When ANSTO's first research reactor, High Flux Australian Reactor (HIFAR), started up in the 1950s, the staff were said to be laying bets on precisely when it would 'go critical'. That is, when the reaction in the core would become self-sustaining.

Dr Greg Storr is one of ANSTO's nuclear experts working on the replacement research reactor (RRR) currently being built at Lucas Heights. He says he'll be ready to take a punt on the new reactor once he knows precisely when and how the fuel rods will be loaded. "Then we can do the calculations and I reckon we could make a good prediction," he said.

Construction of the new facility is due to finish early next year. However, turning on a reactor on involves more than simply flicking a switch.

The first step in the process is called cold commissioning because it tests the systems without fuel being involved. ANSTO personnel check that everything's consistent with the reactor design and specifications. In addition to ANSTO, the Australian Radiation Protection and Nuclear Safety Agency need this information in deciding whether to issue ANSTO with an operating licence.

Once this is demonstrated, hot commissioning can get under way. First, fuel is loaded, then the control rods are slowly withdrawn until the reactor goes critical. The starting power will be perhaps one watt -- much less than the average light globe. If all goes well, the RRR will be allowed to hum along at some tens of watts.

In the next stage ANSTO engineers slowly take the RRR up to full power (20 megawatts), testing and measuring all the while to ensure the reactor's performance is consistent with its design.

The role of the new reactor, like that of HIFAR, is to produce neutrons. But the two reactors are designed very differently. "For a start, you'll be able to see the core of the new reactor," Greg said.

This is because it is an open pool design: the core literally sits on the bottom of a deep pool. The water in the pool is pure water, known in scientific circles as 'light water'. It's used because it soaks up neutrons more effectively than heavy water.

There's heavy water in a special tank built around the core, technically called the reflector vessel. Because heavy water doesn't absorb neutrons as much as light water does, the reflector vessel becomes jam-packed with neutrons.

"This is where the guts of what we do is located," Greg said. "It's where we slide long vertical tubes filled with materials we want to make into radioisotopes, and it's where the tubes that channel neutrons for our beamlines originate."

Like HIFAR, the new reactor will produce what are known as thermal neutrons. "These emerge from the other end of the beam tubes at the same temperature as the heavy water, which is about 45 - 50 degrees Celsius," Greg said.

"What's new is that the new facility will be able to make cold neutrons. Really, really cold, about minus 250 degrees Celsius," he said.

This is important because cold neutrons let scientists study different things from thermal ones. Cold neutrons have a different wavelength from thermal ones and 'see' different structures in materials.

Another major difference is that the replacement reactor has twice the power of HIFAR, which means the amount of neutrons produced will also double.

But the really neat thing about the reactor is that it's extremely efficient neutron-capturing design means ANSTO's capacity to make radioisotopes and generate neutron beams will be much greater than it is today.