How does OPAL work?
The key event in any nuclear reactor is controlled fission, in which a neutron hits the nucleus of an uranium atom and splits that atom, leading to the creation of more neutrons.
During fission, energy is released, some of which is carried away by neutrons released from the atom. These neutrons are what the scientists and engineers use for neutron beam research and the irradiation of materials.
Two or three high energy neutrons are produced when a uranium-235 atom fissions. In order for fission to support a chain reaction, the neutrons need to be moderated (slowed down) and reflected back into the fuel.
In OPAL, those tasks - moderation and reflection - are performed by the cooling water flowing through fuel assemblies in the core and heavy water (D20) contained in the reflector vessel surrounding the core.
Core and fuel
The compact core of the reactor is a notable feature. The whole core size is only 35 cm square and just over 60 cm high, about the size of a two-drawer filing cabinet. The small size maximises the flux of neutrons available for radioisotope production, irradiation of materials and research.
The core is an arrangement of 16 fuel assemblies, in a four by four square matrix. Each of these assemblies is 8 cm square and holds 21 fuel plates. The fuel plates contain slightly less than 20 percent uranium-235 and 80 percent uranium-238. The uranium is sandwiched between aluminium alloy plates.
There are five neutron absorbing control rods (or plates) of hafnium within the core located between the fuel assemblies. The control rods control the rate at which fissions occur and hence the reactivity of the reactor. The control rods are raised and lowered out of and into the core via a drive mechanism. One of these control rods is cruciform in shape and centrally located within the core, whilst the other four are plates that divide the core into four quadrants. The rods also provide a means by which the reactor can be rapidly shutdown if required.
The core and control rods are located approximately 10 metres below the surface of the pool.
The open pool design allows operators to see directly into the reactor core and the irradiation facilities in the surrounding reflector vessel. This enables the operator to perform various tasks such as refuelling the core and unloading/loading the irradiation facilities.
The 13 metre deep reactor pool contains approximately 200 cubic metres of demineralised ordinary water (H2O), which acts as both a coolant and a radiation shield.
The reflector vessel is a cylindrical tank of heavy water (D2O) sitting at the base of the pool of lighter water containing the core. It is used as a neutron reflector and as a location for the irradiation facilities. It is made of zirconium alloy and is 2.6 m in diameter and 1.2 m high. This vessel is vital in operating the reactor, as the heavy water reflects high energy neutrons released during fission back into the core, maintaining the nuclear chain reaction.
The majority of neutrons enter the reflector vessel at high energies. As they pass through the heavy water, they lose energy and are eventually reflected back into the core. Meanwhile, some neutrons escape and are absorbed by irradiation targets located in the reflector vessel, so not all the neutrons find their way back to the core. Some neutrons also find their way into neutron guides, where they are channelled through mirrored guides to various research instruments. Other neutrons go into detectors that the Reactor Operators use to measure reactor power.
While its main purpose is to sustain the nuclear reaction, draining the vessel provides an efficient second means of quickly shutting down the reactor if required.
The reactor pool is linked through a transfer canal to a service pool with a moveable gate for isolating the pools from each other. The service pool is used for the handling and storage of irradiated silicon, radioisotopes and the storage of spent fuel. There is sufficient capacity to store up to ten years of spent fuel.
When operating, water circulates through coolant channels between the fuel plates to remove heat produced by the fission reaction. Approximately 2000 m3/h of water is circulated per hour.
The primary OPAL cooling system operates at approximately 37°C. Two main pumps circulate water through the core and heat exchangers. Another set of pumps then circulate secondary cooling water through these heat exchangers, transferring the 20 MW to the cooling towers. Core cooling is maintained either by the main pumps during power operation or via natural circulation when the reactor is shutdown.
Natural circulation occurs when pool water passes through the core in convection currents; that is, warmer water rises and is replaced by cooler water. People will often see steam clouds near a reactor when it's operating and this is a result of evaporation of secondary water from the cooling towers.
The control plates can either be lowered or dropped under gravity depending on the speed of reactor shutdown required. The quickest shutdown possible from full power takes less than a second. The hafnium rods absorb neutrons, thereby reducing the fission rate and effectively stopping the nuclear chain reaction.
The second shutdown system is also available as an independent and diverse means of reactor shutdown. This system drains some of the heavy water from the reflector vessel, preventing neutrons from being reflected back into the core and effectively stopping the nuclear chain reaction.
The reactor is housed in a steel reinforced building designed to withstand external events, including a one in ten thousand year seismic event and impact from a light aircraft. In addition to providing structural integrity, the mass of re-enforced concrete forms a structural base for the reactor.