# OPAL: Wikis

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# Encyclopedia

OPAL (Open Pool Australian Lightwater reactor) is a 20 megawatt (MW) pool-type nuclear research reactor that was officially opened in April 2007 at the Australian Nuclear Science and Technology Organisation (ANSTO) Research Establishment at Lucas Heights, located in South Sydney, Australia.

The main reactor uses are:

• Irradiation of target materials to produce radioisotopes for medical and industrial applications
• Research in the field of material science using neutron beams and its sophisticated suite of experimental equipment
• Analysis of minerals and samples using the neutron activation technique and the delay neutron activation technique
• Irradiation of Silicon ingots in order to dope them with phosphorus and produce the basic material used in the manufacturing of semiconductor devices

The reactor runs on an operation cycle of 28 days nonstop at fullpower, followed by a stop of 2 days to reshuffle the fuel.

## History

The Argentine company INVAP was fully responsible through a turn key contract for the delivery of the reactor, performing the design, construction and commissioning. The facility is currently in operation.

OPAL was opened on 20 April 2007 by Australian Prime Minister John Howard[1] and is the replacement for the HIFAR reactor. ANSTO received an operating license from the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) in July 2006, allowing commencement of hot commissioning, where fuel is first loaded into the reactor core. OPAL went critical for the first time on 13 August 2006 and reached full power on November 3, 2006. [2]

## Facility details

The OPAL reactor pools. Made of stainless steels and 4.5m wide, it contains demineralised water used for shielding and cooling.

The reactor core consists of 16 low-enriched plate-type fuel assemblies and is located under 10 metres of water in an open pool. Light water (normal H2O) is used as the coolant and moderator while heavy water (D2O) is used as the neutron reflector.

OPAL is the centrepiece of the facilities at ANSTO, providing efficient and rapid radiopharmaceutical and radioisotope production, irradiation services (including neutron transmutation doping of silicon), neutron activation analysis and neutron beam research. OPAL is able to produce four times as many radioisotopes for nuclear medicine treatments, and a wider array of radioisotopes for the treatment of disease than the old HIFAR reactor. The modern design includes a cold neutron source (CNS).

## Neutron scattering at OPAL

The Bragg Institute at ANSTO hosts OPAL's neutron scattering facility. It is now running as a user facility serving the scientific community in Australia and around the world. New fundings have been received in 2009 in order to install further competitive instruments and beamlines. The actual facility comprises the following instruments:

### ECHIDNA

Engineering drawing of the ECHIDNA High-Resolution Powder Diffractometer (August 2003)
The Ge-115 monochromator has been acquired from the Brookhaven National Laboratory.

ECHIDNA is the name of the high-resolution neutron powder diffractometer. The instrument serves to determine the crystalline structures of materials using neutron radiation analogical to X-ray techniques. It is named after the Australian monotreme echidna, as the spiny peaks of the instrument looks like an echidna.

It operates with thermal neutrons. One of the main features is the array of 128 collimators and position sensitive detectors for rapid data acquisition. ECHIDNA allows for structure determinations, texture measurements and reciprocal space mapping of single crystals in most different sample environments serving the physics, chemistry, materials, minerals and earth-science communities. ECHIDNA is part of the Bragg Institute's park of neutron scattering instruments.[4]

#### Components

• Neutron guide
The instrument is located on the TG1 thermal neutron guide of the OPAL reactor. The distance from the reactor is 58 m. The position is the second on the guide after the WOMBAT instrument. The size of the guide is 300 mm height X 50 mm width and it is plated with supermirror coatings.
• Primary collimator
There are Söller collimators prior to the monochromator in order to reduce the divergence of the beam and to increase the angular resolution of the instrument. Since this is an intensity compromise, two items of 5' and 10', respectively, can be interchanged or fully removed by an automated mechanism. The collimators cover the full size of the beam delivered by the neutron guide.
• Monochromator
The monochromator is made by slabs of [115] oriented Germanium crystals which are inclined towards each other in order to focus down the Bragg reflected beam. The device has been acquired from the Brookhaven National Laboratory in the USA after the shutdown of their neutron facility.
• Secondary collimator
Optionally a secondary collimator with 10' angular acceptance and 200 mm X 20 mm can be placed in the monochromatic beam between the monochromator and the sample, which again influences the resolution function of the instrument.
• Slit system
Two automated sets of horizontal and vertical pairs of absorbing plates allow to cut down the size of the monochromatic beam prior to the secondary collimator and sample size. They remove unwanted netrons and reduce the background near the detector. In addition, they allow to select the sample position to be studied.
• Beam monitor
A 235U fission monitor measures the amount of neutrons incident to the sample. The efficiency is 10-4 and most neutrons traverse the device undisturbed. The monitor counts are important to correct for beam flux variations due to changes in the reactor or at the upstream instrument.
• Sample stage
The sample is supported by a heavy load goniometer consisting of a 360° vertical omega rotation axis, x-y translation tables and a chi - phi cross tilt stage of ±20° range. It can hold a few hundred kilograms in order to support heavier sample environment, such as cryostats, furnaces, magnets, load frames, reaction chambers and others. A typical powder sample is filled into vanadium cans which give little, unstructured background. The mentioned sample environment allows to measure changes of the sample as a function of external parameters, like temperature, presser, magnetic field, etc. The goniometer stage is redundant for most powder diffraction measurements, but will be important for single crystal and texture measurements, where the orientation of the sample plays a role.
• Detector collimators

A set of 128 detectors each equipped which a 5' collimator in front are arranged in a 160° sector focusing to the sample. The collimators select the scattered radiation into the well defined ranges of 128 angular positions. All the collimator and detector setup is mounted on a common table which is scanned in finer steps around the sample, to be combinded further to a continuous diffraction pattern.

• Detector tubes
The 128 linear position sensitive 3He gas detector tubes cover the full opening height of 300 mm behind the collimators. They determine the position of the neutron event by charge division over the resistive anode towards each end of the detector. Overall and local count rates lie in the several 10000 Hz range.

### PLATYPUS

PLATYPUS is a time-of-flight reflectometer built on the cold neutron. The instrument serves to determine the structure of interfaces using highly collimated neutron beams. These beams are shone on to the surface at low angles (typically less than 2 degrees) and the intensity of the reflected radiation is measured as a function of angle of incidence.

It operates using cold neutrons, with a wavelength band of 0.2–2.0 nm. Although up to three different angles of incidence are required for each reflectivity curve, the time-of-flight nature means that timescales of kinetic processes are accessible. By analysing the reflected signal one builds a picture of the chemical structure of the interface. This instrument can be used for examining biomembranes, lipid bilayers, magnetism, adsorbed surfactant layers, etc.

### WOMBAT

WOMBAT is a high-intensity neutron powder diffractometer. The instrument serves to determine the crystalline structures of materials using neutron radiation analogical to X-ray techniques. It is named after the wombat, a marsupial indigenous to Australia.

It will operate with thermal neutrons. It has been designed for highest flux and data acquisition speed in order to deliver time resolved diffraction patterns in a fraction of a second. WOMBAT will concentrate on in-situ studies and time critical investigations, such as structure determinations, texture measurements and reciprocal space mapping of single crystals in most different sample environments serving the physics, chemistry, materials, minerals and earth-science communities.

### KOWARI

KOWARI is a neutron residual stress diffractometer. Strain scanning using thermal neutrons is a powder diffraction technique in a polycrystalline block of material probing the change of atomic spacing due to internal or external stress. It is named after the kowari, an Australian marsupial.

It provides a diagnostic non-destructive tool to optimize e.g. post-weld heat treatment (PWHT, similar to tempering) of welded structures. Tensile stresses for example drive crack growth in engineering components and compressive stresses inhibit crack growth (for example cold-expanded holes subject to fatigue cycling). Life extension strategies have high economic impact and strain scanning provides the stresses needed to calculate remaining life as well as the means to monitor the condition of components since it is non-destructive. One of the main features is the sample table that will allow to examine large engineering components while orienting and positioning them very accurately.

### Others

• TAIPAN - Thermal 3-Axis Spectrometer
• KOALA - Quasi Laue Diffractometer.
• QUOKKA - Small-Angle Neutron Scattering
• PELICAN - Time of Flight Polarisation Analysis Spectrometer
• SIKA - Cold Neutron 3-Axis Spectrometer

## July 2007 shutdown

Following the discovery of loose fuel plates during a routine inspection, the ANSTO announced on July 27, 2007 that the reactor would be shut down for 8 weeks to fix the fuel plates and a minor fault causing light water to seep into the reactor's heavy water.[5][6]

According to reports, during the examinations no radiation leakages were detected, although it took much longer than 8 weeks to obtain the necessary clearances to complete repairs and readjustments. ANSTO announced on 25 October 2007 that the reactor would remain shut down until early 2008 while it sought approval from ARPANSA to restart the reactor.[7]

As of March 2008, the reactor remained shut down. According to an ANSTO news release on 22 February 2008, "Late last week, ARPANSA submitted a series of questions on the application, which ANSTO will respond to as quickly as possible. This is expected to take some weeks."[8]

The supply of radiopharmaceuticals was rationed, causing the postponement of some treatments for patients; this caused some concern amongst doctors in nuclear medicine.[9]

OPAL returned to full operational power on 23 May 2008, after a 10-month shutdown, following approval by the nuclear regulator, ARPANSA to use a modified fuel design.

## References

1. ^ ANSTO (20 April 2007). "PM Opens Australia’s New Nuclear Reactor". Press release. Retrieved 2009-07-03.
2. ^ INVAP (10 November 2006). "Sydney Opal reactor at full power". Press release. Retrieved 2009-07-03.
3. ^ INVAP (14 November 2006). "The OPAL reactor already has received seven awards in Australia". Press release. Retrieved 2009-07-03.
4. ^ Liss, L.; Hunter, B.; Hagen, M.; Noakes, T.; Kennergy, S. (2006). "Echidna—the new high-resolution powder diffractometer being built at OPAL". Physica B 385-386: 1010. doi:10.1016/j.physb.2006.05.322.
5. ^ "Sydney nuclear reactor to shut down". ABC News. 27 July 2007. Retrieved 2009-07-03.
6. ^ ANSTO (27 July 2007). "Reactor to shut down for about eight weeks". Press release. Retrieved 2007-10-25.
7. ^ Sabra Lane (25 October 2007). "Lucas Heights to stay closed till 2008". ABC News. Retrieved 2007-10-25.
8. ^ ANSTO (22 February 2008). "Reactor fuel submission". Press release. Retrieved 2009-07-03.
9. ^ Richard Macey (22 February 2008). "Idle reactor keeps sick waiting for treatment". Sydney Morning Herald. Retrieved 2009-07-03.

General Opal Mineraloid Hydrated silica. SiO2·nH2O White, black, red, orange, most of the full spectrum, colorless, iridescent Irregular veins, in masses, in nodules Amorphous[1] None[1] Conchoidal to uneven[1] 5.5–6.5[1] Subvitreous to waxy[1] White opaque, translucent, transparent 2.15 (+.08, -.90)[1] Vitreous to resinous[1] Single refractive, often anomalous double refractive due to strain[1] 1.450 (+.020, -.080) Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[1] none[1] None[1] black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave. May also phosphoresce; common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce.[1] green stones: 660nm, 470nm cutoff[1] darkening upon heating hot saltwater, bases, methanol, humic acid, hydrofluoric acid [2][3]

Opal is a mineraloid gel which is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl and basalt. The word opal comes from the Latin opalus, by Greek opallios, and is from the same root as Sanskrit upálá[s] for "stone", originally a millstone with upárá[s] for slab.[4]

The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Opal is Australia's national gemstone.

## Precious opal

Precious opal shows a variable interplay of internal colors and even though it is a mineraloid, it does have an internal structure. At micro scales precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[5] It is the regularity of the sizes and the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.

Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.

The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem.  An opal doublet is a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian.  The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.

Combined with modern techniques of polishing, doublet opal produces similar effect of black or boulder opals at a mere fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.

The triplet-cut opal backs the colored material with a dark backing, and then has a domed cap of clear quartz or plastic on top, which takes a high polish and acts as a protective layer for the relatively fragile opal. The top layer also acts as a magnifier, to emphasise the play of color of the opal beneath, which is often of lower quality. Triplet opals therefore have a more artificial appearance, and are not classed as precious opal.

## Common opal

Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality), resin opal which is honey-yellow with a resinous luster, wood opal which is caused by the replacement of the organic material in wood with opal[6], menilite which is brown or grey, hyalite is a colorless glass-clear opal sometimes called Muller's Glass, geyserite, also called siliceous sinter, deposited around hot springs or geysers and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.

## Other varieties of opal

Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not show any play-of-color. The most famous source of fire opals is the state of Queretaro in Mexico and these opals are commonly called Mexican fire opals.

Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.

## Sources of opal

File:Opal from Yowah, Queensland,
Polished opal from Yowah, Queensland, Australia

Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal.[citation needed]

The town of Coober Pedy in South Australia is a major source of opal. Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.[7]

The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement. Miocene age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried. The largest black opal in the Smithsonian Museum comes from the Royal Peacock opal mine in the Virgin Valley.[citation needed]

Another source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.

Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.

In late 2008, NASA announced that it had discovered opal deposits on Mars.[8]

## Synthetic opal

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974.[5] The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous.

Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed "imitation opal", as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewelry are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.

## Local atomic structure of opals

The lattice of spheres of opal that cause the interference with light are several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, there is no unit cell that describes the structure of opal. Nevertheless, opals can be roughly divided into those that show no signs of crystalline order (amorphous opal) and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal.[9] Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2·nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.

### Microcrystalline opal

Opal-CT has been interpreted as consisting of clusters of stacking of cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny microcrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Lussatite is a synonym. Opal-C, also called Lussatine, is interpreted as consisting of localized order of $\alpha$-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%.

### Non-crystalline opal

Two broad categories of non-crystalline opals, sometimes just referred to as "opal-A", have been proposed. The first of these is opal-AG consisting of aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing. The second "opal-A" is opal-AN or water-containing amorphous silica-glass. Hyalite is another name for this.

Non-crystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed.[10]

## Historical superstitions

In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.[11] Victorian superstitions were created by the established gem dealers to stop the rush to buy opals. They paid an author to attribute bad luck to the stone, though some believed this is avoided if opal is the owner's birthstone or if the stone was a gift. Even as recently as under the last czar at the beginning of the 20th century, it was believed that when a Russian of any rank saw an opal among other goods offered for sale, he or she should not buy anything more since the opal was believed to embody the evil eye.[11] Opal is considered the birthstone for people born in October.

## Famous opals

• The Andamooka Opal, presented to Queen Elizabeth II, also known as the Queen's Opal
• The Aurora Australis Opal, considered to be the most valuable black opal
• The Black Prince Opal, originally known as Harlequin Prince
• The Empress of Australia Opal
• The Fire Queen Opal
• The Flame Queen Opal
• The Flamingo Opal
• The Halley's Comet Opal, the world's largest uncut black opal
• The Jupiter Five Opal
• The Olympic Australis Opal, reported to be the largest and most valuable uncut gem opal ever found
• The Pride of Australia Opal, also known as the Red Emperor Opal
• The Red Admiral Opal, also known as the Butterfly Stone
• The Roebling Opal, Smithsonian Museum [12]

## References

1. ^ a b c d e f g h i j k l m Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
2. ^ http://webmineral.com/data/Opal.shtml Webmineral: Opal
3. ^ http://mindat.org/min-3004.html Mindat.org: Opal
4. ^ Sanskrit, Tamil, and Pahlavi Dictionaries. U. of Cologne.
5. ^ a b Klein, Cornelis, and Hurlbut, Cornelius S.; 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
6. ^ Gribble, C. D. (1988). "Tektosilicates (framework silicates)". Rutley's Elements of Mineralogy (27th ed. ed.). London: Unwin Hyman. pp. 431. ISBN 0045490112.
7. ^ http://www.nrw.qld.gov.au/mines/fossicking/opal.html Queensland opal
8. ^ "NASA probe finds opals in Martian crevices". Retrieved on 2008-10-29.
9. ^ Graetsch, H., "Structural Characteristics of opaline and microcrystalline silica minerals", "Silica, physical behavior, geochemistry and materials applications". Reviews in Mineralogy, Vol. 29, 1994. Editors PJ Heaney, CT Prewitt, GV Gibbs, Mineralogical Society of America
10. ^ (Cady et al., 1996)PDF (2.03 MiB)
11. ^ a b Fernie M.D., W.T. (1907). Precious Stones for Curative Wear. Bristol, John Wright & Co.. , cpages 249
12. ^ http://www.mnh.si.edu/earth/text/dynamicearth/6_0_0_GeoGallery/geogallery_specimen.cfm?SpecimenID=4038&categoryID=1&categoryName=Gems&browseType=name

# Simple English

The Opal is a fairly white rock mineral. It is the birthstone of someone whose birthday lands in the month of October.Opals are made from tiny spheres of silica (another mineral) and lots of water.