Kr-85 and Xe-133 monitoring at MRI, Tsukuba and its importance

Atmospheric Kr-85 and Xe-133 have continuously been monitored since 1995 (w eekly averages) at the Meteorological Research Institute, Tsukuba. Elevated Kr-85 concentrations observed in spring and fall of 1995-1997 are the resu lt of two factors, the operation of the PNC Tokai nuclear plant and secondl y the seasonal change of prevailing wind directions. Elevated Kr-85 levels arising from releases of the Tokai plant are interpreted in a semi-quantita tive fashion using the daily Kr-85 release data and available meteorologica l data. During periods without the influence of the Tokai plant, Kr-85 back ground concentrations (1.3 Bq/m(3) of air) are similar to those for latitud es between 35 and 40 degrees N. Background concentrations show a continuous increase with time and pronounced seasonal variations. The yearly increase rate of about 2.5% observed between 1996 and 1998 is very similar to Europ ean observations. The features of the seasonal variations are minimum value s during summer and maximum values during winter. Xe-133 concentrations are not influenced by major local sources. One observation of a high Xe-133 co ncentration in June 1996 can be interpreted as originating from a Chinese u nderground nuclear test conducted earlier in the same month. (C) 2000 Elsev ier Science Ltd. All rights reserved.