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

Atmospheric 85Kr and 133Xe have continuously been monitored since 1995 (weekly averages) at the Meteorological Research Institute, Tsukuba. Elevated 85Kr concentrations observed in spring and fall of 1995–1997 are the result of two factors, the operation of the PNC Tokai nuclear plant and secondly the seasonal change of prevailing wind directions. Elevated 85Kr levels arising from releases of the Tokai plant are interpreted in a semi-quantitative fashion using the daily 85Kr release data and available meteorological data. During periods without the influence of the Tokai plant, 85Kr background concentrations (1.3 Bq/m3 of air) are similar to those for latitudes between 35 and 40°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 European observations. The features of the seasonal variations are minimum values during summer and maximum values during winter. 133Xe concentrations are not influenced by major local sources. One observation of a high 133Xe concentration in June 1996 can be interpreted as originating from a Chinese underground nuclear test conducted earlier in the same month.