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This web site addresses design issues relating to containers for safe storage and transport of nuclear waste.
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 going critical if water penetrates the container.
A shipping container is a 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 provides 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 a 24 wheel 120 ton road truck.
A porcelain container is a 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 an accessible naturally dry naturally ventilated deep sub-terranian granite repository.
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 spent CANDU fuel transport, radioactive material containers and radioactive material storage can all be reduced more than 10 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 be insoluble in water and 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 containers can vent accumulated internal gas pressure from time to time to release inert gas decay products and low molecular weight hydrocarbon gases resulting from gamma irradiation of the liquid dielectric.
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 and to allow periodic removal of water and/or dielectric testing and replacement.
The main purposes of the 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 an apparent 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 dielectric evaporation or 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 cap on the stainless steel container is much heavier than the vent cap 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 very much better understanding of ceramics, especially porcelain. 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 lends itself to remote monitoring.
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.
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 must either be removed or chemically balanced by addition of further SiO2 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 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.
Each storage container assembly consists of an outer porcelain container, an inner stainless steel container and a heavy oil dielectric separator between the inner and outer container walls. While in storage fission daughter inert gases will form in the inner stainless steel container and light hydrocarbon gases will form within the liquid dielectric. For pressure safety gas accumulations in the inner stainless steel container must automatically vent into the porcelain container and gas accumulations in the porcelain container must automatically vent into the vault space. The vault space should have reliable natural ventilation.
The outer container and the vent caps should be made of porcelain. Porcelain is an almost unique ceramic that does not take on water of hydration. Porcelain rarely occurs in nature. Porcelain consists of randomly oriented needle like mullite crystals in a silica matrix. The porcelain should vitrify all the way through and not require an external glaze. The randomly oriented needle like mullite crystals give the porcelain immense strength and toughness. The porcelain actually used should be tested to ensure that it does not take on water of hydration and that the required toughness is actually realized.
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.
The porcelain container vent seal material can be a fluorocarbon liquid. The porcelain container perimeter top seal can be wax enhanced by an external 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 faces. This seal fabrication requirement impacts the choice of stainless steel alloy used to fabricate the inner container. This alloy must be low carbon for good welding characteristics. On the drawing the gasket thickness allowance of 0.25 inches is really an overall length tolerance in the stainless steel container that includes cutting, welding, machining, polishing and gasket thickness tolerances. The gasket seal can be leak tested with helium.
OIL LEVEL SENSOR:
One cm of Pb reduces 1 MeV gamma rays by about a factor of 2.0. Hence to protect the oil level sensor from gamma ray damage requires about:
11 cm X 1 inch / 2.54 cm = 4.33 inches thickness of lead shielding.
The dielectric fills the space between the inside wall of the porcelain storage container and the outside wall of the stainless steel non-fissionable material container. One of the functions of the dielectric is to seal the stainless steel non-fissionable container vent valve which allows controlled release of fission product decay daughters that are inert gases, primarily krypton and xenon. This inert gas will flow out of the inner stainless steel container through the stainless steel vent tube, bubble up through the heavy oil and accumulate between the top surface of the heavy oil and the lower surface of the porcelain container top. The 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 dielectric must not be easily flammable and must not transport water into the container.
The chosen dielectric is a heavy hydrocarbon oil that is a stable high molecular weight low vapor pressure liquid with an initial density of about 0.85 gm / cm^3.
The heavy oil achieves several objectives. The heavy oil allows controlled release of inert gas products that are fission product decay daughters. This inert gas will flow out of the inner stainless steel container through the stainless steel vent tube, through the porcelain vent cap, will bubble up through the heavy oil and will accumulate between the top surface of the heavy oil and the lower surface of the porcelain container top.
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 bitumen with a density as high as 1.05 gm / cm^3.. The lowest 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 oil density change process low molecular weight hydrocarbon gases must be vented.
The higher molecular weight components have a higher density and will gradually will gradually turn into wax and accumulate at the bottom of the oil where they can be periodically removed by vacuum suction and replaced by lower density oil. There will be gradual 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 replacement or top up. Due to the decreasing gamma flux these time intervals will become longer as the time in storage increases.
PORCELAIN CONTAINER DESCRIPTION:
Evaporation of the oil dielectric is prevented by the porcelain container top which has a wax seal on its perimeter.
The porcelain container top has its own gravity sealed pressure relief vent. This vent has a hat to protect it from overhead condensation.
The porcelain container's lid should have an external shape that will shed water dripping from overhead.
The buoyancy in water of a filled porcelain container must be negative so that if the DGR vault floods the container will not float and will remain upright.
A suitably shaped asterisk 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 denser than the heavy oil can accumulate and become trapped over time. The porcelain container has a flat external bottom so that the porcelain container will be stable when placed upright 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.
The porcelain outer container lid has a combined wax and ground porcelain seal.
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 bulk ielectric. The vacuum extraction tube going to the bottom passes loosely through a hole in the stainless steel container flange, a matching hole in the stainless steel container top 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 outer 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 valve permits container isolation, container evacuation and container back filling.
Once in place the position of the porcelain container can be stabilized by surrounding the porcelain container with low density gravel. Gravel can also be used to prevent porcelain container fracture if there is 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 fluorocarbon grease 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.
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 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 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 insetion and removal of stainless steel containers from either porcelain storage containers of 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. 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.
The inside stainless steel container has a lifting ring welded to its top.
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.
The inner stainless steel container is well sealed except via its vent tube. If 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.
The inside stainless steel container has a lifting ring welded to its top.
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.
Chris Hatton of the NWMO has correctly pointed out that to withstand a large external pressure the inner stainless steel container should be cylindrical with hemispherical ends. Under severe external pressure hemispherical container ends minimize stress within the inner container material. A stainless steel cylindrical container with hemispherical ends is also highly resistant to internal pressure that can accumulate if the container is not adequately vented.
Disadvantages of domed end inner stainless steel container design are that the stainless steel ends require special fabrication (casting, forging and machining) and the useful containment volume is reduced. Provided that the storage space remains accessible and the containers are suitably vented no such pressure buildup will occur. Hence in these circumstances doming of the stainless steel container ends is likely unnecessary.
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 (39.69 inch) high with a wall thickness of 12.7 mm (0.500 inch). The ends are stainless steel, 0.5 inch thick. The bottom is attached to the cylinder by welding. The cylinder overlaps the bottom by about 0.5 inch. The top is attached to the cylinder with 3 inch wide bolted flanges. Each lower flange is made in two half circular parts out of 1.00 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 96 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 distortio?ns.
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 container 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 0.25 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 within 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 2 inch diameter symmetrically spaced radial holes suitable for attachment of lifting 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 six lengths, each length nominally 39.69 inches long. Then the overall stainless steel container length is:
40.00 + 21 + 21 = 82.00 inches
STAINLESS STEEL END DOME THICKNESS:
Let the inner stainless steel container end dome thickness be Tm. Let the external radius of the end dome be Rm.
Let P be the interior pressure in the stainless steel container.
The hoop stress Sc on the cylindrical container walls is:
Sc = P 2 (Rm - Tm) dL / 2 Tm dL
= P (Rm - Tm) / Tm
On the end dome the wall stress Sd is given by:
Sd = P Pi (Rm - Tm)^2 / [Tm Pi 2 (Rm - (Tm / 2))]
~ P (Rm - Tm) / 2 Tm
Hence the wall stress on the domes is only half the wall stress on the cylinder side walls.
Let Sm = the maximum allowable metal working stress for safety (Sm = 10,000 psi).
Hence the end domes can be 0.500 inch thick low carbon stainless steel. To provide an increased margin of safety choose a flange plate thickness of 1.00 inches.
The stainless steel container flanges should be 1.00 inch thick. The top dome should permit lifting the steel container using a 5 inch high lifting ring welded to or integral with the top dome. 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 0.5 inch. The upper flange is integral to the top dome.
OVERALL STAINLESS STEEL CONTAINER DIMENSIONS:
This design yields a minimum stainless steel container inside length of:
(39.69 inch + 42 inch - 1.0 inch = 80.69 + 0.31 inch gasket thickness) = 81.00 inch
Then the stainless steel container outside length is:
(81 inch + 2 (0.5 inch) = 82 inch
Volume of steel exclusive of fastening material is given by:
(Pi X 42 inch X 39.69 inch X 0.500 inch) + (4 X Pi X (21 inch)^2 X 0.5 inch)
+ 2 (Pi X 45 inch X 3 inch X 1.00 inch)
= (Pi)[(42 X 59.75 X 0.500) + (2 X 21^2) + 270 + (61 X 80)] inch^3
= (Pi)(833.49 + 882.00 + 270) inch^3
= 6237.6 inch^3
The weight of steel is given by:
6237.6 inch^3 X (.0254 m / inch)^3 X (7850 kg / m^3) = 802.39 kg
INTERNAL PRESSURE WITHSTAND CAPABILITY:
The maximum safe working pressure Pw within the stainless steel container is limited by the hoop stress.
Pw (41 inches) dL = 10,000 PSI 2 (0.5 inch) dL
Pw = 10,000 PSI / 41
= 243.9 PSI
= 243.9 PSI/ (14.7 PSI / bar)
= 16.592 bar
The internal volume Vol of the stainless steel container is given by:
Vol = Pi (20.5 inch)^2 40 inch + (4 / 3) Pi (20.5 inch)^3
= Pi (20.5 inch)^2 [40 inch + (4 / 3)(20.5 inch)]
= 1320.25 inch^2 [67.333 inch]
= 88,896.83 inch^3
= 88,896.83 inch^3 X (.0254 m / inch)^3 X 1000 lit / m^3
= 1456.76 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 is limited to:
[1456.76 lit / (24.04 lit-bar / mole)] X 16.592 bar = 1005.43 moles
If the only inert gas emitting substance is Cl-36 changing to Ar-36 then the maximum safe quantity of Cl-36 in the container is:
36 gm / mole X 1005.43 moles = 36.195 kg
This is about 1.1% of a typical container load.
LEAD SHIPPING CONTAINER DESCRIPTION:
The purpose of a shippiing container is to provide biossafety shielding and in the event of a transportation accident toprovide mechanical protection for the contained stainless steel container.
To realize the required biosafety 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 82 inches.
The lead shipping container is simply a ~ 40 ton domed end lead cylinder with 12 inch thick lead walls that fits snugly around the stainless steel container to provide biosafety 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 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 1.0 inch clearance on radius and hence a 2.0 inch clearance on overall length gives:
Combined end dome volume = (4 / 3) Pi [34^3 - 22^3] inch^3
= (4 / 3) Pi [39,304 - 10,648] inch^3
= Pi (38,208 inch^3)
Lead cylinder side wall volume = Pi [(34 inch)^2 - (22 inch)^2] X 40 inch
= Pi (26,880 inch^3)
Hence the total lead volume in the lead shipping container, exclusive of the flange ring, is:
Pi (38,208 inch^3 + 26,880 inch^3)
= 204,480 inch^3
= 204,480 inch^3 x (.0254 m / inch)^3
= 3.351 m^3
The density of lead is:
11,340 kg / m^3
Hence the mass of the shipping container, exclusive of the flange ring, is about:
11,340 kg / m^3 X 3.351 m^3 = 37,998 kg
= 37.998 tonnes
= 41.798 tons.
The actual weight of the lead shipping container is slightly larger due to the flange ring and 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 flange ring is additional external lead used to make up for lead cut out of the notchs for the flange ring and the 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 rail car. These rail cars must have a 100 ton cargo rating and 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 mishap or accident.
Note that the lead shipping container OD is 68 inches. On a rail car the lead shipping containers should be positioned directly over the wheel boggies.
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 90% by weight pure uranium oxide and 10% by weight CANDU spent fuel concentrates. The CANDU spent fuel concentrates are a mixture of remaining uranium oxide, fission products and trans-uranium actinides. After this separation process there are three categories of spent fuel nuclear waste: pure uranium oxide, CANDU spent fuel concentrates and radioactive non-fissionable materials.
The radioactive non-fissionable materials can be placed in sealed stainless steel containers which in turn are inserted into lead shipping containers. These composite containers can then be transported off-site to various locations for further reprocessing, interim term storage, medium term storage or long term storage. At a storage location the stainless steel container is removed from the lead shipping container and may be inserted into a porcelain storage container.
The 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 prior to use as FNR blanket rod material. Much of it is placed in interim storage for future use in a FNR. The uranium oxide should not go into long term storage because it will eventually be needed as FNR fuel.
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 going 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.
The concentrates are transported to a remote fuel reprocessing location where the concentrates are separated into uranium oxide, fission products and core rod material. This reprocessin location is located far from any metropolitan area.
Some of the uranium oxide is reduced to uranium metal and is then alloyed with zirconium to form blanket rods which are then shipped to fast neutron reactor (FNR) sites. The remaining uranium oxide is transported to a storage location where it is held in inventory for future use.
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 an accessible safe naturally dry naturally vented Deep Geologic Repository (DGR).
At the remote reprocessing site 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 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.
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 the remote fuel reprocessing site (eg Chalk River) for reprocessing. At this remote site 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.
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 weight 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 such as a hydrocarbon high molecular weight heavy oil 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 non-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 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 40 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 40 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 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 fission product 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 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 an unanticipated temperature change. Each storage vault is naturally ventilated to maintain a low temperature and is individually monitored. Over time the containers will release heat 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.
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:
(2 layers / container) X (61 fuel bundles / layer) = 122 fuel bundles / container
Assume that each fuel bundle weighs 22.5 kg. Then the maximum spent nuclear fuel bundle weight is given by:
140 bundles / container X 22.5 kg / bundle = 3150 kg / container
LOADED STORAGE CONTAINER WEIGHT:
The loaded container weight is given by:
CANDU fuel residue + steel + porcelain + lead
= 3150 kg + 802.39 kg + 2220 kg + 3355 kg
= 9527.3 kg
= 9.527 tonnes x 2200 lb / tonne X 1 ton / 2000 lb
= 10.48 tons
Thus a storage container fully loaded with residue from 1400 spent CANDU fuel bundles weighs about 9.53 tonnes.
The empty porcelain container weight including the lead shields is given by:
(porcelain weight) + (lead shield weight)
= 2220 kg + ____ kg
= ____ kg
= ____ tons
This weight indicates that:
(100 tons / box car) / (6.13 tons / empty container) = 16 empty porcelain containers
fitted with lead rings for protecting the fluorocarbon gasket can be transported by a single railway boxcar that is rated for a 100 ton load. The load limit is not the container size. The load limit is the container weight.
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.
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 / 1400 bundles) X (50 ft^2 / container) = 107,143 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 March 29, 2017.
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