31 Oct 13

Dungeness B offline - Background note



Background


Dungeness B is a twin reactor advanced gas-cooled station situated on the Kent coast approximately 30km south-west of Dover.

It was the first commercial advanced gas-cooled reactor, build started in 1966 and the site started generating power in 1983.

The station consists of two reactor units with a shared services unit for services, instrumentation and control, and a shared turbine house.

Each reactor drives a single 660 MW turbine generator set.

Advanced gas-cooled reactors use carbon dioxide as the coolant to remove heat from the fuel (the primary circuit).

At Dungeness B the carbon dioxide is circulated around the fuel using four electrically driven motors which enable the heat to be transferred to boiler assemblies, known as the secondary circuit.

Water fed to the boilers is heated and converted to steam where it is then used to drive turbines and generate electricity.



Electrical supplies


Electricity produced by Dungeness B power station is exported from the station turbines by a high voltage alternating current (ac) transmission system to supply the national grid.

This transmission system can act as a two-way system allowing the power station to draw electricity from the grid.

Under normal operating conditions the electricity taken from the grid by the station is used to power the various systems around the station.

If loss of off-site power happens - as it did on October 28 - the power station is capable of operating independently until grid connections are restored.

The decision is always taken to shutdown the reactors if the site loses grid connection, electricity is then provided by the on-site diesel generators which power the essential on-site plant.

These diesel generators provide the equivalent power for around 3000 homes.

There are several sets (groups) of diesel powered generators designed to provide power to safety critical systems, which will automatically start when the grid connection is lost.

Generators produce power at different voltages which are suitable for the systems they serve.

The site has many days back up diesel supplies on site, and once grid supply is lost a key role for the station’s team is to source new diesel supplies.

The reliability of on-site power is assured by providing sufficient independence and redundancy of diesel generators and batteries. The on-site electrical distribution systems are capable of performing essential safety functions even if a single failure occurs.



Consumables and re-supply


Sufficient stocks of consumables such as water for the boilers and diesel fuel for operation of the diesel powered generators are stored on site.

It is a normal requirement within EDF Energy to have sufficient stocks of essential consumables on each site for independence from offsite support for a minimum of 24 hours. Usually sufficient stocks are held on site to allow operation for a much longer period.

Following an emergency, station staff will continuously review stocks of essential consumables and make whatever arrangements for their supply/replenishment that are considered appropriate.

If a significant incident occurs at any of EDF Energy’s sites the Central Emergency Support Centre – based in Barnwood - is set up to manage off-site technical resources in conjunction with the on-site Emergency Control Centre and the local emergency services.

If required the Central Emergency Support Centre can contact specific suppliers, with which there are pre-existing arrangements, to deliver additional supplies to the site within the required time period.

With delivery of consumables to site, successful post trip cooling of the reactors can be maintained indefinitely.