One of the most important utilization areas of research reactors is the neutron beam research by means of beam tubes such as the material structure analysis, material irradiation test and inspection of complex structural materials, which is usually not possible with X-ray or gamma- ray. For the implementation of effective research on these fields, it is essential to design the beam tubes in a way that thermal neutron flux can be made available as high as possible at the beam tube exit. Therefore, the radiation streaming phenomenon in the beam tube should be thoroughly scrutinized so as to be reflected in the beam tube design.
Fluid enters and leaves the reactor core to cool the nuclear fuel at an elevated temperature and pressure during the normal operation of reactor. To make such operation possible and effective, provision is made in and out the reactor for the installation of many components and auxiliary systems, and some of them are instrumentation and control systems which are usually encased in mechanical conduits. In addition, there are also many ducts and cavities in the structure of power reactor, which are provided to accommodate large-sized coolant pipes and cables leading to the reactor core through shield and external structure. In particular, penetrations are embedded in primary shield, and, above all, spacious cavity exists in the upper plenum of the pressure vessel of reactor.
The neutrons produced in reactor core normally stream out through the ducts and cavities causing nuclear heating and activation of the reactor components and surrounding materials. The resulting radiation also induces radiation exposure to reactor operators and maintenance crew. In fusion reactors, a number of ducts and penetrations are installed in the blanket and shield assembly so that the neutrons from the deuterium-tritium reaction in a plasma region may easily leak out through the cavities. Hence it is very important to shield these ducts, cavities and penetrations for mitigating the radiation damage to the reactor components and for minimizing radiation hazard to working personnel. The ducts, cavities and penetrations can be shielded efficiently only when the phenomenon of radiation streaming through those is well understood.
A radiation streaming analysis for the beam tubes of a research reactor has been carried out herein using the three-dimensional Monte Carlo transport code entitled MCNP. To evaluate the calculational results, measurements have taken place at a 250 kW TRIGA (Training, Research and Isotope Production reactor manufactured by General Atomic) which is equipped with three radial and one tangential beam tubes. The length of all these beam tubes is about 3 meters. The neutron fluxes are measured using gold-aluminium foils, while dose rates of neutron and gamma-ray are obtained by using thermoluminescent dosimeter.
When compared with the experimental results, the calculated thermal neutron flux has turned out to reproduce the measurement well, i.e., within 2-90% at both beam tubes. The calculated nonthermal neutron flux and gamma-ray dose rates show about the same distribution along the beam tube as the measurements. For the neutron dose rate, however, there is a considerable discrepancy between the calculation and measurement in case of the radial beam tube but good agreement in the tangential tube. Based on the calculational results, it is taken for granted that the MCNP may well be utilized for the analysis of the particle streaming phenomenon in the beam tube of a research reactor and eventually for its design improvement.
The pivotal consideration in designing a beam tube of research reactor is to make an optimal arrangement of beam tube so as to obtain high thermal neutron flux and low nonthermal & gamma-ray fluxes at the beam tube exit as much as possible. The interior of beam tube is preferred to be designed into vacuum in order to preclude probable airborne activation problem. Since experimental results have been few, the beam characteristics in the radial and tangential tubes are investigated by the MCNP calculations resulting in a useful database for the future reference in the beam tube design. The thermal neutron fluxes are found to be almost identical in both beam tubes, whereas the ratios of the thermal-to-nonthermal neutron flux and the thermal neutron-to-gamma-ray flux in the tangential beam tube are identified to increase from only 12% and 18% higher at the nose to 2.4 times and 2.8 times higher at 130 cm from the nose, respectively, than those for the radial tube. Thus, the tangential beam tube is proved to give a better neutron beam quality, i.e., the same thermal neutron flux and lower nonthermal neutron & gamma-ray fluxes at the beam tube exit as compared with the radial one.
