Headspace gas analysis of closed radioactive waste vaults in a near surface disposal facility of Hungary

As a part of a national safety assessment research programme about the Centralised Radioactive Waste Treatment and Disposal Facility of Hungary, near Puespoekszilagy village, two near surface radioactive waste container vaults were opened at 15th March 2000. This A type vaults were closed in 1972, when they had been full. Before removing the lids sampling of the headspace gas of the vaults was performed by a special gas outlet system through the bed of concrete. One of the cells (Code: A5) contains not conditioned radioactive waste. The other one (Code: A6) contains cemented waste. Using the same method qualitative gas analyses of the two headspace gases were carried out on the spot, in the site of the disposal facility. During the gas-analyses with an OMNISTAR quadrupole mass spectrometer gas samples were collected from both cells into 7 Lvolume stainless steel bulbs with a special gas-handling system. Helium, carbon and oxygen isotope ratios, respectively, '1'4C and tritium contents of gas samples were measured in the laboratory of INR of the HAS. Compositions of the headspace gases were very similar to the ones of air. No significant hydrogen production or oxygen depletion was detected. The carbon-dioxide content was sixty-times more in the gas of A5 cell and six-times more in A6 cell compared to the air. The C and O isotope-ratios show that this surplus is the result of decomposition of organicmaterials. The specific radiocarbon activity of carbon dioxide was about 3 Bq/cm'3 in both cells. The tritium activity-content of headspace gas was two orders of magnitude more in A5 cell (8.8 Bq/L) than in A6 cell (0.04 Bq/L). These results are in good agreement with the total tritium contents of contained waste in the individual cells. The higher '3He content in the vaults coming from tritium decay show that gas-migration from the cells must not be considerable.(author)