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XYLENE POWER LTD.

PORCELAIN

By Charles Rhodes, P. Eng., Ph.D.

STORAGE CONTAINER MATERIAL:
The material used to fabricate nuclear waste storage containers must be highly stable against long term corrosion, durable, water resistant and inexpensive to both minimize the cost of nuclear waste storage and to discourage future thieves. While in principle copper, gold and platinum are easy to fabricate and can be made quite corrosion resistant the corrosion resistant metals are almost useless as nuclear waste container materials because these metals are relatively rare, relatively expensive, are consistently in high demand and attract thieves. The damage caused by such thieves usually far exceeds the scrap value of the stolen material. It is both impractical and uneconomic to heavily guard the storage facility far into the future.

Copper, the least expensive of the corrosion resistant metals, is also unstable against sulfur corrosion on a million year time scale.

A natural material with the appropriate properties is quartz. However, making large containers out of pure quartz would be very expensive. Making such containers would involve both complex and expensive chemical purification and attaining a working temperature of at least 1700 C.

A much more practical container material is porcelain. Porcelain has been used for making high quality stable containers for about 3500 years, as indicated by porcelain containers in museums that date from about 1500 B.C.

In this respect today the name "porcelain" is frequently misused. If a material is not translucent or if the material requires glazing to prevent water absorption then it is not "good porcelain". For example, toilets and most dish ware are not made of "good porcelain" even though the parties selling these items say that they are made of porcelain. Similarly, most floor and wall tiles are not made of "good porcelain". Making good porcelain requires source material chemical and particle size control and a high firing temperature that makes the material price uncompetitive in the dish ware, bathroom fixture and tile markets.

For nuclear waste storage the outer container should be made of good porcelain, chemically known as aluminum silicate [Al2(SiO3)3]. Good porcelain has the feature that it does not take on water of hydration. Good porcelain rarely occurs in nature. Good porcelain consists of randomly oriented needle like mullite crystals in a silica matrix. The randomly oriented needle like mullite crystals give the porcelain immense strength and toughness. If the source material is suitably ground and then fired at 1400 C the crystals arrange themselves and bind together in a manner that almost eliminates light reflection from crystal grain boundaries, making the material translucent. The resulting porcelain should be tested to ensure that it is correctly compounded and fired, that it is translucent, that that it does not take on water of hydration and that the desired toughness is actually realized.

Porcelain when correctly fired vitrifies all the way through so that it will not absorb either oil or water. Good porcelain does not need to be glazed. Good porcelain is used as a high voltage electrical insulator.

From a bulk chemical balance perspective good porcelain is aluminum silicate or Al2(SiO3)3. However, scanning electron microscopy reveals that good porcelain is actually a mixture of (3 Al2O3 + 2 SiO2) + 7 SiO2. The (3 Al2O3 + 2 SiO2) is known as mullite. At 1400 degrees C mullite forms needle like crystals with random orientation. These needle like crystals give strength and toughness to the porcelain. The additional 7 SiO2 gives porcelain glass like qualities and prevents the Al2O3 taking on water of hydration.

The randomly oriented mullite crystals have a larger thermal coefficient of expansion than does silica. As porcelain cools after firing the mullite goes into tension and places the surrounding silica in compression. The effect is analogous to use of prestressed steel reinforcement in concrete. The resulting porcelain has extraordinary strength and toughness in all directions. The individual crystals are small. In good porcelain there are no extended crystal planes along which impurities can diffuse or along which the porcelain can easily fracture.

The tensile yield stress of porcelain is typically:
(900 kg / cm^2) X (100 cm / m)^2 X (9.8 m / s^2) X (1 kPa / 1000 Pa)
= 88,200 kPa
= 88.2 MPa

The porcelain actually used should be tested to ensure that it: has the required optical properties, does not take on water of hydration and has the required toughness.
 

PORCELAIN SOURCE MATERIAL:
Porcelain is made from a finely ground mixture of SiO2 and Al2O3. In theory beach sand could be used but it is too course, contains too wide a range of impurities, and grinding it finer would be too expensive. However, certain mining operations that are targeted at extracting gold, silver and other valuable metals embedded in quartz involve grinding the quartz into particles that are less than 75 um in diameter. This finely ground quartz is a potential health hazard because if inhaled it can cause silicosis. To prevent this finely ground quartz becoming air borne and hence a public health hazard mining companies have dumped it into natural or man made lakes known as tailing ponds. In Canada there are over 11 million tonnes of silica rich material in tailing ponds of which about 3 million tonnes are easily accessible. Of the accessible 3 million tonnes, about one million tonnes are believed to be chemically suitable for production of good porcelain. That is sufficient material to form containers for all of Canada's nuclear waste for centuries into the future.

The accessible material needs to be further processed to concentrate the aluminum silicate and remove impurities, but this type of processing is well known to the mining industry.

At present the major obstacle to production of porcelain containers for nuclear waste containment is lack of will on the part of the Nuclear Waste Management Organization (NWMO). For at least 7 years that organization has ignored informed advice from the Canadian hard rock mining industry. In the view of this author any solution to this problem likely involves terminating the existing upper management of the NWMO. That organization has been guided by perceived politics rather than by science.
 

WATER OF HYDRATION TEST:
If a representative porcelain test tile gains more than 0.5% weight during a 5 hour immersion in boiling water followed by a 24 hour immersion in cool water then the representative test tile is not true porcelain.
 

CHEMICAL COMPONENTS OF PORCELAIN:
Alumina (Al2O3) as a container material offers long term stability at room temperature. Alumina melts at 2072 degrees C. It is radiation resistant. It can easily be fabricated and machined. Alumina can be formed into large diameter cylinders. However, if anhydrous alumina is exposed to water it gradually takes on water of hydration according to the chemical equation:
(Al2O3 + 3H2O = 2 Al(OH)3)

For use in a damp environment alumina Al2O3 should be compounded with silica (SiO2) at 1400 degrees C to form porcelain in accordance with:
3 Al2O3 + 9 SiO2 = (3 Al2O3 +2 SiO2) +7 SiO2
Porcelain does not absorb water. The component (3 Al2O3 +2 SiO2) is known as 3:2 mullite. This 3:2 mullite forms randomly oriented needle like crystals in porcelain that give porcelain some of its favorable mechanical characteristics.

Silica (SiO2) as a container material offers very long term structural stability at room temperature. Pure silica melts at 1600 degrees C. However, mullite forms at 1400 degrees C and melts at 1840 degrees C. If the green ceramic is fired at 1400 degrees C the silica will vitrify all the way through forming porcelain that does not need glazing to prevent absorption of water.

Fabricating large containers out of porcelain is far less expensive than fabricating similar size containers out of pure silica.

Some porcelain contains small amounts of clay impurities such as CaO, MgO, K2O or Fe3O4. There may also be toxic impurities such as arsenic, lead and NaCN left over from prior ore concentration processes. These impurities must be removed and/or chemically balanced by addition of further SiO2 to prevent absorption of water of hydration. The required processing and chemical balancing requires the use of sophisticated analytical equipment such as is available at Simon Fraser University in Burnaby, British Columbia.
 

COMPONENTS OF GOOD PORCELAIN:
Chemical Composition:
When examined under a scanning electron microscope (SEM) or a scanning helium ion microscope (SHIM) good porcelain consists of randomly oriented needle like mullite crystals surrounded by silica. The SiO2 is transparent. The Al2O3 is slightly opaque. The resulting porcelain is translucent.

Silica is SiO2.

Anhydrous alumina is Al2O3

Hydrated alumina is Al2O3 + 3 H2O

3:2 Mullite is:
(3 Al2O3 + 2 SiO2)

Good porcelain is believed to be:
(3 Al2O3 + 2 SiO2) + (7 SiO2)
which superficially is:
3 Al2(SiO3)3
which is known as aluminum silicate even though microscopic examination shows it to be a mixture of mullite crystals surrounded by silica.

The atomic weights of the component elements are given by:
Atomic weight of Al = 26.981
Atomic Weight of Si = 28.0855
Atomic Weight of O = 15.9994

The relative molecular weights of Al2O3 and SiO2 in mullite are given by:

Weight of [3 Al2O3] = 6 (26.981) + 9 (15.9994) = 161.886 + 143.9946 = 305.8806
Weight of [2 SiO2] = 2 (28.0855) + 4 (15.9994) = 56.171 + 63.9976 = 120.1686

In mullite (3 Al2O3 + 2 SiO2) the ratio of these weights:
(weight of silica) / (weight of alumina) = 120.1686 / 305.8806 = 0.3929

This weight ratio is no where near the weight ratio of the material described by McDanel as mullite on its data sheets.

For material MV20 McDanel claims that:
(weight of silica) / (weight of alumina) = 41.9 / 55.2 = 0.759

For material MV-HR1 McDanel claims that:
(weight of silica) / (weight of alumina) = 36.0 / 59.0 = 0.610

For material MV30 McDanel claims that:
(weight of silica) / (weight of alumina) = 37.9 / 60.0 = 0.632
 

Assume that the material that McDanel is actually selling is (3 Al2O3 + 4 SiO2). Then:

Weight of [3 Al2O3] = 6 (26.981) + 9 (15.9994) = 161.886 + 143.9946 = 305.8806
and
Weight of [4 SiO2] = 4 (28.0855) + 8 (15.9994) = 112.342 + 127.9952 = 240.3372

For this presumed molecular ratio:
(weight of silica) / (weight of alumina) = 240.3372 / 305.8806 = 0.785
which is fairly close to:
(41.9 / 55.2) = 0.759
for material MV20
 

Now assume that the material that McDanel is actually selling is (3 Al2O3 + 3 SiO2). Then:

Weight of [3 Al2O3] = 6 (26.981) + 9 (15.9994) = 161.886 + 143.9946 = 305.8806
and
Weight of [3 SiO2] = 3 (28.0855) + 6 (15.9994) = 84.2565 + 95.9964 = 180.2529

For this presumed molecular ratio:
(weight of silica) / (weight of alumina) = 180.2529 / 305.8806 = 0.5893
which is fairly close to:
(36.0 / 59.0) = 0.610
for material MV-HR1
 

Thus it appears that the material that McDanel Advanced Ceramics actually sells is ceramic mixtures in the range:
(3 Al2O3 + 3 SiO2) to (3 Al2O3 + 4 SiO2).
 

GOOD PORCELAIN:
I believe that "Good Porcelain" is actually:
(3 Al2O3 + 9 SiO2) = (3 Al2O3 + 2 SiO2) + (7 SiO2)
= 3:2 mullite + 7 SiO2
but this belief remains to be proven by accurate measurement.

The 3:2 mullite is believed to form needle like crystals and in good quality porcelain these mullite crystals are surrounded by pure SiO2. There is sufficient excess SiO2 that the material is translucent.

Thus in good porcelain:
Weight of [3 Al2O3] = 6 (26.981) + 9 (15.9994) = 161.886 + 143.9946 = 305.8806
and
Weight of [9 SiO2] = 9 (28.0855) + 18 (15.9994) = 252.7695 + 287.9892 = 540.7587

For this molecular ratio:
(weight of silica) / (weight of alumina) = 540.7587 / 305.8806 = 1.7678

Thus good porcelain has a much higher silica fraction than does the material sold by McDanel Advanced Ceramics.

In good porcelain the fractional weight of aluminum is:
6 (26.981) / [305.8806 + 540.7587]
= 161.886 / 846.6393
= 0.1912
 

ION MOBILITY IN PORCELAIN:
The object of a porcelain container is to isolate nuclear waste from the environment for about 1 million years. It is important that the porcelain container wall be thick enough and the storage temperature be low enough to achieve this end. Clearly we cannot do an experimental test for 1 million years to prove the suitability of porcelain. However, we can identify the most problematic waste isotopes and we can measure their diffusion rates through porcelain at elevated temperatures. From this data we can calculate the porcelain container wall thickness required to safely contain the problem isotopes at room temperature.

The diffusion rate through porcelain decreases as the size of the diffusing atom increases. Hence hydrogen and helium diffuse more quickly than does sodium and calcium and chlorine diffuse less quickly than does sodium. The isotopes of primary containment concern are Cl-36 and Ca-41. Heavier isotopes diffuse less quickly. Lighter isotopes have shorter half lives.

An upper bound on the calcium and chlorine diffusion rate is provided by sodium. The diffusion of sodium through SiO2 has been extensively studied by the electronics industry. Thus if the porcelain thickness is sufficient to contain sodium for one million years at room temperature then the porcelain thickness will likely be sufficient to contain both calcium and chlorine. This issue of minimum porcelain wall thickness required to realize one million year containment at ambient temperature needs further study. This issue also points to the importance of natural ventialtion in a Deep Geologic Repository to prevent the storage room ambient temperature rising to the extent that the containment is compromised.

An experimental plan for measuring the diffusion rate of calcium and chlorine through porcelain as a function of temperature is currently being formulated. The minimum required porcelain wall thickness will likely determine the optimum size for the nuclear waste containers. The container wall thickness together with the physical properties of porcelain will determine the maximum heating and cooling rate during firing which in turn will determine the porcelain container production rate per kiln.
 

MATERIAL PROPERTIES
PROPERTYAl2O3Fused SiO2MV20PorcelainMild Steel
Density3.92 g / cm^32.2 g / cm^32.5 g/cm^32.4 g / cm^37.85 g / cm^3
Tensile Yield Stress Sy @ 20 C248 MPa110 MPa145 MPa88.2 MPa206.12 MPa
Compressive Yield Stress2500 MPa690-1380 MPa655 MPa206.12 MPa
Youngs Modulus Y370 GPa73 GPa151 GPa210 GPa
Thermal Conductivity @ 20 C30.0 W / m-K1.4 W / m-K2.4 W / m-K5 W / m-K
TCE @ 20 C8.2 X 10^-6 / K0.4 X 10^-6 / K5.4 X 10^-6 / C12.0 X 10^-6 / C
[Sy^2 / (2 Y)] @ 20 C83.11 kPa82.88 kPa69.62 kPa101.1 kPa
Max strain = [Sy / Y] @ 20 C0.670 X 10^-31.507 X 10^-30.9603 X 10^-30.9815 X 10^-3
Melting Point2015 C1830 C
Light Transmissionopaquecleartranslucentopaque

 

TOUGHNESS
A measure of a material's toughness is its elastic ability to absorb energy without breaking.

Another indication of toughness is the maximum amount of elastic strain that the material can accept.

Low carbom mild steel, with which most people are familiar, is a good benchmark for toughness.

Consider a rod of material with initial length Xo and cross sectional area A.

Apply an axial tensile force F

The axial stress is (F / A)

The axial strain is (X - Xo) / Xo

Let Xm be the material length at the yield stress Sy.

Within the materials elastic range:
Young's Modulus = Y = (stress) / (strain) = (F / A)[ Xo / (X - Xo)]
or
F = Y (X - Xo) A / Xo

Hence the maximum allowable force Fm is given by:
Fm = Y (Xm - Xo) A / Xo

Note that Young's modulus Y is constant until:
(F / A) = (F / A)max = Sy = yield stress
or Fm = Sy A

Hence:
Sy = Y (Xm - Xo) / Xo
or
(Xm - Xo) = (Sy Xo / Y)

The maximum stored elastic energy is:
Integral from X = Xo to X = Xm of F dX
 
= Integral from X = Xo to X = Xm of
Y (X - Xo) A dX / Xo
= Y (Xm - Xo)^2 A / (2 Xo)
= Y (Sy Xo / Y)^2 A / (2 Xo)
= (1 / Y) Sy^2 (Xo A) / 2

Thus the maximum elastic energy per unit volume that the material can absorb is:
(Sy^2 / 2 Y)

Recall that:
Y = (Fm / A) /[(Xm - Xo) / Xo]

Hence the maximum allowable strain is:
(Xm - Xo) / Xo = (Fm / A Y) = (Sy / Y)

The toughness of a material can be expressed in terms of the maximum elastic energy per unit volume and the maximum allowable strain.
 

This web page last updated September 25, 2016.

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