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Radioactive decontamination by ultrasonics
RADIOACTIVE DECONTAMINATION BY ULTRASONICS by H. WELLS
Radioactive contaminant can be almost any element in the periodic table and takes many physical forms. It can be contained in metallic dust, in grease, oil or cutting liquid often mixed with swarf or powder, and even in an oxide or corrosion layer. A tenth of a microgram on a surface may be hazardous. If the contaminant is airborne, the significant amount is much smaller. Laboratory studies in three countries have shown that ultrasonics can accelerate decontamination and raise the decontamination factor
“Cleanliness is next to Godliness” is a precept we often hear very early in life, and which we just as frequently question. In industrial processes time given to cleaning, being usually non-productive, and therefore expensive, must be both shown to be necessary, and be carried out in the most efficient manner. Some of the more common reasons for cleanliness are I. Health e.g. the retention of pathogenic bacteria on surfaces in hospitals. 2. Cross-contamination, which happens in a variety of manufacturing processes, especially those in the food and drug industries. 3. Mechanical efficiency and precision of instruments and mechanisms. 4. Adhesion in the metal finishing industry. Most of the above reasons can be advanced to some extent for radiological decontamination, as cleaning is rather grandly termed in nuclear establishments, but of them the health implications of radiological contamination are by far the most important. Compared to conventional soiling the amount of radioactive contaminant is usually extremely small: 0.1 I*g of certain radioisotopes retained on a surface can emit sufficient ionizing radiation to constitute a health hazard to personnel in the vicinity. For loose and airborne contamination the maximum permissible levels are many orders of magnitude less because of the possibility that they may enter the body by ingestion and inhalation and give a large radiation dose to the surrounding tissues. It is understandable therefore, that considerable effort has been devoted in nuclear establishments to the design of buildings and equipment which can be easily cleaned, e.g. by covering skirtings, boxing in of essential services, and the use of flush fitting light and electrical fittings. Similarly the formulation of cleaning agents, and the development of efficient cleaning methods, has received much attention. THE NATURE OF CONTAMINATION
In certain industries it is possible to define the type of contamination which is proving troublesome, and with this knowledge predict the reagent and method which should be most effective. With ultrasonic techniques a chlorinated
hydrocarbon would probably be used for contamination by oil or grease, and an inhibited acid for the removal of contamination including scale or a corrosion layer. The frequency used would depend on the physical nature of the contamination, e.g. its particle size and density. In nuclear establishments the contaminant can be almost any element in the periodic table and it may be present as dry metallic dust; as contaminated grease, oil, or cutting fluid mixed with powder and swarf; or as a component of the oxide or corrosion layer on a surface. Where contamination consists of particles held loosely at a surface, decontamination is not generally a difficult operation except where the particles are trapped in pores, cracks, and other irregularities, or where they have penetrated into an intricate mechanism. When the contaminant is fixed in an oxide layer, and where contaminating isotopes are firmly held by ion exchange, chemical combination, or other adsorptive processes, decontamination is more difficult: the surface layer often has to be removed. There are many processes which have been identified as being part of the contamination process. The kinetic exchange between a metal and the radioactive ions of its isotopes has been observed, for example an exchange between the ions of Fe5s and Crj5 and inactive iron and chromium ions in stainless steel. The fixation or adsorption by chemical mechanisms of niobium, zirconium and particularly ruthenium, all constituents of fission products, on the surface of stainless steel, is all too frequently met with in practice in nuclear establishments, where stainless steel has been used extensively in chemical separation plants. Although, as has been stated, the contaminant can be almost any element, the most common contamination is that resulting from fission products, the waste products formed in nuclear fuel by the fission or splitting of certain isotopes, e.g. IJ235, by neutrons. The fission products, which are retained in the fuel by the cladding, build up as the fuel is burned, and ultimately tend to “poison” the reactor by capturing the neutrons necessary for continued fission. At this stage fuel is unloaded from the reactor, “cooled” to allow short-lived isotopes to decay, dissolved, and processed in a chemical plant to separate the unused fuel from the contaminating fission products. The relative abundance of the various elements present in fission products depends on reactor conditions, time of irradation
uLTRASONICS/Januar_v1966
29
of the fuel. and the decay period after the fuel has left the reactor. The constitution of l&ion products bix months aftet removal from what may be termed a conventional gas cooled reactor is shown very approximately in Table I Table 1. ~KWN
PROI)UCTS
FROhl
A
Inert gases Alkali metals, c.g. Caesium Alkali earths, e.g. Strontium Rare earths, e.g. Ccritlm e.g. Zirconium e.g. Niobium e.g. Ruthenium
R, AUOR
I0 x and Barium
~
25 35 V :
Contatnination by lission products could occur by failure of the cladding material on nuclear fuel, but in practice this is fortunately extremely rare owing to the extremely efficient inspection methods used. More usually contamination by fission products occurs in the plant and equipment where nuclear fuel is reprocessed, and in the so-called “hot” cells where irradiated fuel is examined by met Illurgists. chemists and engineers to ascertain changes effected by irradiation. Fig. 1 shows hot cells. Finally mention should be tnade of “crud”. a name given to the corrosion products which build up in the primary water cooling circuits of certain reactors. As the particles circulate in the reactor. radioactivity is induced in them by neutron bombardment. and they become a source of contamination whether they finally become part of the oxide layer or are trapped in valves, heat exchangers or filters. The use of ultrasonics in cleaning filters will be referred to later in the paper. REAGENTS
lJXI>
IN IECONTAMINATIOK
The reagents used in decontamination classified under four headings:
can be approxini;itel>
I. Solvents. emulsifiers and detergents used where the contamination is contained in oil and grease. 3. Complexing agents to remove certain ions held at a surface by adsorption or ion exchange or chemisorption. 3. Oxidizing agents, to “condition” components of an oxide layer to a higher valency state. 1. “Pickling agents” to remove the surface layer. In the first category, solvents are only used where emulsifying or detergent solutions are ineffective. This is for a variety of reasons. including toxicity, fire hazard and difficulties in disposing of contatninated solvents. If the use of a solvent is necessary trichloroethane is proferred to the more toxic trichlorethylene. The most satisfactorv detergents are those which are alkaline in nature and wli:ch contain cotnplexing agents such as polyphosphates in addition to the surface wetting, emulsifying and soil suspending materials. Complexing agents. in particular the salts of ethylene diatnine tetracetic acid (EDTA) have found widespread use in decontamination because of their ability to make certain radioactive ions soluble, in particular the alkaline and rare earths, and prevent their redeposition on surfaces. Solutions
30
III
1 I
Jn/?ua/~1~
IMh
of citric acid and ammonium citrate also come into thi\ categrr:. Oxtdtsing agents are usualI) specilic reagent\ IO condition an oxide film prior to the use of a complcxing reagent. A process widely used and very successful in America ih the conditioning of a tenacious oxide film with alkaline pcrmanganate prior IO its removal by ammonium citrate. Similarly. oxalate peroxide mixtures have been used for decontaminating ferrous and non-ferrous alloys in the gas-cooled reactor programme. Pickling agents are similar to those used in conventional metal cleaning processes. the tnost popular being inhibited solutions of phosphoric and sulphuric acids. it is apparent. therefore. that the choice of materials for vessels to store and use these reagents. especialI> at elevated temperatures. has posed certain problems: thi, is especially true in the consideration of ultrasonic technique\. M~.‘rHODS
OF
DtX‘ON~rAMINA’I
IOPI
Several factors are concerned in choosing the method most suitable for the decontamination of plant and equipment. These include : I. Size. This can range from a delicate mechanism or a tnetallurgical specimen in dimension of several inches. to large heavy shielded transport containers weighing several tons (Fig. 2). 7. The radiation emitted by the contamination: if this 14 excessive remote tnethods of decontamination are necessbt y. 3. The location and extent of contamination: if it i\ confined to a small area, local treatment \uch ah scrubbing in a tray is preferred to immersion method\ which could spread the contaminant. Immersion methods also usually result in larger volume\ 01‘ radioactive effluent for subsequent treatment and disposal. .As the purpose of this paper is primarily to discus5 the ti4e
to use ultrasonics in the decontamination of magnox clad fuel elements at the Springfield establishment of the U.K.A.E.A. The tests were a joint programme between various establishments of the Authority and were reported by Appltton and Fairbairn. Briefly, samples of stainless steel, two inches square, were contaminated with acid solutions of fission products, the containment either being allowed to air dry on the surface, or being fired on the surface after air drying to form a more retentive source of contamination. In addition magnox samples two inches square were contaminated with uranyl nitrate, and the contamination allowed to air dry. The ultrasonic equipment used in the test is described as a low frequency unit rated in continuous operation at 2kW output. Four magnetostrictive transducers each rated at 5OOW were fitted and the operating frequency during the tests was approximately 20 kc/s. Certain limitations to the tests are admitted, largely owing to the necessity of preventing contamination and oorrosion of the equipment, and the fact that the tests were carried out in beakers caused considerable energy losses. Energy intensities of O-8 W/cm2 were realized compared with a possible figure of 3 W/cm2 on unshielded samples. To standardize procedures, time and temperature were fixed at 2 min and 25” C. Table 2 is an abstract of the results obtained on the stainless steel samples contaminated by fission products in the unfired condition.
Table2.
Decontaminating
RESULTS ON UNFIRED SAMPLES
solution
_______ Citric acid 10; Built detergent 1 “/, Sulphuric acid 1 % Inhibited phosphoric acid 2.5 “/I Disodium salt of EDTA, 1 %
Percentage contaminarion remaining A, with ultrasonics
0.31 0.64 0.04 0.60 0.50
Gain in decon-
B, without ultrasonics factor, B/A
~
9 8.9 19.5 8.25 3.30
I
29 i3 487.5 13.8 6.6
I
Fig. 2. Transport flasks weighing as much as tons are used to shield the fuel during transfer, for example from reactor to hot cells
of ultrasonics in decontamination, 1 will not dwell on the other methods commonly used in nuclear decontamination centres. They are generally quite orthodox methods, e.g. immersion tanks, steam cabinets, hydrabrasive jets, and more recently electro-polishing techniques, all of which are usuaily adapted or modified to prevent the spread of contamination. Extraction and filtration systems are made ultra-efficient (Fig. 3).
LABORATORY
STUDIES
English An early report by Linsley-Hood in 1953l demonstrated that ultrasonic techniques showed promise in accelerating decontamination rates. The next programme of tests was initiated by the proposal
The results obtained on the fired samples were much less efficient but pre-soaking the samples for 30 min followed by a 2 min ultrasonic treatment was quite effective. As a result of these tests the cleaning of magnox cartridges was recommended as routine procedure at Springfields. In 1960 D. G. Stevenson” reported a test programme using two commercially available low frequency types of ultrasonic equipment, one producing a 20 kc/s continuous wave output and the other a 36 kc/s pulsed output. He draws attention in his report to differences in intensity in the 20 kc/s 2 kW continuous wave equipment feeding into four magnetostrictive transducers and states that variations ranging from O-5 W/cm? at nodal points within the tank, to 14 W/cm2 close to the centre of the transducers. were measured. No power calibration was possible on the other machine, which operated at 36 kc/s delivering 500 W peak with 125 W mean power; the transducers in this equipment were of the barium titanate piezoelectric type. The contaminated samples used in the tests covered a wide
ULTRASONICS/~anuary
1966
31
Fig. 3. Decontamination centre. As well as ultrasonics, all conventional methods of cleaning are installed
range of materials including stainless and mild steels, copper, aluminium, lead, Perspex, rubbers, glass, and painted steel, and the contaminants used included fission products in neutral and acidic media, and plutonium in acid and buffered solutions. A range of decontaminants including detergents, complexing solutions and acids were used and in the actual tests such variables as orientation of the samples, power level (on the 20 kc/s instrument) and time of treatment were included. It is difficult to abstract typical data from such a comprehensive programme but Tables 3, 4 and 5 give some indication of behaviour trends. Tests in the 36 kc/s pulsed tank demonstrated that the higher peak intensity gave more reproducible and uniform improvement factors. Table 3.
ORIENTATION
FFFtCT
20
KC/S
Perpendicular. Adjacent to plate Perpendicular. Midpoint of tank Horizontal. 0.25 in from plate Horizontal. I in from plate Horizontal. Midpoint of tank
32
111TRAsoNlc‘S~Junua,:1~
CONlINUOUS-WAVL
Table 4. liuctiorr oj’powrr ort~put
EFF~CI
~ Nil
OF ULTRASONIC
l/32
I:‘16
POWER
l/8
1
~tv1.1
1:4
I,‘2
Full
Stainless steel contaminated with fission products in a IN solution Decontamination used a detergent solution at 25°C for 2 min.
of HNO,.
Table 5.
EFFECT
STtEL
OF TIME ON CONTAMINATION
CONTAMINATED
WITH
AQUEOUS
RtMOVAL FISSION
FROM STAI’.I.l.SS
PRODUCTS
IANR
Table 5 appears to draw attention to two basic facts. namely the importance of choosing the correct decontaminating solution. the detergent solution showing insignificant gain using ultrasonic aid. and the fact that decontamination gains are greater at room temperature.
1966
Many interesting conciuslons are reported but the overriding factor appears to be intensity, which agrees with the views of the French workers we shall discuss next.
but at higher intensities the decontamination factor for stainless steel appears less frequency dependent. Table 8 considers the influence of intensity on ultrasonic gain, also at 80 kc/s.
French A very systematic study of the factors influencing radiological decontamination was reported by Cerre, Mestre, and de Kerdelleau* in 1962. After an examination of the theoretical aspects of the problem, their programme reported the effect of frequency, intensity and time on contamination removal from stainless and mild steel and cotton fabric. They used an apparatus which could operate at the following frequencies : 16.5, 22, 30, 80 and 175 kc/s with nickel transducers and 300, 500, 1000, 2000 and 3000 kc/s with quartz transducers. The contaminant was a solution of fission products in nitric acid solution, which was dried on the samples for twenty-four hours, rinsed with water and dried. The contaminated samples were then immersed in the bath, the contaminated area parallel to and facing the transducer. The samples were immersed for two minutes and the ultrasonic gain calculated by dividing the decontamination factor with ultrasonics by the decontamination factor obtained with a conventional immersion bath under similar reagent, temperature, and time conditions. Tables 6 and 7 give the results obtained on the stainless steel samples with O-IN sulphuric acid as decontaminant. Table
6.
Frequency
[kc/s1
GAIN
IN DECONTAMINATION
Temperature
[“Cl
I/
FACTORS
Decontamination
BY ULTRASONICS
factor Gain
/ Without 1 ultrasonics
With ultrasonics
16.5 22
Table&
EFFECT OFINTENSITY ON GAIN IN DECONTAMINATION STAINLESSSTEEL
Intensity
Temperature
[ W/c@1 0.35 0.70 I.2 1.6 2.3 ::;
25’
,
:: 30’
,, ,
3;”
FACTOR.
Decontamination fhcror -----;Without I With rilirasonics 1 ultrasonics
~
Gain
I :: 53
, i6”
t:: 17 17 1’::
1 :
I
377 72
The implications of Table 8 are self evident, a minimum intensity of approximately 3.5 W/cm2 seeming necessary to effect efficient decontamination. The ultrasonic gains on the cotton samples were, perhaps understandably, rather disappointing, and the authors conclude that ultrasonics do not appear to be useful in this respect. In the results obtained on the mild steel samples the optimum frequency was reported as 22 kc/s, but again a frequency of 80 kc/s gave excellent results. The minimum intensity for mild steel using O*IN sulphuric acid as decontaminant appeared to be 2.5 W/cm2. The authors have since published a further report, reporting on the ultrasonic decontamination of copper and brass. They again reach the conclusion that the optimum frequency is 80 kc/s, with a minimum intensity of 4 W/cm2. Fig. 4. Ultrasonic cleaner installed inside a “hot” ceil. Metallurgical specimens are transferred straight from the polishing wheel to the cleaning bath in front of it
iz 175 :iC :E 3000
Stainless steel at 4W/cm2 Table7.
GAININDECONTAMINATIONFACTORSBYULTRASONICS
Decontamination Frequency
[kc/s1
factor
Temperature
[“Cl
28 38 68 69
Gain
1
Without ultrasonics
i
With ultrasonics
; 5
91 231 241
> 281 200 82 > 382 >) 318 5 287 32
> j2 32 32 32 32
> > >
ft 17 12 3 12 10 7
Stainless steel at 7W/cmz The authors draw the conclusion that there appears to be an optimum frequency (80 kc/s) at an intensity of 4 W/cm2
uLTR,4soNlcsiJanuar.v 1966
33
Much of the American published work refers rather to the decontamination of items of equipment than to laboratory test programmes of the kind just described. A paper in 1955 by Cortlett and Kolb” described the use of a 2.50 W ultrasonic bath using a 300 kc,‘squartz transducer to decontaminate baskets of 150 components. They reported that ultrasonics enhanced the speed of decontamination by factors of 2-3 in a number of reagents. A report by Henry” in 1957 described the use of ultratonics in decontaminating “crud” contaminated heatexchanger piping. Two frequencies were used: a 300-1000 kc’s quartz transducer delivering 120 W. and a 19-2 I kc’s magnetostrictive transducer. He found the magnetostrictive transducer to be more effective in “crud” removal. Rod’ in 1958 gives further examples of the lield applications of ultrasonics in radiological decontamination. The fact that the source and nature of contamination varies considerably in nuclear establishments explains to some extent the variety of instruments. characteristic frequencies. and power levels which are used in laboratories, in “hot” cells. and in decontamination centres throughout the U.K.A.E.A. Applications of ultrasonic equipment arc numerous, and mostly very efficient. An example will demonstrate the typical usage.
<‘LEANING
OF METALLURGI(‘AL
SPECIMENS
Metallurgical specimens are cleaned in lead shielded metallurgical boxes prior to removal of the specimens for photomicrographic examination. The cleaning ensures that contamination is not carried into the “clean” or “inactive” box where the photographic work is carried out and also ensures a clear image for microscopic analysis. Certain specimens are rather fragile and could be damaged by cavitation under certain frequency and intensity combinations. Present practice is to use instruments operating at 40 kc/s with barium titanate transducers. Fig. 5 illustrates the use of an ultrasonic bath to decontaminate an irradiated fuel element prior to canning it in ;I stainless steel sheath. The operation took place remotely in a shielded “cave”. The element was cleaned in the bath using ;I citric acid solution. transferred to an adjacent cell for the canning process. and the canned element subsequently rinsed in the
34
U~.TUASoNl~S,‘JUnuu/._l~
/!rid
lirst cell. For the final rinsing the contaminated citric acid had been replaced by distilled water. Six transducers were used in the bath which operated at a frequency of 40 kc/s with a pulsed output averaging I kW with a peak of 4 kW. Ultrasonic cleaning is usual (Fig. 6) for sintered stainless steel filters. In circuit in the reactor the filter becomes clogged with the active and inactive corrosion producta referred to in the early part of the paper. Various forms of cleaning. including backwashing, had proved ineffective. Acid solutions had been partially successful but lead to accelerated corrosion rates of the filter thimble. solution has Ultrasonic washing in a built detergent proved effective, clean and speedy and is now standard procedure. The equipment operates at 25 kc!‘>. has a pulsed oup~lt of I50 W with peak value of 300 W. The transducer\ are fabricated from lead zirconate titanate.
( ONC’I.UI)ING
IIl+MARKS
Ultrasonic equipment has established itself in radiological decontamination practice and as the use of atomic powci increases. so progressively will contamination problems and the need for speedy and efficient decontamination methods. The general theme of laboratory evaluations suggests that high intensity machines will be most useful for the removal ol’the more stubborn oxide layer type of contamination we meet so often. There is also a place for a high intensit! “scrubbing brush” probe which we could LISC. if neceysaril> remotely. on large and heavy equipment.
Radioactive contaminant can be almost any element in the periodic table and takes many physical forms. It can be contained in metallic dust, in grease, oil or cutting liquid often mixed with swarf or powder, and even in an oxide or corrosion layer. A tenth of a microgram on a surface may be hazardous. If the contaminant is airborne, the significant amount is much smaller. Laboratory studies in three countries have shown that ultrasonics can accelerate decontamination and raise the decontamination factor
“Cleanliness is next to Godliness” is a precept we often hear very early in life, and which we just as frequently question. In industrial processes time given to cleaning, being usually non-productive, and therefore expensive, must be both shown to be necessary, and be carried out in the most efficient manner. Some of the more common reasons for cleanliness are I. Health e.g. the retention of pathogenic bacteria on surfaces in hospitals. 2. Cross-contamination, which happens in a variety of manufacturing processes, especially those in the food and drug industries. 3. Mechanical efficiency and precision of instruments and mechanisms. 4. Adhesion in the metal finishing industry. Most of the above reasons can be advanced to some extent for radiological decontamination, as cleaning is rather grandly termed in nuclear establishments, but of them the health implications of radiological contamination are by far the most important. Compared to conventional soiling the amount of radioactive contaminant is usually extremely small: 0.1 I*g of certain radioisotopes retained on a surface can emit sufficient ionizing radiation to constitute a health hazard to personnel in the vicinity. For loose and airborne contamination the maximum permissible levels are many orders of magnitude less because of the possibility that they may enter the body by ingestion and inhalation and give a large radiation dose to the surrounding tissues. It is understandable therefore, that considerable effort has been devoted in nuclear establishments to the design of buildings and equipment which can be easily cleaned, e.g. by covering skirtings, boxing in of essential services, and the use of flush fitting light and electrical fittings. Similarly the formulation of cleaning agents, and the development of efficient cleaning methods, has received much attention. THE NATURE OF CONTAMINATION
In certain industries it is possible to define the type of contamination which is proving troublesome, and with this knowledge predict the reagent and method which should be most effective. With ultrasonic techniques a chlorinated
hydrocarbon would probably be used for contamination by oil or grease, and an inhibited acid for the removal of contamination including scale or a corrosion layer. The frequency used would depend on the physical nature of the contamination, e.g. its particle size and density. In nuclear establishments the contaminant can be almost any element in the periodic table and it may be present as dry metallic dust; as contaminated grease, oil, or cutting fluid mixed with powder and swarf; or as a component of the oxide or corrosion layer on a surface. Where contamination consists of particles held loosely at a surface, decontamination is not generally a difficult operation except where the particles are trapped in pores, cracks, and other irregularities, or where they have penetrated into an intricate mechanism. When the contaminant is fixed in an oxide layer, and where contaminating isotopes are firmly held by ion exchange, chemical combination, or other adsorptive processes, decontamination is more difficult: the surface layer often has to be removed. There are many processes which have been identified as being part of the contamination process. The kinetic exchange between a metal and the radioactive ions of its isotopes has been observed, for example an exchange between the ions of Fe5s and Crj5 and inactive iron and chromium ions in stainless steel. The fixation or adsorption by chemical mechanisms of niobium, zirconium and particularly ruthenium, all constituents of fission products, on the surface of stainless steel, is all too frequently met with in practice in nuclear establishments, where stainless steel has been used extensively in chemical separation plants. Although, as has been stated, the contaminant can be almost any element, the most common contamination is that resulting from fission products, the waste products formed in nuclear fuel by the fission or splitting of certain isotopes, e.g. IJ235, by neutrons. The fission products, which are retained in the fuel by the cladding, build up as the fuel is burned, and ultimately tend to “poison” the reactor by capturing the neutrons necessary for continued fission. At this stage fuel is unloaded from the reactor, “cooled” to allow short-lived isotopes to decay, dissolved, and processed in a chemical plant to separate the unused fuel from the contaminating fission products. The relative abundance of the various elements present in fission products depends on reactor conditions, time of irradation
uLTRASONICS/Januar_v1966
29
of the fuel. and the decay period after the fuel has left the reactor. The constitution of l&ion products bix months aftet removal from what may be termed a conventional gas cooled reactor is shown very approximately in Table I Table 1. ~KWN
PROI)UCTS
FROhl
A
Inert gases Alkali metals, c.g. Caesium Alkali earths, e.g. Strontium Rare earths, e.g. Ccritlm e.g. Zirconium e.g. Niobium e.g. Ruthenium
R, AUOR
I0 x and Barium
~
25 35 V :
Contatnination by lission products could occur by failure of the cladding material on nuclear fuel, but in practice this is fortunately extremely rare owing to the extremely efficient inspection methods used. More usually contamination by fission products occurs in the plant and equipment where nuclear fuel is reprocessed, and in the so-called “hot” cells where irradiated fuel is examined by met Illurgists. chemists and engineers to ascertain changes effected by irradiation. Fig. 1 shows hot cells. Finally mention should be tnade of “crud”. a name given to the corrosion products which build up in the primary water cooling circuits of certain reactors. As the particles circulate in the reactor. radioactivity is induced in them by neutron bombardment. and they become a source of contamination whether they finally become part of the oxide layer or are trapped in valves, heat exchangers or filters. The use of ultrasonics in cleaning filters will be referred to later in the paper. REAGENTS
lJXI>
IN IECONTAMINATIOK
The reagents used in decontamination classified under four headings:
can be approxini;itel>
I. Solvents. emulsifiers and detergents used where the contamination is contained in oil and grease. 3. Complexing agents to remove certain ions held at a surface by adsorption or ion exchange or chemisorption. 3. Oxidizing agents, to “condition” components of an oxide layer to a higher valency state. 1. “Pickling agents” to remove the surface layer. In the first category, solvents are only used where emulsifying or detergent solutions are ineffective. This is for a variety of reasons. including toxicity, fire hazard and difficulties in disposing of contatninated solvents. If the use of a solvent is necessary trichloroethane is proferred to the more toxic trichlorethylene. The most satisfactorv detergents are those which are alkaline in nature and wli:ch contain cotnplexing agents such as polyphosphates in addition to the surface wetting, emulsifying and soil suspending materials. Complexing agents. in particular the salts of ethylene diatnine tetracetic acid (EDTA) have found widespread use in decontamination because of their ability to make certain radioactive ions soluble, in particular the alkaline and rare earths, and prevent their redeposition on surfaces. Solutions
30
III
1 I
Jn/?ua/~1~
IMh
of citric acid and ammonium citrate also come into thi\ categrr:. Oxtdtsing agents are usualI) specilic reagent\ IO condition an oxide film prior to the use of a complcxing reagent. A process widely used and very successful in America ih the conditioning of a tenacious oxide film with alkaline pcrmanganate prior IO its removal by ammonium citrate. Similarly. oxalate peroxide mixtures have been used for decontaminating ferrous and non-ferrous alloys in the gas-cooled reactor programme. Pickling agents are similar to those used in conventional metal cleaning processes. the tnost popular being inhibited solutions of phosphoric and sulphuric acids. it is apparent. therefore. that the choice of materials for vessels to store and use these reagents. especialI> at elevated temperatures. has posed certain problems: thi, is especially true in the consideration of ultrasonic technique\. M~.‘rHODS
OF
DtX‘ON~rAMINA’I
IOPI
Several factors are concerned in choosing the method most suitable for the decontamination of plant and equipment. These include : I. Size. This can range from a delicate mechanism or a tnetallurgical specimen in dimension of several inches. to large heavy shielded transport containers weighing several tons (Fig. 2). 7. The radiation emitted by the contamination: if this 14 excessive remote tnethods of decontamination are necessbt y. 3. The location and extent of contamination: if it i\ confined to a small area, local treatment \uch ah scrubbing in a tray is preferred to immersion method\ which could spread the contaminant. Immersion methods also usually result in larger volume\ 01‘ radioactive effluent for subsequent treatment and disposal. .As the purpose of this paper is primarily to discus5 the ti4e
to use ultrasonics in the decontamination of magnox clad fuel elements at the Springfield establishment of the U.K.A.E.A. The tests were a joint programme between various establishments of the Authority and were reported by Appltton and Fairbairn. Briefly, samples of stainless steel, two inches square, were contaminated with acid solutions of fission products, the containment either being allowed to air dry on the surface, or being fired on the surface after air drying to form a more retentive source of contamination. In addition magnox samples two inches square were contaminated with uranyl nitrate, and the contamination allowed to air dry. The ultrasonic equipment used in the test is described as a low frequency unit rated in continuous operation at 2kW output. Four magnetostrictive transducers each rated at 5OOW were fitted and the operating frequency during the tests was approximately 20 kc/s. Certain limitations to the tests are admitted, largely owing to the necessity of preventing contamination and oorrosion of the equipment, and the fact that the tests were carried out in beakers caused considerable energy losses. Energy intensities of O-8 W/cm2 were realized compared with a possible figure of 3 W/cm2 on unshielded samples. To standardize procedures, time and temperature were fixed at 2 min and 25” C. Table 2 is an abstract of the results obtained on the stainless steel samples contaminated by fission products in the unfired condition.
Table2.
Decontaminating
RESULTS ON UNFIRED SAMPLES
solution
_______ Citric acid 10; Built detergent 1 “/, Sulphuric acid 1 % Inhibited phosphoric acid 2.5 “/I Disodium salt of EDTA, 1 %
Percentage contaminarion remaining A, with ultrasonics
0.31 0.64 0.04 0.60 0.50
Gain in decon-
B, without ultrasonics factor, B/A
~
9 8.9 19.5 8.25 3.30
I
29 i3 487.5 13.8 6.6
I
Fig. 2. Transport flasks weighing as much as tons are used to shield the fuel during transfer, for example from reactor to hot cells
of ultrasonics in decontamination, 1 will not dwell on the other methods commonly used in nuclear decontamination centres. They are generally quite orthodox methods, e.g. immersion tanks, steam cabinets, hydrabrasive jets, and more recently electro-polishing techniques, all of which are usuaily adapted or modified to prevent the spread of contamination. Extraction and filtration systems are made ultra-efficient (Fig. 3).
LABORATORY
STUDIES
English An early report by Linsley-Hood in 1953l demonstrated that ultrasonic techniques showed promise in accelerating decontamination rates. The next programme of tests was initiated by the proposal
The results obtained on the fired samples were much less efficient but pre-soaking the samples for 30 min followed by a 2 min ultrasonic treatment was quite effective. As a result of these tests the cleaning of magnox cartridges was recommended as routine procedure at Springfields. In 1960 D. G. Stevenson” reported a test programme using two commercially available low frequency types of ultrasonic equipment, one producing a 20 kc/s continuous wave output and the other a 36 kc/s pulsed output. He draws attention in his report to differences in intensity in the 20 kc/s 2 kW continuous wave equipment feeding into four magnetostrictive transducers and states that variations ranging from O-5 W/cm? at nodal points within the tank, to 14 W/cm2 close to the centre of the transducers. were measured. No power calibration was possible on the other machine, which operated at 36 kc/s delivering 500 W peak with 125 W mean power; the transducers in this equipment were of the barium titanate piezoelectric type. The contaminated samples used in the tests covered a wide
ULTRASONICS/~anuary
1966
31
Fig. 3. Decontamination centre. As well as ultrasonics, all conventional methods of cleaning are installed
range of materials including stainless and mild steels, copper, aluminium, lead, Perspex, rubbers, glass, and painted steel, and the contaminants used included fission products in neutral and acidic media, and plutonium in acid and buffered solutions. A range of decontaminants including detergents, complexing solutions and acids were used and in the actual tests such variables as orientation of the samples, power level (on the 20 kc/s instrument) and time of treatment were included. It is difficult to abstract typical data from such a comprehensive programme but Tables 3, 4 and 5 give some indication of behaviour trends. Tests in the 36 kc/s pulsed tank demonstrated that the higher peak intensity gave more reproducible and uniform improvement factors. Table 3.
ORIENTATION
FFFtCT
20
KC/S
Perpendicular. Adjacent to plate Perpendicular. Midpoint of tank Horizontal. 0.25 in from plate Horizontal. I in from plate Horizontal. Midpoint of tank
32
111TRAsoNlc‘S~Junua,:1~
CONlINUOUS-WAVL
Table 4. liuctiorr oj’powrr ort~put
EFF~CI
~ Nil
OF ULTRASONIC
l/32
I:‘16
POWER
l/8
1
~tv1.1
1:4
I,‘2
Full
Stainless steel contaminated with fission products in a IN solution Decontamination used a detergent solution at 25°C for 2 min.
of HNO,.
Table 5.
EFFECT
STtEL
OF TIME ON CONTAMINATION
CONTAMINATED
WITH
AQUEOUS
RtMOVAL FISSION
FROM STAI’.I.l.SS
PRODUCTS
IANR
Table 5 appears to draw attention to two basic facts. namely the importance of choosing the correct decontaminating solution. the detergent solution showing insignificant gain using ultrasonic aid. and the fact that decontamination gains are greater at room temperature.
1966
Many interesting conciuslons are reported but the overriding factor appears to be intensity, which agrees with the views of the French workers we shall discuss next.
but at higher intensities the decontamination factor for stainless steel appears less frequency dependent. Table 8 considers the influence of intensity on ultrasonic gain, also at 80 kc/s.
French A very systematic study of the factors influencing radiological decontamination was reported by Cerre, Mestre, and de Kerdelleau* in 1962. After an examination of the theoretical aspects of the problem, their programme reported the effect of frequency, intensity and time on contamination removal from stainless and mild steel and cotton fabric. They used an apparatus which could operate at the following frequencies : 16.5, 22, 30, 80 and 175 kc/s with nickel transducers and 300, 500, 1000, 2000 and 3000 kc/s with quartz transducers. The contaminant was a solution of fission products in nitric acid solution, which was dried on the samples for twenty-four hours, rinsed with water and dried. The contaminated samples were then immersed in the bath, the contaminated area parallel to and facing the transducer. The samples were immersed for two minutes and the ultrasonic gain calculated by dividing the decontamination factor with ultrasonics by the decontamination factor obtained with a conventional immersion bath under similar reagent, temperature, and time conditions. Tables 6 and 7 give the results obtained on the stainless steel samples with O-IN sulphuric acid as decontaminant. Table
6.
Frequency
[kc/s1
GAIN
IN DECONTAMINATION
Temperature
[“Cl
I/
FACTORS
Decontamination
BY ULTRASONICS
factor Gain
/ Without 1 ultrasonics
With ultrasonics
16.5 22
Table&
EFFECT OFINTENSITY ON GAIN IN DECONTAMINATION STAINLESSSTEEL
Intensity
Temperature
[ W/c@1 0.35 0.70 I.2 1.6 2.3 ::;
25’
,
:: 30’
,, ,
3;”
FACTOR.
Decontamination fhcror -----;Without I With rilirasonics 1 ultrasonics
~
Gain
I :: 53
, i6”
t:: 17 17 1’::
1 :
I
377 72
The implications of Table 8 are self evident, a minimum intensity of approximately 3.5 W/cm2 seeming necessary to effect efficient decontamination. The ultrasonic gains on the cotton samples were, perhaps understandably, rather disappointing, and the authors conclude that ultrasonics do not appear to be useful in this respect. In the results obtained on the mild steel samples the optimum frequency was reported as 22 kc/s, but again a frequency of 80 kc/s gave excellent results. The minimum intensity for mild steel using O*IN sulphuric acid as decontaminant appeared to be 2.5 W/cm2. The authors have since published a further report, reporting on the ultrasonic decontamination of copper and brass. They again reach the conclusion that the optimum frequency is 80 kc/s, with a minimum intensity of 4 W/cm2. Fig. 4. Ultrasonic cleaner installed inside a “hot” ceil. Metallurgical specimens are transferred straight from the polishing wheel to the cleaning bath in front of it
iz 175 :iC :E 3000
Stainless steel at 4W/cm2 Table7.
GAININDECONTAMINATIONFACTORSBYULTRASONICS
Decontamination Frequency
[kc/s1
factor
Temperature
[“Cl
28 38 68 69
Gain
1
Without ultrasonics
i
With ultrasonics
; 5
91 231 241
> 281 200 82 > 382 >) 318 5 287 32
> j2 32 32 32 32
> > >
ft 17 12 3 12 10 7
Stainless steel at 7W/cmz The authors draw the conclusion that there appears to be an optimum frequency (80 kc/s) at an intensity of 4 W/cm2
uLTR,4soNlcsiJanuar.v 1966
33
Much of the American published work refers rather to the decontamination of items of equipment than to laboratory test programmes of the kind just described. A paper in 1955 by Cortlett and Kolb” described the use of a 2.50 W ultrasonic bath using a 300 kc,‘squartz transducer to decontaminate baskets of 150 components. They reported that ultrasonics enhanced the speed of decontamination by factors of 2-3 in a number of reagents. A report by Henry” in 1957 described the use of ultratonics in decontaminating “crud” contaminated heatexchanger piping. Two frequencies were used: a 300-1000 kc’s quartz transducer delivering 120 W. and a 19-2 I kc’s magnetostrictive transducer. He found the magnetostrictive transducer to be more effective in “crud” removal. Rod’ in 1958 gives further examples of the lield applications of ultrasonics in radiological decontamination. The fact that the source and nature of contamination varies considerably in nuclear establishments explains to some extent the variety of instruments. characteristic frequencies. and power levels which are used in laboratories, in “hot” cells. and in decontamination centres throughout the U.K.A.E.A. Applications of ultrasonic equipment arc numerous, and mostly very efficient. An example will demonstrate the typical usage.
<‘LEANING
OF METALLURGI(‘AL
SPECIMENS
Metallurgical specimens are cleaned in lead shielded metallurgical boxes prior to removal of the specimens for photomicrographic examination. The cleaning ensures that contamination is not carried into the “clean” or “inactive” box where the photographic work is carried out and also ensures a clear image for microscopic analysis. Certain specimens are rather fragile and could be damaged by cavitation under certain frequency and intensity combinations. Present practice is to use instruments operating at 40 kc/s with barium titanate transducers. Fig. 5 illustrates the use of an ultrasonic bath to decontaminate an irradiated fuel element prior to canning it in ;I stainless steel sheath. The operation took place remotely in a shielded “cave”. The element was cleaned in the bath using ;I citric acid solution. transferred to an adjacent cell for the canning process. and the canned element subsequently rinsed in the
34
U~.TUASoNl~S,‘JUnuu/._l~
/!rid
lirst cell. For the final rinsing the contaminated citric acid had been replaced by distilled water. Six transducers were used in the bath which operated at a frequency of 40 kc/s with a pulsed output averaging I kW with a peak of 4 kW. Ultrasonic cleaning is usual (Fig. 6) for sintered stainless steel filters. In circuit in the reactor the filter becomes clogged with the active and inactive corrosion producta referred to in the early part of the paper. Various forms of cleaning. including backwashing, had proved ineffective. Acid solutions had been partially successful but lead to accelerated corrosion rates of the filter thimble. solution has Ultrasonic washing in a built detergent proved effective, clean and speedy and is now standard procedure. The equipment operates at 25 kc!‘>. has a pulsed oup~lt of I50 W with peak value of 300 W. The transducer\ are fabricated from lead zirconate titanate.
( ONC’I.UI)ING
IIl+MARKS
Ultrasonic equipment has established itself in radiological decontamination practice and as the use of atomic powci increases. so progressively will contamination problems and the need for speedy and efficient decontamination methods. The general theme of laboratory evaluations suggests that high intensity machines will be most useful for the removal ol’the more stubborn oxide layer type of contamination we meet so often. There is also a place for a high intensit! “scrubbing brush” probe which we could LISC. if neceysaril> remotely. on large and heavy equipment.