Das Projekt "Use of the variance-covariance method in radiation protection" wird vom Umweltbundesamt gefördert und von Universität Würzburg, Biozentrum, Institut und Lehrstuhl für Humangenetik durchgeführt. General Information: Reductions of the dose limits in radiation protection and changes of the quality factor for densely ionizing radiations will require increased precision in area monitoring and in personal dosimetry. Tissue equivalent proportional counters are increasingly employed for this purpose. However, their routine use in radiation protection requires the variance-covariance method which is an extension of the pulse height determination in two ways: it is not restricted to radiation fields of extremely low dose rate and, unlike the variance methods, it is applicable in the time varying radiation fields that are frequently encountered in radiation protection practice. Achievements: Measurements in the field of a diagnostic X-ray tube have been performed with a 2 pulse generator. The beam was filtered with 1 mm of aluminium. The twin detector consists of 2 cylindrical proportional counters. The plastic detector walls have a thickness equivalent to 13 mm of tissue. The detector currents are integrated on capacitors. The voltage at the capacitors is digitized and the results stored on a computer. Calibration of the proportional counters was performed with an americium-241 alpha source. As a side product to this the Townsend coefficients for methane based tissue equivalent gas have been determined for a broad range of reduced field strengths. In further measurements variations of the dose averaged lineal energy, y d, during the 10 ms time interval of the high voltage pulse of the X-ray tube have been determined. In a theoretical analysis the inherent possibilities of the variance covariance method for suppression of noise and electric pickup have been examined. Several types of disturbances have been considered. Preliminary measurements have been performed in the photon and electron fields of a 20 MV linear accelerator. To cope with the high dose rate 2 improvements of the instrumentation are necessary. Increased sampling frequency will reduce the dose per sampling interval and smaller detectors will reduce the current delivered. In the experimental system, the signal processing has been improved by the implementation of faster and more accurate analogue to digital conversions (ADC) and the mechanical device for pressure stabilization in the gas flow system has been replaced by an electronic pressure control. In addition the gas flow is now adjusted by a mass flow control. Work is being done to transform a system designed for variance covariance measurements in pulsed therapeutic X-ray and electron fields into one appropriate for measurements in continuous cobalt-60 beam, regarded as an intermediate step before measuring in diagnostic X-rays. The proportional counters were found to be unsuitable for continuous beam measurements so it has been decided to apply new detectors. Ionization chambers and charge integration have been chosen to achieve a more satisfactory calibration. 2 Keithley 617 programmable electrometers together with a ...
Das Projekt "Effectiveness and long-term behaviour of cleanable high efficiency aerosol filters" wird vom Umweltbundesamt gefördert und von Technischer Überwachungsverein Bayern Sachsen durchgeführt. Objective: Because of the high quantity of dust generated by various cutting/dismantling processes, frequent replacement of high-efficiency sub-micron particulate air filters is necessary. If such filters could be cleaned during service, costs for the replacement of the filters, radiation exposures and the amount of secondary waste could be reduced. The effectiveness in long-term operation (approx. one year) of high-efficiency sub micron particle air filters will be investigated in the framework of the dismantling of the Niederaichbach nuclear power station (KKN) in Germany. A high-efficiency sub micron particle air filter system will be exposed to heavy dust generation during the remote-controlled dismantling of KKN primary circuit pressure tubes, and therefore must be dedusted periodically. The dust is radioactively charged (essentially cobalt-60 and iron-55). The radioactivity could amount to approximately 1E5 Bq/g (pressure tubes and moderator tank) and the dose rate to 0.1 Sv/h. There is at present no experience on the effectiveness and the long-term behaviour of high-efficiency sub micron particle air filters that are dedustable during operation. General Information: WORK PROGRAMME: 1. Installation of the filters; 2. Determination of the main parameters of the clean filter station; 3. Continuous measurements (pressure pickups, air humidity and temperatures) during cutting of KKN primary cooling circuit (activated cooling channel tubes inside the reactor vessel); 4. Final evaluation including radiation exposure of workers, secondary waste arisings, specific costs, effectiveness and long-term behaviour of the filter system. Achievements: Because of the high quantity of dust generated by various cutting/dismantling processes, frequent replacement of high efficiency sub micron particulate air filters is necessary. If such filters could be cleaned during service, costs for the replacement of the filters, radiation exposures and the amount of secondary waste could be reduced. The effectiveness in long term operation of high efficiency sub micron particle air filters is investigated in the framework of the dismantling of the Niederaichbach nuclear power station (KKN) in Germany. ...