ELSEVIER

J. Environ. Radioactivity, Vol. 33 No. 2, 117-128, 1996 pp.

Copyright 0 1996 Else&r Science Limited

Printed in Ireland. All rights reserved

0265-931X(95)00093-3 0265-931X/96 $15.00 + 0.00

The Skim and Burial Plough: A New Implement for Reclamation of Radioactively Contaminated Land

J. Roed, K. G. Anderson & H. Prip

Riw National Laboratory, DK-4000, Roskilde, Denmark

(Received 8 January 1995; accepted 9 November 1995)

ABSTRACT

Radionuclides accidentally released to the atmosphere in severe nuclear acci- dents can give rise to widespread radioactive contamination of land. If no remedial action is taken, the contamination may constitute a long-term external radiation hazard. Similarly, agricultural and dairy produce from the contaminated land may be a source of internal radiation. A newly developed agricultural implement named the Skim and burial’plough has been built and tested and shown to be an effective countermeasure against both of these potential hazards but without the disadvantages of some other land reclama- tion methods. The new plough skims off a shallow layer of the contaminated top soil and buries it at a depth of about 45 cm without inverting the 5-45 cm horizon. The results are that radiation levels at the soil surface are greatly reduced, the contamination becomes much less available for plant uptake and in most cases there is little or no effect on soil quality. The efficacy of the new plough has been demonstrated under particularly severe field conditions. Copyright 0 1996 Elsevier Science Limited

INTRODUCTION

The Chernobyl accident in April 1986 tragically demonstrated how severe nuclear accidents can lead to heavy contamination of large tracts of land. In such situations, the prime concern is that of the possible adverse health effects (from external and internal radiation) on people living in the contaminated areas. Depending on the levels of radiation, it may be necessary to transfer the local populace to clean areas while decontami- nation is carried out. Decontamination of both urban and rural areas may be called for.

117

118 J. Roed et al.

In the immediate aftermath of the Chernobyl accident, the local autho- rities introduced measures to limit the consumption of contaminated agricultural and dairy products. Such measures inevitably have econom- ical penalties in the form of lost production, loss of revenue and loss of productive farmland. In order to minimize the effect of such penalties, various countermeasures were used to clean up the urban areas and return agricultural land to productive use in the shortest possible time. Many of the countermeasures employed were of a hit-or-miss nature since no contingency plans had been formulated in advance.

In agricultural areas, the main countermeasures used were as follows:

(1) harvesting and disposal of surface-contaminated crops (leaving the land relatively clean)

(2) spraying plastic or rubber-based fixatives on soil and dust to suppress resuspension

(3) application of lime and inorganic fertilizers to soils to reduce root uptake of certain radioelements

(4) removal of surface soil and therefore most of the contamination from the land

(5) normal mouldboard ploughing (to 25-45-cm depth) to suppress resuspension, dilute the contamination and disperse it from the surface of the ground.

Some of these countermeasures were ineffective and others had adverse side effects. For instance, there was no means of predicting in advance the optimum amounts of lime and fertilizers to be added to achieve a known reduction in soil-to-plant transfer of the relevant radioelements (Desmet, 1991). The process was therefore not as cost effective as it might have been.

The application of fixatives in the contaminated land areas has been limited to suppression of resuspension. Fixatives could, however, also be applied for decontamination operations in small land areas (Andersson & Roed, 1994a).

Scraping off just a lo-cm layer of surface soil from an area of 1 km2 generated about 50,000 metric tonnes of radioactive waste for disposal and often removed the fertile soil layers. In the affected areas it was diffi- cult to scrape off thin layers of soil homogeneously due to variations in terrain. Melin et al. (1991) reported great difficulties in scraping off layers thinner than 20cm with a bulldozer. A new technique for removal of surface soil with a turf harvester has been suggested. The advantage of this procedure is that only a few centimetres of soil would need to be removed (IAEA, 1994). However, the method should not be applied for stony soils, and as it requires a mature turf mat, it cannot be considered as a generally applicable method (Lehto, 1994).

The skim and burial plough 119

Roed (1995) achieved a dose reduction factor of about 15 by ploughing to a depth of 30cm in a 86Rb contaminated field (gamma photon energy 1.078 MeV) with an ordinary mouldboard plough.

An advantage of ploughing procedures as large-scale dose-reducing measures is that they can be carried out by local agricultural workers. The local groundwater conditions should, however, be considered prior to any ploughing procedure to ensure that the contamination is not brought too close to the groundwater level. In the Chernobyl case, a fine mist of water has often been applied immediately before ploughing, in order to reduce resuspension.

In terms of cost and availability, normal ploughing is an attractive countermeasure, but it is of limited effectiveness since the normal mould- board plough operates down to a depth of only about 25 cm and much of the contamination remains accessible to plant roots. Further ploughing may not improve the situation since much of the buried contamination would then be returned to the surface.

By deep-ploughing to 45 cm instead, contaminants such as radiocaesium are placed beyond the root-zone of many crops and will remain in a thin layer deep in the soil profile without further intervention, due to the strong soil fixation (Andersson & Roed, 1994b).

Experiments by Menzel et al. (1968) demonstrated an effectiveness of more than 90% in reducing radiostrontium uptake by various subse- quently grown crops by deep-ploughing. The effectiveness was substan- tially increased by the addition of sodium carbonate to the soil. Since sodium carbonate is a root inhibitor rather than a chemical which affects strontium availability, the results should also apply to other radioisotopes.

The reduction of the external radiation in the area will also be larger than by conventional ploughing. Also, the radioactive matter will have been placed sufficiently deep in the soil profile that is not redistributed by subsequent ploughing. The main disadvantage of deep-ploughing is that poorer quality soil is brought to the surface.

In order to overcome the problems associated with both normal ploughing and deep-ploughing, a new type of plough has been developed jointly by Rise National Laboratory and Bovlund Agricultural Engineers, Denmark. This new plough skims off the topmost layer of soil (about 5 cm) and buries it at a depth of some 4&50cm without inverting the intermediate layer; hence the name ‘skim and burial plough’. The removal of only about a 5-cm layer of topsoil rarely affects the fertility of the land, and poorer quality subsoil is not brought to the surface.

Overall, the skim and burial plough greatly reduces radiation levels at the ground surface, the resuspension hazard is eliminated, most of the contam- ination is made inaccessible to plant roots and soil quality is unaffected.

120 J. Roed et al.

DESIGN AND CONSTRUCTION OF THE SKIM AND BURIAL PLOUGH

Examination of various types of common ploughs and attachments showed that nothing was already available which could be easily modified to achieve the desired objective. A machine which lifts a 50-cm layer of soil to a height of about 60cm while simultaneously placing the topmost 5-cm layer in the bottom of the trench was considered but this was rejec- ted on account of its excessive power requirements, making it unsuitable for use with normal-sized tractors. It was therefore decided to design and build an entirely new type of plough, and this has been done.

A skim coulter first places the upper 5-cm layer of soil in a trench made by the main ploughshare. In one movement, the main ploughshare then digs a new trench and places the lifted subsoil on top of the thin layer of topsoil in the bottom of the trench of the previous run. The skim coulter simultaneously places the top layer from the next furrow in the new trench. In this way, the 5-50-cm soil layer is lifted only about 10-l 5 cm and the power requirements are minimized. However, the possibility exists that the 5-50-cm layer will be partially inverted and that not all of the contaminated top layer will be placed in the bottom of the trench.

Initially, a l/ 10 scale model of the skim and burial plough was constructed and tested. After various adjustments and modifications, a full-sized version was constructed. As a result of further attempts to improve performance subsequent to testing, various modifications were made to the original design.

Based on the much modified full-sized model, a prototype full-scale plough was constructed and tested on a soil consisting of a 30-cm layer of light soil overlaying a sandy soil. This field was specially selected for a rigorous test of the performance of the plough on a light soil. The tests showed that it was necessary to build a higher and broader shield in order to prevent the sandy soil from falling into the furrow before the top layer had been placed in the bottom of the furrow. Further testing and modifi- cations resulted in a plough considered to be worthy of extended and intensive field trials. The plough is shown in Fig. 1.

FIELD TRIALS

The plough was tested on two different types of land: a permanent pasture that had not been tilled for 40 years, and a tilled soft soil. Both soils would be characterized as sandy loam. The plough, which weighed about 880 kg, was drawn by a normal farm tractor. The power requirement was some

The skim and burial plough

Fig. 1. The skim and burial plough showing the positions of the main ploughshare (1) and the skim coulter (2).

90 kW. The plough produced a furrow 60-cm wide and 50-cm deep in both types of land. The skim coulter, which was adjustable, was set to skim off the topmost 5-cm soil horizon.

In order to determine the new positions of the original soil layers after ploughing, small metal cylinders (height 2 cm, diameter 1.5 cm) were placed in six vertical columns to a depth of 56 cm and spaced on a row at 20-cm intervals.

Each column of cylinders was painted in a different colour and each cylinder in a column was marked with a number indicating its original vertical position under the soil surface. After ploughing, the cylinders were retrieved using a metal detector. The new positions of the cylinders were then determined by excavation and compared with the original positions.

RESULTS OF FIELD TRIALS

The results are shown in Fig. 2(a)-(f) for pasture land and in Figs 3(a)-(f) for tilled land. In the figures, the lines connect the initial positions of the cylinders with the positions subsequent to skim and burial ploughing. Some cylinders were never retrieved. The reason for this was that the skidding tractor wheels threw some of the surface layer cylinders to rather

122 White series. Pasture.

(b)

cc>

20

10

0

-10

-20

-30

-40

-50

Yellow series. Pasture.

J

0 10 20 30 40 50 60 70 80 SO 100

Red series. Pasture.

0 10 20 30 40 50 60 70 80 SO 100 110 120

Fig. 2. (a)-(c).

Green series. Pasture. 20

10

0

-10

-20

-30

-40

-50

.\ 0 10 20 30 40 50 60 70 80 90 100 110 120

Blue series. Pasture.

(e) 0 10 20 30 40 50 60 70 80 SO 100 110 120

Light blue series. Pasture.

123

0 0 10 20 30 40 50 60 70 80 SO 100

Fig. 2a-f. Displacement of vertical columns of small metal cylinders by skim and burial ploughing on pasture land. The different colours refer to the individual columns of the experiment. The horizontal and vertical displacement from the initial positions are shown

on the horizontal and vertical axes in units of centimetres.

124

w O 10 20 30 40 50 60 70 80 90 100 110 120

20

10

0

-10

-20

-30

-40

-50

@I

Grey series. Tilled land.

0 10 20 30 40 50 60 70 80 90 100 110 120

Yellow series. Tilled land.

J

0 10 20 30 40 50 60 70 80 90 100 110 120 Cc)

Fig. 3. (a)-(c).

20

10

0 1 -10

-20

-30

-40

-50

(d)

Red series. Tilled land. 125

0 10 20 30 40 50 60 70 80 so 100

Blue series. Tilled land.

so 100

Light blue series. Tilled land.

(f) 0 10 20 30 40 50 60 70 80 90 100

Fig. 3a-f. Displacement of vertical columns of small metal cylinders by skim and burial ploughing on tilled land. The different colours refer to the individual columns of the experiment. The horizontal and vertical displacement from the initial positions are shown

on the horizontal and vertical axes in units of centimetres.

126 J. Roed et al.

large distances. The wet weather conditions and the thick, slippery grass added to the severity of the test.

DISCUSSION AND CONCLUSIONS

Since the field trials were carried out with non-radioactive materials, the reduction in radiation levels could not be measured. However, from some work reported earlier it is clear that the reduction in dose rate at the ground surface using the plough on contaminated land will be consider- able. For instance, Roed (1995) reduced the external dose from a *6Rb contamination by a factor of 20 by deep-ploughing to 45cm. The maxi- mum 86Rb concentration after ploughing was found at a depth of about 30cm. The soil profiles are in good agreement with the findings of Crites et al. (1980), who deep-ploughed a sandy soil which had been contami- nated with 241Am.

The reduction in dose rate achievable with the new plough can be calculated analytically using empirical expressions for the so-called build- up factor (relationship between the ‘broad-beam’ dose rate and the ‘narrow-beam’ dose rate) as applied by Hedemann Jensen (1979). However, as many different parameters should be taken into account (e.g. the atomic number of the shielding materials, the thickness of the shield- ing materials and the photon energy), Monte Carlo analysis appeared to be a better approach.

If dose rate factors for infinite, homogeneous isotropic plane sources at different depths in the ground, as found by Jacob and Paretzke (1986) by Monte Carlo analysis, are used with the source energy 662 keV, and it is assumed that the soils are similar, the external dose rate will be reduced by 94% (a factor of almost 17) by skim and burial ploughing, as opposed to 89% (a factor of 9) by ordinary ploughing, where most of the radioactive matter will be placed at a depth of about 15 cm.

The dose-reducing effect of deep-ploughing is similar to that of skim and burial ploughing. However, as described above, the skim and burial ploughing has several important advantages. For instance, it does not bring poorer quality soil to the surface. The cost of a skim and burial plough is at present about 4000 ECU, which is approximately twice the cost of an ordinary plough in Western Europe. However, the annual discount will be dominated by the much more expensive tractor that will be needed for both methods. With the skim and burial plough an area of about 3000cm2 can be treated per hour. The total costs per unit of area treated will therefore be low.

Since the field trials with the skim and burial plough were carried out

The skim and burial plough 127

under particularly severe conditions where the root systems were well developed and the ground was very wet, it is believed that the plough will be even more effective on some of the dry light soils with shallow-rooted plants which are typical of much of the Chernobyl-contaminated land in the Ukraine and Byelorussia. The next field trials will be conducted within the 30-km zone around the Chernobyl nuclear power plant.

SUMMARY

A completely new type of plough called the skim and burial plough has been designed, built and tested, and the results have shown considerable potentials for use in the reclamation of land contaminated by Chernobyl fallout.

The new plough eliminates the resuspension hazard, greatly reduces radiation levels at the soil surface, greatly reduces the availability of the contamination for root uptake and seldom affects the quality of the soil. No radioactive waste is generated. The plough can be equally effective for disposing of non-radioactive materials such as asbestos. Since the plough is drawn by ordinary tractors, the only limitation on its use is the number of ploughs available in an emergency.

REFERENCES

Andersson, K. G. & Roed, J. (1994a). Removal of radioactive fallout from surface of soil and grassed surfaces using peelable coatings. J. Environ. Radioactivity, 22, 197-204.

Andersson, K. G. & Roed, J. (1994b). The behaviour of Chernobyl i3’Cs, 134Cs and lo6 Ru in undisturbed soil: implications for external radiation. J. Environ. Radioactivity, 22, 183-196.

Crites, T. R., Denham, D. H. & Barnes, M. G. (1980). The effect of ploughing on 24’Am contamination in sandy soil. Health Phys., 38, 699-703.

Desmet, G. (Ed.) (1991). Improvement of practical countermeasures: the agri- cultural environment. Post-Chernobyl action, CEC report EUR 12554 EN.

Hedemann Jensen, P. (1979). Dlempningsfaktorer for gammastraling fra depo- neret radioaktivitet opnaet ved plsjning af jord og asfaltpalagning af veje. Report by Gruppen for Anlagshelsefysik, Riss Natl. Lab., Sept. 1979.

IAEA (1994). Guidelines for agricultural countermeasures following an acci- dental release of radionuclides. IAEA Technical Report Series No. 363.

Jacob, P. and Paretzke, H. G. (1986). Gamma-ray exposure from contaminated soil. Nucl. Sci. Engng., 93, 248-261.

Lehto, J. (Ed.) (1994). Cleanup of large radioactive-contaminated areas and disposal of generated waste. Nordic Council of Ministers, TemaNord 1994, 567.

128 J. Roed et al.

Melin, J., Backe, S., Erixon, O., Johnsson, B. & Roed, J. (1991). Sanering efter reaktorolyckan i Tjernobyl (in Swedish), SSI-rapport 91-03, Statens Strlls- kyddsinstitut, Stockholm, Sweden.

Menzel, R. G., Eck, H. V., James, P. E. & Wilkins, D. E. (1968). Reduction of strontium-85 uptake in field crops by deep-ploughing and sodium carbonate applications. Agronomy J., 60,499-502.

Roed, J. (1995). Dose reduction by ploughing down gamma-active isotopes. To be published.