CA1130565A

CA1130565A - Decontamination method

Info

Publication number: CA1130565A
Application number: CA347,699A
Authority: CA
Inventors: Robert T. Marchese
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1979-04-12
Filing date: 1980-03-14
Publication date: 1982-08-31
Also published as: JPS55141700A; KR830002114B1; ZA802191B; ES490511A0; YU63680A; YU42329B; ES8301386A1; EP0018152A1; DE3064799D1; EP0018152B1; KR830003115A; FR2454160A1; JPS6333117B2

Abstract

9 48,392 ABSTRACT OF THE DISCLOSURE
The method comprises using a high pressure water jet containing abrasive grit for removing radioactive oxide film from the surface of nuclear power plant compon-ents. The method provides a means by which components used in nuclear power plants may be decontaminated so that working personnel may inspect or repair the components without absorbing excessive radiation exposure.

Description

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l 48,392 ~ECONT~INATION METHOD

mis invention relates to decontamination meth~
ods and more particularly to methods ~or decontaminating components of nuclear power plants.
During the operation of nuclear power plants and similar apparatus, certain components become e~posed to radiation and may develop a thin redioactive film on the surface o~ the component. From time ~o time, it is neces-sary to either inspect or repair these components o~ the 1~ nuclear reactor power plant~ During the inspection or repair of the components, 1t is necessary for working personnel to enter the compon~nt or to be sta*ioned in cloæe proximity to the component where~y working personnel may be exposed to radiation emitted ~rom the contaminated ;`
component~ In some circumstances~ the radiation ~ield emitted from these somponents is such that a worker would receive the maximum permiss1ble radiation dose in les~
than ~ive minutes of working time. Such a situation means that a given worker may only spend a relatiYely short amount of time working on the inspection or the repair operation ~f the nuolear component. Having each worker spend a relatively ~hort ~mount of time in the repair or .3 36~65 inspection procedure, necessitates the use of many workers with each worker working a short time period in order to accomplish the desired procedure. While this may be an acceptable practice ~or minor inspections or repair proce-dures, this is not an acceptable prac-tice where there is an extensive inspection or an extensive repair job to be performed. Where the procedure to be performed is a time consuming procedure, it is l:ikely that an unusually large number of highly trained personnel would be necessary to carry out -the task. Such a situation may not only be unacceptable from a financial aspect, but may also be unacceptable from a man-power level aspect. Therefore, what is needed is a decontamination method that reduces the radiation field in components of nuclear reactor power plants so that working personnel may perform operations thereon.
SUMMARY OF THE INVENTION
A method for decontamina-ting radioactive compon-ents comprises the use of a high pressure water jet con-taining abrasive grit to remove radioactive oxide filmfrom the surface of the component. The flow rate and pressure of the water jet are controlled so that a thin layer of metal oxide deposits may be removed from the component without damaging the metal componen-t itself thereby reducing the radiation field of the componen-t.
The grit types, size, and concentration in -the water jet are also controlled along with -the distance of the nozzle from the surface of the component and the angle of im-pingement so that an optimum level of decontamination is achieved without deterioration of the metal surface.
BRIEF DESC~IPTION OF THE D~AWING
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the invention, it is believed the inven-tion will be better understood from the following descrip-tion, taken in conjunction with the accompanying drawing, wherein the single fig~re of the drawing is a diagram of apparatus used to perform the decontamination method.

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3L~3~56~i DESCRIPTION OF THE PR~F~RRED EMBODIMENT
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During operation of typical nuclear power plants, certain components of the nuclear power plant such as the nuclear s-team generators, become radioactively contaminated. Since certain repair or inspec-tion opera-tions that must be performecl periodically on the nuclear power plan-t components require working personnel to be present in or near the components, it is important that the radiation field associated with the component be at a level that is compatible w:ith the presence of working personnel for an extended period of time. The invention, described herein, is a method for decontaminating compon-ents of nuclear power plants so tha-t working personnel may perform operations thereon.
15Referring to the drawing, apparatus for perform-ing the decontamination method comprises a platform 10 for supporting container 12 which may be a steel drum having a closure plate 14. Container 12 comprises s-upports 16 for supporting the specimen 18 to be decontaminated. Contain-

2~er 12 also has a vent 20 and a drain 22. Container 12 is arranged such that the front portion thereof is supported by a bar 24 so tha-t container 12 is tilted such that liquid within container 12 will flow through drain 22.
Drain 22 is connec-ted to filter 26 which may be a 5-25 micron filter bag chosen from those well known in the art.
Filter 26 is connected to a pipe 28 which is connected to a drain for disposing of or recirculating -the water used in the decontamination process. A half-inch diameter tube 30 extends through universal joint 32 and closure plate 14 so that the fron-~ end of tube 30 is disposed within con-tainer 12. A nozæle 34 is mounted on the fron-t end of tube 30 and is also connected to flexible hose 36. Nozzle 34 may be chosen from those well known in the art such as a "Dynajector" manufactured by the Aqua-Dyne Engineering, Inc. of Houston, Texas. Flexible hose 36 extends through closure plate 14 and is connected to gri-t supply 38 for supplying g:rit to nozzle 34. Tube 30 is connected to pump 40 which may be a 40 horsepower pump which is also con-.:, ~i3¢3~6 nected to water supply 42 :Eor supplying water to nozzle34. Nozzle 34 provides a mechanism for mixing the grit and wa-ter and for emikting the water-grit mixture from nozzle 34 toward specimen 18. Tube 30 is also attached to drive mechanism 44 which may be a mechanism for controll-ing the horizontal movemen-t of tube 30 and nozzle 34.
Drive mechanism 44 may be chosen from those well known in the art.
The decontamination me-thod comprises introducing water through tube 30 at a pressure between approximately 2,000 psi and approximately 2,700 psi. The water flow rate at -this pressure should be approximately 8 to 9 gallons per minute through nozzle 34. Several types of grit may be used for mixing with the water such as alumina or magnetite. However, the grit size should be approxi-mately 120 to 325 mesh size in accordance with United States Sieve Series Mesh Sizes. It is importan-t to note that the grit concentration in the water spray should be approximately 3% to approximately 7% by weight. In order to provide effective decontamination without excessive metal deterioration, it is important that the nozzle be placed approximately 6 inches to 10 inches from the sur-face of specimen 18. It has also been found that nozzle 34 should be arranged at approximately between 30 to 70 with respect to the longitudinal axis of tube 30 so that the water-grit mixture impinges the surface of specimen 18 at approximately be-tween a 30-70 angle and preferably at about 45.
OPERATION
In operation, a test specimen 18 having a top surface that has been contaminated with radioactivity is placed within container 12 and supported by supports 16 as shown in the drawing. Closure plate 14 is then attached to container 12 such that tube 30, nozzle 34, and ~lexible hose 36 are arranged as shown in the drawing. Universal joint 32 which also comprises a clamping mechanism is arranged such that the flow of -the water-grit mixture from nozzle 34 is direc-ted to the proper elevation of specimen

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18. At this point, nozzle 34 is approximately 6 to 10 inches from the surface of the specimen 18. Next, pump 40 is activated which causes water to be pumped from water supply 42 -through tube 30 and into nozzle 34. The flow of water through nozzle 34 creates a vacuum in nozzle 34 which draws the grit from gr:it supply 38 through flexible hose 36 where i-t mixes with the water in nozzle 34. The water-gri-t mixture is then directed toward the specimen 18. Simultaneously, arive mechanism 44 is activated which causes tube 30 and nozzle 34 to move in a horizontal ].ine across specimen 18 at a speed of approxima-tely 1 foot per minute to approximately 3 feet per minute. The speed of travel of nozzle 34 is correlated with the water-grit flow rate so as to provide effective decontamina-tion witho-ut excessive deterioration of the metal of specimen 18. The water and grit mixture impinges on the surface of specimen 18 and removes a -thin oxide layer from the specimen which is carried away by the wa-ter-grit mixture and drains through drain 22 into filter 26. Once nozzle 34 has made a complete horizontal pass of specimen 18, universal joint 32 is readj~tsted so that nozzle 34 is directed toward a different vertical elevation of specimen 18 and at the same 6 to 10 inch distance. Then, drive mechanism 44 is reversed so that nozzle 34 makes a similar horizontal pass f specimen 18 but at a different elevation. In this manner, an entire sweeping of specimen 18 may be made. Ln addition to the apparatus as shown in the drawing, an .
additional nozzle 34 may be attached to tube and also arranged at approximately a 45 angle and opposing the first nozzle 34 so as to provide a dual nozzle arrangement.
A test of the decontamination me-thod was made with the following parameters: ::
1. Nozzle Pressure - 2400 psig 2. Water Flow ~ate - 8.5 gal/min/nozzle 3. Grit Types - Alumina and Magnetite . ~ .. , . .. . . . ~ .

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4. Grit Size - 220 mesh and 325 mesh

5. Grit Concentration in Spray - 3% to 7% by weigh-t

6. Spray Traverse Velocity across Surface - 16 in/min to 38 in/min

7. NozY.le to Surface Distance - 6 inches

8. Impingement Angle - 45 Two passes were made across the surface of the specimen, one pass each at opposing 45 angles. This ensured clean-ing of bo-th sides of the irregularities in the surface of the specimen. Results of -the tests show that 98.3% to 99.9% of the radioactive contamination was removed from the sample simulating nuclear reactor componen-t surfaces.
None of the samples suffered perceptible surface damage.
Tests and analysis have determined that with the use of the decontamination process, it is likely that less than .001 inch of metal surface will be removed from -the specimen. It appears that the use of an alumina grit on Inconel metal will remove a metal layer of less than .0002-.0003 inches and that the use of a magnetite grit on stainless steel will remove a metal layer of less than .0005-.001 inch.
While the decontamination method has been des-cribed as being used in a test apparatus, the decontamina-tion me-thod may be used on nuclear reactor components such as nuclear steam generators with the equipment adapted to be placed within the nuclear component. Therefore, the invention provides a decontamina-tion method for lowering the radiation field of nuclear reactor power components so -that working personnel may enter the componen-t and perform operations thereon.

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Claims

I claim as my invention:

1. A decontamination method comprising:
spraying a component with a water-grit mixture having a grit concentration of between approximately 3% to 7% by weight and having a grit size of between approximately 120 mesh to 325 mesh for removing contamina-tion from the component without damaging the metal surface of the component;
emitting said mixture from a nozzle at between approximately 2000 psi to 2700 psi and at a rate of between approximately 8-9 gallons/min. per nozzle with said nozzle located 6 inches to 10 inches from the surface of said component and arranged with an angle of impingement of be-tween approximately 30° to 70°; and moving said nozzle along said component at between 1 ft./min. to 3 ft./min.