The detection of radioactive material from a venting underground nuclear explosion

J. Environ. Radioactivity 11 (1990) 1-14

The Detection of Radioactive Material from a Venting Underground Nuclear Explosion Bj6rn Bjurman, Lars-Erik De Geer,* Ingemar Vintersved National Defence Research Establishment, S-102 54 Stockholm, Sweden

Anne Liv Rudjord, Finn Ugletveit National Institute of Radiation Hygiene, Box 55, N-1345 Osterhs, Norway

Hannele Aaltonen, Kari Sinkko, Aino Rantavaara Finnish Centre for Radiation and Nuclear Safety, PO Box 268, SF-00101 Helsinki, Finland

Sven Poul Nielsen, Asker Aarkrog Riso National Laboratory, Box 49, DK-4000 Roskilde, Denmark

&

Walter Kolb Physikalisch-Technische Bundesanstalt, PF 3345, D-3300 Braunschweig, FRG (Received 14 March 1989; revised version received and accepted 10 October 1989)

A BSTRA CT In northern Europe, there are many stations in operation for surveillance of airborne particulate radionuclides at low concentration levels. In August 1987, after a leakage from a Soviet underground nuclear weapons test at Novaya Zemlya, at least five countries could detect the event at their national stations. Observed radionuclide concentrations are reported and isotope ratios and meteorological air parcel trajectories are used to characterise the source.

*To whom all correspondence should be addressed. 1

J. Environ. Radioactivity 0265-931X/90/$03.50 (~) 1990 Elsevier Science Publishers Ltd. England. Printed in Great Britain

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INTRODUCTION At the beginning of August 1987, unusually high concentrations of radioactive iodine (131I and 133I) were detected in northern Europe together with small amounts of radionuclides such as 99Mo, 13'Te and 14°Ba. The radionuclides detected are typical short-lived fission products and therefore indicated a leakage from some kind of nuclear reaction device, such as a reactor or a nuclear explosion. Five to ten days before the radioactive cloud reached the sampling stations, the Soviet Union had performed an underground nuclear weapons test at Novaya Zemlya (2 August 1987 at 02:00 UTC, 73.289 N and 54-713 E, about 15 kt; Bergqvist, 1987, pers. comm.) and a leakage from this test was immediately seen as the most probable explanation for the radionuclides detected. The Soviet Union also confirmed that a leakage had occurred soon after the test (IHT, 1987). The aim of this report is to present a summary of measurements from Sweden, Norway, Finland, Denmark and the Federal Republic of Germany, together with some conclusions that can be drawn from the results. Figure 1 shows the locations of all the air sampling stations employed in this report. The stations are run by different national organisations and descriptions of the air sampling stations can be found elsewhere (Aarkrog et al., 1980; Koib, 1982; Vintersved & De Geer, 1982; Aaltonen et al., 1987). The air sampling stations are equipped for sampling of particlebound radionuclides on glass fibre or polystyrene filters. The station at Nurmij~irvi is also equipped with a carbon filter for total iodine measurements. The reported air concentration values are well above the individual detection limits, even though these may differ considerably between the stations. Table 1 summarises the results of measurements on airborne particulate short-lived fission products. The results for 131I are also graphically presented for continuously running air sampling stations in Fig. 2. The total iodine measurements at Nurmij~irvi showed that the gaseous or desorbable fraction of 131Iwas 90-95%, implying that the total concentrations of 131I and 133I were probably 10-20 times higher than those found in Table 1. The 131I air concentrations are far below the values measured after the Chernobyl accident but apart from this event such high values have only been reported once in northern Europe (Kauranen et al., 1967) since the signing of the Partial Test Ban Treaty in 1963. Small amounts of 131I can occasionally occur in the vicinity of hospitals but in such cases the air concentration is only a few/.LBq/m 3 and is only locally detectable. The 131I is then never accompanied by ]3"-Te andt33I as these nuclides are not used in hospitals. From time to time, 131I at very low levels is detected in large

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areas of northern Europe. The sources of the radioactive iodine detected during these events have, in most cases, not been traced.

R A D I O N U C L I D E ANALYSIS By examining radionuclide concentration ratios it is often possible to draw several conclusions about the source of the activity and time of formation. This technique has been used frequently for air concentration measurements after atmospheric nuclear weapons tests and was used at an early stage to determine the source of the August cloud (Aaltonen et al., 1987; Bjurman et al., 1987). As the air concentration of ~34Cs did not increase during the passage of the cloud and virtually no t34Cs is produced during a nuclear explosion, the ratio between t34Cs and L~-Te is a strong verification that the source of the radionuclides was a nuclear weapons test and not a leaking nuclear reactor, in which significant amounts of ~34Cs are present (e.g. US Nuclear Regulatory Commission, 1975). To date the explosion, the ratio between 133I and 131I can be used. The 14°Ba to 14°La ratio has a potential for dating in the early stage. For this event, however, the debris reached the Scandinavian countries one or two days too late for this ratio to be useful. An iodine ratio calculation based on the first measurements from Ivaio and Kiruna gives a time of detonation which is 11 to 16 hours later than the actual one. This fact is most probably due to fractionation during the venting. We can see from the mass 132 and 133 data that tellurium is suppressed at least by a factor of 25 compared with iodine. We can thus disregard tellurium and other even less volatile mass 131. 133 precursors from the venting analysis and then estimate the release time from the measured 133I to ~3tI ratio, the known age of the sample and the calculated iodine ratio as a function of time. Such an analysis indicates a release centred around 80 + 15 minutes after the explosion, if fission neutron induced fission of 239pu is assumed (independent fission yields taken from Rider, 1980). The result is not especially sensitive to fission mode. Fission neutron fission of 235U indicates a release at around 90 _+ 15 minutes and high energy neutron fission of 23SU indicates a venting at around 50 _+5 minutes. Although we do not know the exact mixture of fission modes, we can, however, conclude that the main venting probably occurred around one hour after the detonation. From the presence of ~4°Ba in the radioactive cloud, it is evident that some venting also occurred within the first few minutes after the explosion. Barium is refractory both in elementary and oxidised forms, so the detec-

10

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tion of 14°Ba in the cloud indicates that one or both of the za°Xe and ~4°Cs precursors, with the short half-lives of 13-6 and 63.7 seconds, respectively, must have been released to the atmosphere.

ESTIMATED RELEASES From the measurements in Skibotn, Ivalo, Rovaniemi and Kiruna, an estimate of the 131I release can be made by assuming a cloud moving at a speed of 10 m/s, with a size of 1000 km horizontally and 2 km vertically. In this cloud, we can estimate a mean integrated gaseous and particulate 131I concentration of 10 000 Bqs/m 3, which yields a release of about 200 TBq. Compared with the amount produced in a 15 kt nuclear explosion, 84 000 TBq (Rider, 1980), this fraction must be regarded as very small (0.002 + 0.001).

AIR PARCEL TRAJECTORIES Various calculations of receptor oriented trajectories to the observation stations and source oriented trajectories from the Novaya Zemlya test site and the nuclear reactors on the Kola peninsula have been made by the meteorological authorities in Sweden, Norway and Finland. To obtain an indication of where to look for the source from the receptor oriented trajectories, it is important to narrow down, as much as possible, the arrival time of the leading edge of the cloud. Judging from the Ivalo and Kiruna data and using the observed wind speed of approximately 10 m/s below 1 km, we can estimate the arrival times at these stations to be 13:00 and 23:00 UTC, respectively, on 6 August (both + 10 hours). The trajectory analysis based on this gives a source region covering the Kola peninsula, the Barents and Kara Sea coastlines in the Soviet Union and the Kara Sea east and northeast of Novaya Zemlya (see Fig. 3). Although Novaya Zemlya is on the outskirts of the source region defined by the receptor oriented trajectories and although the source oriented trajectories starting at the test site show a main air mass transport in a northwest direction towards Spitzbergen, the analysis cannot be regarded as excluding the nuclear explosion as the source. Trajectory calculations in this area can be rather unreliable as the analysed wind fields are based on very few observations. In the early days of August, a high pressure ridge was situated over the Barents Sea and Novaya Zemlya and the fate of leaking fission products from the nuclear test would be very sensitive to the exact position of this ridge at the time.

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DOSE ESTIMATE The passing of a cloud containing the integrated 131I concentration of 10 kBqs/m 3 will result in the extremely small effective dose equivalent of around 20 nSv due to inhalation. On the other hand, 131I was, as expected, also detected in cows' milk in the north of Norway and Finland (Fig. 4). The concentrations in milk of 9 and 16 Bq/1 at two different farms on 11 August in Vads0 and a maximum concentration of 3.4 Bq/l in Ivalo on 14 August show that low iodine concentrations in air can cause relatively high concentrations in milk. An estimate of the 131I concentration in cows' milk from the particulate iodine concentrations in air, a dry deposition velocity of 0.5 cm/s (Erlandsson & Isaksson, 1988), an assumed 0.5 kg of grazed grass per m 2 and a milk to grass concentration ratio of 0.06 Bq 1-~/Bq kg -~ (Hfikansson et al., 1987) yields peak values at all sites of less than 0.5 Bq/1. This is up to a factor of 30 lower than the concentrations observed. The effective dose equivalent due to the local consumption of 1 litre of milk per day can be estimated to 7/zSv for a 1-year-old child (Greenhalgh et al., 1985). In areas where grass is not particularly lush, the grazing area

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required per litre of milk will be greater, thus leading to higher radionuclide concentrations in the milk. A n o t h e r reason for the relatively high concentration measured in milk in northern Norway might be that the iodine had a high deposition velocity due to mist in the area during the passage of the cloud.

CONCLUSIONS The short-lived fission products detected in northern Europe in August 1987 were most likely due to a leakage from an underground nuclear weapons test performed by the Soviet Union on 2 August 1987. The fraction of radioactive iodine released into the atmosphere and disseminated over northern Europe was very small (about 0.2%), but nevertheless easily detected. The results illustrate that the absorbed dose due to the consumption of milk might be several times higher than that which might be expected from available data and measured air concentrations. As a final conclusion, it can also be seen from the present results that no significant amount of gamma-emitting radionuclides could be released into the atmosphere in northern Europe without being detected and the source could most likely be traced. This study illustrates the high sensitivity which a global network of air sampling stations would have as a means of monitoring a Comprehensive Nuclear Test Ban Treaty.

ACKNOWLEDGEMENTS We thank C. Persson at the Swedish Meteorological and Hydrological Institute and J. Saltbones and K. H. Midtb~ at the Norwegian Meteorological Institute for providing us with air parcel trajectories.

REFERENCES Aaltonen, H., Sinkko, K., Rantavaara, A., Savolainen, A. L. & Hatakka, J. (1987). Short-lived fission products in Finland in August 1987. Report STUK-BV A L 0 5 0 . Finnish Centre for Radiation and Nuclear Safety, Helsinki, Finland. Aarkrog, A., B~tter-Jensen, L., Dahlgaard, H., Hansen, H., Lippert, J., Nielsen, S. P. & Nilsson, K. (1980). Environmental radioactivity in Denmark in 1979. Report RisO R-421. Ris~ National Laboratory, Roskilde, Denmark. Bjurman, B., Vintersved, I., De Geer, L.-E., Arntsing, R., Jakobsson, S. & Roos, P. (1987). The detection in Sweden of short-lived fission products probably vented from the underground nuclear test at Novaya Zemlya on 2

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August 1987. Report FOA Report C 20673-9.2. Swedish Defence Research Establishment, Stockholm, Sweden. Erlandsson, B. & Isaksson, M. (1988). Relation between the air activity and the deposition of Chernobyl debris. Environmental International, 14, 165-75. Greenhaigh, J. R., Fell, T. P. & Adams, N. (1985). ReportNRPB-R162. National Radiological Protection Board, Harwell, UK. Hfikansson, E., Drudge, N., Vesanen, R., Aplsten, M. & Mattsson. S. (1987). Transfer of 134Cs, '~'Cs andl3tI from grass to cow's milk. A field study after the Chernobyi accident. Report GU-RADFYS 87:01. University of G6teborg, G6teborg, Sweden. IHT (1987). Moscow reports leak during nuclear test but denies any fallout. International Herald Tribune, August 17. Kauranen, P., Kulmala, A. & Mattsson, R. (1967). Fission products of unusual composition in Finland. Nature, 216, 238-41. Kolb, W. A. (1982). In Proc. of the Third Int. Symp. Soc. for Rad. Prot. Society for Radiological Protection, pp. 132-7. Rider, B. F. (1980). Compilation of fission product yields. Report NEDO-121543B ENDF292. Vallecitos Nuclear Center, Vallecitos, USA. US Nuclear Regulatory Commission (1975). Reactor safety study: an assessment of accident risks in US commercial nuclear power plants. In Report WASH1400 (NUREG 75/014). Washington, DC, appendix VI. Vintersved, I. & De Geer, L.-E. (1982). The Swedish air monitoring network for particulate radioactivity. IEEE Transactions Nuclear Science, NS-29. 827-31.