Irradiations in the nuclear reactor. The neutrons produced in the reactor are used to irradiate different elements and compounds, making both radioactive and non-radioactive products for industry and research. One important non-radioactive (or stable) product is semiconductors. Pure single crystal silicon in the shape of cylindrical ingots is irradiated with neutrons and stable phosphorus atoms are produced homogenously inside the crystal through absorption by the silicon nucleus. This process is known as neutron transmutation doping (NTD). The silicon cylinders are then cut into wafers and transformed into high quality semiconductors that may be used in high-power rectifiers for the electrical power industry, hybrid cars, and smart electric power grids.
By irradiating certain metals we can produce radioactive sources for the industry. Examples of such sources are 60Co, often used in equipment for measuring fluid or dust levels, and 182Ta used for investigating oil-pipelines. We are also producing radioactive sources as tracer compounds. Examples are 82Br and 24Na which when dissolved as ions in water can be used in search for leaks and for calculating flow rates and volumes in water fluid systems. There are numerous possibilities for producing radioactive sources for many other applications, for instance, radioactive compounds used in medicine for diagnostics and therapy of cancer. In research collaboration with the Isotope department at IFE, the reactor is producing 177Lu for new treatment modalities within the field of β-particle therapy of cancer.
An example of source container for industrial use. In the center is a small source of 60Co. The source is of metallic cobalt measuring 4 mm in diameter and 4 mm in height. The rest of the source container is made of lead for shielding of the gamma-rays.
The largest volume of irradiated materials in the reactor is ultrapure silicon as single crystals. By irradiating with neutrons, the crystals are homogeneously doped with phosphorus atoms beginning with the nuclear reaction: 30Si + n --> 31Si which is radioactive and subsequently beta decays to stable 31P within a few hours. The silicon crystals are irradiated until they reach the desired resistivity - at this point they have become semi-conductors. The irradiation is performed in certain water-filled positions in the reactor. The silicon crystals are placed inside aluminum tubes before they are moved into the irradiation positions. In each of these positions a silicon crystal of 40 cm length and 12.7 cm (5 inches) in diameter may be irradiated. The weight of such a crystal is approximately 12 kg (picture to the right). Smaller silicon crystals may also be irradiated.
The irradiation of other materials is performed in dry positions in smaller tubes. The tubes have several positions for placing boxes of 10 cm height with diameter 5 cm. The neutron flux in the irradiation positions is calibrated 3 times a year and has a maximum value of approximately 1.3 * 1013 n/(cm2-s).
The gamma irradiation plant. The gamma irradiation plant is situated in the first reactor building. It contains a highly active 60Co source (1015 Bq) which is used for several tasks in industry and research. Examples are:
• Reduction of the bacteria content in spices to safe levels (Mattilsynet.no)
• Sterilization of medical equipment
• Conservation of food (research only)
• Modification of material characteristics in organic polymers (plastics)
• Testing the tolerance for gamma irradiation of electronic equipment used in satellites
The gamma source is made up of 25 cylindrical rods filled with metallic cobalt containing the radionuclide 60Co. The source strength can be up to 3.5 PBq (3.5*1015 Bq). The source rods decay with the half-life of 60Co, 5.3 years, and are therefore necessary to replace regularly to maintain the desired activity level. At a source strength of 3.5 PBq, the dose rate by the source is 15 kGy/hour and the capacity is approximately 24 000 standard units (a cardboard box) per year (irradiated with 10 kGy). The source is situated in a cellar room, and the material to be irradiated is placed inside special cardboard boxes that are moved into and out of the source room by a conveyer belt.
The gamma irradiation plant is situated inside the building housing the institute’s first nuclear reactor, JEEP I. The gamma source is placed in the cellar right under the former reactor block (to the right in the picture). Part of the conveyer belt and the lift, which move the cardboard boxes down into the irradiation positions and back up again, are shown in the above picture.
This picture shows the irradiation room in the cellar. The gamma source is inside the aluminum frame shown in the center of the picture. When the source is not in use, it is lowered into a shielded position in the floor (the irradiation level in the room is then approximately 5μSv/hour).