• Abstract and Figures
• Public Full-text

The Whole Body Monitor HUGO II at Studsvik Design and Operation L. Devell, I. Nilsson and L. Venner AKTIEBOLAGET ATOMENERGI STUDSVIK, NYKOPING, SWEDEN 1970 AE-378 THE WHOLE BODY MONITOR HUGO II AT STUDSVIK. DESIGN AND OPERATION L Devell, I Nils son and L Venner ABSTRACT « The whole body monitor laboratory at Studsvik is presented with special attention to descriptions of localities, equipment, calibration and types of measurements. The main monitor, shielded by 53 tons of iron, is equipped with a 8 in. diam. x 4 in.Nal(Tl) detector and a 51 2- channel Nuclear Data analyzer and located at ground level in the Health and Safety Laboratory a few hundred metres from the various labora­ tories and reactors at Studsvik. Since the start in June 1 963 and up to January 1 969 more than 4500 measurements have been performed, most of them for radiation dose control purposes. In about 40 % of these latter measurements, in­ ternal contamination exceeding the normal detection limit of a few nCi has been observed. Only a few cases so far have exceeded the maximum permissible quarterly dose set by ICRP. Printed and distributed in January 1970. LIST OF CONTENTS 1. INTRODUCTION 2. THE LABORATORY 2. 1 Iron room 2. 2 Subject handling facilities 2. 3 Monitor operating room 2. 4 Remaining laboratory facilities 2. 5 Ventilation and temperature control 3. GEOMETRIES, DETECTORS AND ELECTRONIC EQUIPMENT 3. 1 Standard chair geometry 3. 2 Scanning bed 3. 3 Equipment for background control 4. RADIATION BACKGROUND 4. 1 Background reduction 4. 2 Stability of background 4.3 Body influence on the background 5. CALIBRATION AND SENSITIVITY 5.1 Plastic phantoms 5.2 Ingestion of radionuclides 5.3 Intercomparisons 5. 4 Minimum detectable amount of activity 6. NORMAL MEASURING PROCEDURES 7. ACTIVITY CALCULATION METHODS 8. TYPES OF MEASUREMENTS 9. EXPERIENCES FROM THE MONITORING 10. ACKNOWLEDGEMENT 11. REFERENCES - 3 - 1. INTRODUCTION Internally deposited radionuclides are normally most effectively studied by means of whole body counters. This technique is therefore well suited for the monitoring of internal contamination of employees working with radioactive materials. The monitor HUGO II was put into regular operation in June 1 963 and was built mainly for measuring internal contamination of the employees at the nuclear research station Studsvik, which includes a 30 MW (t) testing reactor of swimming pool type, three zero power reactors, hot cells and laboratories for handl­ ing of irradiated fuels and materials, centre for radioisotope produc­ tion and facilities for storage and disposal of radioactive waste. Experience from the operation of the first whole body counter (HUGO I in Stockholm) of the company [l ] was exploited for the design of HUGO II at Studsvik. Sensitive equipment was chosen in order to meet different requirements and to facilitate special investigations. In this report the design and normal use and operation of the monitor are de­ scribed. Results from the measurements [2, 3, 4] and special studies L5, 6] are presented elsewhere. 2. THE LABORATORY The ground floor of an existing building, originally designed as 2 an assembly hall with an area of 1 43 ra , could be used for the WBC laboratory and offered some essential advantages. The building was sit­ uated close to the border of the research station area, which decreased the risk of background disturbances from the reactors and other strong activity sources as much as possible in case of incidents. Further, as no internal walls restricted the dimensions of the building blocks for the background shield, the latter could be of simpler construction, and heavy and efficient machines could be used to mount these blocks. A drawback was that no attempts could be made to reduce the background by choosing a special low-activity building material. A plan of the labo­ ratory is shown in Fig. 1 . 2. 1 Iron room The background shield is built of laminated sheets of virgin iron, Fig. 2. The inside dimensions of the monitoring room are 2. 5 x 3. 0 x * 50 MW" (t) since October 1969 - 4 - x 1.9 (height) m. This allows the use of two measuring geometries, one chair and one stretcher. The total thickness of walls, roof and floor is 1 6 cm and the weight 53 tons. The inside surfaces are covered with 3 mm lead sheets, lined to the iron with epoxy resin. The steel­ works produced the sheets directly with the final dimensions, so that no further work on them was necessary before mounting. The thickness of the iron sheets is 4 cm. One sheet covers the whole area of the walls, but in the roof and floor each layer consists of two sheets. The sheets are mounted to form labyrinths at the corners and other joints to avoid direct radiation from the outside. All joints are tight-welded inside and outside to avoid radioactive contamination between the sheets, which would be unremovable in case of an incident during the calibration work for example. In the inner sheet of the roof, channels are cut out to allow passage for the ventilation air. These channels are also formed as laby­ rinths to protect against external radiation. The door has two hinges with ball bearings and is very easily handled despite its weight of 2. 8 tons. It is laminated of two pieces, 4 and 12 cm thick respectively. The doorway is 0.9 x 1.9 m. A lead glass window is mounted in the door to allow observation of the patient. The window is also thought to be an ef­ ficient anticlaustrophobial feature. Finally the room is painted inside in light colours with a resistant varnish, allowing the use of efficient decon­ tamination treatment if necessary. 2. 2 Subject handling facilities The risk of contamination of the low-activity area in a WBC labora­ tory within a nuclear research station is obvious. To reduce this risk and to eliminate, as far as possible, external skin and hair contamina­ tion, the subjects have to use a special entrance procedure, including undressing and showering. After this they pass into the first room within the low-activity area, which is a dressing-room where they put on a special dress. Then they pass through the monitor operation room and enter the iron room. The personnel working in the laboratory and visitors only have to pass the operators' dressing-room, where they change to special, clean shoes before entering the monitor operation room. - 5 - 2. 3 Monitor operating room The monitor operation room is mainly occupied by the racks and a table for the instruments for the iron room, equipment, but the space is big enough also for an operator's desk and instruments for special measurements, e. g. a thyroid counter which may be used out­ side the iron room. 2. 4 Remaining laboratory facilities Further localities in the low-activity area are a laboratory used for service and special experiments, offices for the personnel, and a room for handling of the phantoms and solutions for calibration. The iron room is connected to the monitor operation room by an intercommunication system. It is also equipped with a separate loud­ speaker connected to the light radio program. 2. 5 Ventilation and temperature control The laboratory is air-conditioned with an ordinary, but high quality, system including particle filters. The ventilation system is designed to give an higher air pressure in the low-activity areas than 3 in general areas. The iron room is ventilated with about 200 m of air per hour. The air is taken from a separate intake and is filtered and conditioned for humidity and constant temperature. The temperature is stabilized within ± 1 C. The air pressure in the room is + 8 mm H?0, which is 3 mm higher than the pressure in the monitor operation room. 3. GEOMETRIES, DETECTORS AND ELECTRONIC EQUIPMENT 3. 1 Standard chair geometry As mentioned, the space in the iron room allows the use of two counting geometries. The one first put in operation and commonly used is a chair geometry, Figs. 3 and 4. The detector is a 8 in. diam. by 4 in.Nal(Tl) crystal equipped with three photomultipliers of 7. 5 cm diameter. This geometry was chosen because of its convenience from the point of view of installation, calibration and service, and of its high sensitivity to isotopes deposited in the essential parts of the body with - 6 - good resolution. The main purpose of the measurements in this geom­ etry is to make a rough determination of the radioactivity and to identify nuclide and not to make a very careful evaluation of the amount and lo­ calization of the activity. A block diagram of the electronic equipment for the standard chair geometry is shown in Fig. 5. The three P. M. tubes are all sup­ plied with the same high voltage. Individual differences in sensitivity are compensated with potentiometers in the individual dynode voltage divider chains. The signal outputs are coupled in parallel and fed dir­ ectly to the input of the pulse height analyser. This is a Nuclear Data type 1 30 A with 51 2 channels, each with a capacity of 1 0 counts. The analyser may be used with 2 or 4 subgroups of 255 or 1 27 channels (one channel is occupied by the timer). Transfer is possible between the sub­ groups, positively or negatively, which allows background reduction and convenient comparison between different measurements. The ana­ lyser is also equipped with a data processing unit which makes it pos­ sible to get the sum of the total counts or any desired interval of a spec­ trum.

Load more

  34

Copy Link