54. Determination of ⁹⁹Tc Deposited on the Ground within the 3Q-km Zone around the Chernobyl Reactor and Estimation of ⁹⁹Tc Released into Atmosphere by the Accident

S. Uchida, K. Tagami, W. Ruhm¹ and E. Wirth² (¹Institute for Radiation Biology Ludwig Maximilians Universitat Munchen, Germany; ²Federal Office for Radiation Protection, Institute of Radiation Hygiene, Germany)

Keywords: ⁹⁹Tc, Chernobyl accident, forest soil, deposition, ¹³⁷Cs, migration

There have been many reports concerntng radionuclides' concentrations in environmental samples due to the accident in Unit 4 of the Chernobyl Nuclear Power Station (CNPS). At present, however there are almost no data on ⁹⁹Tc contamination of environmental samples and consequently, limited information on the ⁹⁹Tc activity released by the Chernobyl accident is available. In this study, results of ⁹⁹Tc and ¹³⁷Cs measurements in samples taken from three forest sites within the 30-km zone around the CNPS are presented.

Soil samples collected within three forest sites around Chernobyl in 1994 and 1995 were used. The sites DI and D3 are located 28.5 km and 26.0 km to the south of the CNPS, respectively, while K2 is Nocated 6.0 km to the southeast of the CNPS. The ¹³⁷Cs activities in the samples were measured with a Ge detector system before chemical separations. Then, the ⁹⁹Tc concentrations in each solution were defermined by ICP-MS after the chemical separatuon.

We plotted our data for ¹³⁷Cs in addition to the data given by Aarkrog et al. (J. Environ. Radioact 6, 151-162, 1988) versus distance from the CNPS. A least squares fit of these data gives a regresston function of DCl_cs=5.l4X10⁴XR^-1.477 where D_cs is the ¹³⁷Cs deposition on the ground (GBq km^-2) and R, the distance from the CNPS (km) (see Fig. 19) Assuming the same horizontal distribution of the ¹³⁷Cs deposition in all directions from the CNPS, we calculate the ¹³⁷Cs release as

₁¹⁰⁰⁰⁰2R x 5.14 x 10^-2 x R^-1.477 dR76 PBq.

A similar procedure was applied in order to estu mate the ⁹⁹Tc release due to the accident, using our ⁹⁹Tc depositions for distances of 6 km (K2), 26 km (D3), and 28.5 km (DI). For a distance of 1000 km, we assumed the ⁹⁹Tc deposition from the ⁹⁹Tc/¹³⁷Cs ratio of 1.01 X 10^-5 given by Aarkrog et al. The resuiting regression function is D_Tc= 4.88 X R^-1.771 where D_Tc is the ⁹⁹Tc deposition on the ground (GBq km^-2) and R, the distance from the CNPS (km) (see Fig. 19). Again, the release may be calculated by integration over R as

₁¹⁰⁰⁰⁰2R x 4.88 x R^-1.771 dR970 GBq.

The total core inventory of ¹³⁷Cs is estimated to range from 210-290 PBq. A ¹³⁷Cs release of 76 PBq, as estimated in this paper, corresponds therefore to a fraction of 26% to 36% of the total ¹³⁷Cs core inventory. From the inventory of ¹³⁷Cs and from the theoretical ratio of ⁹⁹Tc to ¹³⁷Cs of 1.4X10^-4 the total inventory of ⁹⁹Tc can be calculated to range from 30 to 40 TBq. Consequently, the percentage of ⁹⁹Tc released into the atmosphere by the accident could be estimated to range from 2% to 3%. This result is close to the estimate given by Aarkrog et at. who suggested the release percentage of ⁹⁹Tc would be an order of magnitude less than that of ¹³⁷Cs. However, our result is higher by a factor of 3 to 4 compared to the estimate of the release percentage of ¹⁰³Ru and ¹⁰⁶Ru given in the report of Gudiksen et at., which is believed to be similar to the release percentage of ⁹⁹Tc. Since data on ⁹⁹Tc concentrations in environmental samples are quite limited, further investigations are required for a precise estimate of the amount of ⁹⁹Tc released by the accident.

Fig.19. 137Cs and 99Tc deposition as function of distance from Chernobyl. 137Cs deposition data are from this study and from Aarkrog et al. 99Tc deposition data are from this study and from Aarkrog et al. whitch was estimated for a distance of 1000 km.

Publicatton:
Uchida, S., Tagami, K., Ruhm, W. and Wirth, E. Chemosphere, 39, 2757-2766, 1999.