Posts Tagged ‘neutron science’
Australian Greens senator Scott Ludlam has again this week called for Australia’s 20 MW OPAL research reactor to be closed down, following reports that a minor problem with the neutron reflector in this “tank-in-pool” reactor has yet to be rectified.
The facility has been out of operation for 11 of the past 14 months, during which time Australia has had to rely on costly imports of medical and scientific radionuclides from foreign suppliers in South Africa and Canada.
The OPAL reactor core sits in the centre of a heavy water neutron reflector, which itself sits within the reactor’s large pool of light water, as we see in the above diagram.
In the centre of the circular heavy water vessel is the nuclear fuel itself, an array of 16 fuel assemblies. The large and small holes that pass through the entire height of the reflector, into the reactor core, support the generation of products such as transmutation-doped silicon and medical and scientific radionuclides as well as supporting neutron irradiation experiments. Several different neutron beamlines are also installed into the reactor, set up for different neutron spectra, including a liquid deuterium moderated cold neutron source.
Here, the square reactor “core” is clearly visible in the centre of this photograph, illuminated strongly by its own Čerenkov radiation, with the round neutron reflector surrounding it, pierced by the aforementioned ports for the irradiation of samples, with the greater reactor pool, containing light water, surrounding that.
The purpose of the neutron reflector is to improve neutron economy in the reactor, and hence to increase the maximum neutron flux – neutron flux being a fundamentally important metric of the performance and usefulness of a research and isotope production reactor.
To maximise the neutron flux or neutron economy in the reactor, heavy water, being a good moderator, basically a material from which elastic scattering of neutrons readily occurs, is used to construct a neutron reflector, immediately surrounding the reactor.
You’ve got light water from the pool seeping into the heavy water neutron reflector that surrounds the reactor. So, the light water from the pool is “leaking” into the reactor components, in towards the reactor. The reflector vessel is kept at a lower pressure than the light water at ambient pressure in the reactor pool. Any leakage pathway at all will allow light water to seep into the reflector vessel, diluting the heavy water. This issue was first identified in December of 2006, following commissioning of the new reactor, and attempts have been made to address the problem during an extended shutdown, which have been somewhat, but not totally, successful.
The sole consequence of this is that it dilutes the expensive heavy water. Of course, some people, and some media reports, seem to persist in documenting such a “leak” as though it were luminous green radioactive goo tricking out into suburban Lucas Heights.
If the heavy water is diluted to any significant extent, the efficiency of the neutron reflector is diminished, and the neutron flux that is achieved under nominal operating conditions is diminished, making the reactor less efficient for neutron beam experiments, neutron irradiation or radionuclide production. There is absolutely nothing here of any health physics or safety significance, at all, period. This dilution of the heavy water in the reflector vessel has absolutely no significance with regards to safety of the facility.
The Greens have derided ANSTO’s comments on the nature of the fault as “spin” and link these technical concerns to some kind of supposed, imaginary potential for safety concerns in the future. Of course, Ludlam wouldn’t know what a neutron reflector was if it bit him, and he has a proven track record of carrying on fervently about issues of nuclear science and technology, whilst possessing an alarming lack of understanding of such science and technology; especially for a federal politician.
Once the heavy water in the vessel becomes diluted, the only way to un-dilute it is via the same methods of deuterium enrichment such as those originally used to make it – such as distillation, or the Girdler sulfide process. In the case of a high deuterium concentration, as in a tank of somewhat diluted heavy water, distillation is the best option. Apparently, ANSTO are planning to construct a small-scale heavy water re-distillation system for online re-enrichment of some of the heavy water passing through the reflector circulation loop. This will fully counteract the problem, and allow the use of the reactor with the fullest efficiency for research and isotope production.
Anyway, Senator Ludlam and the Greens are not just content with calling for the reactor to be shutdown until the heavy water dilution issue can be rectified or nullified, however – they are quite adamant in calling for the permanent shutdown of the reactor.
“We think the safest solution for this reactor is for it to be shut down and for the waste to be contained properly,” Greens senator Scott Ludlam said this week. Importing radionuclides from international suppliers such as in South Africa and Canada could continue, he said.
In addition to the production of medical radionuclides, the reactor is used to produce neutron-transmutation-doped silicon boules for microelectronics – a valuable commercial service marketed by ANSTO – as well as for the production of radiopharmaceuticals and scientific radiochemicals. The radionuclides, most of them employed in nuclear medicine, typically commonly produced with ANSTO’s reactor, are thus:
Samarium-153 – 1.93 days
Molybdenum-99 – 2.75 days
Indium-111 – 2.83 days
Iodine-131 – 8 days
Chromium-51 – 27.8 days
Iodine-125 – 59.4 days
Half-lives are as indicated. The short half-life of 153Sm, the basis of the onocological radiopharmaceutical Quadramet, in particular means that importation of this radionuclide is difficult and impractical, and it is essentially unavailable in the absence of an operating isotope production reactor in Australia.
We’ve learned from painful experience that the supply of expensive imported radionuclides has been subject to delays or interruptions to supply during shutdowns of OPAL (and HIFAR) in the past. On the basis of ANSTO’s past experience, it can reasonably be assumed that still worse problems would arise if Australia were to be totally reliant upon imported radionuclides. The supply problems arise from a range of causes, such as weather delaying flights, aviation regulations relating to radioisotopes being carried with other goods, or opposition from freight pilots.
The International Atomic Energy Agency has identified the “growing problem of refusal by carriers, ports and handling facilities to transport radioactive material” as a significant problem for nuclear medicine and scientific research involving radionuclide importation across the world, and has initiated processes intended to identify ways in which it can be overcome. A number of international, such as British Airways, no longer accept carriage of radioactive material, and others have imposed tight restrictions. Unless a way can be found to reverse such trends, shipments of radionuclides across the world will become increasingly problematic.
The reactor and its associated neutron guides and instruments are used for neutron radiography, neutron scattering imaging, neutron reflectometry and other advanced neutron-beam based research and technological applications, neutron activation analysis, for example for forensic applications, as well as the analysis and testing of materials under neutron irradiation and research into the potential for Boron Neutron Capture Therapy as a potent weapon against cancer – which requires the patient to be bought to a nuclear reactor to produce the thermal neutron flux required.
Even if some radionuclides can be imported, clearly our research reactors in Australia are of significant importance and usefulness in such fields. If radionuclides are to be imported from foreign suppliers, they are still being produced in similar nuclear reactors – if a research reactor is such a dangerous thing, as is suggested by these groups, why should foreign nations be subjected to such a burden for the production of radiopharmaceuticals which are for the benefit of us? Why shouldn’t we take responsibility for our own reactor, if we have decided that we value the benefits of its products, and we’re not prepared to forgo them?