Working Surfaces for Radiochemical Laboratories GLASS, STAINLESS STEEL, AND LEAD PAUL C. TOMP#INS1 AND OSCAR M. BIZZELL Oak Ridge National Laboratory, Oak Ridge, Tenn. T h e determination of the susceptibility of protective coatings and structural materials to radioactive contamination and their subsequent ease of decontamination is essentially a new field, but one which is rapidly becoming very important. A systematic attempt has been made here to develop simple tests which permit comparisons between surfaces, decontamination reagents, and contaminating conditions. The data are reported in terms of two newly defined quantities called the decontamination index, D.I., and the spill index, S.I. The term decontamination index defined as :
D.I. = log
activity on surface before decontamination activity on surface after decontamination
(
1
is proposed as a measure of the degree of removal of air-
0
NE of the major problems faced in the construction of radio-
chemical facilities is the eelection of structural materials and surface coatings which permit ready cleaning. The requirements for a decontaminable surface which have become accepted among Atomic Energy Commission workers may be listed as follows:
1. Smooth and nonporous (minimum of adsorption area and penetration). 2. Nonionic (minimum exchange). 3. Resistant t o corrosion by acids, alkalies, organic solvents. 4. Resistant t o heat. The contamination results from the attachment of a radionuclide to a solid surface; decontamination involves its subsequent removal. By common usage, a material is considered a “decontaminable material” when it can be cleaned by liquid solvents without undue deterioration (13). The four points mentioned above are the major criteria that relate to the properties of the materials themselves. Many other pertinent points such as cost, durability, and the manner of construction (no cracks, etc.) have been suggested in more extensive discussions of the subject (1, 3, 8, 11). This paper reports preliminary studies made on the decontamination of glass, polished stainless steel (type 347), unpolished stainless steel (type 316), and lead, for they are mentioned among the materials usually suggested for radiochemical facilities (9, 14). These experiments have led to the development of tests by which the suitability of different materials and decontamination reagents for use in laboratories working with Ps2,Ba1@,and 1181 may eventually be estimated. Relatively minor defections such as scratches, etc., on the surface result in rather large variations in both the susceptibility of the surface t o contamination and its subsequent ease of decontamination. Therefore, a n effort was made to select materials that were representative of the optimum conditions that might 1 Present
Calif.
address, Naval Radiological Defense Laboratory, San Frenoisco,
dried radioisotopes as contamination The term spill index defined as:
S.I. = -log
from surfaces.
activity on surfice after decontaminating spill total activity of sample spilled
(
is proposed as a measure of the situation that would be created in case of an accidental spill which is cleaned up within an hour. The presence of isotopic carrier in the decontamination reagent is demonstrated to be of little value, and the justificqtion for the use of complexing agents is shown to rest on erroneous conceptions of the decontamination mechanism. Conclusions regarding the suitability of glass, stainless steel, and lead for radiochemical laboratory surfaces are drawn, and a schedule for safe operation a t various levels of activity is presented.
reasonably be expected under normal laboratory conditions. Further experience under a variety of working conditions must be gained before a true evaluation can be made. Meanwhile, it is hoped that a body of standardized information can be built up which, in conjunction with known chemical, structural, and thermal properties, will aid in the selection of surface materials used in radiochemical laboratories. MATERIALS
NUCLIDES. The nuclides were carrier-free (No isotopic carrier was added deliberately during the production. The total solids were of the general order of 1 mg. per 25 ml.) HsPaZOain hydrochloric acid solution (pH 2 t o 3), Ba’4oC12 in hydrochloric acid solution (pH 2 to 3), and NaIlsl in sodium bisulihe solution and Ca46are widely used in biological and (pH -8). Ps2, medical work. However, Ba140 was used for these experiments instead of calcium because of the more favorable radiation characteristics. This combination also permits differences between anions and cations t o be studied. The active solutions were isotope shipments as received from the Oak Ridge National Laboratory (15). Approximately 100 to 200 microcuries (*e.) of activity were applied as a contaminant in each case. REAGENTS. Acid reagents were prepared by ,diluting the appropriate volume of the concentrated C.P. acid with distilled water. Other reagents were prepared b dissolving the appropriate quantity of solid material in diaticed water and diluting to 100 ml. Thus 10% T.S.P.indicates that there are 10 grams of sodium phosphate per 100 ml. of solution. PLAQUES. The materials for decontamination tests were prepared by cutting 2.5 X 2.5 inch plaques from a sheet of the material (thickness 0.05 t o 0.125 inch) to be tested. These squares were cut in half for the adsorption tests, giving pla ues 1.25 X 2.5 inches. The edges on all plaques were smoothe8 to minimize adsorption. METHODS FOR TESTING
ADSORPTION TESTS.The susceptibility of a surface toward contamination by a particular ion is indicated in part by the adsorption of that ion from an aqueous medium. One tenth (0.1) ml. of the active solution was pipetted onto the surface about 0.75 inch from the end of a 1.25 X 2.5 inch plaque. The entire plaque was covered by a n inverted watch glass t o minimize evaporation of the radioactive solution. It was allowed t o stand 1469
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1470
for 1 hour, after which the solution was carefully sucked off with a transfer pipet. The plaque was washed immediately by immersion in four successive 30-ml. volumes of mater in 50-ml. beakers for 5 seconds each. It was then soaked 10 minutes in a fifth 30ml. volume of water after which it was dried over calcium sulfate before monitoring. For the purpose of all of these experiments. water, per se, is not considered to be a decontamination reagent.
n
Vol. 42, No. 8
The ratio activity/sq. cin. on the surface before decontamination activity/sq. em. on the surface after decontamination
)
is called the decontaminnt,ion Iactor, D.F. Since the area covered by the radionuclide is unchanged by the cleaning proccss, the decontamination index also represents the fraction of the total radionuclide removed from the surface. The situation that would be faced as the result of an accidental spill has been estimated through the calculat~ionof a spill index,
S.I.:
S.I.
=
-log
Figuie 1. Drying Unit
on surface after spill _ decontaminating __~____ ___ (act,ivitytotal activity of sample spilled
These indexes are considered in greater detail in a subsequent section of this paper. MONITORING.The plaques were monitored with an end-Jvindow Geiger-Muller tube enclosed in a lead ehield with standard shelf positions, and a scaler equipped with a scale of 1024. The plaque was placed on a 2.5 X 3.25 inch card, and inserted into one of the shelf positions in the lead shield. A new card was used for each determination to minimize the probability of cont,aminating the inside of the lead shield. When the activity rcmaining on the plaque was too high for measurement with a Geiger-hluller tube, it was determined in an ionization chamber by comparison with a control of known activity mounted under the samc conditions. RESULTS AND DISCIJSSION
Under the conditions used, the activity per cubic centimeter of the solution applied was directly proportional to the total activity since a constant volume was used throughout. Also, the area contaminated was identical with area covered by the solution. The percentage of the radionuclide adsorbed was derived from the equation: I’ercentage adsorbed = activity adsorbedlsq. cm. of surface ~ _ - _ _ _ . - . ~ (activity/ml. solution applied) (ml./sq. cm. surface covered 100 (1)
[
DECONTAMIXATION TESTS. The square plaques were colitaminated by pipetting 0.1 ml. of t,he active solution onto the center of tmhesurface area. A funnel, 1 inch in diameter, was mounted in such a position that it could be lowered over the drop of solution to within a few millimeters of the plaque (Figure 1). A vacuum was applied through a filter to t,he stem of the funnel. The air flow through the funnel was adjusted to a velocity which prevented the s o l u t i ~ nfrom flowing near the rim of the funnel. By this means, it was possible t,o evaporat,e the solution to dryness in approximately 1 hour a t room temperature with the activity confined to the center of the plaque. The 0.1-ml. sample contained around 5 micrograms of total solids. The solutions spread over an area of less than 1 square cni. on nonwetting surfaces to as much as 2 square cm. on an easily wet surface. Therefore, the general’solid distribution was about 2 to 5 micrograms per square cm. of surface. A stepwise decontamination procedure similar to that used in laboratory cleaning was used for much of this work. Reagent decontamination wit’hout scrubbing (step 1) was used first to lower the activity level as much as possible before personnel exposure became necessary. One fourth (0.25) ml. of the decontamination reagent was applied to the affected area and allowed to stand approximately 3 minutes before being removed with a transfer pipet. When two different reagent,s were applied, approximately equal volumes of each were used, the total reagent volume remaining constant,. A4ftereach decontamination step, the plaque was rinsed with a spray of water, blotted dry, and monitored. Each subsequent attempt a t decontamination (steps 2, 3, etc.) was accomplished by applying 2 ml. of reagent, and brushing manually with a 1.5 X 1.5 inch surgical brush for 1 minute. Radioautographs have demonstrated that the activity is rather uniformly deposited over the contaminated area, and that, Iese than 1%of the activity is spread to other portions of the plaque during the brushing process (4). The data are reported in terms of a newly defined quantity called the decontamination index, D.I., which is the common logarithm of the ratio of the activities per unit area of the surface before and after decontamination. D.I. = log
(-
activity/sq. cm. on surface before decontamination activity/sq. cm. on surface aftcr decontaminat’ion
ADSORPTIOS. The adsorption test. described in the t>spcrimental section is used to investigate the susceptibility of the various surfaces to contamination-Le., the extent t o which the radioelement becomes att,ached t o the surface. The method by which the attachment occurs is immaterial. The radioelement, in t,he general case, may be held by chemisorption--o.g., ion eschange-wit,h surface valences, by physical adsorption, or mechanical means (diffusion into cracks and pores on the surface). The form in which the radioelement occurs on the surface-Le., ionic or colloidal aggregatcs-depends largely 011 the composition of the medium from miiich it is deposited. Since the radioelements exist as ions in the solutions used as the contaminating medium for this work, and since the surf:we materials were selected because they provide a relatively small specific surface area, ion exchange and chemisorption are the predominant phenomena in the part’icular studies reported here. Several test,s with different shipments of the carrier-free isotopes as received from Oak Ridge showed that the percentage adsorbed froin different solutions of the same element varied by a factor of 2 or 3. The carrier-free samples were used becausr t1ic.y
AND CLEAXING TABLEI. STANDARD D A T - 4 O N SUSCEPTIBILITY PROPERTIES O F GLASS.ST.4ISI.ESS STEEL, AND LE.4D
Material Pyrex glass h-o. 774
Adsorbed in 1 Hour
5 9 4 8 6 4
0.02 0.2 0.04
2 9 3 9 3 6
80.0 2.0 3 0
3.9 5 .Q 4.9
0.6 0.4 0.05
:2 . ?
I131
P 32 Bal&o
4 2 6.2
0.5 0.2
3.9 5,3 3.9
Bald0 1131
Lead
%
Spill Index, 8.I.
Ssotope P 32 P32 ~ a 1 4 0
11”
D.I.* 4 2
4A
4 8 4.2
4.1
Polislied stainless steel, type No.
347
Unpolished stainless steel, type No. 316 Bonded stainless steel, type N o . 302 llicrorold stainless steel, type KO.302
psn
~a140
Iiai
4.9
0.1
0.a
3.6
August 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
represent the highest dilutions of the element likely to be met and should give the highest percentage adsorption. The data are presented in column 4 of Tablc I. The other entries in Table I are explained later. Except for the fact that the adsorption of Bald0 on lead was unexpectedly high, the comparative figures for each element on Pyrex glass KO. 774, polished stainless steel, unpolished stainless steel, and lead seem reasonable. The general similarity between glass and polished stainless steel has been known for some time, and is usually attributed to the very smooth surface which they present. The difference between polished and unpolished stainless steel probably reflects the relative specific surfaces of the two materials more than differences in composition. 8 The increased adsorption for both &PazOa and I- on lead a? compared to glass or either variety of stainless steel was anticipated on the basis that the formation of insoluble salts should increase the apparent adsorption. Since the procedure used to derive the adsorption figures has been adopted in the authors' laboratory as a routine test for the susceptibility of different materials to contamination, i t seems worth while to consider its derivation in greater detail. Differentiation between porous and nonporous surfaces by this procedure is not difficult. Several preliminary experiments demonstrated that the susceptibility of a porous surface to contamination by a solution was reflected by an apparent adsorption of greater than 1%. The nonporous materials take up much less than 1%,often 0.01% or less, except when specific chemical reactions occur between the surface material and the radioelement in the solution. A 1-hour contact time seemed sufficient to classify (within an order of magnitude) the relative adsorptive power of nonporous materials because of the general character of the adsorption isot h e r m which are followed. To illustrate, a typical curve for the adsorption of &Pa20aon frosted glass (see Figure 2) has been chosen as the basis for discussion since this system will be used later in the report to show the basic principles of the subsequent decontamination cycle. (Frosted glass was used for this particular experiment so that less active Pszsolutions could be used. The frosted glass provides a better adsorptive surface than smooth glass.) In this experiment, the frosted end of a microscope slide was immersed in a solution of &Pa204, removed a t intervals, rinsed in successive changes of distilled water, and monitored. A very rapid phase of adsorption was deposited within a matter of seconds after the solution and surface were brought together; this amount was equivalent to a few tenths to a few per cent of the ultimate equilibrium amount of P32. This is a typical feature of the adsorption of polar ions on different surfaces and is attributed to electrostatic adsorption or ion exchange at the outer surface of the electric double layer at the solid-solution interface (5). Two slower phases of adsorption were found when the isotherm in Figure 2 was replotted as log (I F ) versus time, where F is the fraction of the equilibrium value. (It is sufficient to consider the radioelement to be in exchange equilibrium with the surface when the activity adsorbed maintains a constant value for 10 hours with the plague immersed in the solution. This value was reached a t about 20 hours' immersion.) These phases were represented by two straight lines with half-times of 10 minutes and 9 hours. The 10-minute step was essentially complete after the first hour, the total activity a t this time being about 25% of the equilibrium value. In the adsorption of polar ions, such steps commonly follow the rapid initial adsorption a t the surface of the double layer and are usually attributed to successive steps, one of which is probably due to diffusion of the radioelement through the double layer (5, 7 ) . The fact that the semilog plots were straight lines for each of the two phases indicates an exponential equation for the lines. This is in accord with the observation of Boyd et al., that an exponential equation is obtained when the reaction kinetics of the
-
1471
exchange occurring between a solution and an ion exchange resin are calculated for either a diffusion-controlled or a chemicalexchange mechanism ( 2 ) . An exponential equation is also obtained for the exchange between a radionuclide and its stable isotopes in the same ionic form when the stable ion is initially in exchange equilibrium in two or more compartments, and the radionuclide is introduced into one of them (6, 10). Because of the I, 5widespread occurrence of stepwise exponential adsorption (? 7 , 1 0 ) it is logical to expect that adsorption quite generally will be initially rapid with later slower stages and that each stage will be exponential in character-i.e., the form of the adsorption curve observed in this experiment will prove to be the rule rather than the exception for the adsorpt,ion conditions used in these studies.
II
0
Figure 2.
4
8
HOURS
I2
Adsorption of Hap3204 on Glass
By selecting an adsorption time of 1 hour, one is making comparisons a t a time when the adsorption is proceeding rather slowly, although only around 10 to 25% of the equilibrium distribution may have been reached. A more exact comparison a t a later time is not necessary because the important point to the laboratory worker is the ratio of the adsorbed activity to the tolerance activity permitted on the surface. For example, if the maximum activity permitted on a given surface is 100 counts per minute with a particular instrument, and the maximum activity which can be adsorbed is 106 counts per minute, about lo4counts per minute will be taken up in a matter of seconds. This will rise to about 2 or 3 X 106 counts per minute during the first hour. When one is choosing between two possible materials, it makes little difference if one must reduce 106 to 100 or 5 X 106 counts per minute to 100. The two must differ by a factor of 10 or more before one surface has a real practical advantage over the other. A difference this large can be identified by the 1-hour test. DECONTAMINATION. The results of the decontamination test are given in Table 11. The twts were run in duplicate and were repeated in several instances with different shipments of the radioelement. The over-all precision obtained was '0.3 decontamination index unit. Several observations of interest are evident in the general pattern of the results. The most consistent feature is the fact that the cleaning action of any given reagent is, for all practical purposes, completed a t the
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
1472
TABLE 11.
REMOVAL O F
AIR-DRIEDCONTAMINATION BY
STEPWISE PROCEDURE
D . I . per Step 1 Step 2 Step 3 Step 4 Step 5 From Pvrex Glass S o . 774 1% Mulsor 224 2.9 1.5 0.23 (>2.5In ... 3Satd. N "08-3 01 .. 6 0 ..2.5. 0.11 0.08 Calgon N HsPOa 33 .. 06 0 6 N "0s 3.4 0.4 0.3 0.'13 ( > 2 . 7 ) . 1% Sapamine 2.7 0.4 ... ... ... 1% Sequestrene 1.9 2.1 0.5 0.4 ... 6 N "0s 2.8 1.3 (0,l)b ... 1% Mulsor 224 1.9 1.8 0.2 i0.3)C ... 107' ammonium citrate 2.5 0.6 (0.15)d ( > 2 . V a ... 56% H I 4.1 0.9 0.3 ... 10% NaHSOa, satd. XI 3.5 1.0 0.18 (0.2): 17 Sapamine 3.0 1.1 0.11 (0.8) ... Saed. KaHSOs 3.2 0.8 0.2 (0.33)c . . . Reagent
...
Total D.Z. Initial All reagent reagents 4.6 4 6 4 .. 0 4.2 3.1 4.9 4.1 3.9
>7.0 44 .. 06 >7.0 3.1 4.9 4.2 4.2
3.1 5.3
>6.0 5.3
Vol. 42, No. 8
process. One would expect that the first action of a liquid reagent ~ o u l dbe simply to dissolve any salts of the radioelement that might be lyillg loose on the surface. Another immediate action would be the rapid adsorption of solute ions originally present in the decontamination solution, and exchange between the surfacc and solution where such exchange is possible. Part of the cleaning effect will depend on how fast these ions can reach the sur-
face, and on t,he degree t'o which they can react Tvit,h t8he surface valences in competition with the ions of the radioFrom Polished Stainless Steel element. Thus, the effect' of acids is 5.7 3NHNO3-3NH3POa 2.7 0.5 0.04 (1.5)f 0.6 3.6 primarily due to the hydrogen ion and 6 A' "01 2.3 0.4 0.3 ... 3.0 3.0 25 .. 0 3 00 .. 27 3 (0.8)Q 2.5 35 .. 97 secondarily due to the anion. 61% A' Sapamine HNOI ... .0.' .3 .0.. .3 5.7 10% ammonium citThe radioelenlent, will ultimat,ely be(O.2)h 0.18 4.0 4.4 0.6 rate, 107, K a O H 2.3 1.1 come distributed bet'vveen the solut,ion 56% H I 4.7 1.8 ... ... ... 6,5 6 5 6 >\' "08, 10% and the surface in the proport,ions de2.7 2.5 ... ... ... 5.2 5.2 NaHSOa termined by the distribut'ion coefficient, F r o m Unpolished Stainless Steel i . . . between the decontamination reagent, 6 iV "03
