IN DECOMMISSIONING NUCLEAR INSTALLATIONS, VARIOUS TYPES OF WASTE MATERIALS WHICH ARE EITHER FREE OF ACTIVITY OR ACTIVATED/CONTAMINATED HAVE TO BE RELEASED. UNRESTRICTED USE OF THESE MATERIALS MAY BE PERMITTED IF THE RESIDUAL ACTIVITY CONCENTRATIONS ARE BELOW LIMITS SET BY THE LICENSING AUTHORITY WITH REGARD TO THE RADIOLOGICAL RISK. IN ORDER TO PROVE COMPLIANCE WITH THESE LIMITS RESIDUAL ACTIVITY CONCENTRATIONS HAVE TO BE MEASURED ON EVERY SINGLE PIECE OF MATERIAL, WHICH CAN BE VERY COMPLICATED AND TIME-CONSUMING. THE DERIVATION OF DEPENDABLE RESULTS IS DIFFICULT BECAUSE OF THE NON-IDEAL CONDITIONS USUALLY PREVAILING AND THE HIGH DEGREE OF PRECISION REQUIRED.
THE AIM OF THIS RESEARCH PROGRAMME IS TO ASSESS ELIGIBLE MEASURING TECHNIQUES AND TO OPTIMISE THEM WITH RESPECT TO ACCURACY, TIME AND COST.
In order to determine and quantify the radiological relevance of certain waste nuclides in release measurements, relative radiotoxicities of all waste nuclides were defined and calculated according to the limits set by the German Radiation Protection Ordinance (Strahlenschutzverordnung, StrSchV). It was found that the hard to detect nuclides such as iron-55, nickel-59 and nickel-63 are of minor radiological relevance and therefore their explicit measurement is not necessary even if their abundance in certain waste categories is predominant. Standard beta or gamma measurements are highly sufficient if alpha contamination can be excluded. For determination of the suitability for release measurements, 20 different detectors were tested and their detection efficiencies, limits of detection and minimum measuring times determined using standards of carbon-14, prometheum-147, cobalt, caesium, strontium-90 and yttrium-90, chromium-51 and americium-241 in 5 geometries representative of contaminated waste material. It was found that in most geometries proportional counters are fully adequate for release measurements, while for the detection of alpha radiation small detectors such as surface barrier detectors, photodiodes or Geiger-Mueller counters are needed in difficult geometries.
B.1. GENERAL BASIC STUDIES TO DETERMINE THE SOURCE-DEPENDENT FREQUENCY DISTRIBUTION FOR THE NUCLIDE CONTENT OF RADIOACTIVE MATERIAL.
B.2. COMPILATION OF RADIOLOGICALLY AND METROLOGICALLY RELEVANT PARAMETERS.
B.3. ASSESSMENT OF PARAMETER IMPORTANCE BY MEASUREMENTS ON REPRESENTATIVE GEOMETRIES USING VARIOUS DETECTORS.
B.4. PROCUREMENT/PRODUCTION OF REPRESENTATIVE SAMPLES, OF VOLUME-RELATED AND AREA-RELATED ACTIVITY STANDARDS AND OF SUITABLE DETECTORS.
B.5. EXPERIMENTAL DETERMINATION OF DETECTOR EFFICIENCIES AND DETECTION LIMITS FOR VARIOUS RELEVANT GEOMETRIES AND NUCLIDES.
B.6. EVALUATION OF RESULTS SUPPORTED BY COMPUTATION IF NECESSARY, IN ORDER TO SET UP A GUIDE FOR SELECTION OF THE OPTIMUM MEASURING TECHNIQUE ACCOUNTING FOR MATERIAL, MEASUREMENT TIME AND COST.