Clearance of buildings and land after decommissioning of nuclear facilities at the Riso site

摘要

The nuclear installations at Riso National Laboratory have all been closed for decommissioning. The DR 1 reactor was closed in 2001 and the decommissioning began in 2004. It was planned to use the reactor building for other purposes and therefore clearance measurements for both the reactor building and the surrounding land were needed to document an unconditional clearance of the reactor site. The clearance condition for buildings were the nuclide- and surface-specific clearance levels recommended by the European Commission and derived from a dose criterion of 0.01 mSv/a, and exposure scenarios in which the buildings are used for other purposes. Clearance levels should be applied to the total activity on and below each single square meter of the building area. International recommended clearance levels for land areas do not exist. Therefore, such levels had to be derived. This was done using similar considerations as for the buildings, i.e. a dose criterion of 0.01 mSv/a, and exposure scenarios for future use of the area. As the future use of the land area was unknown, clearance levels were derived from combined external and internal exposure pathways assuming that milk, meat, vegetables and grain products were produced and consumed within the area. Clearance levels for ~(60)Co, ~(90)Sr and ~(137)Cs were determined to be 3,000 Bq(centre dot)m~(-2), 11,000 Bq(centre dot)m~(-2), and 7,000 Bq(centre dot)m~(-2), respectively. The measurement strategy and the measurement results for the DR 1 reactor building and the surrounding land area are presented. Both gross-beta and gamma-spectrometric measurements were used. It is shown how the measured values and the associated uncertainties were used to demonstrate compliance with the clearance levels.