Research reactors as sources of atmospheric radioxenon

Abstract

Radioxenon emissions of the TRIGA Mark II research reactor in Vienna were investigated with respect to a possible impact on the verification of the Comprehensive Nuclear Test-Ban-Treaty. Using the Swedish Automatic Unit for Noble Gas Acquisition (SAUNA II), five radioxenon isotopes 125Xe, 131mXe, 133mXe, 133Xe and 135Xe were detected, of which 125Xe is solely produced by neutron capture in stable atmospheric 124Xe and hence acts as an indicator for neutron activation processes. The other nuclides are produced in both fission and neutron capture reactions. The detected activity concentrations ranged from 0.0010 to 190 Bq/m³. The source of the radioxenon is not yet fully clarified, but it could be micro-cracks in the fuel cladding, fission of 235U contaminations on the outside of the fuel elements or neutron activation of atmospheric Xe. Neutron deficient 125Xe with its highly complex decay scheme was seen for the first time in a SAUNA system. In many experiments the activity ratios of the radioxenon nuclides carry the signature of nuclear explosions, if 131mXe is omitted. Only if 131mXe is included into the calculations of the isotopic activity ratios, the majority of the measurements revealed a “civil” signature (typical for a NPP). A significant contribution of the TRIGA Vienna to the global or European radioxenon inventory can be excluded. Due to the very low activities, the emissions are far below any concern for human health.