Opal, Opal shining bright... Giving 10 MW, half its nominal operational power outpout, during OPAL's commissioning process, brilliant Cerenkov's radiation outlines quite clearly the very compact design of the reactor core.

The reactor core design is very compact, so as to maximize neutron flux, and located under 10 metres of water, near the bottom of the main pool, surrounded by a zircalloy cooling chimney.

The core consists of an array of 16 fuel assemblies made of low-enrichment uranium silicide plates, interspersed by channels through which cooling water circulates to carry away the heat generated by the nuclear fission reaction.

The coolant water is demineralized, and it circulates upwards forced by pumping during normal operation, but with the reactor shut down, natural convection guarantees the removal of the residual decay heat. Reactor reactivity is controlled by means of neutron absorbing plates, which move vertically.

The core is surrounded by a reflector tank containing heavy water. This acts as a neutron reflector and ensures the availability of high neutron fluxes over a large volume. The reflector tank constitutes an extra safety feature as well: its quick depletion, in an emergency, starves the core of the reflected neutrons needed to keep criticality. The facilities for the irradiation of targets and the guides for the extraction of neutron beams are within this reflector tank.

		Within the reactor pool, close to the core and the reflector tank, are the cooling channels, the nuclear and non-nuclear instrumentation as well as the irradiation facilities. Waste Handling Nuclear and non-nuclear waste materials are controlled, classified, isolated and monitored in a special installation of the reactor. The radioactive liquid waste produced during the operation of the reactor, its installations and laboratories is classified, collected and temporarily stored. The system has a special refill pool, of sufficient capacity to collect the reactor water. The irradiated fuel elements are stored in baskets which are placed within the auxiliary pool. The design of these baskets and the storing conditions are such as to ensure the preservation of the integrity of the fuel cladding. The service pool has provisions for the reduction of the volume of solid metallic waste.
With OPAL churning out a modest 30 KW at the start of its commissioning, the uranium-silicide fuel assemblies begin to light-up. At the bottom of the pool, engraved in the innards of the reactor, you can read OPAL's proud identity and ancestry: owned by ANSTO, designed and built by INVAP.