For the adsorbents used in radioiodine monitoring in nuclear reactor accidents, high retention efficiencies for radioiodine species and low retention efficiencies for noble gases are required under various conditions such as relative humidity, sampling time and bed depth. To find the suitable adsorbents for air monitoring, retention efficiencies for methyl iodide and xenon were examined for Ag-zeolite, Ag-silicagel, Ag-alumina and TEDA-charcoal. In Ag-alumina and Ag-silicagel-A, the retention efficiencies for methyl iodide were more than 97%, and the retention ratios, defined as the ratio of the retention efficiency for methyl iodide to that for xenon, were more than 105 under the conditions of face velocity 19cm/sec, relative humidity ≤20-90%, loading time 5-60min and bed depth 2cm. For other two adsorbents, with lower retention ratios, the retention ratio could be raised by means of post-test ventilation of the adsorbent by clean air.
The migration and distribution of radionuclides (60Co, 85Sr and 137Cs) in an aerated sand layer are examined with a sand layer model apparatus. The adsorption ratio of each radionuclide in the sand layer decreases exponentially in the upper part, and more slowly with increase in the depth of the sand layer. Distribution of 137Cs in the vertical cross section is such that its migration is uniform within the whole section, while that of 60Co is such that the migration is larger in the center than in the perimeter. Distribution of 85Sr shows intermediate character between that of 137Cs and 60Co. Those distributions observed are compared with calculated ones on the basis of the dispersion equation, and it is found that the agreement is relatively good only for 85Sr. For 85Sr, the migration in the aerated sand layer can be practically estimated with the dispersion equation.
A method has been studied of estimating the minimum detectable dose equivalent of fast neutrons for measurement of recoil proton tracks in NTA film. Poisson distribution was applied in estimation of the minimum detectable number of tracks. Factors such as sensitivity of the film for fast neutrons, energy dependency of the film response, and latent image fading of the tracks were taken into consideration in the estimation. The minimum detectable dose equivalent was affected by fluctuation in the number of tracks due to background radiation and degree of the affection depended on the measured area. The minimum detectable dose equivalent decreased by increasing the measuring area of background radiation even if the measuring area of neutron-irradiated film was small. The relation between the minimum detectable dose equivalent and the measuring area of neutron-irradiated film was obtained considering the effect of latent image fading.
The DBM (Discrimination Bias Modulation) method is one of electronic energy weighing techniques for carrying out the Spectrum-Dose conversion automatically. In the DBM method, the energy weighting is performed by passing the energetic pulses through the comparator (discriminator) of which the bias is modulated by a function wave. In an original circuit, it was difficult to reduce the energy weighting error to less than ±5%, since the generated waveforms were not so accurate. Here, by using a Programmable Read Only Memory (PROM) for storing and for generating the waveforms, its error has been reduced to one tenth of the original one, and accurate operation of better than ±0.5% has been attained in the range from 100keV to 3MeV. In addition to the above, the reproducibility and the stability have been much improved, and the adjusting and testing processes of the function waveforms have been extremely simplified.
Collection performance of a HEPA filter and a new type filter which was recently developed for removing fine particles below 0.3μm was verified by using a laser aerosol particle counter in the particle size range from 0.12 to 0.42μm diameter in which the filter performance was not sufficiently clarified. Tested filter sheets were 14.5cm by 14.5cm in dimensions. DOP particles ranging from 0.12 to 0.17μm exhibited higher penetration of HEPA filter than DOP particles from 0.27 to 0.42μm by a factor of 20 at the standard face velocity of 2.4cm/sec. The penetration of HEPA filter was 6.6×10-4 for 0.12 to 0.17μm DOP particles, and that of the new type filter was 3.1×10-6. It was confirmed that overall system penetration of two HEPA filters in series could be described as the product of each stage penetration.
An automatic radioactivity measurement and data analysis system has been developed for radiation monitoring. It consists of ten detectors and a minicomputer system. Eight detectors are used for spectroscopy measurement and two detectors are used for only counting alpha and beta-rays. This system can fulfill (1) easy and parallel operation with ten detectors, (2) quick analysis of measured data, (3) evaluation of the radioactivity released from each facility, and (4) automatic measurement and data analysis for numerous samples. As a result of developing this system, up to 30, 000 samples a year can be dealed with economically and effectively.