Capabilities

OPAL utilisation facilities may be sub-divided into:

• Neutron Beam Research Facilities; and
• Irradiation Facilities for radioisotope production, neutron-activation analysis of materials and silicon ingot doping, for the semiconductor industry

Neutron Beam Research Facilities

The reactor is especially designed to sustain neutron-related research, with three different sources supplying neutrons with varying ranges of energy.

• One cold neutron source, for very low energy neutrons. Cold neutrons with accordingly large wave lengths are important for biological application of neutron scattering. In this field, larger objects can be studied, rather than objects of atomic size.
• One thermal neutron source, for medium range energy neutrons. Thermal neutrons are neutrons in the energy range comparable to room temperatures. Two examples of use for thermal neutrons are the investigation of fluids, or therapy of tumours.
• There is a further provision for a future option to provide a hot neutron source for higher energies. Hot neutrons are needed especially for scattering experiments on fluids. Their scattering image has to be a snap shot of the atomic order; the operator needs short interaction time and high velocity of the neutrons.

Five assemblies for the extraction of neutron beams are grouped around the reactor core. From these, the neutrons are led towards the neutron guides.

The neutron guides conduct the neutrons from the sources with minimum attenuation towards the neutron beam hall instruments, which can be as far as 40 metres away from the reactor core.

At present OPAL has two thermal and two cold neutron guides extending into the neutron guide hall. There is capacity for further expansion, including potential for a second neutron guide hall. The guides are operated under vacuum, because neutrons are scarce: there are 10 billion more air molecules than neutrons in a cubic centimetre of a guide.

The neutron guides begin 1.5 metres from the reactor core and continue through beam shutters, without windows, until the outer perimeter of the reactor. These neutron guides are 50 mm wide and between 50 mm and 300 mm high. The guides are curved between the reactor face and the exit of the guide bunker, to beyond line–of–site, to reduce contaminating radiation.

Cold Neutron Source

In order to slow down neutrons produced at the core, there is a special device, a cold neutron source (CNS), installed in the reflector vessel. The CNS uses liquid deuterium (an isotope of hydrogen) and operates at very low temperatures (about –250⁰C). The CNS is supported by a cryogenic plant and a 500 kW compressor. The CNS moderator is cooled by Helium vapour circulating through the heat exchanger and in an outer cooling jacket.

With this device, neutrons are “moderated” to lower energies: the neutrons have around 3 times less energy than in thermal guides. The cold neutron source is located as near as practical to the peak in the thermal neutron flux (~50 cm from the core, centre to centre).

Neutron Beams

The best way to use neutrons for research investigations is by conducting them through a tube to special equipment on which scientists can do their neutron research.

Guides

At present OPAL has two thermal and two cold neutron guides extending into the neutron guide hall. There is capacity for further expansion, including potential for a second neutron guide hall.

There is a neutron beam transport system from OPAL to the beam hall consisting mainly of super-mirror guides.

Neutron super-mirrors are devices for transporting, bending and focusing neutron beams. Neutrons are reflected off the surfaces of the inside of these guides, made by sputtering layers of nickel and titanium onto a surface. The guides are operated under vacuum, because neutrons are scarce: there are 10 billion more air molecules than neutrons in a cubic centimetre of a guide.

The neutron guides begin 1.5 metres from the reactor core and continue through beam shutters, without windows, until the outer perimeter of the reactor.

These neutron guides are 50 mm wide and between 50 mm and 300 mm high.

The guides are curved between the reactor face and the exit of the guide bunker, to beyond line–of–site, to reduce contaminating radiation.

Irradiation Facilities

There are provisions in OPAL for the following irradiation facilities:

• General purpose irradiation facilities.
• Bulk irradiation facilities.
• Large volume irradiation facilities used for irradiating silicon.
• Neutron activation delayed neutron activation facilities.

The irradiation facilities are contained in various tubes within the reflector-tank moderator that access neutron fluxes of varying wavelengths as required for the material being irradiated and its end purpose. For example, the large volume irradiation facilities used for irradiating silicon so that it may be "doped" by neutron transmutation are located around the outer part of the reflector vessel.The irradiation facilities are supported by extensive pre- and post–irradiation facilities handling stations in the reactor service pool and a complex of hot cells. However, no processing of irradiated material takes place within the OPAL facility. Instead, there is a pneumatic shuttle system between OPAL and ARI processing and transport centre.