Take it all in18 May 2018
As traction gathers on the Hinkley Point C project, Ovivo UK Ltd discusses the work it is doing to provide a safe and environmentally friendly water intake system.
Hinkley PoInt C is the first of the new generation of nuclear power plant in the UK for many years. It has numerous unique features to ensure it is safe, environmentally friendly and operates reliably and efficiently. As with all thermal power plants, a key strategic element is the design of the heat sink and the cooling water system. Like other nuclear power facilities in the UK, the system selected is a once through intake design. The drum screens for the intake are some of the largest units in the world, to be supplied by Ovivo UK Ltd based in Essex. Ovivo has a long history of supplying intake screening systems globally since the 1950s.
The cooling water supply to nuclear power stations is of vital importance, both for safety-related purposes and to provide a heat sink for condenser cooling. In the UK, the most obvious source of this cooling water is the ocean surrounding the island. In the case of Hinkley C, cooling water is drawn through two 6m diameter intake pipes tunnelled 3.6km offshore into the Severn Estuary. This distance is required because of the large tidal variations in the area the intake screening system needs to be located deep in the ground.
The volumes of water used are large to ensure the temperature rise in the environment is kept within strict limits to minimise the environmental effects. The system is equipped with several measures to prevent damage to fish and aquatic life.
It is essential that the water be screened to remove debris that could cause fouling in the station. This is done in three stages:
- The offshore inlet;
- Coarse raked bar screens;
- Fine Mesh Screens.
The offshore inlet has vertical bar screens that are not cleaned. The approach velocity is kept low to permit fish to escape. This deters sensitive species that could be damaged if they enter the intake.
The coarse raked bar screens remove large debris that has been brought in through the offshore intake tunnels.
This reduces the load on the fine mesh screens and any potential damage from larger objects in the flow. The Hinkley Point C design uses 20 cable-driven raking systems with pneumatically actuated tines to penetrate between the bars and ensure all debris is lifted to the surface for disposal.
Two types of fine mesh screens are commonly used on seawater intakes and both are employed at Hinkley Point C to remove debris of over 5mm for two separate applications; band screens and drum screens.
The mesh panels on a band screen are mounted in frames on an articulating chain, driven by two sprockets at ground level. The two screens per unit are mounted in a dual flow configuration such that the mesh panels are parallel to the channel and the water flows through both sides of the band and out through the centre. This leaves the debris on the outside of the band. These screens run continuously and are provided with lifting buckets to raise bulk debris and any live fish that have entered the intake to be removed safely.
The extremely high volumes of cooling water and the depth of the intake make it necessary for the drum screens to be some of the largest ever installed. Four drum screens are provided in total, two for each unit, with each one measuring 27m diameter, 6.7m wide. They can remove all particulates over 5mm in size at a flowrate of 44m3 per second each. To put this into context, the drum screens extend nine stories down (in double decker buses, six tall by three long). Each one treats half the average flow of the River Thames in London. The flow on these units is such that the water enters through the side and out through the back of the panels, leaving debris on the inside of the screens. Debris and fish are again raised to the surface by hoppers mounted on the screen frame and elevated to ground level where they are washed off separately. These screens are equipped with three separate dedicated drive motors for each speed. This means they rotate at between 2.66 and 21.74m/min at the circumference to handle varying debris conditions.
The debris from the coarse bar screens and fish return streams are fed to a sump from where the fish are elevated to return to the sea using an Archimedes screw to prevent them from being harmed. This, in turn, is protected by an automatic, robotic Brackett Bosker raking machine.
The drum screen features a ‘unique’ nuclear approved drive arrangement with three separate drive motors, weighing approximately eight tonnes, developed by Ovivo specifically for the requirements of the Hinkley Point C project. The drive assembly can be removed in its entirety, in case of a failure, and be replaced relatively easily by the plant operators.
To prevent corrosion of the system the band screens are made from duplex stainless steel. The large drum screens are constructed from painted carbon steel and protected by an impressed current cathodic protection (CP) system. This CP system will protect the drum screens from corrosion during operation in areas where the paint has been damaged by floating debris.
Ovivo are also providing automatic self-cleaning, in line debris filters to prevent fouling of heat exchangers in the component cooling water system. These ‘RRI’ filters are specifically designed to remove debris on a semi hemispherical mesh panel. This enables a higher screening area to be provided in a smaller pipe diameter and results in a pressure drop reduction of some 30% over conventional conical or half round designs. When the pressure drop increases above the set point, the cleaning arm removes the fouling debris using upstream water pressure, preventing the need for additional pumping. These units ensure debris does not foul these vital heat exchangers.
As with all nuclear power plants, safety is always the key priority and several measures have been factored into the design of the pump house screens. This includes demanding seismic spectra, aeroplane crash loading which replicates the shock caused by an aircraft hitting the building, and exceptional loads from other events including hydraulic loads on the bar screens to cope with extreme debris conditions.
The screens are designed for a tidal range of almost 16m from high to low water levels. These levels incorporate an allowance for extreme tidal considerations and make allowance for climate change up to the year 2110.
Hinkley Point C has a design life of 70 years with equipment designed accordingly, having a projected 60-year operating life and 20 years for decommissioning. Throughout the design the demanding French standards Regles de Conception et de Construction des Materiels Mecaniques (RCCM) for mechanical design and RCCE for electrical design are adhered to, together with ASME standards as appropriate.
The Hinkley Point C schedule is controlled using Primavera project planning software and scheduled to be operational in 2025. The design phase for Ovivo is nearing completion and evaluation for procurement is now well underway. It is intended that fabrication will start in Q2 2019, with deliveries commencing in June 2021.
Ovivo is also involved in other capacities on the Hinkley Point C project, supplying the condensate polishing plant to GE and tendering for other water treatment packages through both EDF and other awarded contractors.
Ovivo sees a great future for new nuclear in the UK and is already starting work on designs for Wylfa Newydd, as well as having early discussions on Bradwell and Sizewell C.