Patent Description:
Storage of spent nuclear fuel, spent ion exchange resins, and special nuclear materials in closed containers can result in generation of mixtures of hydrogen and oxygen, whose worst case condition is a stoichiometric proportion. The generated gases need to be removed via filtered vent paths in order to prevent container pressurization and at the same time to contain contamination. A stoichiometric mixture is highly dangerous because its combustion can result in super-sonic shock waves that could destroy the container and associated confinement boundaries, thus not only causing extensive damage to anything nearby, but also resulting in the undesired release of radioactive material into the environment. Such gas mixtures have resulted in explosions in operating nuclear power stations in situations where the gases accumulated.

A key challenge is that the containers are thick-walled to provide shielding of their contents. A vent path through the shielding presents an unacceptable resistance to removal of the flammable gases, because the vent path resistance is very large compared to the filter resistance. Document <CIT> discloses a storage container for hazardous material such as transuranic waste having a body covered by a lid and being equipped with a venting means.

Embodiments of the present invention provide a means to safely and passively remove stoichiometric flammable source gases from shielded containers through a filtered vent path, such that the actual gas mixture in the container is not even flammable.

As one aspect of the disclosed concept a passive venting arrangement for use in venting of gases produced by radioactive materials is provided. The venting arrangement comprises: a source gas region structured to receive the gases produced by the radioactive materials; a filter ullage region disposed above the source gas region and segregated therefrom except for a plurality of bore holes which each extend between, and fluidly couple, the source gas region and the filter ullage region; and a plurality of filters disposed in contact with the filter ullage region, wherein each filter is structured to provide for the exchange of gases from the filter ullage region through the filter to an ambient environment.

The plurality of bore holes may comprise at least three bore holes.

The source gas region may be structured to house the radioactive materials.

The source gas region may be structured to receive the gases produced by the radioactive materials which are contained in a source gas location separate from the source gas region.

The passive venting arrangement may further comprise a vent pipe which is structured to fluidly couple the source gas region and the source gas location.

The source gas region may be defined, in-part, by a cone shaped region surrounding an opening of the vent pipe to the source gas region.

As another aspect of the disclosed concept, a containment vessel for use in storing radioactive materials is provided. The containment vessel comprises: a body defining a source gas region therein which is structured to house the radioactive materials; a filter ullage region defined in the body above the source gas region and segregated therefrom except for a plurality of bore holes defined in the body which each extend between, and fluidly couple, the source gas region and the filter ullage region; and a plurality of filters disposed in contact with the filter ullage region, wherein each filter is structured to provide for the exchange of gases from the filter ullage region through the filter to an ambient environment.

The plurality of bore holes comprises at least three bore holes.

The body may comprise a removable lid coupled to the body, wherein the filter ullage region and the plurality of bore holes are defined in the lid.

As yet another aspect of the disclosed concept, another containment vessel for use in storing radioactive materials is provided. The containment vessel comprises: a body defining a source gas region therein which is structured to house the radioactive materials; a first filter ullage region defined in the body above the source gas region and segregated therefrom except for a first plurality of bore holes defined in the body which each extend between, and fluidly couple, the source gas region and the first filter ullage region; a plurality of first filters disposed in contact with the first filter ullage region, wherein each first filter is structured to provide for the exchange of gases from the first filter ullage region through the first filter to an ambient environment; a second filter ullage region, independent from the first filter ullage region, defined in the body above the source gas region and segregated therefrom except for a second plurality of bore holes defined in the body which each extend between, and fluidly couple, the source gas region and the second filter ullage region; and a plurality of second filters disposed in contact with the second filter ullage region, wherein each second filter is structured to provide for the exchange of gases from the second filter ullage region through the second filter to an ambient environment.

A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:.

In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms.

The following description consists of an example application of a venting arrangement in accordance with the present invention, followed by an alternative application that shares the same common key features. The example venting arrangement is shown in <FIG>.

Referring to <FIG>, consider a thick-walled (shielded) vessel <NUM> comprising a vessel body <NUM> and a top lid <NUM> whose contents are the source of hydrogen and oxygen produced in stoichiometric proportion, or with less oxygen than in stoichiometric proportion, with stoichiometry being the worst case. The interior of the vessel <NUM> is called the source gas region, with the source gas emanating from a source gas location, which in the present example is also within the interior of the vessel <NUM>. The atmosphere of the source gas region <NUM> consists of air plus the source gases hydrogen and oxygen, where the proportions of each gas are controlled by proper design of this invention as described below.

In such example, the contents of the thick-walled vessel <NUM> in the source gas region/location <NUM> may be spent nuclear fuel, damaged spent nuclear fuel, highly damaged fuel debris, special nuclear materials, ion exchange resin loaded with radionuclides, or other radioactive waste. The radioactivity of these contents causes liquid water and hydrocarbon materials also in the container to decompose into hydrogen, oxygen, and possibly other hydrocarbon gases.

In the top lid <NUM> of the vessel there are a plurality of bore holes 120a-d, preferably at least three bore holes (four are shown in the example), which join the source gas region <NUM> to a second gas region called the filter ullage region <NUM>. The filter ullage region <NUM> is a very small region located at a higher elevation than the source gas region <NUM>, for reasons discussed further below. Thus, the bore holes 120a-d and the filter ullage region <NUM> are located within the vessel top lid <NUM>. The purpose of the filter ullage region <NUM> is to receive gases from the source gas region <NUM>, and allow these gases to contact filters 130a-c which are positioned in contact with the ambient environment <NUM>. The gases may then diffuse from the filter ullage region <NUM> through the filters 130a-c to the ambient environment <NUM>. Hence, a set of two, three, or more (three are shown in the example) sintered metal filters 130a-c are connected to the top of the filter ullage region <NUM>. These filters 130a-c may be commercial filters such as commonly fitted to threaded bung holes of thin-wall drums or any other suitable filters. Gases are exchanged between the ambient environment <NUM> and the filter ullage region <NUM> through the filters 130a-c. The purpose of the filters 130a-c is to provide a barrier to prevent contamination release from the container <NUM>. The top lid <NUM> of the container may have more than one of such vent arrangement provided therein.

When the venting arrangement is properly designed, the gas mixture in the gas source region <NUM> has a lower density than the gas mixture in the filter ullage region <NUM>. This causes the less dense gas to flow up one or more of the bore holes 120a-d from the gas source region <NUM>to the filter ullage region <NUM>, and it also causes the more dense gas to flow down the remaining bore holes 120a-d from the filter ullage region <NUM> to the gas source region <NUM>. Because the concentrations of hydrogen and oxygen in the filter ullage region <NUM> are greater than their respective concentrations in the ambient environment <NUM> outside the filters 130a-c, hydrogen and oxygen diffuse through the filters 130a-c from the filter ullage region <NUM> to the ambient environment <NUM>. This is ultimately how the hydrogen and oxygen source gases leave the thick-walled vessel <NUM>.

Proper design of such venting arrangement requires the appropriate selection of: (<NUM>) the number of bore holes 120a-d, (<NUM>) the diameter of the bore holes 120a-d, (<NUM>) the number of filters 130a-c, (<NUM>) the number of sets of bore hole/filter ullage/filter groups, and (<NUM>) the intrinsic ability of the filters 130a-c to pass hydrogen and oxygen. When properly designed, the hydrogen concentration in the source gas region <NUM> is below <NUM>°a by volume, which guarantees that the gas mixture is not flammable.

In an alternative application such as schematically illustrated in <FIG>, which shares a number of aspects similar to those of <FIG>. Thus, referring to <FIG>, consider a thick-walled (shielded) vessel <NUM> comprising a vessel body <NUM> and a top lid <NUM> whose contents are the source of hydrogen and oxygen produced in stoichiometric proportion, or with less oxygen than in stoichiometric proportion, with stoichiometry being the worst case. The interior of the vessel <NUM> is called the source gas region, with the source gas emanating from a source gas location <NUM>. For example, the source gas region <NUM> is actually the upper termination of a vent pipe <NUM> which proceeds from the gas source region <NUM> downwards through a water pool <NUM> to a submerged container (not shown) holding any of the contents mentioned above for the thick-walled vessel <NUM>. In such example, the submerged container and the vent pipe <NUM> are filled with water which is contaminated with radionuclides whose source is the contents of the container. The water line of the system exists within the gas source region <NUM>. In some aspects, the water line may be controlled to remain between a high water level <NUM> and a low water level <NUM>. Shielding exists on top of the gas source region <NUM> in order to protect workers from the radioactive source within the gas source region and within the vent pipe <NUM>. In some aspects, the portion of the vessel body <NUM> connected to the vent pipe <NUM> may have a conical cross section. The conical cross section may have a diameter about the size of that of the vent pipe <NUM> at its lower extent. The conical cross section may also have a dimeter about the size of that of the vessel body <NUM> at its upper extent. The atmosphere of the source gas region <NUM> consists of air plus the source gases hydrogen and oxygen, where the proportions of each gas are controlled by proper design of this invention as described below.

In such example, the contents of the thick-walled vessel <NUM> in the source gas location <NUM> may be spent nuclear fuel, damaged spent nuclear fuel, highly damaged fuel debris, special nuclear materials, ion exchange resin loaded with radionuclides, or other radioactive waste. The radioactivity of these contents causes liquid water and hydrocarbon materials also in the container to decompose into hydrogen, oxygen, and possibly other hydrocarbon gases.

In the top lid <NUM> of the vessel there are a plurality of bore holes 220a-d, preferably at least three bore holes (four are shown in the example), which join the source gas region <NUM> to a second gas region called the filter ullage region <NUM>. The filter ullage region <NUM> is a very small region located at a higher elevation than the source gas region <NUM>, for reasons discussed further below. Thus, the bore holes 220a-d and the filter ullage region <NUM> are located within the vessel top lid <NUM>. The purpose of the filter ullage region <NUM> is to receive gases from the source gas region <NUM>, and allow these gases to contact filters 230a-c which are positioned in contact with the ambient environment <NUM>. The gases may then diffuse from the filter ullage region <NUM> through the filters 230a-c to the ambient environment <NUM>. Hence, a set of two, three, or more (three are shown in the example) sintered metal filters 230a-c are connected to the top of the filter ullage region <NUM>. These filters 230a-c may be commercial filters such as commonly fitted to threaded bung holes of thin-wall drums or any other suitable filters. Gases are exchanged between the ambient environment <NUM> and the filter ullage region <NUM> through the filters 230a-c. The purpose of the filters 230a-c is to provide a barrier to prevent contamination release from the container <NUM>. The top lid <NUM> of the container may have more than one of such vent arrangement provided therein.

When the venting arrangement is properly designed, the gas mixture in the gas source region <NUM> has a lower density than the gas mixture in the filter ullage region <NUM>. This causes the less dense gas to flow up one or more of the bore holes 220a-d from the gas source region <NUM> to the filter ullage region <NUM>, and it also causes the more dense gas to flow down the remaining bore holes 220a-d from the filter ullage region <NUM> to the gas source region <NUM>. Because the concentrations of hydrogen and oxygen in the filter ullage region <NUM> are greater than their respective concentrations in the ambient environment <NUM> outside the filters 230a-c, hydrogen and oxygen diffuse through the filters 230a-c from the filter ullage region <NUM> to the ambient environment <NUM>. This is ultimately how the hydrogen and oxygen source gases leave the thick-walled vessel <NUM>.

Proper design of such venting arrangement requires the appropriate selection of:
(<NUM>) the number of bore holes 220a-d, (<NUM>) the diameter of the bore holes 220a-d, (<NUM>) the number of filters 230a-c, (<NUM>) the number of sets of bore hole/filter ullage/filter groups, and (<NUM>) the intrinsic ability of the filters 230a-c to pass hydrogen and oxygen.

Example <NUM> - Underwater storage of spent nuclear fuel - this example application involves underwater storage of spent nuclear fuel that has failed, so the failed fuel is sequestered into closed storage containers within the pool. This prevents the release of contamination to the pool at large, and thereby allows normal operations by personnel above the pool.

If the fuel is in a closed container, the gases derived from the radiolysis of water (H<NUM> and O<NUM>) will pressurize the container, and therefore the container must be vented. However, the gases to be vented are highly combustible, bounded by the obvious stoichiometric proportion of hydrogen and oxygen. Solutions to the problem involve either a passive trap-style gas release design that can accumulate and vent the stoichiometric mixture while allowing for natural changes in the system volume, or an actively vented design that introduces an inert gas at the proper rate to prevent combustible mixtures. The trap-style design allows for the potential for detonation, while the latter option requires continuous operation and monitoring.

Example <NUM> - Interim shielded storage of damaged fuel and fuel debris - in this example, damaged fuel and fuel debris are placed in a shielded container for interim storage, and for practical reasons it is desirable to tolerate an arbitrary water content in the container, so that stoichiometric gases are generated by radiolysis. The container must therefore be vented.

Clearly in both cases a passive solution that prevents the potential for accumulation of a flammable mixture is a superior solution.

Examples of passive vent designs which may be employed on such examples are illustrated schematically in <FIG> and <FIG>. Essential elements of the design corresponding to example application <NUM> are as follows:.

The combination of (a) The number of holes in the shield, (b) The diameter of holes in the shield, (c) The thickness of the shield, (d) the number of filters, and (e) The filter performance specification are crucial to the acceptable performance of the system. In particular, we know that the filter performance is dependent upon its actual application and it is not the same as given by manufacturers' specifications.

Performance Model. The source gas is hydrogen plus oxygen at a worst case rate that is stoichiometric, although the model can vary the proportion. The key to the model is that excess oxygen is represented, so the variable that is tracked is the mole fraction of oxygen in excess of the normal proportion in air. The model considers the densities of the gases flowing both up and down as a combination of excess hydrogen and oxygen. The model is extended to include continuity of both gas species. Filter experiments and manufacturer's specifications provide an important input, the rate at which hydrogen is removed from the filter as a function of the hydrogen mole fraction difference across the filter. Crucially, we do not know the same value for oxygen. In the absence of data we can assume that oxygen removal is proportional to hydrogen removal based upon the ratio of their respective binary diffusion coefficients in air.

Gas density p is defined by the mole fractions of hydrogen "x" and excess oxygen <MAT> <MAT> where ω is molecular weight and the subscript "a" refers to air, and subscripts H2 and O2 refer to hydrogen and oxygen respectively.

The driving pressure for bore hole flow due to buoyancy is <MAT> <MAT> where H is the shield thickness and the subscript "<NUM>" is for the lower gas volume and "f" is for the filter gas plenum. The friction and form loss pressure drop is <MAT> where <NUM>. is the bore hole length and d is the bore hole diameter, and KTOT is the form loss. The first term is for upward flow from the lower gas volume to the filter plenum, and the second term is for downward return flow. The two pressure drops are of course equal, and a non-dimensional version of the equation is <MAT>.

Continuity of total gas flow in equilibrium is <MAT> where Q<NUM> is the volume flow rate upward from the lower gas volume, Qf is the volume rate of return flow, and QH2 and QO2 are the hydrogen and oxygen gas source rates. The velocities used in the pressure drop equation are found from the volume flow terms <MAT> where Ni bore holes carry upward flow and Nf bore holes carry downward flow.

Continuity of the hydrogen and excess oxygen is given by <MAT> <MAT>.

Lastly, from the definition of the filter performance specification <MAT> where the number of filters is Nf and the units of the filter performance constant are volumetric flow per mole fraction.

Given the gas source rates QH2 and QO2, the mole fractions xf and yf in the filter plenum are immediately defined. The three continuity equations plus the pressure drop equation provide four equations to find the values of the upward and downward volume flow rates Q<NUM> and Qf and the lower gas volume gas concentrations x<NUM> and y<NUM>.

Predictions: Demonstration of a Successful Design. Consider a customer application that requires removal of source gases supplied at a rate up to about <NUM>/hr of hydrogen and with oxygen in stoichiometric proportion, therefore up to about <NUM>/hr of oxygen. The goal of the design is to maintain the source gas region hydrogen concentration below about <NUM>%, which is the lower flammability limit (LFL) for hydrogen in air. This is also the LFL for hydrogen in air with excess oxygen.

The model has been applied to yield the following design values that succeed:.

The hydrogen performance value is based upon filters already tested. The oxygen performance value is conservatively assumed to be about ¼ that of the hydrogen value, corresponding to the ratios of the respective binary diffusion coefficients in air.

Performance results are shown in <FIG>. In this figure "up" refers to gases flowing upward in bore holes from the gas source region to the filter ullage region, and "down" refers to the return flow downward.

Under the parameters of the simulation, it appears that this design can handle slightly more than about <NUM>/hr of hydrogen (with stoichiometric oxygen) and maintain the hydrogen mole fraction in the lower gas volume to less that <NUM>% (the lower flammability limit). This represents the mole fraction of hydrogen capable of diffusing upward through the bore holes. The hydrogen mole fraction in the filter plenum (that is, hydrogen capable of diffusing downwards) is slightly less than half the value in the lower gas volume. Crucially, it should be noted that the source gas mole ratio is about <NUM>:<NUM> hydrogen:oxygen, while the gas source region mole ratio is about <NUM>:<NUM> oxygen:hydrogen. Because of oxygen accumulation in the source region, and oxygen being heavier than air, it is not immediately obvious that the design will work, but the model proves that it will work.

The calculation assumed one bore hole carrying up flow and three carrying down flow, because this yields a slightly higher hydrogen mole fraction compared to results with an equal number of up and down holes. Sensitivity analysis shows that the bore hole diameter should not be reduced below about <NUM>, so the value of <NUM> is a good choice to allow for any possible occlusion. Results are not sensitive to shield thickness.

The value of the filter coefficient for oxygen removal was pessimistically assumed to be about ¼ the value of the hydrogen coefficient because that is the ratio of the binary diffusion coefficients for the two gases in air. However, it is known that mass transfer should dominate the actual gas removal performance, so that the actual rate of removal of excess oxygen should be greater.

Variation of the oxygen removal coefficient does not noticeably affect hydrogen removal performance as shown in <FIG>. It may be observed that the relative oxygen removal coefficient versus hydrogen of about <NUM>%, about <NUM>%, and about <NUM>%) are all align in <FIG> throughout the range of the rate of hydrogen source production. There is of course a variation in the excess oxygen in the lower gas volume as shown in <FIG>. As depicted in <FIG>, as the relative oxygen removal coefficient versus hydrogen increases, from about <NUM>% to about <NUM>%, the percent lower volume excess oxygen concentration decreases, again across the entire range of values of the hydrogen source production rate.

Claim 1:
A passive radioactive material venting arrangement to vent gasses produced by radioactive materials, the venting arrangement characterized by:
a source gas region (<NUM>) structured to receive the gases produced by the radioactive materials;
a filter ullage region (<NUM>) disposed above the source gas region (<NUM>) and segregated therefrom except for a plurality of bore holes (120a-120d) which each extend between, and fluidly couple, the source gas region (<NUM>) and the filter ullage region (<NUM>); and
a plurality of filters (130a-130c) disposed in contact with the filter ullage region (<NUM>), wherein each filter (130a-130c) is structured to provide for the exchange of gases from the filter ullage region (<NUM>) through the filter (130a-130c) to an ambient environment (<NUM>).