-- HW 36734 O 0 - 8 EXPERIMENTAL BIOLOGY UNCLASSIFIED AND MEDICINE ERR

BIOLOGY SECTION '7 1.7 :l CSY RADIOLOGICAL SCIENCES DEPARTMENT 0

THE ABSORPTION OF FISSIONPRODUCTS BY PLANTS RERBSITORY

8 0 e COLLECTION hd

0 May 17, 1955 F*LDE*

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HANFORDATOMK PRODUCTSOPERATION 0 RICHLAND. WASHINGTON

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@ Q 6.13 UNCLASSIFIED HW -36734 Experimental Biology and Medicine

--THE AESORPTION OF FISSION PRODUCTS BY PLANTS

BY

J. H. Rediske, J. F. Cline, and A. A. Selders

Biology Section Radiological Sciences Department

May 17, 1955

HANFORD ATOMIC PRODUCTS OPERATION F?CBLAND , I.?AS HLNG TOK

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INTERNA L DISTFUB UTION Copy Number 1 H. M. Parker - M. L. Mickelson 2 W. D. Norwood - P. A. Fuqua 3 D. W. Pearce - 2. E. Carey 4 W. A. McAdams - A. J. Stevens 5 S. M. Smith 6 S. W. Healy 7 L. K. Bustad - R. F. Foster 8 R. C. Thompson - R. W. Wager - D. E. Warner 9 F. P. Hungate 10 J. H. Rediske - J. F. Cline - A. A. Selders 11 H. A. Kornberg 12 300 File 13 Yellow File 14 - 20 Extra Copies

EXTERNAL DISTiiZBUTION Distriblltion according to M-3679 (16th ed. )

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ABSTRACT

The absorption of the important fission products by plants is quantitatively presented as a concentration factor which is defined as the ratio of the fission product concentration found in the leaves to the fission product concentration found in the nutrient substrate. Of the fission products, the isotopes of strontium were found to be the most important by virtue of their high concentration factor, long half-life, and low maxi- mum permissible amounts for animals. Iodine and barium follow in importance, with cesium moderately important in some soils. All other fission prodmts have concentration factors less than strontium by 100 or more. The effect on the concentration factor of different agricultural plants, as well as different organs of the same plant, causes variations of about a factor of ten or less for each isotope. The concentration factor tends to increase as the pH of the nutrient substrate is decreased. Addition - of stable carrier to the substrate does not decrease the amount os the radioactive isotope that is absorbed into the plant. The presence of iodine and yttrium carrier actually causes a significant increase in the concentra- tion of the respective radioactive isotopes in the plant tissue. Qb ;9

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THE ABSORPTION OF PRODUCTS BY PLANTS FISSION e

INTRODUCTION 00 *e ne am ., 0 0 Fission products appear to be the most important group of radioactive . materials potentially available to contaminate food crops. There are two important methods by which food crops may be contaminated with fission products. First, fall-out may occur directly on crops, resulting in an immediate contamination problem from the fission products iodged on the plant surfdces. Second, the fall-out products may contaminate agricultural soils and be absorbed from these3 soils into plants. Unlike the method of direct contamination, where fission products are only on the one year's crop, fission products absorbed from the soil will be present in each successive year's crop. Furthermore, successive fall-outs may cause a fission product build-up in the soil. Problems inherent in measuring a plant's potential to remove fission products from a contaminated soil are many because of the extreme variance of the situations encountered. The soil itself is a variable medium for plant growth. Many physical and chemical factors enter into 1 .\ the ability of a soil to supply fission products to a plant. Such environ- , mental factors as light, temperature, humidity and rainfall can influence the remvsl of fission products by plants from a soil. The species of plant is likewise important.

In this paper the more important factors affecting fission product absorption studied at this laboratory will be discussed, as well as material from other sites which contributes to this proglem. In order to compare the uptake of differeiit elements from? the root environment, the incorpora- tion of a fission product by a plant will be expressed as a concentration

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factor. This is defined as the ratio of the concentration of fission product in the leaves to concentration of fission product in the root environment.

FISSION PRODUCTS OF POTENTIAL SIGNIFICANCE TO AGRICULTURE

0 84 Most of the isotopes produced by fission occur in small yield or have very short half-lives. In order to determine which of theh may prove 1 to be agriculturally significant, Table I was compiled from the curves of ,3 Hunter and Ballou. (l) Only those fission products which contribute more ! than one per cent of the total activity at any one time are included. The inert gases are excluded, since they would not be expected to have any .BA consequences agriculturally. Furthermore, the data of this table is for

a. slow neutron fission and thus may not apply with the same precision in all &4 cases of fission product production. ,I The utilization of this table for agricultural purposes will be facilitated if thc isotopes are grouped SCCoidiZg to similar chemical f “I! characteristics. Thus the rare earths, with yttrium, may be considered as the most important fission product group. They are strongly represented ,4 in all time groups. The group of strontium-barium isotopes is second in 44 4 importance in contr ibuting radioactivity to the fission product mixture. n The remaining elements in order of their importance are zirconium- niobium, which appear most prominently in the period of 90 days to one year, and ruthenium and daughters, which play a role up to three or four Q years. Iodine is important only in the first few weeks, and 33-year cesium, 0 with its daughter metastableaarium, is important in long-term considera- tions of more than five years.

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i TABLE I

Significant Fission Products 5

Contribution to Total Fission Product Activity

at Indicated Times after Fission Per Cent 1 Isotope 10 Days 30 Days 90 Days 1 Year 3 Years 10 Years Half-Life ~r89 2. 8 6. 7 10.5 2. 7 - - 53 d. ~r90 - - - 1. 8 8. 7 22.0 19.9 y. . Y90 - - - 1. 8 8. 7 22. 0 61 h. Y 91 3.5* ZJ 7. 6* 12.5* 3.9* - - 61 d. ~r95 3. 7 8.2 14. 7 7. 3 - - 65 d. 4% - 4,1* 18. O* 15* - - 35 d. 6. 8 @ - - - 67. 0 h. M?03 2.6 5. 7 7. 2 - - - 39.8 d. RU103m 2.5 5.5 7. 0 - - - 57 m. Rh106 - - - 2.4 3.0 - 1.0 y. Ru106 3 - - 2.4 3. 0 - 30 s. Rh132 - 5.1 - - - - - 77. 7 h. 331 6. 8 3. 7 - - - - 8.14 d. I132 5. 3 - - - - - 2.4 h. I 137 - - - 1. 5 6. 8 18. 0 33 y. CS137m 1. 5 6. 8 18. 0 2.6 *'140 - - - rn. 10. 6 10.8 1. 6 - - - 12.8 d. Ba140 - - - 40 h. La141 12. 0 12.5 1. 8 6. 3 11.2 a. 5 - - - 33.1 d. e144 - 2. 0 6. 0 26. 5 21. 0 - 282 d. Ce144 - 2.0 6. 0 26. 5 21. 0 - 17. 5 m. ' "143 2. 6 - - - 13. 7 d. "147 10.0 11.2 ' 4. 8 4.1 - - - - 11. 3 d. ' Nd 147 - d - 5.7 19.0 16. 0 2.6 y. I Sm'951 - - - - - 1. 1 2. 5 73 y. *Contribution from both metastable and ground state. The data in the above table are for slow neutrons and were inter- preted from a paper, 'IFission Product Decay Rates," by H. F. Hunter and N. E. Ballou in Nucleonics 9 (5): C2-C7, 1951. The half-life values are taken from the "Table of Isotopes, 'I Reviews of Modern Physics 25, 469-651, 1953. These are at variance in some cases with the older values used to calculate the data by the above authors. 8 All fission products contributing more than one per cent of the total activity at the above times are listed, with the exception of the inert gases, which are considered to have little agricultural significance.

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The grouping of the fission products in the above manner is based on the following fundamental assumptions: 1, that plants will make no distinction between isotopes of the same element as far as absorption is concerned; 2, that plants will tend to absorb elements of similar chemical characteristics in a similar manner, as with the rare earths; and 3, that the activity of short-lived daughters should be included with the parent regardless of the chemical identity of the daughter isotope. This follows 0 because any short-lived daughter isotopes found in plant tissue will have resulted from absorbed parent isotope. This will cause an apparent doubling of the activity of the parent isotope at equilibriun? 8 CONCENTRATION F -4 C TORS 8

To make a corqarative study of the uptake of fission products, a simple quantitative test was needed to evaluate the amount of absorption of representative isotopes of each fission product group. The Neubauer seedling test, in out experience, satisfies this requirement. The test has been previously described, (2) as well as the modifications necessary for fission product absorption studies. (3) Essentially, it is the growth of 100 . barley seedlings in 100 grams of a soil contaminated with about 0.1 pc/g of fission products. The seedling growth exhausts the available nutrient supply such that the fission product concentration in the leaves rises to a maxixnum in a period of about 18-20 days. Q %he dose or concentration of the fission product added to the soil0 has little effect on the concentration factor at levels below those which .a might cause radiation damage to the absorptive mechanism. The threshold value to produce such damage is known to be greater riian that delivered by 0.1 pc Sr ''Ig of soil,(3) probably many times this value. 8

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Table I1 is a list of concentration factors for some common soils and representative isotopes. It may be seen from this table that strontium has the greatest concentration factor. Iodine and bar'um& are also important, with cesium of possible secondary importance. All other elements studied have a concentration factor less than strontium by a factor of one hundwd or more, and thus are of little importance in a natural mixture of fission . products.

FACTORS AFFECTING FISSION PROD'IJCT ABSORPTION

Four factors have been investigated which will tend to modify the 9 concentration factor for a particular soil. These are plant species, plant organ, carrier concentration and pH of the nutrient environment. All of these four factors were studied by means of nutrient solution cultures, 0 the methods of which have been previously outlined.(3) It is not suggested that these data be used in a direct comparison with those obtained from soils, since nutrient solution cultures represent a highly controlled system. Instead, the data should be used only to modify concentration factors obtained 8 from soils. The nutrient solution concentration factors are calculated by taking into account the adsorbed material on the roots as a source of nutrient ions. A discussion of this method has been previously presented.(3,5) Plant Species 8 From preliminary@ examination, conducted at this laboratory as well as elsewhere, (6D7, it has been found that in nearly all cases the effect on the concentration factor contributed by species differences amounts to abo& on& order of magnitude or less.

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Table 111 indicates no simificant correlation In uptake between

44 isotopes and various species of plants. With strontiums the difference *k between all plants tested was only a Pactor of four, with the difference between the cultivated plants being negligible. Plant Organs- 0 The leaves have a concentration €actor far the elements studied -3 as great or greater than any ather organ. Thus in Table III the concentratto-E factor for those fruits and seeds investigated is always less than that for -a the leaves, but usually by not more than a factor of ten. Likewise the roots have concentration factors about the same or slightly less than in' the leaved?) All underground organs, however, may have considerable radioactive material adhering to the outer surface, which is usually readily removable,

Because of the higher concentration found in leaves, they are used . for a general expression of the concentration factor. This usually all-ows a safety factor when other organs are concerned. Furthermore, the leaves are mast convenient andnearly always avziilable if the plant is .in.a growing condition. All the concentration factors in this report are based on leaf tissue unless otherwke identit'-:&, 8) Element C onc entra tion @ The effect of the elernen? concentration m &sorption of fission products is indicated in Figure 1. In general the concentration factor for a fission product increases as the concentration of the stable isotopes B -1 of that element is increased% the nutrient substrate. This is apparent Q for all the isotopes of Figure 1, with the exception of Ba14'. Barium exhibits an initial increase in concentration factor and finally decrezses as the carrier concentration continues to be increased. The concentration factor for all these isotopes may ultimately decrease as the carrier con- centration reaches excessively high values. The point at which this decrease 7

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TABLE III

The Nutrielt Solution Concentration Factors for Several Fission Products and Plutonium - onc entra -3n E .ctor 90 v91 131 106 Plant ;r -CSl3? -Ru -c Pu239 Sean Leaves 3 .7 .0005 3 .0002 001 ,0003 Pod and Seeds 02 .2 .OQ004 04 Tomato Leaves ,4 .3 .002 ,2 .0002 ,0001 .oooz Fruit ,009 .2 .0009 ,02 Russian Thistle Leaves and Stem .1 .os .00006 .5 .0002 ,0002 .00002 Fruit ,008 .000003 .4 Grasses Barley Leaves .5 ,0001 .0000' .0002 Wheat Leaves .3 .02 .00003 Head . 01 .000005 - - I

The pH of the nutrient environment was 6. 0, with 1. 0 pg of carrier/ml in all cases except pU239, when a concentration of 2.2 x pc of the isotope was used per ml of solution.

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FIGURE 1 The cancex-+=.atianfactors of several fission products for bean plants grown in nutrient solutioa culture. The culture solutions weie maintained at pH 6. 0 with various concentratims of the stable isotopes.

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occurs would be expected'to vary with the different elements. The rate ' of initial increase is not uniform and is dependent on the element, being particularly pronounced for yttrium and iodine but less so for cesium and stroiltiurn. Although teresdts reported are those from plants grown on nutrient solution substrate, essentially the same results were obtained for the uptake of iodine (unpubliskd data) and cesium(6) from soil. pK It is apparent from Figure 2 that as the pH becomes more acid the concentration factor tends to increase in all cases. This is a common reaction and is discussed in previous papers. (3a8) his increase is very marked for some fission products, as Ygl, with very little variation for others, as Cs13? and Sr90 .

DLSCUSSION

The question as often presented(') is, "What plants can absorb whlt fission products from what soil types and store them in what edible portions in such chemical forms and amounts that the fissim products can be transferred to man or domestic animals and with what effects?" . 9 This question differentiates between external contamination from 3 fall-out and those fission products absorbed internally. The effect of ' :g 'I fall-out on a nearly mature crop should not be minimized. However, the questions of utilization of this crop can be made at the time of contamination with the knowledge that the frission products associated with the crop will 0 be in the same relative concentx@ions as in the fall-out. Furthermore, it may be possible to utilize crops contaminated too heavily for human consumption as animal feeds in such a manner that the resulting meat

products will be acceptable. ' The over -all effects of an externally con - tarninatL.4 crop can be determined as primarily that of a single. exposure,

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FIGURE 2 The concentration factors of several fission products and plutonium for bean plants grown ir~nutrient solution culture. The culture solutions ware main- tained at 1. 0 pg/d of the stable isutope and in a pH range of 4 to 7.

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Q with some residual contamination cf the soil. Conversely the over-all effect of a crop contaminated by absorption from the soil will be a series of exposures as each successive crop is harvested and consumed. Obviously each yearly exposure will be much smaller than the initial fall-out because 0 of both decay and the selective abs8ption by plants. As has bee3 shown, those fission products which will be absorbed with any consequence by plants are isotopes of strontium, barium and iodine. Cesium apparently can be moderately important with certain soil types. However, for practical considerations, taking into account the half-life: percentage composition of a natural mixture of fission products and the potential effect on animals, the isotopes of strontium remab as the most important fission products. The absorption of the strontium isotopes from se?era1 representa- tive agricultural soils has been indicated in Table U. The capacity of any unknown soil to supply strontium to plants can be evaluated by a Neubauer seedling test. This test is normally conducted with a cultivated grass, as barley or rye. The difference between this test plant and other species of agricultural plants has been found to be small :or strontium. A factor of two or t :e is the general variation as determined at this Q laboratory (Table XU) and at otner laboratories. (6J7, It must be borne in mind, however, that data approach a maximum figure for 2Dsc1-p- IQ these tion. Conditions in the field will rarely cause such maximum uptake. Adverse factors of climate and vertical distribution of thMission products wifl tend to lower the concentration factor. No ready means is now available to assess these variables.

0 The concentration factor itself is somewhat idealized. The soil con- 0 centration used m calculating this factor is the average concentration of 0 the isotope only in the root zone. Obviously fission products on top of the 0 soil or below thr root zone will not enter into absofition by the plant. Like-

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The application of the stable element to depress uptake is apparently not as effective as has been supposed in the past. By reducing the specific activity of a fission product in the soil, within practical limits, the uptake is not decreased but may be increased. Some depression of uptake might

be achieved in acid soils by reducing- the acidity. Reduced absorption of all* of the elemen\$ was observed when the substrate was made more alkaline.

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BIBLIOGRAPEFY

1. Hunter, €1. F. , and N. E. Ballou, "Fission product decay rates," Nucleonics -9 (5), C2-C7 (1951). 2. Vandecaveye, S. C., "Biological methods of determining nutrients in e soils,t1in Diagnostic Techniques for Soils and Crops, The American Potash Institute, Washington, D. C. (1948). 3. Rediske, J. H., and A. A. Selders, :'The absorption and translocation of strontium by plants, Plant Physiol. -28, 594 (1953). 4. Nishita, H., B. W. Kawalewsky, and K. H. Larson, "Fixation and extra tabilit of fission products contaminating various soils and clays. I. Sr§o, Y9 I, Ru106, Cs137, and Ce144,tt Document UCLA-282 (1954) (UNCLASSIFIED). 5. Rediske, J. H., and A. A. Selders, '!The uptake and translocation of yttrium by higher plants, It Document HW -29091 (1953) (UNCLASSIFIED). 6. Neel, J. W., J. H. Olafson, A. J. Steen, B. E. Gillooly, H. Nishita, and K. H. Larson, ItSoil-plant interrelationships with respect to the up a e of fission products. 1. The uptake of SrgO, Cs137, Ru106, Cet144J and Y91, It Document UCLA-247 (1953) (UNCLASSIFIED). 7. Romney, E. C. , W. A. Rhoads, and K. Larson, "Plant uptake of SrgO, Rulo6, Cs137, and Ce144 from three different types of soils,t' Document UCLA -294 (1954) (TJNCLASSIFIED).

8. Rediske, J. H., and A. A. Selders, '!The uptake and translocation of yttrium by higher plants," Am. J. Bot. 41, 238 (1954). 9. Heslep, J. M., and A. W. Bellamy, ttTherecycling of fission products in the biotic cycle, I' J. Chem. Educ. -30, 421 (1953).

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