NNL Support to Highly Active Waste Work at Sellafield

One of our proudest achievements over the past few years has been the combined work we have carried out to support the ongoing operation of the evaporators in the Highly Active Liquor Evaporation and Storage (HALES) plant at Sellafield. The HALES plant is an essential part of reprocessing and therefore of key strategic importance to Sellafield Limited and the NDA. Work we have carried out in close consultation with our customer has, over the past few years, directly ensured that the evaporators and several other key plants have been able to continue operating. Our work includes:

  • Inspection devices development and deployment
  • Evaporator boiling rig
  • Evaporator structural assessment
  • Evaporator thermal assessment.

Work in each of these areas was focused on providing an updated corrosion rate assessment and accurate prediction of remaining operating life of the evaporator.

Future Work Programme for Modelling of Flow and Temperature in HALES Evaporator C

Previous Computational Fluid Dynamics (CFD) and process modelling work has suggested a potential flow regime in Evaporator C at Sellafield. Applying this flow regime helps to bring predicted wall temperatures within the evaporator into better agreement with plant observations. To integrate the separate flow and temperature assessment approaches, additional rig work needed to be carried out in order to produce a robust remnant lifetime assessment.

This matter is of significant importance for our customer - given that reprocessing cannot continue without an operational evaporator. As such, NNL developed an issue resolution strategy by working with both our own experts and external academics who support and peer review the HALES programme.

This strategy has identified the necessary rigs and experimental analysis required to deliver a robust understanding of temperatures and flows in Evaporator C. Six separate trials were identified.

NNL's work has provided a route map of what is required and the key decision points. A clear programme of work required to underpin modelling work predicting temperatures within Evaporator C has been developed speedily, ensuring our customer has the required information to continue with its operations.

Vitrification of Uranium-Bearing Liquors

High Active Storage Tanks (HASTs) 1 and 2 were filled between 1955 and 1975 and contain highly active liquors from the earliest production scale reprocessing operations to take place on the Sellafield site. The composition of the liquors is different to current wastes because of technical advances. NNL was asked to carry out a number of experiments to determine how the materials behaved and how best to treat and vitrify them.

NNL established facilities at the Central Laboratory to manufacture and test glasses containing uranium. Much of the equipment required to carry out the experiments was located in our Non-Active Laboratory. NNL determined that the safest and most cost efficient way to carry out the experiments was to set up a special nuclear material accountancy area with additional security and safety measures.

NNL provides a safe, secure and cost effective capability to perform research utilising existing equipment. The results of the work have allowed Sellafield Limited to develop a strategy for the vitrification of the contents for HASTs 1 and 2 and to reduce highly active liquor stocks with the knowledge that the resulting glass product will be of an appropriate quality.

Modelling of Flow in HALES Evaporator C

Plant observations from Evaporator C were not in line with current theories for flow and boiling. This meant there was uncertainty around the predictions for heating jacket temperatures and therefore the remnant life of the evaporator. This in turn had potential implications for reprocessing.

NNL deployed a combined process/Computational Fluid Dynamics (CFD) modelling approach. A CFD model was used to indicate the likely flow regime in the evaporator up to the boiling point. However, developing a CFD model to represent boiling would have been too time consuming so a process model was developed.

Building on the CFD results, an existing process model of the medium active salt-free evaporator was modified to represent Evaporator C. The effects of boiling could then be predicted.

This innovative solution allowed an answer to be generated much more quickly than was possible using experimental trials or either modelling technique in isolation.