The nuclear power plant Wu¨rgassen in Germany entered into decommissioning in 1995. The project has currently advanced to the clearance of the buildings. Clearance of buildings has commenced in 2007 with establishing the clearance procedure in pilot rooms. It has entered the “productive” phase in 2010 and will last until about 2014. In 2007, Brenk Systemplanung has been commissioned by E.ON / NPP Wu¨rgassen to perform clearance of the pilot rooms in the turbine building. It has been one of the goals of this work to establish an innovative clearance procedure which would not rely on 100% surface measurements but which should be based on the use of in situ gamma spectrometry and the application of approaches to minimise the number of clearance measurements. In 2010 and 2011 Brenk Systemplanung has been commissioned by E.ON / NPP Wu¨rgassen to perform clearance of rooms with partly considerably higher contamination levels in other buildings. Central points of the concept for clearance of buildings which is presented in this paper were the establishment of averaging areas of up to 10 m2, which is an ideal size for the use of in situ gamma spectrometry, and the application of statistical methods, which allow an efficient performance of the clearance measurements. As a first step the measurement instrumentation necessary for the performance of clearance measurements had to be qualified. The innovative concept was first tested and implemented in the pilot rooms and has now been used also in various other parts of the NPP Wu¨rgassen. The procedure which has been established by Brenk Systemplanung for NPP Wu¨rgassen allows rapid measurements making best use of the averaging area and measurement area of collimated in situ gamma spectrometry and minimising the number of single measurements in comparison to other clearance procedures for nuclear installations of similar size. The paper highlights the relevant approaches and the conceptual differences and innovations.

Clearance of buildings of nuclear installations in Germany has to comply with general requirements laid down in Section 29 of the German Radiation Protection Ordinance (RPO; Strahlenschutzverordnung), clearance levels as stipulated in Table 1 of Annex III RPO and technical guidance given in the technical standard DIN 25457. Clearance procedures are usually proposed by the licensee and are made part of a decommissioning licence after appropriate review by the authorities. The clearance procedures consist of a radiological characterisation of the various parts of the buildings, a decontamination step if necessary, followed by measurements for checking the success of decontamination, and the actual decision measurements for demonstrating compliance with clearance levels. These three steps rely on various measurement methods, selected depending on the relevant radionuclides to be measured, like sampling with subsequent gamma spectrometry (and if necessary separate evaluation of beta and alpha emitting nuclides), measurements with surface contamination monitory and in situ gamma spectrometry. For a nuclear power plant, the number of samples taken during this procedure can easily reach a few 10,000, while the number of single measurements can reach several 100,000 single surface measurements and several 10,000 measurements with in situ gamma spectrometry. This large number of data together with the correct interpretation according to the valid nuclide vector, the penetration depth of the contamination, the correction for radioactive decay etc have to be managed, which is a error-prone process if carried out manually or with insufficient support by customised software. For this reason, a versatile software tool has been developed by Brenk Systemplanung GmbH that supports the operator in all aspects of clearance of buildings, based on the extensive experience with various clearance procedures in nuclear power plants that Brenk Systemplanung has carried out in the past. Many steps of the data evaluation and report generation during the clearance process outlined above can now be carried out automatically. This includes the time-consuming steps of: • support during the acquisition of the geometry data for a room, • generation of the lists for sampling and measurements, • automatic acquisition of the measurement and sampling results from the instruments, • evaluation of compliance with clearance levels, both for measurements on the entire surface and for statistically distributed measurements or samples, • generation of the documentation on clearance, • storage of all data, generation of nuclide vectors, correction for radioactive decay, connection with photo, CAD drawings etc.