and consolidated by a number of Source Collection Agencies (SCAs). The route that the SCAs used was closed because of decommissioning activities,” explains International Nuclear Services’ (INS) Martin Williams. “SRS are used in a wide range of industrial, medical and academic devices. Eventually, SRS reach the end of their useful life. If the SRS cannot be repatriated or recycled, then it is deemed disused and has to be prepared for long-term storage and ultimate disposal. The UK needed a new solution and route for this type of waste,” he says.
For two years INS worked on that solution, but it wasn’t like any other they’d been involved in before. With a history stretching back to the 1970s, INS originally began life as the Spent Fuel Services and Transport Division of the then British Nuclear Fuels (BNFL). Now a wholly owned subsidiary of the NDA, since 2008, it continues to support the authority by providing specialist nuclear transport, design, and licensing services. As well as representation in the UK, it also has operations in France and Japan and works with a mix of international partners at a governmental and commercial level, sharing its expertise with the likes of the US Department of Energy on nuclear non-proliferation projects as well as France and Japan.
Despite that extensive background and wealth of expertise, this challenge was something new for INS to satisfy the UK regulatory requirements. “In seeking a solution,” Robb wrote, “I turned to our specialist transport subsidiary INS.” INS identified the 1648C transport flasks that had previously been used at the UK’s Magnox Chapelcross site near Annan in the Scottish borders for transporting metallic wastes, but had since become redundant. “We had a look at those flasks and thought they would be suitable,” says Williams, “but we had to do substantial engineering and licensing work to make sure they were suitable for all the types of waste that the SCAs may have to process.”
The intention was for the 1648Cs to be used to move the consolidated SRS wastes from the SCAs to the Miscellaneous Beta Gamma Waste Store (MBGWS) at Sellafield. Although each flask had already been used in similar roles, modifications were critical because of the unique nature of their newly intended purpose.
The 4t flasks are 1.3m tall, cylindrical steel with aluminium shock absorbers, have a wall thickness of 232mm, and a disposable inner steel liner with 28L capacity. They have their own bespoke transport trailer, but one particular difficulty was unloading the flask from it. Originally they had been designed to be lifted by large cranes. However, cranes weren’t readily available at the SCA sites, meaning adaptations that would allow forklift trucks to unload if required. Other modifications included designing different sized liners to accommodate the varying types of waste they would be transporting. Because of the mix of medical, industry, and academic sources the flasks have to be able to accommodate a varied waste stream in terms of shape, size, and type of radioactive isotope.
The modifications, although critical to the success of the project, were just one part of this unique and complex task as Williams explains. “The challenge has been the licensing of these flasks, that required a novel approach.” It was developing a transport Package Design Safety Report (PDSR) for such a wide list of potential isotopes that posed the most significant challenge according to Steve Middleton, INS Project Manager.
“In particular, Paragraphs 617 and 648 of the IAEA’s Regulations for the Safe Transport of Radioactive Material (SSR-6) place the emphasis on the package designer to demonstrate the transport package design complies with the radiation level limits. This becomes difficult to demonstrate, as in this case, where the radioactive contents cannot always be well defined in advance,” he says. Paragraph 648 states: ‘A package shall be so designed that if it were subjected to the tests specified in paras 719–724, it would prevent loss or dispersal of the radioactive contents; and more than a 20% increase in the maximum dose rate at any external surface of the package.’
Remedying this was a painstaking process which required a new way of thinking. INS began by developing a bounding case for more than 300 radioactive isotopes listed in the Transport Regulations. It then carried out what is known as a “point source” shielding calculation for every isotope, checking this value against the temperature limit of the flask. If required, the values were adjusted to take heat generation into account, ultimately arriving at a specific safe value for every isotope that may need to be transported, and formulas for how to manage mixtures of the isotopes. “The nature of the analysis meant the values arrived at were completely bounding so that, in practice, no matter what the shape was, or where the isotopes were placed in the transport package, they could never breach the regulatory safety limits specified for radioactive dose rate measured at the surface of the loaded transport package,” Middleton adds.
INS took what it called a “staged approach” to obtaining the Type B licence for the transport package. Because INS is a Competent Body, it was able to issue a Type A licence in 2015, meaning it could transport some wastes with lower levels of radioactivity. This provided the team with the opportunity to establish the waste route, begin to transport materials from the SCAs for disposal and generally obtain experience before developing the Type B licence application.
“As you would expect from a rigorous regulatory system, the ONR (Office for Nuclear Regulation) had many questions about the approach and came to witness operations that were in place to consign the Type A packages,” says Middleton. “They were particularly interested in how Paragraph 648 would be satisfied. After 12 months of review the ONR was satisfied the licence could be granted for the Type B application.”
The first transportation of radioactive waste under the Type B licence took place in November 2017, something Williams believes might not have happened so quickly had the redundant 1648C package not been identified as a solution to what was a growing problem for the UK and its SCAs. “If we’d had to design and license a new flask then that would have taken a lot of time, possibly up to five years, and would have required a significant budget,” he says. “Instead of consigning these flasks to the scrap yard we’ve found another use for them to be put in place in the shortest amount of time possible. So it’s a really good use of equipment that had become redundant. We found another use for them on behalf of UK PLC,” he says.
What are the benefits of reusing the 1648C package?
- Small enough to be handled by SCA facilities (some minor additional handling equipment).
- Compatible with existing long-term storage and maintenance facilities at Sellafield.
- Relatively large payload.
- Substantial inherent shielding.
- Existing fleet of packages maintained and available for immediate use.
Source: International Nuclear Services
Author information: Andrew Tunnicliffe is a Freelance writer and editor