The Australian Nuclear Science & Technology Organisation (ANSTO) has closed its Open Pool Australian Lightwater (OPAL) reactor at the Lucas Heights nuclear medicine precinct to undergo necessary upgrades and maintenance. OPAL is a state-of-the-art 20 MWt multi-purpose reactor that uses low enriched uranium (LEU) fuel. The $300m OPAL reactor, supplied by Argentina’s INVAP, was opened in 2007 with an estimated design life of 60 years.

OPAL is used by members of the scientific, medical, environmental and industrial communities, as well as Australian Universities. Its main uses are:

  • Irradiation of target materials to produce radioisotopes for medical and industrial applications;
  • Research in the field of materials science using neutron beams and associated instruments;
  • Analysis of minerals and samples using neutron activation techniques and delayed neutron activation techniques; and
  • Irradiation of silicon ingots (termed Neutron Transmutation Doping or NTD) for use in the manufacture of electronic semiconductor devices.

One of the main purposes of the planned shutdown is to replace an important reactor component, the cold neutron source (CNS), which lowers the energy of neutrons for some of the scientific instruments. The CNS has reached the end of its operational life of 15 years. The replacement cold neutron source offers increased scientific performance.

Although the reactor is routinely shut down about 10-11 times a year, this pause in operations will last a little longer than usual because of the complexity of the task, ANSTO noted.

The shutdown of a nuclear reactor can be done manually by an operator following a well-established operating procedure or in a matter of seconds via an independent automated system that responds to any parameter that might be out of the acceptable range. OPAL has five control rods, made of the element hafnium, which is a neutron absorber. These can be used to stop the fission of reaction.

Four of the flat plate control rods can be moved into the core (in a shutdown) and out of the core (during a re-start). A cruciform-shaped control rod that acts as a regulator remains in the centre of the core. The control rods are within the Control Rod Guide Box, which extends past the top of the core, so the control rods are protected when withdrawn from the core. The control rods are cooled by the same light water as the reactor.

During a controlled shutdown, a reactor operator manually engages an independent electric motor to drive the controls into the core slowly. In an automatic shutdown, a shot of compressed air forces the control rods to drop within two and 300 milliseconds.

Once the control rods are inserted, fission will cease and the temperature of the reactor will decrease. Even following shutdown, the reactor will continue to produce some heat because of the ongoing radioactive decay in the fuel. At about 1 minute after shutdown, the reactor power reduces to about 400 kilowatts, which doesn’t require extra forced cooling.

The nuclear analysis team undertakes a great many complex computations and calculations to ensure the reactor core always remains safe. Reactor operators also manage a process known as xenon-135 decay, which is an element that builds up when a reactor is shut down. Xenon is a neutron absorber and the process is known as poisoning out and can impact the re-start of a reactor soon after a shutdown.

ANSTO says the Nuclear Operations team is now preparing for the replacement of the cold neutron source., which is a highly complex task. To prepare for this, ANSTO built a full-size replica of the reactor pool to use as a training scenario for the event among other extensive measures. Ensuring the safety of the operation has been paramount. The level of the light water in the pool will be lowered from 13 metres to five metres. A maintenance technician will be lowered in a shielded workbox to install the new CNS.

Image: Simplified graphic illustrates how the control rods are inserted into the core of the reactor (courtesy of ANSTO)