An Fuinnirnh Nuicleiqh the Nuclear Energy Board a Report on The
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INIS-mf—11218 an fuinnirnh nuicleiQh The Nuclear Energy Board A Report on the Levels of Radiocaesium Activity in Mountain Sheep October - December 1987 P.A. COLGAN, B.A. (Mod.), Ph.D. February 1988 an bopd fuinrrirnh nuicleiqJi The Nuclear Energy Board A Report on the Levels of Radiocaesium Activity in Mountain Sheep October - December 1987 P.A. COLGAN, B.A. (Mod.), Ph.D. February 1988 A Report on the Levels of Radiocaesium Activity in Mountain Sheep CONTENTS Abstract 5 Introduction 6 Choice of Measurement Sites 7 Equipment 8 Survey Procedures 9 Results 10 Statistical Considerations 13 Slaughterhouse Monitoring 15 Discussion 16 Final Conclusions and Recommendations 18 Acknowledgements 20 References 21 Tables 23 Figures 29 Appendices 52 ADDENDUM Page 20: The list of Nuclear Energy Board staff should include Jack Madden. CORRIGENDUM Page 6, paragraph 3, line 5: Replace "of sheepmeat" with "by sheep grazing". ABSTRACT This report details the in vivo monitoring of mountain sheep undertaken by the Nuclear Energy Board during the period 5th October to 18th December 1987. A total of 636 farms in upland areas in eight counties were visited and 7,429 sheep were monitored. In addition, monitoring of sheep for the home and export market was undertaken at five slaughterhouses where a further 779 animals were examined. The farm monitoring identified 989 sheep with radiocaesium activities above 600 Bq/kg, of which 162 were above 1000 Bq/kg. Statistical testing was used to identify areas deemed to be "reserved" and "cleared" in each of the counties visited. It is concluded that the imposition of restrictions on the movement and slaughter of sheep may not be the most effective means of preventing animals with unacceptably high radiocaesium concentrations reaching the market. A programme of farmer education in stock management, allied to in vivo monitoring at slaughterhouses serving both the domestic and export markets, is put forward as the most effective method of ensuring the protection of the public. INTRODUCTION Following the accident on 26th April 1986 at reactor number 4 of the Chernobyl nuclear power station, an increase in the levels of airborne radioactivity was first detected in Ireland during the period 2nd-4th May 1986. Heavy rainfall resulted in significant ground deposition of this activity, and the environmental monitoring programme set up by the Nuclear Energy Board determined the distribution and level of caesium, and other radionuclides in the major food items consumed by the Irish population [1]. During the six month period which followed the Chernobyl accident the food intervention level of 1000 Bq/kg adopted by Ireland [1] in line with the recommendations of the International Commission for Radiological Protection [2] was exceeded in only one sample and so it was unnecessary to introduce control measures to restrict the sale or consumption of foodstuffs produced within Ireland. For medium and longterm exposure, specific pathways have been identified which may result in enhanced levels of radiocaesium (caesium-137 and caesium-134) in certain foodstuffs or food products. One such pathway involves the uptake of radiocaesium of sheepmeat in mountain areas. Heather and other upland plants, because of the low concentration of nutrients in the soil where they normally grow, are known to uptake and retain radiocaesium preferentially [3], and consequently sheep whose diet contains a large percentage of such plants are likely to show enhanced level of activity in their flesh. It is important to make the distinction between sheep grazing in upland areas and the radioactivity in sheepmeat available to the consumer through normal retail outlets. Mountain sheep are normally moved to pasture for between 4 and 12 weeks prior to slaughter, and when their normal diet of heather is replaced by grass the levels of radiocaesium in their flesh is quickly reduced in line with the biological half-life of 10-12 days. Controls on the movement and slaughter of sheep were first imposed by the Ministry of Agriculture, Fisheries and Food of the United Kingdom in June 1986 [4]. Following similar actions by the Department of Agriculture, Northern Ireland fifteen months later in September 1987 15], the Nuclear Energy Board initiated a programme of in vivo sheep monitoring to augment its existing laboratory analyses of sheepmeat entering the foodchain through slaughter houses and commercial outlets. Some limited monitoring had previously taken place in counties Louth and Wicklow, and the purpose of this extended programme was to provide additional data on the levels of radiocaesium in mountain sheep with a view to assessing what action, if any, would be necessary. CHOICE OF MEASUREMENT SITE When it came to selecting the locations in which the in vivo monitoring of sheep should be undertaken, three selection criteria were considered:- The first of these was heavy rainfall during the period 2-5 May 1986, at which time the Chernobyl cloud is known to have been passing over Ireland. This resulted in the deposition of radionuclides on soil and pasture and their subsequent transfer to herbage and plants. However, because Ireland is served by only two air monitoring stations (located at Glasnevin, Dublin and Valentia, Co Kerry), one cannot rule out the possibility of certain parts of the country having been affected by additional fallout at a later date. Figures 1A and 1B, which indicate the distribution of rainfall over the period May 3-5th 1986, should not therefore be taken as a mapping of radionuclide deposition attributable to Chernobyl. These maps can, however, be taken as an indicator of how pasture was effected. Secondly, the large body of data collected by the Board immediately following Chernobyl was considered. The concentration of radioactivity in agricultural products can be taken as an indicator of the degree of fallout on pasture. Where these concentrations are highest or where they persist for a longer than expected period of time there is an indication of an enhanced level of deposition. The third consideration was the presence and number of mountain sheep grazing on heather in areas previously deemed to be at the greatest risk. Ireland has a total breeding ewe population of 2,800,000 animals, with over 30% of these divided between counties Galway, Mayo and Donegal. Obviously very high levels of radiocaesium in these areas could have major implications for both the domestic and export markets. On the basis of these criteria, the following areas were selected for investigation:- 1. All upland areas of Co Donegal, 2. The Ox and Glencar mountain ranges in Co Sligo, 3. The northern half of Co Leitrim, 4. That part of Co Mayo west of a line between L. Mask and Ballina, and the area around Bonniconlon at the foot of the Ox mountains, 5. That part of Co Galway west of a line between Galway city and Tuam, and the Slieve Aughty mountain range on the border with Co Clare, 6. The Knockmealdown and Comeragh mountain ranges in Co Waterford, 7. The upland part of Co Kilkenny between Inistioge and Graiguenamanagh, 8. The north-east corner of Co Roscommon between Boyle and Arigna. These areas are shown on the map in Figure 2. EQUIPMENT All measurements were made using portable scintillation detectors. The specific models used were the John Caunt SN1A, the Ludlum 2220 and the Nuclear Enterprises PSR8 with appropriate probe attachments to which additional lead had in some cases been affixed in order to reduce the background count rate. Each piece of equipment consists of a 2" x 2" Nal crystal coupled to a photomultiplier tube, the signal from which is fed into a single channel analyser (SCA). A digital display registers pulses in any preselected part of the full energy spectrum and a range of pre-set counting times are available. One of the John Caunt detectors was initially calibrated in conjunction with the Department of Agriculture, Northern Ireland, using the same Cs-137 and Cs-134 standard sources, as well as a range of milk powders from May/June 1986 which showed activities up to 3000 Bq/kg. The threshold and window of the SCA were set so as to measure the Cs-137 gamma peak (662 KeV) and both Cs-134 peaks (605 KeV and 796 KeV). Because of the difference between laboratory and field conditions, it was not initially envisaged that this laboratory calibration should provide anything more than a course conversion factor between the count rate above background, as recorded by the detector, and activity per kilogramme of sheepmeat. A more accurate calibration was obtained by the slaughter and subsequent laboratory analysis by high resolution gamma spectrometry of animals whose activity was estimated as being in the range 500-1500 Bq/kg. For each animal slaughtered, four samples were analysed : the right and left hindquarter, and the right and left forequarter. Because radiocaesium concentrates uniformly in muscle [61 and because of the predominance of muscle tissue in the hindquarter, the hindquarter samples consistently showed activities 15%-20% higher than those in the related forequarter. In order to err on the conservative side the mean of the two hindquarter samples for each animal was used to determine the appropriate conversion factor. It was found that the two conversion factors - one calculated using radioactivity standards and the other obtained by comparing the results of in vivo measurements with the activity measured in the flesh of the slaughtered sheep - agreed within experimental error. Subsequently the theoretical approach was used as the basis for assigning a conversion factor to the other detectors as it was not possible to obtain suitable carcasses for laboratory analysis. SURVEY PROCEDURES Every county is divided into District Electoral Divisions (DEDs) which are the basic units of demography throughout Ireland. All records of the Department of Agriculture relating to sheep numbers, their geogrr ihical distribution and the names and addresses of flock owners are also compiled by DED within each county.