Hunterston Nuclear Generating Station, 'b' Power Station

Produces 1288MW of electricity from two advanced gas-cooled reactors.

R Paxton and J Shipway 2007

This project was undertaken to input site information listed in 'Civil engineering heritage: Scotland - Lowlands and Borders' by R Paxton and J Shipway, 2007.

Hunterston ‘B’ power station, commissioned in 1976, produces 1288MW from two advanced gas-cooled reactors developed from the original Magnox reactors. It includes a visitor centre which offers tours of the station and displays. From this station and Torness nuclear power station, British Energy Plc generates up to 55% of Scotland’s electricity.

R Paxton and J Shipway 2007

Reproduced from 'Civil Engineering heritage: Scotland - Lowlands and Borders' with kind permission of Thomas Telford Publishers.

HUNTERSTON B POWER STATION

Hunterston B Nuclear Power Station is an electricity generating, second generation nuclear power station. It is located on the Firth of Clyde on west coast of Scotland, some 50km south west of Glasgow. It has two Advanced Gas Cooled Reactors or AGRs, supplying steam to two 660MW turbo generators. It was built by The Nuclear Power Group consortium and operated by the South of Scotland Electricity Board. It is now owned and operated by EDF.

Along with Torness in East Lothian (1), Hunterston B generates up to half of Scotland’s electricity. Hunterston B stands adjacent to Hunterston A, a decommissioned Magnox type nuclear power station commissioned in 1962 whose two reactors (Reactors 1 and 2) were shut down in 1989 and 1990.

Electricity was first generated at Hunterston B in February 1976, although it was not officially opened until 1980. Reactor 4 is due to be decommissioned (shut down) on 7th January 2022.

The basic components needed in any commercial nuclear reactor are uranium fuel, a moderator (slows down the neutrons to allow fission), control rods (to control the chain reaction by removal or addition) and coolant (transfers the heat from the reactors to the boilers to generate steam). At Hunterson B the fuel is uranium dioxide (in the form of small ceramic pellets packed into stainless steel cannisters one 1.0m in length), the moderator is graphite, the 81 control rods in each reactor are of boron and steel and the coolant is carbon dioxide. The AGR is a development of the Magnox reactor, and is a design only built in Britain.

Electricity Generation using Nuclear power

In a nuclear power station, heat from nuclear fission is transferred from the fuel elements to the boilers by the coolant gas. Steam is produced in the boilers and is supplied to turbines at high temperatures and pressures. The turbines, in turn, drive generators which produce electricity just as in any other type of power station.

The driving force at Hunterston B as in any nuclear power station is the vast energy released during nuclear fission – the splitting of uranium atoms. Neutrons in the reactor as slowed down by collisions within a moderator, the graphite core in the case of the AGR. These slow neutrons split the uranium atoms releasing heat and two or three more neutrons. Control rods absorb excess neutrons and are adjusted to leave sufficient neutrons available to cause further nuclear fission reactions – known as a controlled chain reaction. Uranium dioxide is the only naturally occurring material which will sustain such a chain reaction.

The Reactors

The two reactor units (Reactor 3 and Reactor 4) and turbines (7 and 8) are housed in a single building. The central block of this building is where fuel handling, instrumentation and control take place. Reactor 3 and Reactor 4 are situated at each end of the single block and are served by a refuelling or charging machine weighing 600 tonnes and standing 30 metres high which operates within the charge hall. The accommodation of the refuelling machine explains the height of the reactor control building.

Reactor Core

Each reactor core is a 16-sided stack of 25000 graphite blocks containing 308 vertical fuel channels arranged on a square lattice. Access to boilers and all plant other than the core is made possible during nuclear shutdown by provision of a radiation shield wall surrounding the core and a separate shield above it. The core and the shield are completely enclosed by a gas baffle – a device to restrain the flow of the coolant. This is a vertical cylindrical area topped by a welded dome provided with holes correspond-ing to all the 308 channels in the core. The core support structure or diagrid supports the core, shield wall and the gas baffle which in turn rests on rocker type supports. Sitting on bearing plates in the pressure vessel floor, these allow for expansion of the diagrid.

The heat produced during the chain reaction within the reactor is carried by CO2 gas pumped through the core at high pressure by the eight gas circulators (pumping at 500kg gas per second) and leaves the reactor at about 650 degrees Celsius. The gas baffles (containing carbon dioxide coolant) produce a two-way flow of gas through the core. About half the flow passes up between the baffle and the core, cooling the boiler shield wall as it rises. It then re-enters the graphite at the top of the core and further cools the core as it passes downwards (re-entrant flow). The other half goes below the diagrid and all flows combine at the bottom of the core. The full flow then passes up the 308 fuel channels (the insides of the graphite fuel sleeves) and discharges from the upper fuel element guide tubes into the area between the gas baffle dome and the roof. It then flows down through the boilers before re-entering the gas circulators.

Each reactor is contained in a cylindrical, pre-stressed concrete pressure vessel with a wall thickness of 5 metres, the top cap 5.5m and base cap 8.5m in thickness. The pre-stressing and post tensioning system has 3000 steel pre-greased tendons in helical formation threaded through 8cm diameter mild steel tubes embedded in the concrete during construction. These tendons can be accessed for monitoring, restressing and replacement at upper and lower stressing galleries. The pressure vessel has an inner liner fabricated from mild steel plates which operates as a gas tight containment membrane cooled by water pipes on its outer surface. The inside surface of the liner is insulated and the combination of insulation and the cooling system maintains the constant temperature. There is access through the walls of the pressure vessel for boiler feedwater, main and re-heat steam pipework and instrumentation.

Boilers

There are 12 boilers containing steel tubing carrying pure water arranged three to a quadrant around the core of each reactor through which pure water is pumped (from the mains and treated). Once-through boilers were chosen to minimise the number of penetrations through the vessel. Mild steel is used for the boiler tubes in the low-temperature region up to 350 degrees Celsius; 9% chrome in the mid-temp region (350-500 degrees Celsius) and austenitic steel above 500 degrees Celsius.

Gas circulators

There are two centrifugal gas-circulators in each of four boiler quadrants (so eight per reactor). Each gas circulator has its own motor and control gear and is a totally enclosed unit located in a horizontal penetration at the bottom of the pressure vessel wall. The hot CO2 heats up the waterfilled tubes to produce superheated steam. This steam is piped to the turbine hall. The secondary cooling system using fresh water cools the bearings of the gas circulators.

Fuel

Single road-borne delivery AGR fuel provides electricity equivalent to third of a million tonnes coal or 16000 lorries.

Each fuel element has 36 tubes containing hollow pellets of enriched uranium dioxide, canned in stainless steel. Each 36-tube unit is encased within a graphite sleeve and eight of these are linked together by a tie bar running down the middle forming a fuel assembly.

There is a single charge machine and gantry for fuel handling. The charging machine (the main shielding of which is iron shot concrete) contains three turret tubes. One is for withdrawing a used fuel unit, one to carry new fuel unit for insertion and one spare plug unit. On each reactor, the refuelling programme requires about three channels of fuel to replaced per fortnight. Fuel is loaded while the reactors are in operation unlike other designs of nuclear station.

Spent fuel

After discharge, irradiated fuel is transferred to one of 25 pressurised buffer storage tubes for a decay period of about two days. On removal from the decay storage tube, the fuel stringer is broken down into its component parts in a dismantling facility and the irradiated fuel elements are stored in water ponds at the bottom of the control building to allow further decay of fission product heating for several weeks. The elements are then transferred to steel transport flasks for road and railway journey to Sellafield for storage and processing. Fuel throughput of the twin reactor station about 40 tonnes per annum.

Turbo Generators

Turbo generators are generators connected to the shaft of steam turbines to generate electricity in a thermal power station. Hunterston B has two turbo-generators, each run by one of the two reactors and each has a water-cooled stator and hydrogen cooled rotor. The generator supplies power at 23.5 Kilovolts.

Each turbine (C. A. Parsons and Co.) comprises one high pressure (HP) cylinder, one intermediate pressure cylinder (IP) and three low pressure (LP) cylinders and integral condenser assemblies. The HP, IP and three LP rotors are coupled to form a single shaft supported by housed journal bearings. The turbines make 3000 revs per minute. The steam leaving each turbine is condensed back to water by a separate sea-water cooling system and recirculated to the boilers. Hunterston’s coastal position means that it has no need of cooling towers as with inland power stations.

The condensers contain 8,700 titanium tubes. There is a feed flow from the condensers to the boilers that is maintained by a turbine feed pump, with two stand-by electric pumps.

Central Control Room

Automatic, semi-automatic and manual operation of the plant can be selected from the control room. The control room controls the reactor and boilers to match steam requirements of turbo-generators. The reactor follows the demand of the turbine with automatic control loops. The principal loops are: (a) automatic regulating rods controlling reactor power by maintaining the reactor gas outlet temperature of each reactor constant; (b) boiler -feed control loops regulated to maintain the turbine stop-valve pressure constant; and (c) coolant gas flow loops regulating circular guide.

Reactor Shutdown

Each reactor can be shut down a reactor automatically if temperature, coolant flow or reactivity exceed fuel tubes safety limits. The automatic protection system ensures the neutron absorbing boron and steel rods fall under gravity into the reactor core and shut down the reactor

Decaying of fission products still produces heat, so cooling of the reactor core is still required and a greatly reduced percentage of the full power heat. There are stand-by diesel generators in the event of loss of electricity supply to the national grid.

Transmission

Hunterston B has a purpose built 400 kilovolt transmission switchhouse.

(1) Torness AGR power station is some 35 miles from Edinburgh and operated by the South of Scotland Electricity Board from 1980 and commissioned in 1988. It is currently owned and operated by EDF Energy.

Information from Heritage Research Service (M McDonald), Heritage Directorate, August 2021.