All radioactive waste in the Netherlands is collected by COVRA (acronym for Central Organisation for Radioactive Waste) that operates a facility for treatment of waste including interim storage buildings for HLW, LILW and TENORM (Calcinate and U3O8). Like many other waste management organisations, COVRA developed and adopted different waste storage strategies for different types of waste. The basis of all strategies is Isolation, Control and Surveillance (ICS), a principle aimed at minimization of dose to operators and the public alike. The stacking of waste in the storage buildings directly follows from this principle. To minimize radiation exposure of employees, waste is stacked in blocks. In these block the packages with low dose rates are placed on the outside and are shielding packages with higher dose rates and neutron sources inside. The packages with the lowest dose rate are stacked against the outer walls to minimize radiation into the environment. In 2004, a novel Non-Destructive Assay (NDA) method was used to validate the applied waste storage strategies in terms of spatial dose rate distribution. With this method measurements were performed in one of the interim storage modules for LILW. The dose rate at a height of 6m, mainly responsible for the sky-shine at the site boundary, was somewhat higher then expected. Based on the experience and feedback, the NDA method was developed further, into the present INDSS-R (acronym for INDoor Survey System – Radiation) method. This new method was put to the test in 2006 with a second series of spatial radiological measurements. The main aim was now to verify the reproducibility of the method. In 2008, a third series of measurements was carried out and the following data was collected: • dose rate (using a pressurized ionisation chamber) • nuclide depended gamma photon flux (by means of a 3*3″ NaI detector ) • thermal neutron flux measurements (by means of LiI(Eu) detector) The results of the third serial of spatial radiological and thermal neutron flux measurements will be presented and compared to the 2004 and 2006 data on the operational dose for operators and for the public.
Thermal neutron flux measurements in the STAR experimental hall
