NIGU-5 self-contained research irradiator

Radiat. Phys. Chem. Vol. 46, No. 4 - 6 , pp. 5 1 5 - 5 1 8 , 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/95 $9.50 + 0.00

Pergamon

0969-806X(95)00206-5

NIGU-5 SELF-CONTAINED RESEARCH IRRADIATOR

K. KREZHOV, M. CHRISTOVA, D. GENOV, N. GENCHEV, V. SECHKARIOV Institute for Nuclear Research and Nuclear Energy at the Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee blvd., 1784 Sofia, Bulgaria

ABSTRACT The basic principles implemented in the design of a new model of a self shielded gamma irradiation facility for research purposes are presented. NIGU-5 features a large irradiation volume of 5.61, a satisfactory irthomogeneity of the gamma-field distribution better than 20 % in full volume and 10 % in 2.51 central volume, a fool-proof radiation protection, an optimized relatively light biological shield ensuring a dose rate on the surface less than 14/~Gy/h (1.4 mrad/h), not complicated mechanics and high operational reliability.

KEYWORDS Gamma irradiation equipment; i37Cs source; gamma dose rate distribution; gamma field inhomogeneity.

INTRODUCTION The development and application of gamma radiation technology is closely connected with the design of the necessary irradiation equipment. A number of manufacturers from several countries as Canada, France, the United Kingdom and Russia offer irradiation facilities for industrial and research purposes. However, in many cases there is a need for a specific design in order to meet some specific conditions and requirements. Thus, the Institute for Nuclear Research amd Nuclear Energy launched a large scale program in the field of gamma irradiators. It started as early as 1973 when the ground type irradiator GOU-1 (Pandev et a/..,1976, M. Hristova et al., 1985) has been built and put in operation. Nowadays, there are more than 10 large research and industrial irradiators operated in Bulgaria including a portable equipment for pro-sowing irradiation for stimulation of seeds and other products (Pandev et al., 1979), a unit for irradiation of biological objects (Pandev et aL, 1980), a portable gamma-irradiator (Pandev et al., 1981) and others. Three dedicated units of a single design for agricultural purposes were supplied to Albania, Zambia and Pakistan under the supervision of the IAEA-Vienna. Here, the new model of a transportable self shielded gamma irradiation facility for research purposes designed and produced on the basis of experience gained by previous work in the field is shortly described.

GENERAL FEATURES The irradiation chamber is cylindrical and during operation it is surrounded by linear gamma-sources parallel to its axis. The vessel with the material irradiated is rotated in order to achieve a better homogeneity of the gamma-dose absorbed. During operation the access to the vessel is reliably prevented both mechanically and electrically by means of two independent interlocks. This makes impossible any direct personnel exposure 515

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Fig. 1. Schematic drawing of the gamma irradiator NI(_IU-5

to radiation. Power supply failure does not affect the experiments since the control block power supply is switched over automatically to accumulator batteries until completion of the preset time of irradiation. The duration of irradiations is preset by a programmable counter in steps of 1 s within the range from 1 s to 999 h. Loud audible warning reminds the operator that the irradiation has ended. The irradiation can be interrupted at any time if necessary. The elapsed time is memorized by the control block, so that one may easily assess the dose obtained. After opening and closing again the irradiation chamber, the irradiation can proceed further until the preset time duration is achieved. The NIGU5 facility is designed to be loaded with 137Cs radioactive sources (mean energy 0.661 MeV) up to an overall activity of 1.5.1015 Bq (40 kCi). Such an activity provides a dose rate of 5 kGy/h (0.5 Mrad/h) in the center of the irradiation chamber.

BRIEF DESCRIPTION OF THE CONSTRUCTION The main body of the gamma irradiation facility NIGU-5 is shaped as a cylinder 1165 mm high and 692 mm in diameter, Fig. 1. The body consists of two removable parts: upper (11) and lower one (15) coupled together by a flange bond. In between the flanges there is a biological shielding ring. Both parts of the main body are playing the role of a biological shielding. They represent a bearing construction with double walls made of sheet steel and the space between them is filled with lead. The cylindrical gate (stopper (6)) is placed within the upper part of the body while the ring-like block (12) with attached radioactive linear elements (14) is inserted inthe lower part. The lower ends of the radioactive elements are linked together by a common ring. A shielding cap imbedded in the ring-like block is placed above each radioactive element. The caps are pressed altogether by the fixing ring (18). The two blocking rods (17) are fastened firmly upon the upper side of the ring-like block. The construction elements (12), (14) and (18) form the irradiator block.

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The irradiation volume (K) is in the cavity of the upper part of the body in between the linear active elements (when elevated in working position). For irradiation the cylindrical vessel (9) loaded with the materials enters the irradiation chamber. The vessel (9) is attached to the cylindrical steplike shielding lid (6) by means of a coupler (7) with cylindrical bond. The cylindrical shielding body (beam catcher, (13) in the middle of the lower part is affixed firmly to the bottom of the NIGU-5 body by means of a special bolt. The linear radioactive elements surround this body when the facility is off. Two couples of identical hydraulic cylinders are mounted diametrically on the outer side of the main body. The first couple (10) is fixed to the outside face while the other couple (5) is attached to the lid by means of the fixing ring (16). The couple (10) moves up and down the cylindrical shielding lid (the gate) together with the irradiation vessel by means of a mobile cross-beam (3) guided by two vertical rods (8) which are stabilized by the immobile cross-beam (2). The uppermost position is the storage one ("off") while the lowest position is the working position and the safety cylindrical gate together with the irradiation vessel are inserted in the irradiation chamber. In working position the facility is ready to start the irradiation. The couple (5) moves the ring-like block together with the radioactive elements. The operation mode is at the uppermost position of the irradiator block while its lowest position is the off position for the facility. The two couples of hydraulic cylinders and the other hydraulic elements are fed from a hydraulic station situated at the rear side of the body. The cylindrical safety gate through a step-like vertical rod (4) hangs on radiax beatings. The beatings are mounted in a special bearing box supported by the mobile cross beam. An electric motor (1) mounted on the moving cross beam drives the irradiation vessel with loaded material into uniform rotational motion by means of a cam-gear.

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The hydraulic cylinders and the space over the cross beams are covered by a metallic lid. A ventilation system is provided for the ventilation of the inner space of the body. Over the main body and below the pulse electromagnetic clutch there is a sliding platform which takes the irradiation vessel in and out. There is an electric board (see Fig. 2) located at the back of the facility. The automatic control of the mechanics and the irradiation process is performed by means of a dedicated computerized electronic unit. It is mounted on a rotating console attached to the main body of NIGU-5. All systems and mechanisms of the facility are enclosed by three steel sheet casings: upper, lower and a lid.

RADIATIONAL CHARACTERISTICS The calculated characteristics such as the exposure dose rate, homogeneity of the gamma field and others were verified experimentally. A number of reference points were chosen in the volume of the irradiation chamber where the exposure dose rate was periodically checked. The necessary data from the reference points in

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Fig. 3. Distribution of the dose rate [kGy/h] in the cylindrical irradiation vessel: A-with rotation; B-without rotation

the irradiation volume were obtained by means of Fricke and ethanol-monoehlorbenzene dosimeters. The dose rate distribution was measured in both the stationary and rotating vessel modes of operation. Figure 3 gives a schematic view of the cylindrical vessel with the data for the dose rate distribution in air. The source activity at the time of carrying out the measurements in late November 1991 is 13.1 kCi. The figures given in Fig. 3 show that the inhomogeneity of the gamma field is within the limits of + 18 %.

CONCLUDING REMARK It is worth to note that the prototype of the gamma irradiation facility NIGU-5 has been installed in Pakistan and since 1991 proved to be a highly reliable tool in many fields of research related to gamma-rays effects on materials and biological objects, as well as for practical purposes.

REFERENCES Hristova, M., V. Stenger and A. Kovacs (1985). Proceedings of an International Symposium on High-Dose Dosimetry, 8-12 October 1984 IAEA, Vienna. Pandev, I., H. Hristov, M. Hristova, S. Stefanov and D. Genov (1976). Yad. Energy, 3, 68. Pandev, I., M. Hristova, S. Stefanov, N. Genchev, S. Bakardghiev and H. Hristov (1981). Yad. Energy, 15, 39. Pandev, I., M. Hristova, S. Stefanov, N. Genchev, S. Bakardghiev, H. Hristov and D. Genov (1980). Yad. Energy, 12, 62. Pandev, I., M. Hristova, S. Stefannov, N. Genchev, S. Bakardgghiev, H. Hristov and D. Genov (1979). Yad. Energy, 10, 38.