METHOD AND APPARATUS FOR IMPROVED REMOVAL AND RETENTION OF RADIOACTIVE PARTICULATES FROM FLUIDS

Abstract

A method and apparatus for improved separation and containment of radioactive particulates from liquids by filtration. The improvements are achieved by utilizing more than one stage of filtration to remove radioactive particulates from a fluid. The first stage of filtration is designed for high liquid flowrate, low differential pressure across the filter medium, and reversibility of flow through the medium to facilitate backwashing. The second or more stages of filtration receive the backwash flow and captured particulates from the first stage at a lower flowrate, but at high pressure using a high-pressure pump configured between the stages.

Claims

1. A filter apparatus for filtering particulate materials found in aqueous liquids or solutions comprising: a first filter configured for low differential pressure (DP) at high flowrate; a second filter configured to accommodate high DP at a low flowrate; a pumping system to move the liquids or solutions through and between the first and second filters; and a periodic regeneration system that regenerates the first filter and pumps resulting particulate-laden effluent through the second filter.

2. The filter apparatus of claim 1, wherein the apparatus is further equipped with at least one of pressure transducers; gauges or transmitters; flow meters or flow totalizers; radiation monitors to assess filter loading or backwashing efficiency; operators and controllers for pumps and valves.

3. The filter apparatus of claim 1, wherein the periodic regeneration system reduces pressure in the first filter caused by the pumping system with a valve that throttles flow into the filter.

4. The filter apparatus of claim 1, wherein the periodic regeneration system comprises devices to reverse the flow through the filter and/or ultrasonic energy producing transducers.

5. The filter apparatus of claim 4, wherein the frequency of the ultrasonic energy is in the range of 20 kHz to 1.0 MHz.

6. The filter apparatus of claim 5, wherein the frequency of the ultrasonic energy is in the range of 20 to 40 kHz.

7. The filter apparatus of claim 1, wherein the first filter comprises filter media that is resistant to corrosion and cavitation erosion induced by ultrasonic energy.

8. The filter apparatus of claim 1, wherein the first and second filters' dimensions resemble those of a spent nuclear fuel assembly to accommodate transport, storage and disposal in systems configured for a spent fuel assembly.

9. The filter apparatus of claim 1, wherein the second filter is configured to flow in one direction, and comprises structures compatible with high pressure support, and wherein said structures are robust media or one or medium or layered media with supports selected from screens, wires or wedge wires.

10. The filter apparatus of claim 1, wherein the second filter comprises media of high radiation resistance.

11. The filter apparatus of claim 1, wherein the particulate materials targeted for filtration are selected from non-crystalline, crystalline or amorphous materials.

12. The filter apparatus of claim 1, wherein the first filter comprises pore sizes ranging from 0.1 to 100 microns.

13. The filter apparatus of claim 10, wherein the first filter comprises pore sizes ranging e sizes range from 0.2 to 5 microns.

14. The filter apparatus of claim 11, wherein the first filter comprises pore sizes ranging from 1 to 5 microns.

15. The filter apparatus of claim 1, wherein the second filter comprises pore sizes ranging from 0.1 to 100 microns.

16. The filter apparatus of claim 13, wherein the second filter comprises pore sizes ranging from 0.1 to 1 micron.

17. The filter apparatus of claim 1, wherein the backwashing regeneration system further comprises a pulse pot comprising a reservoir connected to the first filter, wherein the pulse pot produces pressure pulses of compressed gas to assist backwashing.

18. The filter apparatus of claim 1, wherein the backwashing regeneration system further comprises an ultrasonic transducer configured to provide ultrasonic energy to the first filter.

19. The filter apparatus of claim 1, wherein the first filter comprises at least one cavitation resistant layer.

20. The filter apparatus of claim 19, wherein the at least one cavitation resistant layer comprises a cavitation resistant material.

21. The filter apparatus of claim 19, wherein the at least one cavitation resistant layer comprises a cavitation resistant coating.

22. A method for retention of particulate materials found in aqueous liquids or solutions comprising the steps of: pumping the aqueous liquid or solutions through a first filter configured to accept high flow and low differential pressure (DP); pumping the aqueous liquid or solutions through a second filter configured to accept high DP at low flowrate; and regenerating the first filter and pumping the resulting particle laden effluent through the second filter.

23. The method of claim 22, wherein during regeneration the pressure in the first filter caused by the pumping is reduced with a valve that throttles flow into the first filter.

24. The method of claim 22, wherein the regeneration is accomplished by reversing the flow, and/or applying ultrasonic energy producing transducers.

25. The method of claim 24, wherein the frequency of the ultrasonic energy is in the range of 20 kHz to 1.0 MHz.

26. The method of claim 24, wherein the frequency of the ultrasonic energy is in the range of 20 to 40 kHz.

27. The method of claim 24, further comprising supplying the ultrasonic energy from outside the filter housing exterior to the filter.

28. The method of claim 24, further comprising supplying the ultrasonic energy from inside the filter housing interior to the filter.

29. The method of claim 24, wherein particulate materials targeted for filtration may be selected from non-crystalline, crystalline or amorphous materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 illustrates the overall process of the improved method of removal and retention of radioactive particle materials.

[0064] FIGS. 2A and 2B illustrate the main components of embodiments of the apparatus.

DETAILED DESCRIPTION

[0065] A method and apparatus are provided for improved filtration of fluids containing particulates or amorphous/colloidal materials for the purpose of one or more of: improving the fluid purity, improving fluid clarity, retaining the particulates for disposal, reducing the fluid radioactivity, treating fluid that results from cleaning of systems or components, or modifying fluid chemistry. The method and apparatus are particularly suitable for the filtration of fluids containing radioactive particulates. The particulates removed are retained in a manner that allows for safe and economical disposal or storage of the particulates. Benefits of the disclosed subject matter include: (1) improved filtration process efficiency/throughput and reduction in number of filters consumed or required for a given fluid volume to be processed or particulate mass to be removed and retained, (2) reduced overall energy requirements, (3) potential to operate at a wide range of fluid flowrates, (4) reduction in size of the filters and filter housings required for retention of the particulates in a manner and form suitable for storage or disposal as radioactive waste in new or existing facilities, (5) more effective regeneration, cleaning or backwashing of the system by ultrasonic cleaning, back pulsing, or back pressure pulsing, and (6) improved integrity and longevity of filter media and filtration system components.

[0066] The improvements are achieved by using more than one stage of filtration, with each stage providing an independent function. The first stage or stages are designed for high flowrate allowing for filtration of large volumes of fluids in a reasonable amount of time. As noted earlier, the volumes of fluids requiring filtration at nuclear facilities can be large and hence high flowrate is desirable. High flowrate, high filtration “flux,” is achieved using a high flowrate pump capable of 75 to 1000 gallons per minute (gpm) or more either discharging to the inlet of the filter or configured to take suction from the discharge side of the filter to pull liquid through the filter.

[0067] The filter medium of the first stage or stages exhibits high specific surface area to achieve low pressure drop. Example fluxes of such media are 1 to 10 gpm fluid/ft/psid in clean state (see for example Purchas, 2002; Pall, 2002). One example is obtained by pleating a porous metal medium. Low pressure drop is further achieved by employing a filter medium with a pore size small enough to retain the vast majority of the mass of particulate, typically 90-99% of the particulates of interest, but not so small as to result in rapid fouling of the filter by clogging that compromises depth filtration behavior (retention of particulates with the medium or media) or excessive caking (formation of layers on the filtration medium or medium surfaces), either of which can result in cessation of flow or unacceptably high pressure through the filter itself. The pore size or filter “rating” may be larger than the smaller particulates in the fluid, but these small particulates may only represent a small percentage of the overall particulate mass. For example, for a particle size distribution of 0.5 to 5 microns (typical of some nuclear plant particles, see Berry, 1979), 0.5 micron particles exhibit only 0.1% of the mass of 5 micron particles. The smallest particles may also be captured as the filter begins to retain particulates and effectively “pre coats” the filter such that the pore size of the retained filter cake is smaller than the normal pore size of the medium, a common behavior of depth or surface filters. This is particularly relevant where the volume of fluid is recirculated many times through the first stage or stages of filtration such as during cleanup of pools of liquids where the fluid is drawn from and returned to an open source of fluid.

[0068] Low pressure drop reduces the energy required to pump large volumes of fluids containing particulates and reduces the structural integrity requirements for the first stage filter and filter medium that might be caused by high differential pressure at high nominal flowrates. A typical motor rating affixed to a 150 to 600 gpm pump used for such filtration is the range of 20-50 horsepower (HP) for a nominal 1 micron cartridge type cylindrical filter that is three feet high by six inches in diameter (see, for example, Trinuclear Corporation, 2019). The volume of the filter in this hypothetical example is about 0.6 cu. ft. (about 5 gallons).

[0069] The integrity of the media must be maintained to prevent fluid from bypassing the medium through damaged areas such as cuts, perforations, tears, or holes. The media used for filtration are often thin and non-structural (not self-supporting) porous synthetic, natural, or metallic sheet like materials (thickness typical of 0.02 to 0.06 inches) and not able to withstand high pressure forces in either the normal flow direction or under reverse flow, such as would be required for backwashing to be discussed below. Various layers of high porosity materials (perforated plates, wedge wire screens, barrier layers, etc.), helical steel wires, or forms are often used to sandwich and support the delicate filter media that permit high flowrates with targeted retention of particles in the range of 0.1 to perhaps 100 microns, but most commonly 0.5 to 5 microns (see, for example, manufacturers' datasheets and catalogs, such as Trinuclear, 2019, several citations). As described elsewhere, differential pressure limits in the normal direction of flow may be limited to 0.7 to 10 bar (about 10 to 150 psid) but may be lower in the reverse direction (less than 3 bar) (see, for example, Pall, 2016; Porvair, 2018; and Gross, U.S. Pat. No. 8,052,879). Clean pressure drop with no particulate loading may be about 1 bar (Purchas, 2002), hence the filter at high flow can be operated until the differential pressure increases about 3 to 10-fold. Other mechanisms by which the filter medium may be damaged is cavitation erosion caused by ultrasonic backwashing or damage caused by back-pulse type backwashing.

[0070] The first stage or stages of filtration is operated intermittently or continuously for hours, days, weeks, or months at high flowrate until the pressure in the filter approaches or exceeds a level that which might compromise the integrity of the filter (e.g. 0.7-10 bar). Alternatively, the integrity of the first stage or stages may be preserved by reducing flowrate to reduce pressure drop and demands on the filter, but at the undesirable expense of longer filtration process times. If the first stage or stages is not intended for long term storage containing radioactive particulates, it can be fabricated with a medium that is less radiation resistant including non-metallic materials such as nylon or cellulose. Hence the cost of first stage filters may be greatly reduced over a radiation resistant metal medium filter.

[0071] The second stage of filtration configured downstream of the first stage is designed to exhibit a high degree of structural integrity to accommodate high pressure drop relative to the first stage, but at the expense of sacrifices in allowable flowrate (lower filtration flux). It is operated for relatively short periods of time (e.g., minutes to hours during backwashing of the first stage as described later in this specification) in the normal direction only—generally from the inside to outside in the case of a cylindrical filter geometry but can be operated from outside to inside if the filter housing provides adequate structural integrity. The second stage of filtration is used to capture particulates released from the first stage during periodic first stage regeneration or backwashing. The second stage of filtration can also be used independently of the first stage if the high flowrate filtration capability of the first stage is not required. The particle size rating of the second stage can be smaller than the first stage thereby capturing finer particles liberated quickly from the first stage during once-through regeneration cycles, the small rating being acceptable given the higher-pressure capacity which counteracts issues with reduced flow (e.g. less than 1 micron for particles discussed in Berry, 1979). Finer particle retention is also advantageous as regeneration by ultrasonic backwashing can result in particle size reduction (attrition) due to disruptive cavitation implosion events at the particles during backwashing with ultrasonic energy.

[0072] The operation of the system summarized in FIG. 1 involves filtering fluid containing particulate materials through a first stage 50. Flow and differential pressure are monitored 51 until they reach a termination criterion: excessive differential pressure across the first stage filter, or cessation of flow in the normal direction through the first stage of filtration. If the differential pressure exceeds a limit, a flow is established and maintained in a reverse direction through the first stage to achieve regeneration or “backwashing” 52. This backwash flow is directed through a pump (preferably a high-pressure pump, capable of operating at above 150 psi, preferably up to 2000 psi and most preferably up to 3000 psi) aligned to discharge to the second stage 53, or the backwashing cycle is ceased after the particulates previously captured in the first stage are captured in the second stage 56.

[0073] While the first stage of filtration is designed for operation at high flow and low differential pressure (using a relatively inexpensive and commercially available high flowrate pumping system with low energy demand such as those described in Trinuclear 2018 and 2019), the second stage is designed for low flow but high differential pressure (using a high head but low flowrate commercially available pump used in many industries such as the chemical and petrochemical industries). As described elsewhere, the second stage is used primarily during backwashing which requires a short period of time (typically minutes and generally only periodically required). More specifically, it is envisioned that the time required for backwashing is on the order of minutes so that the time that the first filter is offline for backwashing may only represent <1% of system operating time if backwashing is required one time per day. A pump used to transfer the fluid and backwashed particulates from the first stage filter is a high head pump capable of delivering fluid at 10 to 200 bars at low flow (1-10 gpm). The motor required to drive such a high head, low flow pump is comparable to or less than that required to drive pump supporting the first stage, or about 20 HP or less. One ordinarily skilled in the art would recognize that the same motor could be used to drive the pumps that supplies the first stage or stages as well as the pump that supplies the second filter stage and that the first and second stage pump can be a single pump with a high turndown ratio.

[0074] Effectively, the second stage accumulates the particulates captured in the first stage over many cycles of regeneration/backwashing 57, 58, 59, 60 and is capable of high specific loading (e.g. in g/cm.sup.2) and therefore captures a large amount of particulate in a small volume device (rather than many comparable sized or larger first stage filters). Specifically, after the end of the backwash 56, filtration is resumed through the first stage or stages of filtration 57. Flow and differential pressure are monitored through the first filter stage or stages 58. For regeneration purposes, backwash is repeated 59. Backwash continues until the differential pressure in the second stage filter exceeds a predetermined criterion 60. Such criteria can be: (1) the second filter 26 would be operated until the maximum allowable DP of the filter set forth by the filter manufacturer is reached, such as up to 1000 psid or up to 1850 to 3000 psid; (2) alternatively, until some percentage of the maximum specified DP for the filter is obtained, say 80% as a margin of safety (80% of 3000 psid would be 2400 psid, for example); or (3) until the activity of the accumulated radioactive material in the second filter 26 reaches some predefined limit in curies, for example, to keep it below a limit that would permit disposal in non-geologic repository—waste that is “Greater Than Class C” or GTCC is defined in 10 CFR Part 61. Although that limit depends on what nuclides are present, it is typically greater than 7 curies per cubic foot (see Anonymous, “Classification of Nuclear Waste . . . ”, 1996). At that point, the second stage filter 61 is replaced.

[0075] The surface areas of the second filter stages can be targeted to be similar to the first stage, but it may not need to be, given its higher loading capacity (g/cm.sup.2). Based on common methods for calculating loading of surface or depth type filters, the second stage loading is roughly proportional to filter cake thickness (neglecting the volume of the filter cake in cylindrical form whose mass would be inversely proportional to the square of the thickness). Hence, for a first stage operating at a termination criteria pressure of about possibly 3 bar, a second stage operated at a differential pressure of 100 bar would have about 30 times the loading capacity of the first stage for the same surface area. As such, a far fewer number of highly loaded and potentially highly radioactive filters would need to be stored or disposed of. A preferred embodiment is to construct the filter with materials including medium which is radiation tolerant (metal).

[0076] In another example, a 10 gallon (volume) first stage filter operated at 150 gpm for an extended period (hours to years) that captures 750 grams of particulates on 15 ft.sup.2 of medium (50 grams particulates/ft.sup.2) up to a limiting DP of say 50 psid, can then be backwashed to and captured by the second stage filter using 1/20th (i.e. 50/1000th) the amount of filter medium or 0.75 ft.sup.2 of medium if the limiting DP of the second stage medium is 1000 psid. This is again in accordance with Darcy's Law which states that filter DP is generally proportional to loading for depth filtration and caking. The backwashing performed for a time no less than the time required to receive one volume of fluid from the first stage or 10 minutes at 1 gpm, but can be longer if backwashing occurs over a longer time duration than the residence time (liquid volume inside the filter in the case of outside to inside flow path during backwashing divided by fluid flow rate) of the fluid in the first stage during backwashing. As described elsewhere, backwashing by ultrasonic energy may take longer than one residence time. In any case, the volume of the second stage filter can be reasonably assumed to be reduced in an amount similar to the reduction in medium area. In this example, the smaller second filter is replaced after one backwashing.

[0077] In a final example, if the surface area of the second stage medium is the same as or similar to the first stage, the second stage could be used for 20 backwash cycles and therefore hold 20 times as much particulate as the first stage. One can further appreciate that the surface area of the second stage could be larger and therefore hold even more than 20 times the amount of particulate at the first stage in this example.

[0078] One can appreciate that it would be less advantageous to design and operate the second stage as the primary filtration with high flowrate and ability to withstand high pressure drops. A 500 gpm filter capable of greater than 100 bar pressure differential would require a very large motor for continuous operation, in excess of 500 HP.

[0079] In one embodiment, in addition to directing the backwash flow to the second stage of filtration using a pump 53, the pump used to transfer fluid from the first stage to the second stage of filtration can be operated in a manner such that it reduces the pressure within the first stage of filtration thereby improving the ultrasonic cleaning action by allowing for easier cavitation of the fluid 54. Backwashing of the first stage is achieved with pressure pulsing (back pulsing) or with ultrasonic energy 55 (which disrupts, dislodges and flushes the particulates from the medium by cavitation or acoustic streaming). Thereafter, steps 56-61 are carried out as described above. This is particularly relevant when the overall filtration system is submerged in, for example, a pool of water and the filter system is under pressure due to hydrostatic head.

[0080] For both the first and second stage, one embodiment is to design the filters such that their dimensions resemble those of a spent nuclear fuel assembly to accommodate storage and disposal in system already available for spent fuel assembles (such a spent fuel pool racks or dry cask storage facilities). A typical 17×17 Pressurized water reactor (PWR) fuel rod assembly is 9 inches on a side, 8 to 14 feet long, while a typical 9×9 Boiling water reactor (BWR) fuel rod assembly is 6 inches or so on a side (pins is a box tube “channel”) and typically 12 feet long. PWR fuel rods are exposed in the assembly and not “channeled”. In both PWR and BWR cases, the configuration of the topmost “nozzles/lifting handles” and lower end “nozzle” geometry of the fuel rod assembly would be replicated in the filter to allow it to mate with existing vessels, containers or racks designed to lift, move, position, and store spent nuclear fuel assemblies (Nuclear Fuel and its Fabrication, 1996). The filters in the first and second stage may consist of segments of filters which are stacked.

[0081] Finally, when ultrasonic energy is used to enhance backwashing, it is important to maintain the integrity of the filter medium in the first stage by rendering it less susceptible to cavitation erosion which could result in loss of filter function if a defect becomes “through wall”. To achieve this goal, the filter medium may be selected to be cavitation erosion resistant. This can be achieved by material selection or coating of the filter medium with a cavitation erosion resistant material layer.

[0082] In the context of this invention, filtration includes the processes of: (1) transferring, passing or pumping fluid through a porous medium in the form of a depth filter, examples of which would be cartridge filters, disc filters, leaf disc filter, a filter press, or a dead end filter such as a candle filter with one element or multiple filter elements configured in parallel, or through a cross flow filter to sequester or retain particulates, (2) removal/replacement of the filter medium/element after the process is complete or filter loading capacity has been met, and (3) optionally employing devices or methods for periodic regeneration of the filter so that it does not need to be removed or replaced. The pore size, pore size distribution, thickness, and other aspects of the filter medium such as structural integrity or chemical or environmental (temperature, pressure) compatibility with the fluid stream are chosen to reliably sequester, hold and retain particulates. Filtration may be ceased if the need for filtration disappears (fluid is clean), other filtration goals have been met (solids concentration goals achieved, for example, in cross flow filtration), or the filtration process is terminated due to excessive filter loading, excessive radioactivity level on the filter (in the case of filtering radioactive particles), or there is reduced filtration throughput, reduced efficiency, or excessive pumping power required due to fouling. In the context of this document, the term filter is meant to include the filter housing, filter hosing and appurtenances such as fluid connections, access for instrumentation, and access for regeneration devices such as by ultrasonic energy cleaning or pressure pulsing.

[0083] Typically, the filter medium used depends on the user's specific goals and objectives. Filter medium can include natural material (cellulose or cotton), polymers (urethanes, nylons, plastics, olefins), ceramics, or metals and metal alloys. Magnets may also be used for filtration if the particulates exhibit enough magnetic susceptibility.

[0084] Filtration may also be used to remove soluble species and contaminants if the filtration medium is configured to do so and contains in total or in part medium that absorb, adsorb, or otherwise sequester the soluble species. Medium amenable to sequestration or removal of soluble species includes organic ion exchange medium, inorganic ion exchange medium, or electrochemical devices.

[0085] The physical and chemical mechanisms involved in filtration are well documented in the literature and not repeated here (see, for example, Ullmann's, 2000).

[0086] An exemplary embodiment is the filtration of particulates from liquids at nuclear power plants or other nuclear facilities which may or may not be radioactive. As described later, the particulates at such facilities are often highly radioactive or toxic and the cost of disposal of filters used to capture these particulates can be very high.

[0087] Example sources of particulates at nuclear facilities, the mechanism of filtration, mechanisms for optimizing filtration system, and a description of the various features of this invention are described below.

[0088] Particulates—While there are many aqueous fluids in nuclear facilities that contain particulates and many particulate types, the following six types are noted herein as examples.

[0089] A. CRUD—Corrosion products in pressurized water reactor (PWR) and boiling water reactor (BWR) fluid medium include CRUD (CANDU Reactor Unidentified Deposit) formed in primary (nuclear side) of the plants as well as corrosion products on the secondary or non-radioactive side of PWRs (see, for example, Berry et al., 1979 and Varrin, 1996). CRUD may contain metallic species (iron, nickel or cobalt particles), corrosion products or oxides of metals such as iron, nickel or cobalt. Particles sizes as characterized by mean diameter can be from <0.1 micron to 100 microns or more. When present on the primary side of the power plant, they may become radioactive or activated during plant operations. Their radioactivity is in many cases so high that exposure to CRUD or removal from primary coolant, or other aqueous systems at plants, is hazardous and costly.

[0090] B. Accumulation of CRUD on fuel surfaces of some plant equipment is also undesirable. In the case of CRUD on fuel surfaces, it can lead to “under deposit corrosion” of the fuel surfaces or a phenomenon known as crud induced power shift (CIPS). This CRUD can be removed from fuel surfaces (see for example, Frattini '892), but disposal of any on filtration or other medium upon which CRUD has been collected is expensive and can result in undesirable exposure of personnel to radiation.

[0091] C. Silicates—Other contaminants present in nuclear plant water include silica species. These may be naturally present in make-up water supplies (all fossil and nuclear plants must have a source of “make-up” water to replace water lost due to evaporation, plant operations or sampling), or from specific sources such as degradation of fuel storage racks that contains contain silicon compounds in neutron absorbing medium. This degradation, results from a combination of reaction of the silicon compounds with water and exposure to radiation fields. While they may not be in themselves radioactive, the fluids in which they exist typically contain other particulates that are radioactive. Suspension of particulate silica species in spent fuel pool water can lead to severe turbidity problems. Hence their removal is desirable. Silica particle sizes can range from <0.001 micron to >100 microns, with typical colloidal particles in the range of 0.1 to 1 micron (se Lambert, 2004). Particulates resulting from degradation of spent fuel pool storage racks may be amorphous or crystalline (Lambert, 2004).

[0092] D. Particulates Associated with Waste Treatment at Nuclear Material Production Facilities—The treatment of wastes generated during the production of fissile materials, particularly those at government owned and operated sites, is challenging. The filtration and collection of particulate waste species is often desirable including aluminum species like gibbsite or boehmite and other oxides.

[0093] E. Settled Sludge—Particulate material that has settled to the bottom of tanks and vessels at nuclear power plants can be removed by vacuuming, a process that mixes the particulates with a fluid to produce a sludge. The species in the sludge may include corrosion products, debris, and detritus. The particulates are often amenable to capture by vacuuming devices where the vacuumed material is directed to a filter system (see Trinuclear Corporation, 2019).

[0094] F. Cutting, Grinding or Machining Debris and Particulates—The maintenance, repair or decommissioning of nuclear facilities often includes underwater cutting, grinding or machining operations. These processes produce particulates or kerf that can result in the spread of radioactive contamination within a fluid or above the surface of an aqueous fluid reservoir. These materials also can reduce water clarity which may hinder the cutting, grinding or machining operation. The removal of such particulates by filtration is often necessary.

[0095] G. Liquid Wastes at Plants that Experience Accidents—The collection and storage of liquids wastes at nuclear sites and plants that have experienced radiological or other accidents is critical to the remediation of the plant site and to protect workers and the public. The treatment of such wastes often include filtration to remove radioactive or non-radioactive particulate species to isolate them for processing or permit downstream processing or release of the fluids after removal of the particulates.

[0096] One ordinarily skilled in the art would recognize that there is a wide range of particulate material in nuclear fluid systems in terms of chemical composition, size, physical properties.

[0097] It is often desirable to remove collected particulates and to re-use the filter used for first stage or stages of filtration. Release of particles to regenerate or clean the filter, such as by backwashing, is achieved by imparting form drag, shear forces, or other forces on the entrapped particles sufficient to overcome attachment to the filter surface s within the medium itself, or interparticle attraction between materials collected at the surface of the filter. Regeneration, such as through backwashing, also disrupts and dislodges layers of particulates that have formed on the filter medium (filter cake). Backwashing can be achieved by reversing flow direction at high or low flowrates (with consequent higher or lower pressure differential through the medium), by other mechanical methods or devices (pressure pulses, ultrasonic cleaning, vibration, acoustic cleaning), or a combination of methods. In one instance reservoir 24, valve 25, and conduit 261 represent a “pulse pot” (typically a gas charged back pulse vessel; see Daniel R. C., et al., 2010, section 3.7). Pulse pots are used more often in gas filtration but can be used with liquids. Valve 25 is a fast-acting valve that discharges liquid to clear the filter. The dislodged particulate material is often directed to another vessel, reservoir or filter for use or disposal.

[0098] Chemicals such as cleaning agents, film forming agents, surfactants, or pH adjusting agents can be used to augment backwashing by changing the surface charge or adherence of the filter medium or particulates.

[0099] Regarding current filtrations technologies and materials for nuclear fluids, the literature is replete with examples of radiation tolerant filtration medium, usually pleated sintered metal or pleated sintered fiber, or austenitic or other corrosion resistant materials (see Pall, 2018; Porvair, 2018; Mott, 2019). These filters are restricted in differential pressure in normal flow direction (inside to out) and even more restricted in reverse flow (outside to in such as during regeneration)—typically in the range of 45 to 150 psi. They are almost exclusively porous cylindrical cartridge type filters often with pleated media to increase surface are per unit volume of filter. Traditional pleating, however, is not advantageous with regard to tolerating high pressures of DPs. Regarding particulate filters and medium, other porous metallic systems are available that can withstand differential pressures as high as 3000 psig (Mott, 2019) and 1850 psid (see Pall, “Unipleat Plus” for high integrity pleat mesh filters 2018; Pall Accusep Selection Guide E78a for multi-tube tubular filters, 2002). Available thick wall tubular filters maintain structural integrity at high DP owing to greater wall thickness and their cylindrical as compared to woven, porous metal or sintered metal thin filter medium in tubular or pleated configurations. However, the flux achievable through these higher-pressure tolerant tubular filters is lower than standard filters and would not be preferable for the first stage of filtration or backwashing as described herein but are suited as the second stage filters as described herein due to lower required flux in the overall filtration process and system described herein. Non suitability of for example multi-tube tubular filters in the first stage is also related to the overall thickness of the medium (much higher than that selected for the in the first stage in the process and system described herein) and its tendency to tightly capture the particle and compress them into a hard filter cake, as well as some inherent geometry restrictions such as penetration of ultrasound to the interior of the tube “bundles” during backwashing in filters like the Pall Accusep multi tube candle filter designs. The configuration of the second stage, however, is not restricted to cylindrical candle type filters but may include cartridge filters, leaf disc filters, a specific design being suitable for higher pressure operation such as are used in the polymer processing industry.

[0100] Regarding regeneration, ultrasonic energy is an accepted method of regeneration of porous metal filters at power levels of on the order of 60 watts/gallon for 10 to 60 minutes (see Mott “Porous Metal Filter Elements”, 2019). Other references describe ultrasonic cleaning times at ambient pressure of 15 minutes (see Uppili, S., WO 2011/163030, 29 Dec. 2011). Hence for a cleaning process that can last days, weeks, or months, there is little schedule disadvantage to performing backwashing 10 to 30 times over the course of the process. Higher ultrasound densities in terms of watts/gallon can be delivered albeit at the risk of causing cavitation erosion damage to standard filter medium.

[0101] Regarding ultrasonic cleaning in general, typical frequencies are in the range of 20 to 150 kHz for conventional ultrasonic cleaning, and up to 1 MHz for mega sonic cleaning, the latter being more amenable to cleaning of small particles in tight crevices or geometries. Mechanisms of cleaning and the role of frequency are described in Busnaina, 1995. Mechanisms include cavitation and imparting forces on fluids by streaming currents which require line of sight between the ultrasonic transducer and the part to be cleaned (or, in this case, regenerated). Ultrasonic energy can pass through solid barriers but at the expense of attenuation of the energy.

[0102] Regarding the mechanism of ultrasonic cleaning, the amplitude of the cyclical (often sinusoidal) high frequency pressure waves transmitted through the fluid must be sufficient to overcome the vapor pressure of the fluid (about 0.5 psia at 25° C. for water). Accordingly, a component to be cleaned, or a filter to be regenerated by a system submerged in water, such as a submerged filter system wherein submergence is used for radiation shielding purposes, will experience less cavitation events at a given ultrasonic energy level (and corresponding pressure wave) because the hydrostatic head pressure increases the barrier to cavitation. Lowering the pressure of the system will enhance cavitation induced cleaning at a given energy level. Higher frequency ultrasounds are even more impeded by ambient pressure than ultrasonics in the range of 25-150 kHz as described by Busnaina, 1995.

[0103] Ultrasonic regeneration can also be incorporated into the apparatus if the apparatus is not submerged by either attaching or contacting the ultrasonic transducer to the filter housing or by inserting an ultrasonic transducer into the center of the fluid filled housing.

[0104] With regard to side-effects of ultrasonic energy cleaning as envisioned herein for backwashing, cavitation erosion can compromise the integrity of the filter (housing, filter media, filter medium supports). Accordingly, it is desirable to use cavitation erosion resistant materials for, as a minimum, the medium (see Thiruvengadam, 1964; Szkodo, 2015). Cavitation erosion materials include austenitic stainless steels and other nickel alloys, stainless steel with high molybdenum content, titanium and titanium alloys, or high hardness materials. Coating may also be applied to materials to improve their resistance to cavitation erosion including diamond-like coatings and nitrides (see Aperador, 2014). Processes such as low temperature surface carburization of stainless steels (LTCSS) may be used as they do not adversely interfere with the ductility of the stainless filter material but can significantly improve erosion resistance (see Collins, 2007).

[0105] Regarding features of current methods of filtration at nuclear plants, several popular systems are described by the website of Trinuclear Corporation (2019). These systems are single stage and can employ filters in parallel, a concept easily adopted by this invention. Systems available from companies such as Trinuclear are single stage, high flowrate, low differential pressure systems and do not incorporate a high pressure second stage filter that can be used to capture larger quantities of particulates.

[0106] FIGS. 2A and 2B depict embodiments of the apparatus, where FIG. 2B is a continuation of the apparatus of FIG. 2A. The apparatus comprises a first filter 1, including a housing 2 and filter element or elements 3, a fluid inlet 4, and a fluid outlet 5. The filter element may be a porous medium, or layers of porous medium supported by one or more structural features such as a coarse filter or mesh 6. The fluid may be introduced into the first filter via pump 7 or pulled through the filter by pump 8 though valve 15 and conduits 16 and 17, and valve 23. Typical flowrates would be <100 up to 1000 gpm, but preferably in the range of 150 to 600 gpm. The discharge pressure of the pump 7 may be in the range of 15 to 100 psig. Alternatively, the first filter may be supplied by a pressurized source of fluid or by gravity.

[0107] The fluid containing particulates is supplied from a source or inventory container 9 to the first filter 1 by pump 7, which pulls the fluid through an input line connected between the source container 9 and the pump 7 via conduits 10 connected between the pump 7 and the first filter 1 with flow control through valve 11. Alternatively, the fluid containing particulates is pulled through the first filter 1 by pump 8—so called suction mode, as defined in the industry, which is used to avoid contamination of pump 7 (see Trinuclear Corporation, 2019). In this case, the fluid containing particulates is pulled via the conduit 16 through valve 15 into the first filter 3, and then through valve 23 via conduit 17. In the case where the fluid containing particulates is pulled through the pump 7, filtrate fluid optionally exits the filter 1 through conduit 12 via valve 13 in the case of release back into an open pool 100 of fluid being treated or can be recycled back through filter 1 (conduits not shown) for subsequent cycles of filtration in the case where the apparatus is submerged below the surface of a pool of aqueous fluid. In the case where the fluid containing particulates is pulled through the first filter 1 by the pump 8, the filtrate fluid exists through an output of the pump 8 to the open pool 100.

[0108] Valves 11 and 13 may serve to isolate the filter, for example to retain and capture particulates. Not shown in the figure are conduits or vents that permit draining of the filter, as their placement is within the skill of the ordinary artisan. Not shown in the figure is a configuration of the apparatus where the all parts or a part of the apparatus, such as pumps 7 or 31, are not submerged below the surface of an open pool 100 but still filters the fluid containing particulates and discharge permeate to another location such as a tank, vessel, reservoir of other apparatus such as waste pool 22. One skilled in the art would appreciate that the exact location of the apparatus relative to fluid containing particulates 9 could be application specific, (e.g., on the “deck” by a pool 100 in a nuclear facility, next to a tank of fluid containing particulates or the system to which permeate is transferred, or remote from a tank containing particulates or the system to which permeate is transferred).

[0109] During filtration, particulates 14 are captured on the medium 3, where the pore size or “rating” of 3 is chosen to be smaller than at least some and possibly all the particulates. Pore sizes would range from 0.1 to 100 microns, but preferably 0.2 to 5 microns, and most preferably 1 to 5 microns to avoid flow resistance that would compromise the ability to achieve target flowrates if finer porosity filters are used.

[0110] Filter 1 may be located underwater in a pool 100 of fluid at depth h below the water level/surface 102. The hydrostatic pressure at the midplane of the filter is P.sub.0, where P.sub.0 is 7 to 15 psi (about 0.5 to 1 bar) typically representing submergence in about 15-30 feet of water; optionally 0 psig if located near the surface or above a body of water. If the filters would be submerged near the bottom of the pool which is about 30 feet deep the pressure would be about 15 psi. The pressure on the receiving (particulate laden fluid) side of the filter is P.sub.1. If pump 7 is used as source of water, P.sub.1 is P.sub.0+5 to 15 psi at the beginning of its life (a clean filter with actual pressure dependent on flow; this value is typical for pleated filters based on the published specs from Pall, 2016) to P.sub.0+40 to 150 psig corresponding to the typical allowable pressure tolerance of the “dirty” filter at which time you would backwash or change the filter 1. The pressure on the completely or partially cleaned fluid side of the filter 1 is P.sub.2, so the corresponding pressure drop across the filter 1 is P.sub.1-P.sub.2 (first filter differential pressure, DP). Pressure P.sub.2 is just above P.sub.0; its value depends on the pressure loss through conduit 12 and valve 13 that is generally P.sub.0+5 to 10 psi. Once the first filter DP exceeds a predetermined limit or other criteria are met, such as a limit on radioactivity (or total activity) of the collected particulates, filtration is terminated, and regeneration can be conducted.

[0111] In a simple backwashing embodiment, flow is reversed through the filter 1 via conduit 18, valve 19 and out of the filter 1 via conduit 20 through valve 21. Alternatively, the backwashed fluid containing the backwashed particulates can be directed to a waste receiving tank, vessel or other system 22 rather than being sent through the second filter stage through conduit 291. This may be selected as the fluid path for a number of reasons, for example if it is determined that the particulates that a have been collected on filter 1 exhibit a specific radioactivity, for example as measured by a radiation detection device placed adjacent to the filter 1 (a common nuclear industry practice), that is below a level that would fully realize the advantages of the collection and concentration on filter 26 as described in this application. One skilled in the art would appreciate that the specific radioactivity where such a decision to divert the particulates to other system 22 would depend on factors such as the relative disposal costs of filter 26 versus processing the particulates by other waste system 22 (e.g., blending, grouting, encapsulation, and vitrification being among the options commonly available in the nuclear industry), as well as local, state or national laws that pertain to the generation, transport and disposal of nuclear wastes.

[0112] Pump 7 can be used to establish and maintain the backwashing flow. Particulates 14 are dislodged by viscous drag forces. Backwashing is terminated using one or more predetermined criteria such as P.sub.2-P.sub.1 reaching a low steady state value (close to clean DP), or the radioactivity levels if the filter have decreased to a desired point indicative of removal of the collected radioactive particles (radiation detectors adjacent to the filter housing or within the housing or conduits are not shown to simply the figure). Filter 1 may also feature a conical bottom 181 to prevent any retained particulates from settling on horizontal surfaces during backwashing.

[0113] In one embodiment, backwashing is assisted by pressure pulses (compressed gas) in reservoir 24 via conduit 261 through opening of valve 25, which may be a “pulse pot” as described above.

[0114] A second filter 26 (or filters 26, 27 connected in parallel to one another by valves 36, 361) are used to collect backwashed particulates. Filters stacked in series (not shown) may also be used (e.g. two 3-foot-long sections connected in series—or stacked—to achieve a six-foot-long filter element). In this embodiment overall, the second filter 26 includes a filter medium 28, optional supporting structures 29, and a housing 30. The medium 28 in the filter 26 may exhibit a pore size or “rating” of between 0.1 and 100 microns, but preferably 0.1 to 1 micron, which is optionally smaller than the preferred pore size of filter 1 for reasons already described and discussed further below. Filter medium 28 may be layers of filter media. The housing 30 and filter medium 28 may be compatible with operating at pressure P.sub.3 (inlet pressure) and pressure P.sub.4 (outlet pressure). For this high-pressure tolerant filter, P.sub.3 could be P.sub.0+125 psi or less with an absolutely clean filter. For the clean filter 26, for example, P.sub.3=P.sub.0+5 to 15 psi just like a clean filter 1. In all cases, the value for P.sub.0 assumes that both filters are at relatively same depth. In FIGS. 2A and 2B, filters 1, 26 and 27 are not pictured all at the same depth, however, the figure is only representational to show interconnections and does not show the actual placement location of the filters. One ordinarily skilled in art would recognize that actual pressure is depth dependent. P.sub.4 typically would be similar to P.sub.2 since the first and second filters would likely be packaged together, but this is not a requirement (e.g., filter 26 could be in an entirely different elevation with respect to 102 or location, and be remote from filter 1

[0115] The structural design of the filter 26 and the filter medium 28 and support 29 is compatible with pressures P.sub.3 and P.sub.4, and differential pressure P.sub.3-P.sub.4 that is much higher than that used as a design basis for filter 1.

[0116] One ordinarily skilled in the art would also recognize that flow through filter 26 can be normally arranged as outside to inside if housing 30 is designed to accommodate the full discharge pressure of pump 7 or 31.

[0117] Note that the housing 30 and filter 26 are graphically shown to be smaller than filter 1 for illustrative purposes but also to identify that filter 26 is rated for higher differential pressure and higher particulate loading as compared to filter 1 and may indeed be more compact than filter 1 or larger.

[0118] Regeneration is achieved by directing flow to filter 26 from filter 1 via conduit 291, valve 301, using pump 31. Alternatively pump 7 may also be used to assist or provide the flow. A coarse debris trap such as a Y strainer 32 can be used to protect pump 31 (typically about 0.5 mm to 1 mm screen traps). Pump 31 is a high-head, low-flow-rate pump. It may a centrifugal pump or a positive displacement pump such as a gear pump, progressive cavity pump, or piston pump, all of which can be designed to be tolerant of particles in the backwashed fluid. The pumping flowrate is typically in the range of 0.5 to 10 gpm, but preferably about 1-2 gpm which would allow a complete flush of filter 1 in about 2 to 5 minutes assuming a typical volume of a particulate filter used in nuclear facilities (see prior discussion in this specification). The outlet pressure of pump 31 may be in the range of 15 psi to 3000 psi, generally greater than 150 psi, with a preferred developed head of 1000 to 2000 psi. Pressure may be gradually increased as the filter media is loaded. During backwashing, particulates 321 collect on the filter medium 28. The medium 28 of filter 26 can be smaller than that in filter 1 owing to the higher pressure supplied and lower required flowrate, thereby assuring greater captures of all sizes of particulates. Cleaned fluid permeate is discharged from filter 26 via conduit 38 and valve 35, or from filter 27 through valve 43. Pump 31 can also be bypassed using conduit 39 (isolation valve not shown) especially in the case where pump 7 is providing flow. If additional removal of particles that not captured on filter 26 is desired, such as those exhibiting a particle size less that the rating of filter 26 or those that pass thru filter 26 due to a filter failure (e.g., due to a hole or breach in medium) the permeate may be directed to a downstream stage of filtration.

[0119] Filter 26 and 27 can be detached from the system by disconnects at flanges or connectors 33 so that the filters 26 and 27 may be transported away together or individually for long term storage or disposal, with the preferred form factor and dimensions of filters 1, 26 and 27 resembling that of a spent nuclear fuel assembly. Particulates are captured and retained inside filters 26 and 27 by upstream and downstream check valve 341. The overall system can be isolated for instance by closing valves 11, 21, 19, 25, 23, 36, 15, 13, and 35 and valves 361 and 362, and other vents and drains. For illustrative purposes, the figure shows more accumulation of particulate in a thicker filter cake on filters 26 and 27 as compared to filter 1. Filter 1 and filters 26 or 27 may also be separated and deposed of independently.

[0120] Ultrasonic energy may be used to enhance backwashing. Ultrasonic transducers may be placed at the very interior of the filter (e.g., a sonotrode 381), or outside with a conventional submersible ultrasonic transducer 37. When a sonotrode 381 is used for ultrasonic energy, it may be removable and able to translate along the axis of the filter 1 centerline to cover the length of the filter 1 should the sonotrode be shorter than the filter. When the ultrasonic transducer is placed at the very interior of the filter, ultrasonic energy is not lost or absorbed by the filter housing or filter medium before disrupting the accumulated particulates. Direct line of sight delivery of the ultrasound also allows cleaning to be achieved by both cavitation and acoustic streaming. The frequency of the ultrasonic energy may be in the range of 20 kHz to 1 MHz, but most preferable 20 to 40 kHz. Nominal energy density inside Filter 1 may be 60 watts per gallon but may be 200 watts per gallon or higher. Higher energy density may improve cleaning but increases the risk of damage to the filter, including the filter medium, by cavitation erosion. As described earlier, cavitation resistant materials and coating can be used/applied to improve the integrity of the medium. In particular, the filter 1 may comprise at least one filter layer that has been coated with cavitation resistant materials. Alternatively, at least one filter layer in the filter 1 may have a cavitation resistant coating. Examples of such materials and coatings are discussed previously.

[0121] In a further embodiment, ultrasonic disruption of the particulates accumulated on filter 1 may be enhanced by operating pump 31 but partially closing valve 36 which throttles flow to filter 1 at pressure P.sub.5 which is below P.sub.0, hence eliminating the burden on the ultrasonics of overcoming hydrostatic head and cavitation threshold for a submerged filter system (typical submerged depths in pools 100 are similar to depths of spent fuel pool or up to 30 ft deep or more). In reduced pressure operation of filter 1, P.sub.5 could be no less than 0.4 psia (vapor pressure of water at 25° C.) but can be anything less than P.sub.0 (say from 30 psia (15 psig) to as low as about 1 psia) to improve ultrasonic regeneration.

[0122] While FIGS. 2A and 2b show several individual parts and components, the apparatus can be configured in an integral package 40 (shown in a dot-dashed line) to simplify deployment and transport.

[0123] Finally, the power required to drive pumps 7 and 31 may be similar so that one motor or drive, preferably a variable speed drive 41, could be used to provide the motive force to both pumps 7 and 31, for example on a single shaft 42.

[0124] Not shown in the figures, but understandable by one skilled in the art, are the instruments that may be incorporated in the system including pressure transducers, gauges or transmitters; flow meters or flow totalizers; radiation monitors to assess filter loading or backwashing efficiency; or operators and controllers for pumps and valves.

[0125] Having now fully described this invention, it will be appreciated by those skilled in the art that the same can be performed within a wide range of equivalent parameters, concentrations, and conditions without departing from the spirit and scope of the invention and without undue experimentation.

[0126] While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications. This application is intended to cover any variations, uses, or adaptations of the inventions following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth as follows in the scope of the appended claims and any combinations of the various disclosed embodiments as may be appropriate.

[0127] All references cited herein, including journal articles or abstracts, published or corresponding U.S. or foreign patent applications, issued U.S. or foreign patents, or any other references, are entirely incorporated by reference herein, including all data, tables, figures, and text presented in the cited references. Additionally, the entire contents of the references cited within the references cited herein are also entirely incorporated by references.

[0128] Reference to known method steps, conventional methods steps, known methods or conventional methods is not in any way an admission that any aspect, description or embodiment of the present invention is disclosed, taught or suggested in the relevant art.

[0129] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art (including the contents of the references cited herein), readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance presented herein, in combination with the knowledge of one of ordinary skill in the art.

REFERENCES

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