Dounreay

Dounreay (/ˌduːnˈreɪ/;Scottish Gaelic: Dùnrath) (Ordnance Survey grid reference NC982669) is on the north coast of Caithness, in the Highland area of Scotland and west of the town of Thurso. Dounreay was originally the site of a castle (now a ruin) and its name derives from the Gaelic for 'fort on a mound.' Since the 1950s it has been the site of two nuclear establishments, for the development of prototype fast breeder reactors and submarine reactor testing. Most of these facilities are now being decommissioned.

History

Dounreay formed part of the battlefield of the Sandside Chase in 1437.

The site is used by the United Kingdom Atomic Energy Authority (Dounreay Nuclear Power Development Establishment) and the Ministry of Defence (Vulcan Naval Reactor Test Establishment), and the site is best known for its five nuclear reactors, three owned and operated by the UKAEA and two by the Ministry of Defence.

The nuclear power establishment was built on the site of a World War II airfield, called RAF Station Dounreay. It became HMS Tern (II) when the airfield was transferred to the Admiralty by RAF Coastal Command in 1944, as a satellite of HMS Tern at Twatt in Orkney. It never saw any action during the war and was placed into care and maintenance in 1949.

Fast-neutron reactor

A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons. Such a reactor needs no neutron moderator, but must use fuel that is relatively rich in fissile material when compared to that required for a thermal reactor.

Introduction

Basic fission concepts

In order to sustain a fission chain reaction, the neutrons released in fission events have to react with other atoms in the fuel. The chance of this occurring depends on the energy of the neutron; most atoms will only undergo induced fission with high energy neutrons, although a smaller number prefer much lower energies.

Natural uranium consists mostly of three isotopes, U-238, U-235, and trace quantities of U-234, a decay product of U-238. U-238 accounts for roughly 99.3% of natural uranium and undergoes fission only by neutrons with energies of 5 MeV or greater, the so-called fast neutrons. About 0.7% of natural uranium is U-235, which undergoes fission by neutrons of any energy, but particularly by lower energy neutrons. When either of these isotopes undergoes fission they release neutrons around 1 to 2 MeV, too low to cause fission in U-238, and too high to do so easily in U-235.