BIOLOGICAL IRRADIATION FACILITY   

Description

An irradiation facility existed at the BRR from 1968 for 18 years. During the reconstruction of the reactor a new system for biology and dosimetry research was designed and completed in 1995. The final tests and the investigation of the beam quality were performed in early 1996. Since that time the system is in continuous operation and improvement.

The channel lock consists of 3 steel and heavy-concrete segments turnable by an excentrical axis to open and close the channel. There is an internal rateable filter holder at a distance of 262 cm from the core which has six windows with the following materials: four Bi disks of 5, 10, 15 and 20 cm thick and one Pb disk of 20 cm, the 6^th one is an open hole. At the orifice of the beam tube two cylindrical tanks were constructed of aluminum to serve as a water shutter and its emergency water storage, respectively. The water can be pumped up from and released to a larger buffer tank located outside of the reactor shielding block by pressurized air. A microprocessor controlled electronic unit connected to a PC operates the two shutters and the internal filter systems. The construction materials inside the beam tube work as internal, not removable filters with total thickness of 18 mm Pb and 15 mm Al.

The irradiation cavity is situated outside of the shielding block of the reactor in a distance of 1400 mm; thus its surface-to-reactor core distance is 3100 mm including the exchangeable core window (65 mm) made either of beryllium (rolling as the fast neutron reflector, too) or of aluminum. This window can be changed only during the maintenance or refueling period. The use of the aluminum window results in a hard neutron spectrum (See Fig. 5). Between the reactor shielding surface and the cavity there is a borated water shielded collimator with a useful diameter of 10 cm. It is possible to use this collimator as a holder for outer filters of about 800 mm length. Presently, filters of Lucite, polyethylene, iron, aluminum and lead are available to decrease the gamma and neutron intensity or to modify the neutron spectrum and the neutron-to-gamma ratio. There are two changeable filter disks of boron carbide working as thermal and epithermal absorbers. The collimator is movable on a rail. The samples to be irradiated can be rotated to achieve a uniform, homogeneous irradiation. Cadmium or Boron carbide filters are used, if required, for decreasing the thermal neutron contribution. A large variety of irradiation geometry can be configured inside or outside of the collimator depending on the state, shape, weight of the material to be exposed.

The cavity is surrounded by a borated water shield, which can be moved on a rail, as well. The whole construction is covered and surrounded by shielding elements, like a bunker, made of borated water and paraffin wax, heavy concrete and lead.

Three levels of the dosimetry system were developed: real time, active beam monitors, passive activation, track etch and TL detectors and computer codes for spectrum and dose calculations. Some typical dose and flux values are presented in Table 1 and the schematic view of the system are presented in Figure 6 and 7.

Responsible: J. Pálfalvi

KFKI Atomic Energy Research Institute
Radiation and Environmental Physics Laboratory