Reactor Safety
"Defense in depth" is the design philosophy of OPAL's building and systems.
There are three independent monitoring systems automatically looking for signs of trouble all the time, each of them able to trigger shut-down instantaneously if needed.
All design and construction features of OPAL guarantee effective protection of the reactor personnel, the general public and the environment against radiological hazards.
The construction of all facilities is extremely robust and within strict safety margins.
The design is fully compliant with all norms and requirements for research reactor safety established by ARPANSA (the Australian Radiological Protection and Nuclear Safety Agency) and IAEA (the International Atomic Energy Agency).
The conception of this one reactor applies two well proven design criteria in nuclear installations: "Defense in Depth", and ALARA ("As Low As Reasonably Achievable"). It thus establishes several successive protection levels and multiple physical barriers to prevent radioactive escapes. The two criteria have been applied to a huge extent, making of OPAL one of the safest research reactors anywhere.
The inherent safety of the reactor is ensured by the open pool concept and the negative reactivity coefficents of the reactor core.
An open pool means non-pressurized circuits, which in its turn hugely reduces the possibilities of leaks or other types of pipe failure. And negative reactivity means that the fuel in the core is not enough to sustain a chain reaction by itself, so the reactor can be very easily and quickly shut down at the merest sign of abnormal operation.
The reactor features passive safety, based on natural phenomena rather than on technological systems. The coolant circulation system, forced by pumps in standard operations but guaranteed by convection in other circumstances, is the principal example of this design philosophy, but there are many others, mostly in the redundant shut-down systems.
The protection systems monitor all variables relevant to reactor safety. Safety responses are triggered automatically whenever set limits are attained.
The nuclear fission chain reaction can be interrupted at any time by the action of redundant and independent shutdown devices, each based on a different physical principle and each functionally non related to the other.
The reactor safety and protection systems take the reactor to a safe shut-down condition should any anomalous operating condition be detected.
Reactor Shutdown: The first shutdown system quickly inserts five neutron-absorbing plates into the reactor core. Upon release, they just fall by gravity. The second shutdown system empties the reflector tank of its heavy water, which -again- flows by gravity into a storage tank placed underneath the core. No energy is to be supplied to make any of these two systems act.
Core cooling: The pumps of the primary cooling system feature intertial flywheels for the continuing cooling during the coastdown period of the shutdown process. No flow reversal is necessary to dissipate the reactor heat by natural circulation, as both pumping and convection make water flow through the core always upwards. The reactor pool itself is a large heat sink, able to cool down the core in any circumstances. In the case of a LOCA ("loss of coolant accident") water in the pool is replenished by gravity close to the core chimney to cool the core.
Contention: During normal operation, the ambient air circulates and its quality is continually checked. If neccessary, the reactor building can be isolated from the environment. Under these conditions, all parameters such as air pressure, temperature and moisture are then kept constant by a special energy dissipation system.