Australia's first reactor

HIFAR reactor at ANSTO

Some benefits from HIFAR included:

1. production of radioisotopes for medical purposes and for industry
2. silicon transmutation doping
3. neutron Activation Analysis and Delayed Neutron Analysis for the mining industry and forensic purposes
4. the production of gamma ray sources for sterilisation purposes, cancer therapy, industry etc.
5. neutron diffraction purposes for the study of matter.

General introduction - the reactor

HIFAR (High Flux Australian Reactor) was Australia's national research reactor. It was central to the research that occured at ANSTO and it operated safely and reliably for over 50 years.

The purpose of HIFAR was to produce neutrons for the production of nuclear medicine and for scientific use. Neutrons are those subatomic particles found in the nucleus of all atoms. HIFAR produced neutrons through the process of fission, the splitting of a large atom, such as uranium, into two smaller ones. Fission occurs when a heavy nucleus absorbs a neutron and splits. Neutrons are given off in the process of fission and, after slowing down (losing energy), are used to keep the fission chain reaction going.

When operating at rated power, HIFAR produced billions upon billions of neutrons. Materials in the reactor absorb many of these tiny particles. Outside the nucleus of an atom a neutron cannot exist alone; it is unstable and will decay within minutes. So a reactor must continually produce more neutrons to serve its purposes.

HIFAR history

HIFAR was originally built to test materials for use in future power reactors. The idea was to test materials in HIFAR by subjecting them to a neutron flux; a relatively quick assessment could be made of their suitability for use in a power reactor. With the decision not to pursue a nuclear power program in Australia, there was a gradual change in how the reactor is used over the years.

The construction of HIFAR commenced in February 1956, it first went critical on 26 January 1958 and routine full-strength operation commenced in January 1960.

HIFAR was a copy of the DIDO reactor in the United Kingdom, which was, in turn, modelled after the CP5 reactor built near Chicago.

Operating program

HIFAR was manned continuously 24 hours a day, seven days a week and staffed by highly trained technical officers. The reactor operated on a 35-day cycle, which was termed an operating program. The first operating program commenced in January 1960.

The operating program commenced with a routine shutdown. During the following three days the reactor underwent fuel changes and any necessary maintenance and development work which could not be done when the reactor was operating. The reactor was started up and operated at about 10 MW thermal for the next 32 days. The reactor was then shutdown and the cycle repeated.

A major shutdown was scheduled approximately every five years for major overhaul, inspection, and modification work.

The reactor core

The core contained 25 fuel elements in a symmetric array and was approximately 60 cm high by 91 cm in diameter. The elements contained approximately 280 grams of uranium enriched with U235 (approximately 170 grams U235 per element).

The uranium was contained in four inner concentric tubes in the element. These tubes were actually a sandwich with the "bread" being pure aluminium and the meat being uranium / aluminium alloy. The outer aluminium was referred to as the cladding. Fuel consumption resulted in the need to replace approximately 30 elements each year.

Reactor power was proportional to the number of neutrons in the core. Neutron absorbers called coarse control arms controlled the neutron flux. Six coarse control arms, which resembled railway signal arms, moved in unison in an arc between the rows of fuel elements. They were made of cadmium sheet (the neutron-absorbing medium) encased in stainless steel. These control arms had a service life of several years.

The core was cooled by heavy water (D₂O) which was pumped upwards through the annular gaps between the fuel tubes. The heavy water circuit contained about ten tonnes of this water, six tonnes of which resided in the Reactor Aluminium Tank (RAT) - the tank that housed the core. The entire circuit was sealed such that there was no leakage. The heat of fission was removed by heat exchangers situated under the reactor which transfered it to a secondary cooling circuit from where it was transferred to the atmosphere by six cooling tower fans. The temperature of heavy water leaving the RAT after first passing through the fuel elements was maintained at about 50 degrees Celsius (the temperature of a hot bath).

The neutron reflectors

The heavy water not only cooled the core, but also performed the function of slowing down the neutrons to an energy that was best suited to maintaining the fission process.

Steps were taken to prevent neutrons born in the fission process escaping from the core region. The core was located centrally in the RAT, which is 3.3 m high and 2 m in diameter, the tank being filled with heavy water. The few neutrons that may have escape from the core were reflected back into it by the heavy water, which surrounds it. This water which surrounds the core is termed "The Heavy Water Reflector".

Those neutrons which penetrate the heavy water reflector encountered another reflector made of graphite (about 57 cm thick) and this also reflected neutrons back to the core region. The neutrons present in the reflector regions are well-thermalised (ie slow or low energy) and there were experimental facilities in the reflectors to make use of such neutrons.

Shielding

Several kinds of materials were used to keep radiation within the reactor. The boron in the lining of the inner steel tank captured neutrons that penetrated the outer (graphite) reflector. Gamma radiation was attenuated by the thermal shield, which consisted of a 10 cm layer of lead. The thermal shield drew heat generated in the graphite and the concrete biological shield. This heat was then removed by a cooling system, which had cooling coils passing through the lead.

The final shielding, which brought the radiation levels down to negligible levels for personnel working in the area, was the concrete biological shield. This was made up of high-density concrete .

The containment building

HIFAR was designed with three barriers, each of which is capable of protecting the health and safety of the public from radiation. The fuel cladding and the coolant pressure boundary were the first two. The third boundary consisted of the containment structure. The containment building was 21 m in diameter and 21 m high and constructed from steel plate. It was designed and constructed as a pressure vessel. It was the third barrier to the release of fission products to the environment in the very unlikely event of release of radiation.

Fresh air continually entered the building during normal operations. It was filtered entering and leaving the building. If for any reason the system detected radiation, all air ducts would close automatically and the containment would prevent the release of radioactivity.

The reactor control room

This room, situated in the containment building, was the nerve centre of the reactor. The monitoring of the most important parameters, reactor control and communications were carried out from this room. This room was continually manned 24 hrs per day, 365 days per year.

Fuel element and rig handling flasks

Heavily shielded flasks, which are fabricated from steel and are lead lined, were used to load new fuel elements and unload radioactive used fuel elements from the reactor core. Rig movements were also done with these flasks as well as with rig flasks. A polar crane that rotated on a circular track above the reactor hoisted these flasks.

Decommissioning

HIFAR is currently being decommissioned and will be totally decommissioned by 2018.