Thermal and intermediate-energy reference neutron flux irradiation facilities development at the Romanian TRIGA Reactor

The development of the nuclear energy imposed the knowledge with high accuracy of the neutron characteristics in the nuclear reactors, for control and safety, for the calibration of the equipment working in the neutron fields. In order to standardize the neutron measurements different nuclear facilities, with different neutron characteristics, producing standard neutron fields, were created. The aim of our project is to develop thermal and intermediate energy neutron irradiation facilities at the steady state core of the Romanian TRIGA Reactor. The Romanian TRIGA Reactor is a dual core reactor having a 14 MW steady-state core and a pulsating annular core. The steady-state TRIGA reactor is a pool type reactor, using high-enriched U-ZrH fuel, water moderated and cooled. At present a mixed core, using high-enriched and low-enriched uranium, is operated. The Reactor is under operation since 1979 at the Institute for Nuclear Research, Pitesti. The neutron flux spectrum measurements, performed in TRIGA Reactor thermal column, show some discrepancies due to the contribution of high-energy neutrons (En > 3 MeV). The thermal flux measurements show the presence of a significant epithermal contribution (about 10%). The Cadmium ratio for Gold is below 20. The TRIGA Reactor thermal column is significant smaller than in other similar facilities. Therefore, the graphite thickness around the cavity decreases up to 10 cm on the bottom and upper sides. Also, the presence of the water layout between the graphite cells affects the spectral composition, in comparison with a pure graphite thermal column. By analyzing a few solutions to improve the thermal flux cavity spectral characteristics, we have chosen to move away the central cavity with 14.3 cm from the core (one row of graphite bricks). So we obtained acceptable values for the Cadmium ratio for Gold (> 50) as well as a flux attenuation of about 2 times, in the present configuration. The analysis is based on computational results, obtained also by using the MCNP-4C code. The thermal column reconfiguration will be done in the near future. The spectrum computation for this new configuration does not show significant spectral modifications, as compared with the actual measurement results. The thermal flux cavity and the intermediate-energy reference spectrum irradiation facilities are now operational at the TRIGA Reactor at Pitesti. The computational and measurement program will be continued in order to improve the spectral characterization, to determine the neutron spatial distribution inside the irradiation systems, the gamma dose and the temperature