Looking to the future to delay the past12 August 2016
The management of highly active liquid waste from the reprocessing of used nuclear fuel is one of the most challenging areas the industry faces. Sellafield has developed some innovative approaches to keep its highly active waste facilities operating past their design life as Penny Hitchin discovered on a visit to the site.
Sellafield is one of the most complex and oldest nuclear sites in the world. In its 70-year history hundreds of nuclear facilities have been constructed within a crowded footprint of two square miles (five square kilometres) in north west England.
Sellafield has been home to many ‘first of a kind’ nuclear engineering projects. The site’s earliest job was to produce plutonium for nuclear weapons, a role that paved the way for the civil production of nuclear power and for spent fuel management services. Now times have changed. The days when Sellafield generated nuclear power are long gone and the emphasis is on hazard reduction and decommissioning.
Reprocessing of spent nuclear fuel has been the site’s main commercial activity since 1994. The Highly Active Liquor Evaporation and Storage (HALES) department conditions nuclear waste streams arising from fuel reprocessing. This involves a number of facilities working in a carefully choreographed sequence to ensure that the volumes of hazardous radioactive waste are kept within rigorously defined levels and processed in a timely fashion.
Spent fuel rods are delivered to Sellafield and stored in ponds for a predetermined period of time while short-lived fission products decay. Reprocessing involves removing the cladding, chopping the fuel into sections and dissolving it in nitric acid. Chemical extraction is used to separate the uranium and plutonium, which are converted to oxide powders and stored. A hazardous liquid waste stream containing fission products and waste actinides remains. This is dispatched to the high level waste plants where the so-called raffinate goes through a process of evaporation, which recovers the nitric acid and leaves behind concentrated highly active liquid (HAL) waste.
HAL stocks at Sellafield represent a significant proportion of the radioactivity stored on the site. The management of this highly active liquid waste is one of Sellafield’s most challenging operational areas.
The self-heating HAL is transferred to highly active storage tanks where it awaits treatment in the waste vitrification plant (WVP). Vitrification involves drying the liquid waste to a powder, mixing it with glass and heating it to a temperature of around 1200°C. The mixture is transferred to stainless steel containers where it solidifies. The process immobilises the waste and reduces the volume to about a third of the original liquid waste size. The dense solid glass blocks of high-level waste are stored pending decisions about final disposal. Sellafield’s vitrification plant has been in operation for 25 years.
Two streams of fuel are reprocessed at Sellafield. The elderly magnox reprocessing plant has been dealing with spent fuel
from the UK’s first generation magnox reactors since the 1960s. The Thermal Oxide Reprocessing Plant (Thorp), built to reprocess irradiated oxide nuclear fuel from both UK and foreign reactors, came into operation 30 years later. Both reprocessing streams are scheduled to finish their work by 2020.
Evaporators: reprocessing pinch point
Managing the waste stream from reprocessing involves volume reduction followed by immobilisation. Access to evaporative capacity is essential in reducing the volume of raffinate from reprocessing.
Sellafield’s evaporators are unique in operating under low pressure (less than 90mmHg) in order to control corrosion and increase throughput rates. Each evaporator has internal coils within an external jacket designed to be filled with steam to heat and water to cool the raffinate. The jacket is enclosed within a thick concrete cell that provides shielding. The evaporator operates at low pressure and the bulk liquor temperature is around 60°C. Ensuring sufficient evaporative capacity to deal with the volumes of raffinate from reprocessing has been an ongoing challenge for Sellafield.
Advances in technology
Sellafield’s engineers and consultants have been able to use advances in technology to gain a detailed understanding of the interior of facilities that were designed and built with an assumption that they would remain sealed throughout their operational life. Remote technology has made possible an inspection and monitoring regime that has been invaluable in giving assurance about continuing the operational life of elderly assets. It has also enabled vital structural repairs.
The umbilical technology deployed in the evaporator inspections is a bespoke design by the National Nuclear Laboratory. It was essential to the deployment and retrieval in pipe work with many tight bends. The inspection heads are also a novel engineering design employing some off-the-shelf technology, such as transducers.
Results from the inspections have informed the theoretical models on corrosion and, along with other data from the operating evaporators, this has fed into the design of the latest evaporator, which will play a vital role in hazard reduction and clean up at Sellafield.
Sellafield has three evaporators with a fourth due to be commissioned next year. The two oldest, A (commissioned in 1970) and B (commissioned in 1984) were designed to process raffinate from magnox reprocessing. Evaporator C entered service in 1990 and was configured to treat both Thorp and magnox waste liquors. The evaporators were built with a design life of around 25 years but all three remain in service, albeit operating at reduced capacity. Once reprocessing is completed, A and B will be retired while C will no longer be in routine use as the new evaporator D will be take over dealing with liquors from the remaining reprocessing and the post operational cleanout (POCO) liquors.
Coils inside the evaporator are used for heating and cooling the raffinate. A and B were each designed with four coils while C has six. The original thickness of the coils ranged from 12.0m (inner) to 24.5m (outer) when installed. However, the combination of heat, highly active liquor and nitric acid make for a very corrosive environment and the safety case does not permit the coils to be used once they have thinned to a minimum specified thickness. The original evaporator design allowed no way of checking the internal elements, so a theoretical corrosion allowance was used to judge when to retire individual coils. Starting in 2000, the coils in Evaporator A have been successively taken out of use, leaving the jacket as the only source of heating in the evaporator. Three coils in Evaporator B have been retired, leaving one coil and jacket in operation. Evaporator C was designed with six coils, of which three have been retired.
As the evaporators have aged and coils have been taken out of service, operational efficiency has declined, creating bottlenecks in the reprocessing programme which have been a cause of concern and focus of attention by the nuclear regulators.
With the three evaporators all operating below their original design efficiency, there is severe pressure on the reprocessing schedule and Sellafield sought a way of physically checking the state of the coils. Inspecting the evaporator components would involve finding a safe way to introduce inspection equipment into areas that had not been examined since they were originally installed decades ago.
Application of advances in technology
In the decades since the evaporators were designed advances in technology have introduced new techniques that can be used for inspection and monitoring. The challenge for Sellafield was to find ways to retrospectively apply these to some of its older facilities.
A team of engineers developed a snake-like transducer device, with a camera, which could be put down the evaporator coils via newly installed inspection ports. The device was tested and refined on a full-scale test rig at the National Nuclear Laboratory’s Workington site and it was introduced on site in 2006.
Since then, regular coil inspections have enabled the evaporator team to build up a better understanding of the way the coils corrode. In 2009 a CCTV inspection of Evaporator C’s cell detected penetration on the wash lines into the evaporator feed lines. This had resulted in nitric acid spraying onto structural steelwork causing significant corrosion damage. The evaporator was withdrawn from service, jeopardizing the tight reprocessing schedule.
This posed a particular threat to the imperative to complete magnox reprocessing in a timely manner. To manage this Evaporator C was diverted from full time Thorp work, to working on blended batches of Thorp and magnox liquors. Following discussion with the regulator Sellafield agreed that it would reserve enough of C’s coil life to complete the magnox reprocessing.
Only a handful of countries reprocess spent nuclear fuel and thus expertise in the field is limited. When the scheduled arrival of the new evaporator was delayed, Sellafield brought together a team of specialists to identify ways to bridge the capacity gap. This included contacting Areva (France) and URS (US), established nuclear companies which had formed part of Sellafield’s management consortium at the time.
A number of approaches were identified. One of these was to get Evaporator B back into action, which would mean addressing the issue of the damage to the structural steel in the evaporator cell.
Sellafield’s lead system engineer for highly active evaporators, Jim Bell, talked to NEI about the complex task. Constructing a full-scale replica was itself a challenge as the records of the original design were not accurate. The legacy information for the in-cell feature positions were inaccurate by up to 300mm and existing records did not include some components. In order to build up an accurate picture of the cell layout, the team decided to drill small holes through the shielding to access the cell using lasers. A tent was constructed around the drilling sites and a pressure differential maintained which ensured radioactive contamination remained within the cell.
Bell says that drilling into a highly active evaporator cell to determine the layout had never been done anywhere before. The carefully planned inspection navigated the vessels, pipe work and support structures to revealed two positions of steelwork deep inside the cell that needed support. Having meticulously mapped the layout, a full size test rig of Evaporator B was built and the team started looking at options to repair or support the steelwork. The solution that emerged was to drill a hole into the thick protective walls in order to pass through a boom with chains to support the steelwork. Having refined and proved the scheme on the test rig, the team sought approval from the regulators and site owners the Nuclear Decommissioning Authority to go ahead.
The actual work on the facility was carried out remotely using visual support from separate cell access positions. With the boom introduced into the cell, the chain was lowered down and a c-shaped grab on the end lodged under the lug on the structural steel diverter, supporting the structure. New strain gauges were installed to monitor any changes.
The reinstatement was two years in planning. Bell reflects: “It was critically important for the project to get this right.
A lot of the work was assuring ourselves that we would get this right first time.” The painstaking planning paid off and Evaporator B was brought back into operation in 2013. The reinstatement cost £3 million and paid for itself within one batch of magnox raffinate.
Studying the coils
The coils are a key life-limiting feature of the Sellafield evaporators, but all three evaporators were designed without any provision for monitoring conditions inside the shielded facility. Retrofitting inspection ports to the evaporators has enabled regular inspection, and this has allowed the team to gain a better understanding of the way that the coils corrode. It is now possible to push an inspection device snake down through 30m of coiled pipe work to examine the entire coil bundle.
The understanding and information gained from inspection about the remaining thickness of the operational coils has enabled technical teams to come up with a technique of ‘condensate flooding’ – an innovative way of extending coil life. The greatest corrosion occurs at the bottom of the coils, where operating temperatures are highest. By controlling the temperature, operators can make condensate start forming higher up the coils. This means the bottom coils are flooded with condensate, which has reduced the corrosion rate by a factor of five. While condensate flooding protects the thinnest part of the coil, it reduces throughput. However, the team have found that reducing the pressure in the evaporator from 70mmHg to 50mmHg has enabled throughput to be maintained.
Bell explains: “Boiling at a lower temperature reduces the corrosion rate. The steam condenses out higher up, protecting the bottom of the coils. If we had not done this the life of the coils would have been finished now.”
The corrosion rates from the coil inspections also raised concern about the thickness of Evaporator C’s base. The theoretical calculation suggested the base, exposed to Thorp liquor which is more corrosive than magnox liquor, was nearing its safety case limit. The desk study predicted that only six to 12 months of life remained to it. In order to test this, the team designed a bespoke umbilical inspection unit that can be inserted into the evaporator jacket and dropped down to the base to measure the minimum thickness. The device – a transducer on an umbilical with camera on the front – had to navigate a 90° turn to reach its target. A major concern was that if it became stuck then the evaporator would be rendered inoperable, which would be a serious setback to the reprocessing programme. A full-scale offsite test rig was built to carry out trials of the device and provide assurance to the site operators, owners and the regulators that the device could be reliably inserted and withdrawn.
The inspection went ahead when Evaporator C was off line. The minimum thickness of the base was found to be significantly more than calculated, sufficient to exceed the planned lifetime extension. Bell points out that the predicted base corrosion rates had been very conservative. “The inspection results confirmed what we believed to be the case: that the profile recorded was expected and explainable.”
Installing a fourth evaporator
Technical problems with the three older evaporators led to a decision in 2006 to construct a fourth evaporator, D. This is
a massive civil engineering project and it has been subject to soaring costs and delays. However the modules have finally been installed on the Sellafield site and the newest evaporator now occupies a large block alongside its three predecessors. Work is underway to commission the new facility and once it is proven it will take over processing magnox and Thorp waste streams. A and B will move into post operational cleanout, while C will remain on standby. By 2020, when reprocessing is completed, the HAST tanks will be empty and D will be switched to evaporating post operational cleanout liquors.
Modelling based on the results from the evaporator inspections has provided baseline data and a methodology to predict thinning in the heating components of the evaporator in future. The life-limiting features of the older evaporators are the coils, not the base. This empirical evidence has been used in the design of D, which includes built-in inspection ports and thicker and more numerous coils. Mindful of the problems experienced in understanding corrosion rates in the old evaporators, Bell notes that the commissioning team for Evaporator D is amassing as much baseline information as possible before it enters service.