KR20090109051A - Chemical enhancement of ultrasonic fuel cleaning - Google Patents

Abstract

PURPOSE: A chemical enhancement of an ultrasonic fuel cleaning is provided to clean a deposit inside a nuclear fuel assembly without any interference when a nuclear reactor is re-started. CONSTITUTION: A chemical enhancement of an ultrasonic fuel cleaning is comprised of the steps: locating a nuclear fuel assembly into a housing containing a liquid or adjacent to the housing(24); supplying ultrasonic energy radiating in all direction from a converter to remove a deposit from the nuclear fuel assembly; and contacting nuclear fuel assembly with at least one chemicals.

Description

Chemical Improvements in Ultrasonic Fuel Cleaning {CHEMICAL ENHANCEMENT OF ULTRASONIC FUEL CLEANING}

TECHNICAL FIELD The present invention relates to techniques relating to maintenance of nuclear power plants, and more particularly to ultrasonic cleaning of irradiated nuclear fuel assemblies of nuclear power plants.

In operation of the reactor, the products and impurities of this reactor coolant are deposited in the fuel assembly. These deposits can affect the operation and repair of nuclear power plants in many ways. E.g,

(a) The nuclear properties of the deposits can adversely affect the nuclear performance of the reactor,

(b) The thermal resistance of deposits can cause the fuel rod surface temperature to rise, resulting in material failure in the fuel rod,

(c) causing radiation exposure during operation due to radioactive decay of the deposit, especially when redistributed through the reactor cooling system during power delivery;

(d) deposits complicate the thorough inspection of the fuel assemblies investigated by visual and eddy current methods;

(e) reduce the visibility of the spent fuel pool in which deposits from the fuel rods are used, significantly delay other operations in the fuel reservoir upon refueling stop,

 (f) Once reloaded into the reactor on the assembly to be irradiated twice or three times, it creates an inventory of material that can be redistributed to the new fuel assembly in a detrimental manner.

Axial Offset Anomaly (AOA) has been reported in PWRs. AOA is a phenomenon in which deposits form on fuel rod cladding due to a combination of local thermo-hydraulic conditions and primary fluid impurity characteristics of the reactor and primary system. Such deposits reduce the margin available under any operating condition, resulting in abnormal power distribution along the furnace axis. Due to AOA, some plants reduce reactor output levels for extended periods of time.

The AOA problem has led to the need to develop an effective and low cost mechanism to remove PWR fuel deposits. This mechanism also reduces overall deposit inventory, lowering the dose rate for plant operators, improving fuel inspectability, preparing fuel for long-term dry storage, and also facilitating the collection of analytical corrosion samples. It is desirable to.

Several approaches have been proposed to remove PWR fuel deposits. One method is to chemically clean the assembly after it has been removed from its original location within the reactor or with a separate wash cell. There are a number of problems with this, including cost, potential for corrosion by cleaning chemicals, and consequently difficult placement of highly contaminated chemicals. Perhaps the biggest drawback of this chemical approach is that it is time consuming, including several hours to clean a single fuel assembly.

U.S. Patent No. 6,396,892 and U.S. Patent Application Publication No. 2002-0163990, incorporated herein by reference in their entirety, describe an apparatus and method for ultrasonically cleaning an irradiated nuclear fuel assembly of a nuclear power plant. Ultrasonic fuel cleaning is an effective mechanical method of removing corrosion products from nuclear fuel using cavitation bubbles from induced boiling. Ultrasonic transducers are typically combined in a radial configuration to lower the local pressure wave sufficient to cause nuclear boiling (which immediately collapses immediately) through the water in the fuel assembly. The collapse of the bubbles results in the release of a solid corrosion product which is then pumped into the filtration system.

Mechanical cleaning is effective, but not 100% effective as corrosion products remain on the fuel assembly. Ultrasonic cleaning is expected to remove up to 80% of the total corrosion product inventory on the fuel. Modified ultrasonic cleaning methods include the intermittent use of transducers to cause 'pulsing' or to improve cleaning. This technique has little additional effect on corrosion removal.

In the art, options have been considered for more complete removal of fuel corrosion through complete system chemical purification with in-fuel fuel. It is anticipated that chemical purification with fuel in the furnace can be carried out safely, but this practice can be a long time and this process is very expensive.

A focused chemical purge is provided for supplying mechanical ultrasonic cleaning. There is a significant advantage to applying both physical and chemical cleaning together. After mechanically removing and removing the corrosion, new corrosion surfaces can be exposed and decomposed. The method improves the physical removal of corrosion products by chemical decomposition. In some embodiments, ultrasonic mechanical and chemical decomposition proceeds to synergistically remove corrosion products from the fuel.

The method is less aggressive than chemical interference in the furnace and also focuses on the task of removing additional corrosion products directly from the fuel while preventing chemicals from circulating through the primary coolant system. The method includes an ultrasonic fuel cleaning technique and in some embodiments supplements the ultrasonic fuel cleaning technique by adding a chemical to the top of the ultrasonic cleaning chamber.

According to the method, the chemical addition is limited to the water in the ultrasonic cleaning chamber, not the entire primary system, thereby significantly minimizing the total liquid waste produced. Less aggressive chemicals may be selected that utilize an ultrasonic fuel cleaning environment. Only the fuel assembly is exposed to chemicals, requiring less chemical cleaning of the vessels or off-site piping. In some embodiments, the chemical addition step is applied to selected high flux assemblies with highly corrosive deposits, while other fuel assemblies may be cleaned only by ultrasound.

If corrosion products remain after ultrasonic fuel cleaning, the remaining corrosion products will have exposed surfaces that are susceptible to chemical degradation. Decomposition of the corrosion product will increase fuel performance by eliminating the initial cycle axial offset. This effect is most pronounced in power plants that emit large amounts of corrosion products from steam generators.

The method is suitable for removing both active and inert corrosion products. Active corrosion products (cobalt-60, etc.) with a half-life of more than 150 days will be removed and will also reduce the off-field radioactive field on subsequent shutdown. Parents (Nickel-58, etc.) for shorter-lived nuclides will be eliminated and will also assist in reducing the off-road dose rate in future shutdowns.

According to the method, the mechanism for improving the removal of corrosion product deposits from the fuel may include one or more of modifying the deposit surface, oxidizing the deposits, solubilizing the deposits or complexing or chelating the soluble deposits. Chemical compounds of this type that can be used according to the method include at least one of peroxides, inorganic or inorganic acid salts, organic acids or organic acid salts, oxidizing agents or chelating agents. Chemical compounds useful in reactor down-cleaning and / or in LOMI / NP or LOMI / AP processes can be used according to the present method.

Specific compounds useful according to the present methods include, but are not limited to, at least one of hydrogen peroxide, vanadous formate, nitric acid, potassium permanganate, alkaline potassium permanganate, oxalic acid, picolinic acid or sodium picolinate Do not.

According to the method, it is time efficient, effective, compact, low cost, and very fast compared to conventional chemical methods for removing deposits from fuel assemblies.

According to the method, the fuel assembly is cleaned without disassembly so that no adverse coating displacements occur that threaten the physical integrity of the irradiated fuel pellets. That is, the internal deposits in the fuel assembly can be cleaned without any effect upon subsequent reactor restarts.

According to the method, improved radioactivity management and radiation exposure of power plant workers are reduced. Therefore, by sealing radioactive particles on a filter that can be safely stored in a fuel reservoir for a long time (when radioactive decay) and washing the fuel, it is possible to reduce the dose rate during downtime and the dose to the operator.

The chemically improved cleaning methods herein assist in ensuring fuel reliability by removing excess corrosion products. It also helps to reduce radiation sources by removing radiation from the fuel before the fuel can build up on the surface of the power plant.

1 is a schematic diagram of an apparatus for implementing a chemically enhanced ultrasonic cleaning method for an irradiated fuel assembly. In one embodiment, the chemical storage tank 10 for storing the chemicals used in the method communicates with the device 20 via a chemical injection port proximate the top of the housing 24 (see FIG. 2). More details). The chemical (s) are injected into a liquid, such as water flowing through the housing, in contact with the fuel being cleaned therein, and with the aid of a pump 92, discharged through the filtration pipe 32, contaminated waste Enter filter 94 to remove solid particles from the stream. The filtration liquid is then passed through ion exchange vessel 12 for the removal or regeneration of any possible complexing or chelating agents and also to remove soluble corrosion products such as ions / soluble nuclides and metal ions. The treated liquid can then be sent to a tank to be removed as liquid rad-waste or back to the spent fuel reservoir.

Each chemical entering the fuel reservoir must be carefully selected. In one embodiment, the chemical selected is hydrogen peroxide because this hydrogen peroxide can be used for nickel decomposition at shutdown chemistry and thus eliminate nickel corrosion products.

The ultrasonic device 20 or wash chamber can also function as an isolation chamber from the rest of the fuel reservoir. If the chemical is injected only when water flows through the housing, the chemical does not contaminate the rest of the reservoir because the chemical is concentrated by the filter / ion exchange resin following the system. In this way, considerable flexibility is obtained when selecting chemicals for the purification of fuel assemblies.

2 is a front view of one embodiment of an exemplary ultrasonic cleaning device 20 for practicing the method. The device 20 can include an ultrasonic transducer 22 mounted to the housing 24. At the top of the housing 24 is a guide 28. A nuclear fuel assembly (not shown in FIG. 2) passes through the guide 28 and into the housing 24. Once the fuel assembly is located in the housing 24, it is cleaned by applying ultrasonic energy from the ultrasonic transducer 22 (more described below).

The assembly reaction support 26 can be used to mount the housing 24 to the wall of the wash reservoir. Alternatively, the housing 24 can be supported by a crane or hoist. 2 also shows the filtration piping 32 and emergency cooling holes 30 used in the case of failure of the filtration system. Emergency cooling holes 30 remove sufficient decay heat from the fuel channel through natural convection in the event of equipment failure (eg, loss of the pump). The filtration tubing 32 is used to send the water containing the removed deposit to the filtration unit (described below).

The transducer 22 can be mounted on a transducer mounting plate 34. The transducer mounting plate 34 is used to connect the transducer 22 to the housing 24. The transducer spacer 36 is used to mount the transducer 22 to the mounting plate 34 in a suitable position.

3 shows a converter 22 useful according to the method. This transducer 22 comprises a first piezoelectric transducer or transducer stack 40 and a second piezoelectric transducer or transducer stack 42 mounted on both sides of the rod 44. Converters 40 and 42 receive control signals over line 46. The transducer 22 of this configuration generates radial pressure waves emanating from the rod 44 in all directions. Thus, the pressure wave radiating radially is called omni-directional.

In one embodiment of the method, the omnidirectional pressure wave is contrasted with a conventional ultrasonic transducer (vibrating in the liquid) which generates a single direction pressure wave in the liquid. Unidirectional wavefronts are flat on the surface and are generated by the movement of flat structures, such as walls or floors of ultrasonic baths to which transducers are attached. The energy delivered is dissipated by striking a physical object. Thus, in the case of fuel rods of a fuel assembly, it is difficult to use conventional ultrasonic waves because it is difficult to bring ultrasonic energy to the center of the fuel assembly. The energy required to achieve this will be excessive and possibly damage the fuel.

In one embodiment, the transducer 22 generates an omnidirectional pressure wave. The wavefront is produced by the phase-locked movement of the two piezoelectric transducers 40, 42. Pressure waves generated in the cylinder spaced apart such that the node structure along the bar axis of the pressure wave generated in the cylinder is approximately equal to the fuel rod spacing or multiples of the fuel rod spacing can more easily penetrate rows of fuel rods. . Thus, cleaning the inner rods in the fuel bundle can be accomplished with much less energy input than is necessary when performing such inner cleaning using conventional ultrasonic waves. That is, the transducer, offset position, and its reflector provide a space with sufficient energy inside the fuel assembly to quickly clean deposits from the highly shielded fuel rods without transferring excess energy to the fuel rods where the cladding physically damages the fuel pellets. It operates to generate a space-filling energy field.

The invention can be carried out using a PUSH-PULL converter sold by Martin Walter Ultraschalltechnic, GMBH, Staubbenhard, Germany. Such converters are described in US Pat. No. 5,200,666, which is incorporated herein by reference. Ultrasonic frequencies of 20 kHz to 30 kHz and transducer outputs of 1,000 to 1,500 watts have proven successful. This results in an energy density of 20-30 watts / gallon, which is particularly effective for removing deposits from irradiated fuel assemblies. This energy density is considerably lower than the energy density obtained with the use of conventional ultrasonic transducers.

Other transducers that can be used to radiate omnidirectional energy radially include telsonic radiator (tube) transducers and sonotrode transducers (with transducers on only one side of the rod).

In one embodiment, the transducer body 44 is formed of titanium and a stainless steel end cap is used. Gaskets, cables, and connectors associated with the device must be configured to operate within the spent fuel pool, otherwise all common compatibility requirements and safety requirements customary in nuclear power plants (eg, in the fuel processing area) (FME) requirements of this substance must be met.

4 is a side view of the apparatus 20 of FIG. 2. 4 shows a fuel channel or housing 24, assembly reaction support 26, guide 28, filtration tubing 32, reflector 50 and assembly mounting beam 52. The reflector 50 is used to increase the amount of ultrasonic energy delivered to the fuel assembly. That is, the reflector 50 operates to reflect ultrasonic energy and send it to the fuel assembly. The assembly mounting beam 52 is used to connect the transducer mounting plate 34 to the assembly reactor support 26. The assembly reactor support 26 is pressed against the wall 54 of the fuel reservoir in which the cleaning is performed (described below).

The housing 24, the mounting plate 34, the spacer 36 and the reflector 50 may be formed of stainless steel. Other materials may be used if they meet the general safety and material compatibility requirements typical of operating a nuclear power plant. In particular, the material selected should be compatible for use in the fuel storage and processing zones of the power plant, including the used fuel reservoir and the Cask Loading Pit.

The inner surface of the housing 24 can be electropolished to reduce the chance that radioactive particles will deposit on or stay in depressions or gaps within these surfaces. This allows the operator to detach and ship the housing without being exposed to radiation. The ultrasonic transducer 22 can be used to clean the housing 24. That is, the transducer 22 is activated when the housing 24 is emptied to clean the wall of the housing 24 with deposits.

5 is a plan view of the ultrasonic cleaning device 20. 5 shows the above-described components: transducer 22, housing 24, transducer mounting plate 34, transducer spacer 36 and reflector 50. 5 also shows a housing spacer 60 that operates to allow ultrasonic energy to pass through two sides of the device that are not facing the transducer arrangement. Each reflector 50 includes an inner reflector surface 56 and an outer reflector surface 54 separated by an air gap 56. This configuration has been found to be particularly effective for ultrasonic energy reflection.

5 also shows a fuel assembly 70 located within the housing 24. The fuel assembly 70 includes individual fuel rods 72. The deposit 74 is shown as being stuck to the fuel rod 72. This type of deposit can be removed according to the present method.

5 shows a fuel assembly of 17 × 17. The housing 24 can be configured to receive fuel in hard water of any design. The housing may also be configured for other fuel sources.

The apparatus of FIGS. 2-5 provides high energy density ultrasound to remove tightly fixed deposits from the irradiated nuclear fuel assembly. In particular, the converter 22 generates a power density and sound field that penetrates the center of the fuel bundle 70 and cleans the fuel rod coating located there. The transducer 22 is installed in a vertical arrangement along the two sides of the fuel assembly (see eg FIG. 2) (the axes are oriented horizontally). 2 shows the transducer 22 at the top of the housing 24 because it matches the position of deposits in most pressurized water reactors. The transducer 22 can be located along the entire length of the housing 24 or a limited strategic position.

Typically more than 200 fuel rods in assembly 70 may be arranged in a square pitch arrangement (eg, 17 × 17). In the candidate assembly to be cleaned, the coating containing the fuel pellet stack is covered with the deposit to be removed. In one embodiment, in the vertical arrangement of each transducer, adjacent transducers are laterally offset so that the fracture on one transducer (i.e., the point of zero displacement for excitation mode shape) is at the top and bottom of the system in operation. Aligned with the maximum displacement point on the adjacent transducer. In addition, each transducer can be axially offset from the transducer located on the opposite side of the fuel assembly in this manner. That is, it is desirable to position the transducers so that they are half-wave offsets (or multiples thereof) along the axis of the transducers facing each other. This positioning significantly improves the passage of the tube bundle.

6 shows an apparatus 20 located within a fuel reservoir 80. In this embodiment, the device 20 is mounted using the assembly reaction support 26. Cable 82 can also be used to support device 20. The apparatus 20 has an associated pump and filtration assembly 90. This assembly 90 includes one or more pumps 92 and a set of filters 94. The radiation sensor 96 may be located at the entry point to the pump. The radiation sensor 96 is used to determine when the fuel assembly is to be cleaned. In particular, it is known that when the gamma activity in the sensor 96 drops to a reference value, no further fuel deposited particles are removed and thus the cleaning is completed.

6 also shows ancillary control apparatus 100 in connection with various embodiments. The apparatus 100 can include an ultrasonic output generator 102, a pump and filtration control circuit 106, and a filtration and washing system 108.

7A-7B illustrate positioning the fuel assembly 70 in a housing 24 that is simply depicted. The fuel assembly 70 is positioned using hoist 110. In FIG. 7A, the fuel assembly 70 is in the housing 24. In FIG. 7B, the fuel assembly 70 is partially removed from the housing 24. In FIG. 7C, the fuel assembly 70 is removed from the housing 24. The hoist 110 of FIGS. 7A-7B can be used in the system of FIG. 6 to insert and remove the fuel assembly 70 into the reservoir 80. Hoist 110 may also be used to reposition fuel assembly 70 to clean other areas along the axial length of fuel assembly 70 upon ultrasonic cleaning.

Once the fuel assembly 70 is positioned in the housing 24, ultrasonic cleaning begins. Successful results can be obtained using omnidirectional radial ultrasound operating at a frequency of approximately 20 to 30 kHz and a transducer output of 1,000 to 1500 watts. Referring to FIG. 6, pump 92 draws water through the fuel assembly and thus washes out deposits that are removed by the ultrasonic energy generated by transducer 22. Providing downward flow through the housing 24 eliminates the need to seal the top of the housing 24.

In addition to washing off deposits removed by ultrasonic energy, removal chemicals from the inlet port near the top of the housing are introduced into the water when chemicals are introduced into the water, which modify the deposit surface, oxidize the deposit, solubilize the deposit, Complexation or chelation of the deposit helps to remove the deposit physically and / or chemically.

In LWR operation, active products such as ⁵⁴ Mn, ⁵⁵ Fe, ⁶⁰ Co, ⁶³ Ni and superuranium such as ²⁴¹ Pu may be deposited on the fuel assembly surface by in-house irradiation of fuel rod coatings and subsequent corrosion of these components. . The nitrate permanganate-low oxidation-state transition-metal ion (LOMI / NP) process, in some embodiments, oxidizes chromium into a more soluble form, makes it available for chelating agents, and removes it from the system, It can also be used to purify these components by exposing additional corrosion products for removal by solubilization. Chelating agents, such as but not limited to picolinic acid, form strong complexes with actinium-based, rattan-based, heavy metals, and transition metals and also help to keep them in a dissolved state. Such contaminated solutions can be treated with ion exchange resins after use to extract soluble metals and chemicals. Such resins can be treated and disposed of.

In some embodiments, the chemically improved removal method can use a combination of organic acid and chelating agent (LOMI reagent) to decompose oxide deposits from the fuel surface and suspend the resulting organometallic complex in solution. V ⁺² (bivalent vanadium formic acid) can be used to reduce Fe ⁺³ to Fe ⁺² in the oxide deposit. This mechanism is related to electron stripping rather than attack of deposits by acid. Picolinic acid (as sodium picolinate) can be used as the complexing agent. This reaction causes oxide deposits to become unstable and release metal ions into the solution. Excess picolinate in the reagent complexes with the metal ions and prevents it from depositing again on the fuel surface.

Some oxide deposits have a significant concentration of chromium that adversely affects the decomposition of the oxide layer. In this case, oxidation of such deposits can be used to control the deposits by making chromium soluble through oxidation from the +3 to +6 valence state. This is done by injecting potassium permanganate nitrate (NP) (optionally at 2.5 pH) or alkaline potassium permanganate (AP) (optionally at 10-11.5 pH). In this way chromium becomes soluble and leaves iron-rich deposits that can be broken down using LOMI reagents or other chemicals.

In the washing process, the contaminated release solution passes through a cation exchange resin to remove corrosion and active products and residual metal ions and also to regenerate or remove chemical reagents.

LOMI reagents, degraded radionuclides and metal ions can be removed from the system by treatment with an ion exchange resin column. Strongly acidic cation and weakly basic anion resin columns can be used to treat the LOMI reagent solution used.

NP (or AP) reagents can be removed in an ion exchange resin column. The residual MnO ₂ formed during the oxidation process can be decomposed into oxalic acid, which can be added directly to the permanganic acid solution and also removed from the mixed bed resin column. After oxalic acid cleaning, the earlier LOMI technique can be repeated using a thinner concentration of LOMI reagent. In either case, the used reagents and corrosion products removed in the washing process can be treated via ion exchange resins, which can constitute a clarification waste product.

The fuel assembly 70 can always be supported by the hoist 110 so that the housing 24 does not actually support the weight of the fuel assembly 70 during the cleaning process. As mentioned above, the transducer 22 can be mounted outside the housing 24 to allow ultrasonic energy to pass through the housing wall. The main effect of the interference housing wall was found to be the attenuation of the low frequency portion of the ultrasonic signal. The high frequency portion of the ultrasonic signal (ie, frequency above 10 kHz), responsible for most of the cleaning effectiveness, passes through a properly configured housing with little attenuation.

Typical washing procedures according to the method are as follows. The fuel hoist 110 picks up the fuel assembly 70 from the fuel storage rack. An operating machine associated with the hoist 110 transfers the fuel assembly 70 to the fuel reservoir 80 or some other cleaning location. The fuel assembly 70 may be recorded with video tape when inserted into the housing 24. For example, FIG. 7B shows a camera 120 positioned on top of the housing 24 to record the fuel assembly 70. After that, the converter 22 is energized. Hoist 110 may be used to push the assembly 70 up and down at two minute intervals (ie, up for two minutes down for two minutes). Each push movement can be approximately several inches.

The activity of gamma radiation is observed through the sensor 96. Water with radioactive fuel deposit particles is pumped by pump 92, passes through filter 94, and then returns to fuel reservoir 80. The total radioactivity of the filter 94 can be observed. It is known that once the gamma activity falls back to the reference at sensor 96, the fuel deposit particles are no longer removed and the cleaning is complete. According to some embodiments, the cleaning procedure can be completed in 7-10 minutes. This is in contrast to conventional chemical methods that last for several hours.

Ultrasonic cleaning is generally performed within critical path time. Detailed chemical addition procedures (including the residence time of chemicals in the washer, the resulting flow rate change when adding chemicals, and a suitable lineup for filters or ion exchanges) affect the length of time the fuel assembly consumes in the scrubber. Can give The waste produced may also be of other types.

After cleaning, the fuel assembly 70 is removed from the housing 24, optionally recorded in video. Video tapes before and after cleaning can be reviewed to confirm the success of the process.

The hoist 110 then moves the fuel assembly 70 to a fuel storage rack. The cleaning system is ready to receive the next fuel assembly 70 to be cleaned. In the case of a strongly supported housing 24, a single hoist 110 can be used to mount a set of ultrasonic cleaning devices 20. This configuration improves overall throughput.

The ultrasonic cleaning technique of the method can be cleaned without applying damaging forces to the fuel pellets. The ultrasonic waves used in accordance with the method do not pass through the gas typically found between the inner surface of the coating and the pellets, so that the only means of transmitting harmful vibrational energy to the pellets is to move the coating inner surface relative to the pellets. Experimental results have shown that the vibration spectrum of the coating is comparable to the vibration spectrum of the fuel in operation. The restriction of harmful vibrations by the normal operating conditions in the reactor is not expected to be effective for conventional ultrasonics, because the higher energy input required to clean the inner rods in the fuel bundle is expected to be detrimental to the pellets. .

Those skilled in the art will appreciate that the method may be practiced in a variety of device configurations.

8 shows a channel 160 for receiving an ultrasonic cleaning device and associated fuel assembly. Channel 160 includes integral pump 162 and integral filters 164, 166. Thus, in this embodiment, a single integrated system provides both washing and filtration functions. The filter 164 may be a coarse filter for internal circulation, but the filter 166 may be a fine filter for emissions to the fuel reservoir upon final cleaning. Block 168 shows that fine filter 166 may be implemented with a matrix of pleated filters (eg, nine two inch pleated filters).

Those skilled in the art will appreciate that the present method provides a time efficient, effective, compact, low cost technique for removing deposits from fuel assemblies. This technique is very fast compared to conventional chemical methods.

The method allows for cleaning without disassembling the fuel assembly. The technique does not create adverse coating displacements that threaten the physical integrity of the fuel pellets investigated. That is, the internal deposits in the fuel assembly can be cleaned without any effect upon subsequent reactor restarts.

Other important advantages associated with this method are improved radioactivity management and reduction of radiation exposure of plant workers. Fuel deposit particles removed by the cleaning process are actually the same radioactive material that causes the most important worker dose at downtime when distributed around the coolant loop due to heat / hydraulic transfer in the furnace. Thus, by sealing radioactive particles on a filter which can be safely stored in a fuel reservoir for a long time (when radioactive decay) and washing the fuel, it is possible to reduce the dose rate during downtime and the dose to the operator. Thus, washing fuel as a strategy for dose rate control and dose rate reduction is a competitive new way to reduce radiation management costs.

The chemically improved cleaning methods herein assist in ensuring fuel reliability by removing excess corrosion products. The method also assists in reducing the radiation source by removing radiation from the fuel before the fuel can build up on the surface of the power plant.

It should be understood that the foregoing embodiments can be combined as well as differently.

1 is a schematic diagram of an apparatus for implementing a chemically enhanced ultrasonic cleaning method for an irradiated fuel assembly.

2 is a front view of the ultrasonic cleaning device.

3 is a diagram of an ultrasonic transducer used to generate radially divergent omnidirectional energy.

4 is a side view of the ultrasonic cleaning device of FIG.

5 is a plan view of the ultrasonic cleaning device of FIG. 2 with the nuclear fuel assembly positioned therein;

FIG. 6 illustrates the ultrasonic cleaning apparatus and associated pump and filtration equipment of FIG. 2 used in accordance with one embodiment.

7A-7C illustrate a process for positioning a fuel assembly in a housing.

8 is a diagram of an ultrasonic cleaning device having an integral pump and filtration system.

Explanation of symbols on the main parts of the drawings

10: chemical storage tank

12 ion exchange vessel

20: ultrasonic cleaning device

22: ultrasonic transducer

24: housing

Claims (10)

A method of cleaning an irradiated nuclear fuel assembly, Positioning the nuclear fuel assembly in and adjacent the housing containing the liquid, Supplying the nuclear fuel assembly with radially divergent omnidirectional ultrasonic energy from a transducer capable of respectively supplying the radially divergent omnidirectional ultrasonic energy located in the housing to remove deposits from the nuclear fuel assembly, and Contacting the fuel assembly with at least one chemical that can improve removal of deposits in the liquid, Improving removal of the deposit includes modifying the deposit surface, oxidizing the deposit, solubilizing the deposit, or complexing or chelating the deposit. The method of claim 1, wherein the chemical comprises at least one of a peroxide, an inorganic acid or an inorganic acid salt, an organic acid or organic acid salt, an oxidizing agent, or a chelating agent. The irradiated fuel assembly of claim 1, wherein the chemical agent comprises at least one of hydrogen peroxide, vanadous formate, potassium nitrate permanganate, potassium permanganate, oxalic acid, picolinic acid, or sodium picolinate. How to wash. 10. The method of claim 1, further comprising circulating liquid through the housing during the feeding step. 5. The method of claim 4, wherein said contacting comprises injecting chemical into the liquid while circulating the liquid through the housing. The method of claim 4, wherein the chemical comprises a combination of an organic acid and a chelating agent. The method of claim 4, further comprising isolating the chemical from the fuel reservoir by injecting the chemical only when the liquid flows through the housing and further focusing the chemical by at least one of filtration and ion exchange. A method of cleaning a nuclear fuel assembly. 5. The method of claim 4, further comprising filtering the liquid after the step of contacting. The method of claim 8, further comprising ion exchanging the filtrate from the filtrating step, wherein the ion exchanging comprises passing the filtrate through at least one or both of a strongly acidic cation resin and a weakly basic anion resin. A method of cleaning an irradiated nuclear fuel assembly comprising the steps of: 10. The method of claim 9, further comprising at least one of removing or regenerating complexing or chelating agents, or removing soluble corrosion or active product or residual metal ions.

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