We were tasked to undertake a calculation or analysis to assess the ventilation system of this ILW Dry storage facility. CFD allowed us to explore multiple system design parameters in a matter of days, a massive time and cost saving over practical instrumentation and testing.
Our client is the site licence company (SLC) responsible for the clean-up and demolition of Britain’s former centre of fast reactor research and development.
Located on the northern coast of Scotland, the site has been a centre for nuclear research and power generation since the mid-fifties; the facility is now undergoing decommissioning and will accommodate the safe storage of radioactive materials for many years to come.
Waste material is generally classified as Low, Intermediate and High level (LLW, ILW and HLW respectively) and is processed and stored in different ways. In the case of ILW, the waste material is often immobilised inside a steel drum using a cementitious grout and then held in a suitable storage facility.
The typical criteria for a storage facility will be to maintain security, protect personnel and public from radiation and gaseous emissions and to minimise package degradation due to water or humidity.
Working with a specialist ventilation provider, we supported our client’s evaluation of an ILW Dry Store option, with a focus on maintaining a steady circulation of air around the waste drums.
Our client, in collaboration with their suppliers, undertook a front-end engineering design (FEED) study to identify a concept to build a new ILW store with capacity for 8,000 drums. Various criteria were considered, from cost to heat build-up and moisture emission from the fresh concrete. In parallel a further FEED study focussed upon the re-use of an existing building, that could provide the necessary capacity but without the capital build costs.
Fundamental to the feasibility of this re-use approach would be the capability of the existing ventilation system to provide regular air changeover, maintain appropriate temperature and humidity and avoid the accumulation of non-life sustaining gases such as Hydrogen & Radon, that could be emitted from the ILW drums over extended periods.
We were tasked to undertake a calculation or analysis to assess the ventilation system; having reviewed the store geometry we concluded that a calculation would not provide sufficient detail of air flow rates in the lower areas of the store and hence a computational fluid dynamics (CFD) approach was adopted.
At PDL, we always start any analysis task with an Analysis Plan, it acts as a focus for us to capture all of the clients’ requirements, identifies all pieces of missing data and provides a framework for review and approval processes throughout the task.
Once the client accepted the analysis plan, we set to work building a 3D CAD model of the facility from the supplied architectural drawings and populating it with stacks of waste drums in their stillages. This work was undertaken using PTC Creo; a check print was then generated to allow the model accuracy to be easily checked, before transferring the model into ANSYS DesignModeler in which the fluid domain was extracted, and geometry split and named for subsequent meshing.
One of the significant challenges of the meshing strategy was length scale; the drum separation was in the order of 100mm and needed sufficient cells to resolve the local flow, versus the facility size of 100+ metres. To mesh throughout, in the order of millimetres, would result in lengthy solution times, so we carefully applied size controls, and inflation layers were required to achieve an optimised mesh.
Once we had defined boundary conditions (inlet, outlet, thermal emission from waste etc.) then we could solve the analysis cases;
- to validate the model against empirical operational data for the current as-built ventilation design,
- to assess the air velocity and temperature distribution for varying inlet temperatures,
- to develop new ventilation concepts within the confines of the existing infrastructure and finally
- to assess the new ventilation concepts over an extended operating timeframe.
In each case a series of consistent post-processing outputs and monitor points were captured, thus allowing a ‘like for like’ comparison of each option to be tabulated, compared to the existing system and reported accordingly.
- Multiple system design parameters could be explored in a matter of days in the virtual CFD environment, a massive time and cost saving over practical instrumentation and test.
- The application of computational fluid dynamics CFD allowed us to demonstrate that the existing infrastructure could meet the project objectives through achieving the necessary airflow and thermal management.
- The use of computational fluid dynamics (CFD) rather than calculations vastly enhanced the ease for non-technical staff to visualise results, from spot values to contour plots and streamlines.
- The project as a whole sought to significantly reduce CAPEX costs through the re-use on an existing building.
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