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XYLENE POWER LTD.

NUCLEAR WASTE CONTAINERS

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

SCOPE:
This web site addresses design issues relating to containers for safe storage and transport of nuclear waste.
 

DEFINITIONS:
On this web page:
Interim storage means storage in anticipation of future use in a fast nuclear reactor. Interim storage could be anything from a few months to centuries;

Medium term storage means about 300 years;

Long term storage means much more than 300 years (eg 50,000 years to 1,000,000 + years).

A non-fissionable material container is a cylindrical stainless steel container that is intended to contain about 4 tonnes of non-fissonable radioactive material.

A fissionable material container is a cylindrical stainless steel container that is externally similar to a non-fissionable material container, but the useful internal volume is smaller because part of the the container's volume is occupied by B4C, the purpose of which is to prevent the container's fissionable contents becoming critical if water penetrates the container.

A shipping container is an ~ 40 ton lead container that can hold either one stainless steel non-fissionable material container or one stainless steel fissionable material container. A shipping container surrounds the stainless steel container with about 12 inches of lead biosafety shielding and is intended to be carried by a conventional 80 ton 18 wheel road truck. Two shipping containers can be carried by one 100 ton rated rail car or one 24 wheel 120 ton road truck.

A porcelain container is an ~ 5 ton porcelain ceramic container that can provide long term external protection for either one non-fissionable material container or one fissionable material container after placement in a suitable accessible naturally dry naturally ventilated deep sub-terranian granite repository.
 

ECONOMICS:
The Nuclear Waste Management Organization (NWMO) contemplates transporting unprocessed spent CANDU fuel bundles and storing these spent CANDU fuel bundles in a limestone Deep Geologic Repository. It is shown herein that the costs of transporting spent CANDU fuel can be reduced about 8 fold by selective extraction of uranium oxide from the spent CANDU fuel before the spent fuel leaves a CANDU reactor site.
 

The cost of reusable stainless steel and porcelain containers that in combination hold fission product concentrates from about 1400 spent CANDU fuel bundles is presently estimated to be less than $100,000 per combination container or $71.43 per CANDU fuel bundle. While that may seem expensive the NWNO budget for disposal of spent CANDU fuel bundles is:
$24,000,000,000 / 3,000,000 CANDU fuel bundles = $8,000 per spent CANDU fuel bundle and the present NWMO plan provides no nuclear fuel or material recycling benefits.
 

STORAGE CONTAINER APPLICATION:
The radio isotopes in storage should be isolated from the environment in engineered storage containers until the radio isotopes either spontaneously decay to a safe level or until the radio isotopes are recycled as components of a Fast Neutron Reactor (FNR) fuel bundle. The storage containers contemplated on this web page are intended for use in naturally dry naturally ventilated subterranean granite storage vaults such as are available in some depleted hard rock mines within granite core mountains. Within the vaults the storage containers are manipulated and positioned using remotely controlled equipment. Since there are no personnel in the underground vaults the storage containers do not require individual integral gamma ray shielding for biosafety.

An important issue in storage of nuclear waste is total exclusion of liquids, especially water, from the waste. All nuclear waste must be converted to chemically stable dry solids prior to being placed in containers. Ideally the solids should have an extremely low vapor pressure at room temperature. Wet or liquid radioactive waste is simply not suitable for safe long term storage.
 

STORAGE CONTAINER CONCEPTS:
A very long life radioactive material storage can be realized by using an inner stainless steel container that is completely surrounded by a layer of liquid dielectric within an outer capped porcelain container. The inner stainless steel container must be fitted with a rupture disk or other mechanism that will vent an accumulation of inert gas decay products into th eouter container before the rupture pressure of the inner container is approached. The outer porcelain container must repeatedly vent both low molecular weight hydrocarbon gases that result from gamma irradiation of the liquid dielectric as well as inert gases that may be released by the inner container. The porcelain container vent must be fitted with a nonreturn valve that will reliably exclude air and water.

The main purposes of the outer porcelain container are to: exclude water, exclude air, confine and prevent evaporation of the liquid dielectric, provide structural and corrosion protection for the inner stainless steel container, allow periodic dielectric testing and replacement and allow periodic removal of water.

The main purposes of the liquid dielectric are to prevent corrosion of the inner stainless steel container, to provide a venting gas seal for the inner stainless steel container and to initiate an alarm if there is a drop in dielectric level indicating that:
1) there is a failure of either the inner stainless steel container wall or the outer porcelain container wall; or
2) the storage container is no longer upright; or
3) there has been a significant loss of dielectric volume due to evaporation and decomposition.

The gas pressure inside the stainless steel container is maintained at a higher pressure than the gas pressure inside the porcelain container to prevent oil back flowing into the stainless steel container. Hence the vent on the stainless steel container operates at a higher absolute internal pressure than the vent on the porcelain container.
 

SUPPORT FOR CONTEMPLATED RADIOACTIVE MATERIAL CONTAINER DESIGN:
1. The dead sea scrolls were preserved for over 2000 years in a relatively dry environment by storage in primitive capped ceramic containers;

2.Porcelain containers made over 3000 years ago are on display in museums today.

3.Today we have a much better understanding of ceramics, especially porcelain than was possible even 100 years ago. Porcelain in combination with an internal stainless steel container and a suitable dielectric separator in dry storage should last hundreds of thousands of years and containers fabricated from porcelain can be inexpensively remotely monitored.

4. High tech ceramics are routinely used in electronics and spacecraft.

5. 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

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

7. Silica (SiO2) as a container material offers very long term structural stability at room temperature. Pure fused 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.

8. Fabricating large containers out of porcelain is much less expensive than fabricating similar size containers out of pure silica. Porcelain material feedstock is available as a byproduct of certain Canadian mining activities. These mining activities typically grind the quartz into particles with an average diameter of about 75 um and selectively extract gold.

9. Some porcelain contains small amounts of impurities such as CaO, MgO, K2O or Fe3O4. These impurities usually come from the source clay or may be added to achieve unique color or other characteristics. These impurities should be chemically extracted to prevent absorption of water of hydration.

10. Sacrificial galvanic magnesium-aluminum electrodes in combination with dielectric isolation have been extensively used for successful external corrosion prevention of buried steel pipelines.
 

STORAGE CONTAINER MATERIALS:
It is crucial that the materials used to fabricate the storage containers be inexpensive both to minimize the cost of nuclear waste storage and to discourage future thieves. While in principle copper is more corrosion resistant than stainless steel, copper is almost useless as a practical container material because it is: relatively rare, relatively expensive, in consistently high demand and attracts thieves. The damage caused by such thieves far exceeds the scrap value of the stolen material. It is impractical and uneconomic to heavily guard the storage facility far into the future. For the inner container it is sufficient to use stainless steel with a sacrificial magnesium-aluminum electrode for galvanic protection against water penetraion of the porcelain container.

Each storage container assembly consists of an outer porcelain container, an inner stainless steel container and an oil dielectric separator between the inner and outer container walls. While in storage fission daughter inert gases will form inside the inner stainless steel container and low molecular weight hydrocarbon gases will collect above the liquid dielectric. For pressure safety gas accumulations inside the inner stainless steel container must automatically vent into the porcelain container and gas accumulations above the liquid dielectric inside the porcelain container must automatically vent into the storage vault space. The storage vault space must have reliable natural ventilation to prevent accumulation of a combustable low molecular weight hydrocarbon gases in the vault space.

The outer container and its vent cap should be made of porcelain. Porcelain is an almost unique ceramic that does not take on water of hydration. 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 physical strength and toughness.

The porcelain should vitrify all the way through and not require an external glaze. Porcelain samples should be tested to ensure that they do not take on water of hydration and that the required toughness is achieved.

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 good porcelain.

The porcelain container vent seal material might be a fluorocarbon liquid. The porcelain container perimeter top seal can be a combined wax and ground porcelain seal.

The inner container, being stainless steel, can be sealed with a soft copper gasket. The gasket flange sealing faces must be corrosion resistant and should be machined and polished optically flat. Near the outer perimeter the bolted flange thickness is reduced to transfer pressure onto the flange sealing face. This seal fabrication requirement impacts the choice of stainless steel alloy used to fabricate the inner container. This alloy must be low carbon stainless steel for good welding characteristics. The perimeter top seal should be leak tested with helium.
 

LIQUID DIELECTRIC LEVEL SENSOR:
The liquid dielectric level sensor involves a minimal use of electronics which must be protected from high levels of gamma radiation. One cm of Pb shielding reduces 1 MeV gamma rays by about a factor of 2.0. In order to reduce the gamma ray flux by a factor of:
2000 = 2^11
to protect the dielectric level sensor from gamma ray damage requires about:
11 cm X 1 inch / 2.54 cm = 4.33 inches thickness of lead shielding.
The shielding must be under the sensor and must extend upwards around the sensor to attenuate gamma rays from adjacent containers.  

DIELECTRIC ISSUES:
The dielectric almost fills the space between the inside wall of the porcelain storage container and the outside wall of the stainless steel container. One of the functions of the dielectric is to reseal the stainless steel non-fissionable container vent valve which allows controlled release of inert gas fission product decay daughters, primarily krypton and xenon. This inert gas will flow out of the inner stainless steel container through the stainless steel container vent, will bubble up through the liquid dielectric and will accumulate between the top surface of the liquid dielectric and the lower surface of the porcelain container top.

The liquid dielectric and its products of gamma radiation must not chemically react with the stainless steel inner container, the porcelain outer container, or any gasket or monitoring system materials or the nuclear waste. The liquid dielectric must not be easily flammable and must not be misible with water.

The chosen liquid dielectric is a hydrocarbon heavy oil similar to diesel fuel that is a stable high molecular weight low vapor pressure liquid with an initial density of about 0.85 gm / cm^3.

Gamma irradiation of the heavy oil will gradually break it into higher molecular weight and lower molecular weight components. The high molecular weight components will remain in place and if not serviced will gradually change to wax with a density as high as 1.05 gm / cm^3. The low molecular weight gaseous components will form gas bubbles which will rise to the surface of the heavy oil and accumulate below the lower surface of the porcelain container top. During this long term oil density change process low molecular weight hydrocarbon gases must be vented.

The higher molecular weight components have a higher density and will gradually turn into wax and accumulate at the bottom of the heavy oil where they can be periodically removed by vacuum suction and replaced by lower density oil. There will also be gradual heavy oil mass and volume losses due to venting of low molecular weight gases.

When the combined pressure of the inert gas and low molecular weight hydrocarbon gases becomes sufficiently high the porcelain container will vent these gases via its gravity sealed top vent. The gravity seal may not be perfect but it will economically minimize heavy oil losses by evaporation and will permit easy periodic liquid dielectric service. Due to the decreasing gamma flux the time intervals between successive oil changes will become longer as the time in storage increases.
 

DIELECTRIC VOLUME:
Dielectric Volume = [volume at asterisk] + [Volume around stainless steel container] + [top volume]
= [Pi (2 inch) (22 inch)^2] + [Pi (43 inch) (1 inch) (79 inch)] + [Pi (28 inch)^2 (7 inch)]
= Pi {968 inch^3 + 3397 inch^3 + 5488 inch^3}
= Pi {9853 inch^3}

The mass of oil is:
Pi {9853 inch^3} X (.0254 m / inch)^3 X 850 kg / m^3 = 431.16 kg
 

PORCELAIN CONTAINER DESCRIPTION:
Evaporation of the heavy oil dielectric is prevented by the porcelain container top which has a combioned wax and ground porcelain seal on its perimeter.

The porcelain container top has a gravity sealed pressure relief vent similar to on a pressure cooker. This vent has a heavy hat that excludes overhead dripping condensation.

The porcelain container's lid top surface should have a slightly conical shape that will shed water dripping from overhead.

The buoyancy in water of a filled porcelain container must be negative so that if the storage vault floods the container will not float and will remain upright.

An asterisk shaped porcelain cradle at the inside bottom of the porcelain container supports the weight of the inner stainless steel container and provides a bottom reservoir where corrosive substances and water which are denser than the heavy oil can accumulate and will remain trapped. The porcelain container has a flat external bottom so that it will be stable and upright when placed on a flat floor.

Indentations in the outside of the porcelain container and its lid are provided for lid lifting, porcelain container lifting and lid clamp attachment.

There must be two tubes through the porcelain cap and into the porcelain container, one to the bottom and one to the top such that any high or low density liquids at the bottom or top of the container can easily be removed without removing the container top. The tube going to the bottom passes loosely through a hole in the stainless steel container top, a matching hole in the stainless steel container top flange and is sealed at the porcelain container top via a invar fitting and a compression fitting. At the top of this tube is mounted an easily replaceable acoustic type oil level sensor. This sensor has its own dedicated lead shield.

The lid on the porcelain container is nominally held in place by gravity, has an internal lip to prevent horizontal sliding and is secured against accidental container roll over by an external stainless steel clamp. A sufficiently high internal gas pressure inside the porcelain container, if not vented, will lift the lid, thus relieving internal pressure and complying with pressure vessel safety requirements. Normal evacuation and back filling is via a top center tube which is fitted with an isolation valve. This tube permits container evacuation, liquid dielectric removal and liquid dielectric back filling.

Once the porcelain container is in place in the storage vault the position of the porcelain container can be stabilized by surrounding the porcelain container with low density pea gravel. Pea gravel could also be used to provide the porcelain container additional protection against an overhead rock fall.
 

The following diagram gives an approximate representation of the assembled storage container. This diagram shows the inner stainless steel container, the outer porcelain container, the dielectric and a wax perimeter top seal. This diagram does not show the optional 61 4.0 inch ID, 4.5 inch OD vertical tubes inside the inner container that can be used to position unprocessed CANDU spent fuel bundles within the inner stainless steel container. These tubes are not required for storage of CANDU spent fuel concentrates.
 


 

PORCELAIN CONTAINER DIMENSIONS:
The porcelain container OD is 60 inches above the lifting ledge. This diameter is limited by a railway boxcar internal width. This OD extends 5 inches below the bottom of the stainless steel container top flange

The side wall thickness of the lower part of the porcelain container is 2 inch

The ID of the upper part of the porcelain container is 48.5 inch to accomodate the stainless steel container flanges.

The side wall thickness of the upper part of the porcelain container is:
(60 inch - 48.5 inch) / 2 = 5.75 inch

The OD of the lower part of the container is:
(42 inch OD pipe) + 2(1 inch gap) + 2 (2 inch thick porcelain wall) = 48 inch

The porcelain container bottom thickness = 3 inch

The asterisk thickness = 2 inch

The overall height of the stainless steel container and its lifting ring is 86.625 inch

When the container is vertical there is an additional 2 inches of oil above the lifting ring.

When the container is vertical there is an additional 3 inches of gas space between the oil surface and the bottom surface of the porcelain top.

The porcelain top is 5 inches thick at its center decreasing to 4 inches thick at its perimeter.

The porcelain container top vent and dielectric level sensor projects 5 inches above the porcelain container top.

The overall height of the porcelain container and dielectric level sensor is:
(bottom thickness) + (asterisk thickness) + (stainless steel container overall haight) + (extra oil height) + (gas gap height) + (porcelain top thickness) + (vent projection)
= 3.00" + 2.00" + 86.625" + 2.00" + 3.00" + 5.00" + 5.00"
= 106.625 inch
which will easily fit through a 120 inch high railway boxcar door when on a dolly or fork lift.

The dielectric level sensor lead shield should be recessed into the porcelain container top near its center.
 

PORCELAIN QUANTITY:
(Porcelain Quantity) = [bottom] + [asterisk] + [48 inch OD sidewall] + [60 inch OD sidewall] + [overlap section] + [top]
= [Pi (22^2) 3] + [Pi (22^2) (2 / 2)] + [Pi 2 (23) (5 + 78)] + [Pi 2 (29) (5 + (86.625 - 78.00) + 5)]
+ [Pi (4) (5) (52)] + [Pi (30^2) (4.5)] inch^3
= Pi {1452 + 484 + 3818 + 1080.25 + 1040 + 4050} inch^3
= Pi {11,924.25} inch^3
= 37,461.1 inch^3

The density of porcelain is about:
2.403 gm / cm^3 X 1 kg / 1000 gm X 10^6 cm^3 / m^3 = 2403 kg / m^3
or
2403 kg / m^3 X (.0254 m / inch)^3 = 0.039378 kg / inch^3

Hence the mass of porcelain is about:
0.039378 kg / inch^3 X 37,461.1 inch^3 = 1475.15 kg
= 1.47515 tonnes
 

STAINLESS STEEL CONTAINER DESCRIPTION:
During the FNR nuclear fuel recycling process some of the transported and stored radioactive material is in metallic rather than oxide form. The metallic form is potentially combustible and combustion of a radioactive metal is a situation to be avoided at all costs. To prevent accidents the radio active materials should to be stored in stainless steel containers that are back filled with the inert gas argon. The stainless steel container must be sufficiently rigid to permit complete evacuation for leak detection and argon insertion. The stainless steel containers should have gas tight lids secured by bolted flanges that allow safe material access and allow container reuse. The stainless steel container flanges are sealed with bolted flanges that have soft copper gaskets. The stainless steel containers are size matched to larger enclosing lead shipping containers that permit safe truck and rail transport. The stainless steel containers are also size matched to larger enclosing porcelain containers for long term accessible subterranean storage.

The stainless steel containers have lifting rings welded onto their tops to permit easy insertion and removal of stainless steel containers into either porcelain storage containers or lead shipping containers.

For radio isotope shipment by rail or truck each stainless steel container is enclosed within and protected by a surrounding 40 ton lead shipping container.

One of the issues with the stainless steel containers is potential for internal pressure accumulation due to radioactive decays that yield inert gas daughters.

The inner stainless steel container is well sealed except via its vent tube. If the inner stainless steel container is unvented over time inert gas radio isotope decay products can build up a substantial pressure inside the inner stainless steel container. An example of a problem radio isotope in this respect is Cl-36. It spontaneously decays into stable Ar-36 which is an inert gas. Thus if the inner container is unvented, for safe long term storage the mass of Cl-36 within the inner steel container must be limited to the mass of argon that the inner stainless steel container can contain at a safe working pressure. The parties must face the reality that the cost of safely containing Cl-36 is high and that in future nuclear reactor design every effort should be made to minimize formation of Cl-36. This issue heavily impacts the future application of Molten Salt Reactors.

In this respect an open end stainless steel tube should run vertically from the top center of the stainless steel container to above the oil surface level. This tube has a tall loose fitting negative buoyancy porcelain cap that in conjunction with the oil forms a vent valve. Then inert gas formed inside the stainless steel container will vent into the porcelain storage container. If the spent fuel is stored for a sufficiently long time before being inserted in the stainless steel containers and if the chemical makeup of the stored material is appropriate this vent arrangement might be replaced by a simpler rupture disk.

In the event that water somehow finds its way into the porcelain container bottom a sacrificial magnesium-aluminum sheet attached to the stainless steel container provides additional galvanic corrosion protection.
 

STAINLESS STEEL CONTAINER DETAILS:
1. The body of the inner cylinder is made of stainless steel pipe, 1.067 m (42 inch) outside diameter, about 2.02565 m (79.75 inch) high with a wall thickness of 12.7 mm (0.500 inch).

The cylinder top and bottom are stainless steel, 1.25 inch thick. The bottom fits inside the cylinder and is attached to the cylinder by welding both inside and outside. The cylinder extends beyond the bottom by about 0.5 inch. The cylinder top is attached to the cylinder with 3 inch wide bolted flanges. Each lower flange is made in two half circular parts cut from 1.25 inch thick stainless steel sheet. The upper flange is integral to the top plate.

2. The flanges are 48 inch OD and are attached to each other with 48 X (1.00 inch) diameter stainless steel bolts equally spaced on a 45.75 inch diameter bolt circle. After welding but before further machining and polishing the welds are x-rayed. Then flange recesses and optical flats are machined on to the flanges. The fabrication order is important to minimize weld induced distortions.

3. For containers intended to store unprocessed spent CANDU fuel bundles the inner stainless steel container is stabilized against deformation and collapse during evacuation by the 61 4.0 inch ID, 4.5 inch OD tubes that stabilize the positions of the spent CANDU fuel bundles. If these tubes are not present a central external reinforcing ring may be required to give the container stability against deformation and collapse during evacuation.

4. The inner stainless steel container is supported by a porcelain cradle that in plan view looks like an asterisk.

5. The inner stainless steel container has an attached external 0.75 inch thick sacrificial magnesium-aluminum plate near its bottom center that provides galvanic corrosion protection for the inner stainless steel container if any water accumulates at the inside bottom of the porcelain container.

6. The vent tube of the inner stainless steel container has a valve that permits evacuation and argon back filling.

7. The top of the inner stainless steel container has attached to its upper surface a 0.61 m (24 inch) outside diameter 0.0317 m (1.25 inch) wall lifting ring, 0.127 m (5 inches) high. This lifting ring has 4 X 2 inch diameter symmetrically spaced radial holes suitable for attachment of lifting bars or hooks.
 

STAINLESS STEEL CONTAINER DIMENSIONS:
Assume that the inner stainless steel container cylinder wall is fabricated from 42 inch OD, 0.500 inch wall stainless steel pipe. Assume that the pipe is supplied in 20 foot lengths, so an economical way to cut the pipe with minimal waste is to cut it into 3 lengths, each length nominally 79.75 inches long. Assume that the top flange extends the length by o.50 inch, the top perimeter gasket is 0.125 inch thick, that top plate is 1.25 inch thick and the lifting ring is 5 inches high. Then the overall stainless steel container length is:
79.75 + 0.50 + 0.125 + 1.25 + 5.0 inch = 86.625 inches
 

STAINLESS STEEL END THICKNESS:
Let the inner stainless steel container end sheet thickness be Tm. Let the unsupported radius of the end be Rm.

Let P be the differential pressure between the inside and outside of the stainless steel container.

The hoop stress Sc on the cylindrical container walls is:
Sc = P 2 (Rm) dL / 2 Tm dL
= P (Rm) / Tm

The torque on the end wall at its perimeter is:
Integral from R = 0 to R = Rm of:
P (2 Pi R) dR (Rm - R)
= P (2 Pi) Rm (Rm^2 / 2) - P (2 Pi) (Rm^3 / 3)
= P (2 Pi) Rm^3 / 6

The resisting torque is:
Integral from X = 0 to X = (Tm / 2) of:
2 Sd [(X / 2) / (Tm / 2)] (2 Pi Rm) dX (X / 2)
= [2 (Sd / Tm) (Pi Rm) ] [Tm / 2]^3 / 3
= [(Sd) (Pi Rm) ] [Tm / 2]^2 / 3

Equating the two torques gives:
P (2 Pi) Rm^3 / 6 = [(Sd) (Pi Rm)] [Tm / 2]^2 / 3
or
P = {[(Sd) (Pi Rm)] [Tm / 2]^2 / 3} / {(2 Pi) Rm^3 / 6}
= {[(Sd)] [Tm / 2]^2} / { Rm^2}
= Sd (Tm / 2 Rm)^2

Let Sm = the maximum allowable metal working stress for safety (Sm = 10,000 psi).
Then the maximum allowable pressure P = Pm at Sd = Sm is:
Pm = Sd (Tm / 2 Rm)^2
= 10,000 psi (1.25 / 42 inch)^2
= 8.86 psi
This is the maximum safe internal working pressure.

Evacuation of the container will lead to local material stress as high as:
(14.7 psi / 8.86 psi) X 10,000 psi = 16, 591 psi

Hence the end sheets can be 1.25 inch thick low carbon stainless steel. To provide an increased margin of safety choose a flange plate thickness of 1.25 inches.

The stainless steel top should permit lifting the steel container using a 5 inch high lifting ring welded the top sheet. The lifting ring should be formed from a 24 inch outside diameter thick wall stainless steel pipe section with four equally spaced 2 inch diameter radial holes.

The 0.500 inch thick stainless steel cylinder walls permit a deep weld between the cylinder wall and the lower flange. Note that the lower flange overlaps the cylinder side walls by 0.75 inch and is welded on both the inside and outside.. The upper flange is integral to the stainless steel top. The inside length of the container is:
79.75 inch - 0.50 inch - 1.25 inch + 0.50 inch = 78.50 inch (1.9939 m)

Note that a stack of four CANDU fuel bundles is nominally 1.98 m high. An uncertain issue is whether fuel swelling will make a stack of four spent CANDU fuel bundles too tall to fit inside the stainless steel container.
 

STEEL WEIGHT:
Volume of steel exclusive of fastening material is given by:
Outer cylinder + bottom + lower flange + top + lifting ring
= (Pi X 41 inch X 79.73 inch X 0.500 inch) + (Pi X (20.5 inch)^2 X 1.25 inch) + (Pi X 2 (21.75 inch) X (1.25 inch) X (3 inch)
+ (Pi X (24 inch)^2 X 1.25 inch) + (Pi X 24 inch X 1.25 inch X 5.00 inch)
 
= 5134.82 inch^3 + 1650.32 inch^3 + 512.47 inch^3 + 2261.95 inch^3 + 471.24 inch^3
= 10,030.80 inch^3

The weight of stainless steel is given by:
10,030.80 inch^3 X (.0254 m / inch)^3 X (7850 kg / m^3) = 1290.35 kg
 

INTERNAL PRESSURE WITHSTAND CAPABILITY:
As shown above the maximum safe working pressure Pw within the stainless steel container is about:
8.86 psi = 8.86 / 14.7 = 0.603 bar

The internal volume Vol of the stainless steel container is given by:
Vol = Pi (20.5 inch)^2 (78.5 inch)
= 103,639.9 inch^3
= 103,639.9 inch^3 X (.0254 m / inch)^3 X 1000 lit / m^3
= 1698.35 lit

At one bar at 20 degrees C one mole of a gas occupies:
22.4 lit X (293/273) = 24.04 lit.

Hence the number of moles of gas that one stainless steel container can safely store at 20 degrees C before venting is limited to:
[1698.35 lit / (24.04 lit-bar / mole)] X 1.603 bar = 113.25 moles

If the only inert gas emitting substance is Cl-36 changing to Ar-36 then the maximum initial safe quantity of Cl-36 in the container before venting will be tripped is:
36 gm / mole X 113.25 moles = 4.077 kg
 

LEAD SHIPPING CONTAINER DESCRIPTION:
The purpose of a shippiing container is to provide bio-safety shielding and in the event of a transportation accident to provide mechanical protection for the contained stainless steel container.

To realize the required bio-safety shielding the wall thickness of the lead shipping containers has to be 12 inches, which due to the the praactical transport weight limitations constrains the external height of a 42 inch outside diameter stainless steel container to about 82 inches.

The lead shipping container is simply a ~ 40 ton lead cylinder with 12 inch thick lead walls that fits snugly around the stainless steel container to provide both bio-safety shielding and mechanical/structural protection during shipment by truck or rail. This shipping container is sized so that two such shipping containers can be transported by one 100 ton rated railway car and one such container can be transported by a 80 ton rated road truck. The shipping containers are intended to be loaded and unloaded using 50 ton rated cranes. The choice of 12 inch (30 cm thick) lead reduces the intensity of 1 MeV gamma radiation by a factor of about:
2^30 ~ 10^9.

Assume that a stainless steel container contains fission product concentrates from 1400 CANDU spent fuel bundles. Neglecting the shielding effect of the steel the radiation at the outer surface of the shipping container about 1 m from the container axis would be about:
(1400 X 0.3 Sv / hr) / (10^9) = 0.42 uSv / hr
This radiation intensity is further reduced by about a factor of 2 due to the stainless steel and porcelain container walls.
 

LEAD SHIPPING CONTAINER DETAIL:
The shipping container is nominally a lead cylinder with 12 inch thick walls and domed end caps. Allowing a 0.5 inch clearance on radius and a 0.375 inch clearance on overall length with a slot for the lifting ring gives:
Combined end volume = Pi [33.5^2 - 21.5^2] inch^2 [24 inch]
= (24) Pi [1122.25 - 462.25] inch^3
= Pi (15,840 inch^3)

Lead cylinder side wall volume = Pi [(33.5 inch)^2 - (21.5 inch)^2] X 82 inch
Pi (82) [1122.25 - 462.25] = Pi (54120 inch^3)

Hence the total lead volume in the lead shipping container is:
Pi (54,120 inch^3 + 15,840 inch^3)
= 219,785 inch^3
= 219,785 inch^3 x (.0254 m / inch)^3
= 3.602 m^3

The density of lead is:
11,340 kg / m^3

Hence the mass of the shipping container is about:
11,340 kg / m^3 X 3.602 m^3 = 40,843 kg
= 40.85 tonnes
= 44.93 tons.

The actual weight of the lead shipping container is slightly larger due to the fastening, lifting and mounting accessories. The lead container casting contains recesses and slots for the stainless steel container flange and the stainless steel container lifting ring.

The weight of the external biosafety shielding required for shipping spent CANDU fuel reduces the maximum number of stainless steel containers to two per 100 ton rated rail car. These rail cars must be of a type that permits top loading and unloading with a 50 ton rated crane.

A lead shipping container is essentially a large 2 piece lead casting. The methodology used to fasten the pieces together must be extremely robust to safely with stand any transportation accident.

Note that the lead shipping container minimum OD is 67 inches. On a 100 ton rated rail car the lead shipping containers should be positioned directly over the wheel boggies. In a trailer truck the lead shipping container must be positioned to equally distribute the load over the trailer wheels.
 

TRANSPORTATION AND STORAGE OVERVIEW:
A crucial aspect of the nuclear fuel cycle is safe, simple, economical and flexible transportation of radioactive materials.

Storage needs to be robust, natually dry, naturally ventilated, safe, accessible and economical.

CANDU reactors directly produce two major categories of nuclear waste: spent fuel and radio active non-fissionable materials.

After the spent fuel has been in on-site wet/dry storage for ten years, the CANDU spent fuel can be separated on site into two portions: 90% by weight pure uranium oxide and 10% by weight CANDU spent fuel concentrates. The CANDU spent fuel concentrates are a highly radioactive mixture of remaining uranium oxide, fission products and trans-uranium actinides.

The CANDU spent fuel concentrates are intensely radioactive and should be placed in narrow sealed stainless steel tubes within fissionable material stainless steel containers. These tubes are separated by B4C to prevent the stored material becoming critical if through accident or mishandling water enters the fissionable material stainless steel container. The B4C is locked into a binder material to ensure that it will not be washed out in a worst case accident in which the stainless steel container is both fractured and immersed in flowing water.

For transport each stainless steel container of radioactive concentrates is inserted into a lead shipping container with 12 inch thick lead walls. The primary purpose of the lead shipping container is to provide biosafety shielding during shipment. A secondary purpose of the lead shipping container is to provide physical protection for the inner stainless steel container in the event of a severe transportation accident.

These composite containers can then be transported to Chalk River for further material reprocessing. At an interim storage location the stainless steel container of concentrates can be removed from the lead shipping container and inserted into a porcelain storage container for interim storage.

At Chalk River the concentrates are separated into uranium oxide, fission products and core rod material. Chalk River is far from any metropolitan area.

At Chalk River the trans-uranium actinides are combined with uranium metal and zirconium metal and are formed into FNR core rods. The FNR core rods are transported to FNR sites in fissionable material containers within lead shipping containers. At the FNR site the fuel rods are inserted into fuel tubes, the fuel tubes are assembled into fuel bundles and the fuel bundles are loaded into the FNR.

The pure uranium oxide is only very weakly radioactive. If it is pure it can be stored and transported in simple unshielded stainless steel containers. If it contains a significant concentration of radioactive impurities the uranium oxide can be stored and transported in the same manner as radioactive non-fissionable materials. This uranium oxide is depleted in the isotope U-235 but is still useful as a fast neutron reactor (FNR) blanket fuel feedstock. The uranium oxide must be reduced to uranium metal and alloyed with zirconium prior to use as FNR blanket rod material. Much of the uranium oxide is placed in interim storage for future use as FNR fuel.

The fission products are intensely radioactive but pose no threat of going critical, even if immersed in water. The fission products are transported to a naturally dry site for medium term storage. At a medium term storage site each stainless steel container is removed from its lead shipping container and is transferred into a porcelain storage container. The gap between the two containers is back filled with heavy oil and then the storage container is capped. The storage container is then placed in a permanently accessible safe naturally dry naturally vented Deep Geologic Repository (DGR).

Every year a portion of the fuel bundles in a FNR is removed from the spent fuel storage zone for reprocessing. The extracted blanket rods and core rods are transported in fissionable material containers to Chalk River for reprocessing. At Chalk River fission products are extracted and new core and blanket rods are formed. The fission products are transported to medium term storage in non-fissionable material containers. The new core and blanket rods are transported to FNRs in fissionable material containers.
 

MATERIAL FLOW:
This web site shows that CANDU spent fuel bundles, after ten years in wet/dry storage at the reactor site, should be preprocessed on the reactor site to selectively remove 90% of the spent fuel bundle weight in the form of very weakly radioactive uranium oxide. The remaining spent fuel concentrates can be placed inside stainless steel fissionable material containers and transported to storage or material reprocessing sites within surrounding lead shipping containers at the equivalent rate of about 2800 CANDU fuel bundles per 100 ton rated rail car trip or about 1400 CANDU fuel bundles per 80 ton road truck trip.

At the storage site each stainless steel container is transferred from its enclosing lead shipping container to an enclosing porcelain storage container. A heavy oil low vapor pressure liquid dielectric is used to fill the gap between the porcelain container and the stainless steel container. The porcelain container is then capped and moved into a naturally dry naturally ventilated subterranean granite storage vault. The cap is retained by a stainless steel clamp.

The cost of reusable stainless steel and porcelain storage containers that in combination each hold fission product concentrates from about 1400 spent CANDU fuel bundles is presently estimated to be less than $100,000 per combination container or $71.43 per CANDU fuel bundle. While that may seem expensive the NWNO budget for disposal of spent CANDU fuel bundles is:
$24,000,000,000 / 3,000,000 CANDU fuel bundles = $8,000 per spent CANDU fuel bundle and the present NWMO plan provides no fuel or material recycling benefits.
 

SPENT CANDU FUEL HANDLING:
Spent CANDU fuel bundles are kept in wet storage at the reactor site for about 10 years after ejection from a CANDU reactor until the fission product decay thermal emission falls to less than 5 watts / CANDU fuel bundle and the gamma ray emission falls to less than 0.3 Sv / hour / bundle at a distance of 1 m.

The uranium oxide component of a CANDU fuel bundle emits very little decay energy. The essence of the Ottensmeyer Plan is to start by selectively removing 90% of the CANDU spent fuel bundle weight in the form of weakly-radioactive uranium oxide from the spent CANDU fuel bundles. The outputs from this process are 90% by weight uranium oxide, that has little radio activity, and 10% by weight of highly radioactive concentrates consisting of zirconium, fission products, transuranium actinides and remaining uranium oxide. The radio activity of the concentrates is about 10X higher per unit weight than the radio activity of the original CANDU spent fuel bundle.

The concentrates are then loaded into stainless steel containers, as described herein, each of which holds concentrates from about 1400 spent CANDU fuel bundles. The contents of each such stainless steel container immediately after loading will emit gamma radiation at about:
1400 bundles X 0.3 Sv / hour-bundle = 420.0 Sv/ hour 1 m from the container axis.
Each such filled stainless steel container weighs about 4.0 tonnes (4.4 tons).

For safe shipping and handling each such filled stainless steel container should then be loaded into an enclosing 45 ton lead shipping container that provides a 12 inch thickness of surrounding lead shielding and that is strong enough to withstand any road, rail or crane handling accident.

The lead shipping containers should then be loaded onto suitable trucks or railway cars using a 50 ton rated crane. Typically a suitable railway car is rated for 100 tons of top loaded cargo (2 lead shipping containers) whereas a suitable conventional road truck is rated for 50 tons of top loaded cargo (1 lead shipping container).

Near the storage site another 50 ton rated crane transfers the shipping containers from railway cars to trucks.

Each truck is driven into a shielded truck bay within the storage facility where the shipping container is unloaded using a 50 ton rated crane. The truck is then driven away. When there is no one in the shielded truck bay remotely controlled equipment is used to remove the stainless steel container from the shipping container and to transfer the stainless steel container into a porcelain storage container. The porcelain storage container is then back filled with liquid dielectric, capped and moved by remotely controlled equipment into a subterranean granite storage vault that is natually dry, naturally vented, gravity drained and high above the local water table.

After all gamma emitters are removed from the shielded truck bay the lead shipping containers are loaded back onto trucks and are driven away for reuse. Thus a small number of 45 ton lead shipping containers can support a large inventory of seldom accessed porcelain storage containers. Each porcelain storage container in the storage vault has within it a stainless steel container that can safely contain fission product concentrates from about 1400 spent CANDU fuel bundles or can contain about 140 unprocessed spent CANDU fuel bundles.

If 3,000,000 unprocessed spent CANDU fuel bundles are all stored at one location the required number of stainless steel and porcelain container pairs is:
3,000,000 CANDU fuel bundles / (140 bundles / container pair) = 21,429 container pairs

However, if only the radio active concentrates are stored in porcelain containers the required number of stainless steel and porcelain container pairs decreases to:
3,000,000 CANDU fuel bundles / (1400 bundles / container pair) = 2,143 container pairs

One of the merits of this container handling plan is that the storage facility can be operated by relatively few personnel who need only limited training. Hence the storage facility is economical to operate and can be located at a geophysically advantageous site that is far from any population center. Furthermore, due to multiple redundant radio isotope isolation barriers plus a naturally dry DGR there is no possibility of ground water contamination once the material is in storage.

The number of 100 rail car trains necessary to move the concentrates to the storage site is:
(3,000,000 CANDU fuel bundles) X (1 rail car / 2800 CANDU fuel bundles) X (1 train / 100 rail cars) = 10.7 trains.

Every porcelain storage container in the storage vault is individually remotely monitored to detect out of design range dielectric level. A dielectric level change may indicate a cap failure, an inner wall failure, an outer wall failure, a non-vertical container position or dielectric evaporation or decomposition.. Each storage vault is naturally ventilated to maintain a low temperature and is individually monitored. Over time the containers will release heat and low molecular weight hydrocarbon gases which must be removed by the natural ventilation.

The minimum natural ventilation rate must be sufficient to ensure that even if there is a major heavy oil spill inside the DGR the temperature and vapor pressure of the heavy oil will not support combustion in air.
 

RECORD KEEPING:
Precise permanent records must be maintained regarding the exact contents of each container and their chemical and nuclear history.
 

MAXIMUM CANDU FUEL BUNDLE LOAD:
Assume that a CANDU nuclear fuel bundle has an OD of 4.0 inches and a length of 19.5 inches. The theoretical number of such fuel bundles that will fit in a single layer inside a 36 inch ID metal container is given by:
1 + 6 + 12 + 18 + 24 = 61
We actually have a 41 inch ID so the available container inside diameter per CANDU fuel bundle is:
41 inch / 9 = 4.555 inch.
Hence there is tolerance to allow for the 0.25 inch wall thickness of the fuel bundle guide pipes. Hence the basic number of fuel bundles / container is given by:
(4 layers / container) X (61 fuel bundles / layer) = 244 fuel bundles / container

The steel tube metal volume is:
61 tubes X 2 m X Pi (4.25 inch) X 0.25 inch X (.0254 m / inch)^2
= 0.2627 m^3

The mass of this tube metal is about:
0.2627 m^3 X 7850 kg / m^3 = 2062.41 kg = 2.06241 tonnes

Assume that each fuel bundle weighs 22.5 kg. Then the maximum spent nuclear fuel bundle weight is given by:
244 bundles / container X 22.5 kg / bundle = 5490 kg / container
 

MAXIMUM LOADED STORAGE CONTAINER WEIGHT:
The maximum loaded storage container weight =
[CANDU fuel bundles] + [steel tubes] + [stainless steel container] + [porcelain container] + [oil]
= [5490 kg] + [2062.41 kg] + {1290.35 kg] + [1475 kg] + [431.16 kg]
= 10748.92 kg
= 10.749 tonnes

Thus a storage container fully loaded with 244 spent CANDU fuel bundles weighs about 10.75 tonnes.
 

STORAGE AREA REQUIREMENT:
Assume that the porcelain containers are placed three deep on each side of a 30 foot wide aisle. Due to the centre to centre distance in effect each container takes up a square 5 feet to a side. This a 5 foot length of aisle services 6 containers. The containers take up:
6 X 25 ft^2 = 150 ft^2.
The corresponding asile area is:
5 ft X 30 ft = 150 ft^2.

Thus on average each container requires a storage area of:
300 ft^2 / 6 containers = 50 ft^2 / container.

Each container holds 5.490 tonnes of CANDU spent fuel. Thus storing unprocessed CANDU spent fuel in this matter requires a DGR with a floor space of:
(50 ft^2 / 5.490 tonnes) X 50,000 tonnes = 455,373 ft^2
which is less than (1 / 10) of the storage space presently available at Jersey Emerald.
 

MAXIMUM STORAGE VAULT FLOOR LOADING:
The storage area floor loading where the containers are concentrated is:
11 tons / 25 ft^2 = 22,000 lb / 25 ft^2
= 880 lb / ft^2
 

CANDU SPENT FUEL BUNDLE INVENTORY:
Assume that there are 3,000,000 CANDU spent fuel bundles. The storage space required is given by:
(3,000,000 bundles) X (1 container / 244 bundles) X (50 ft^2 / container) = 614,754 ft^2
 

INTERMEDIATE WASTE FROM RETUBING REACTORS:
When a CANDU reactor is retubed the 4 inch diameter high pressure fuel channels and the 132 mm OD moderator tubes are replaced. Each tube can be cut into six 52 inch lengths. The fuel channel tube sections will fit inside moderator tube sections. One container can store 37 moderator tube cutoffs of lengths varying from 40 inches to 80 inches. Assuming an average cutoff length of 52 inches each container stores (37 / 6) = 6.17 moderator tubes. Thus if a reactor has 380 tubes the number of containers required to store the used tubing is:
(380 / 6.17) = 63 containers

Each tube assembly has two end fittings. Assume that 14 end fittings will fit in one container. Then the number of containers requred for the end fittings is:
(380 X 2) / 14 = 55 containers.

Thus retubing a 380 tube CANDU reactor requires:
63 + 55 = 118 containers.

The average annual storage space requirement triggered by reactor retubing is:
20 reactors X 1 retubing / 20 reactor-years X 173 containers / retubing X 50 ft^2 / container
= 8,650 ft^2 / year
 

TRITIUM FILTER CARTRIDGES
Tritium has a half life of 12.6 years. After 126 years 99.9% of the tritium has become stable helium-3 and its radioactivity has decreased 1024 fold. These low level waste cartridges should be stored in gas helium tight fusion bonded epoxy coated 48 inch diameter steel pipe with an applied DC bias. There is no need to use an elaborate long term storage container. Sand can be poured around these pipes to protect them from a partial overhead cave in. The tritium filter ion exchange cartridges are only of long term concern if there has been major metal corrosion or erosion that has been captured by the cartridge. Generally the weight of metal captured by these ion exchange cartridges is microscopic. After 252 years in dry storage the radioactivity of these cartriges should be checked before the stored helium-3 is made available for reuse and the cartridge material is released to the environment.
 

This web page last updated July 21, 2017.

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