Study On Void Fraction And Structure Optimization Of Moderator Cell In Cold Neutron Source Of China Advanced Research Reactor

China Advanced Research Reactor (CARR), which will be built up in 2006 by the China Institute of Atomic Energy (CIAE), is a multi-purpose research reactor with neuclear power up to 60 MW. The maximum thermal neutron flux of the reactor core will reach up to 1×1015n/(cm2·s).Cold neutron source (CNS) is the main part of neutron scatting experiment system in CARR. The principle of CNS is that high-gain cold neutron is obtained by decreasing the velocity and increasing the density of neutron beams which produced from reactor core moderator. The cold neutron is a perfect research tool for the physical structure and microscopic movement at molecule and atom dimension.Basically speaking, the nuclear power of CARR is huge, and only can be designed by USA, Russian, Germany, Japan, and France, so the relevant datum is lack respectively.In this thesis, a systematic study is carried out on the void fraction of moderator cell, the structural optimization of moderator cell, and several related problems such as the influence of fluid physical properties on gas bubble rising velocity and void fraction of moderator cell.1) Comparison and optimization are carried out on the moderator cell structure. A new crescent-cylindrical annulus moderator cell with an auxiliary cooling channel is selected for CARR-CNS in the present paper. This moderator cell decreases the nuclear heat and void fraction to a considerable degree, increases the neutron brightness significantly.2) The strength of moderator cell is computed on crescent-cylindrical annulus moderator cell with an auxiliary cooling channel by commercial software. And the strengthening methods for stress concentration point are proposed. From the point of strengthening, the moderator cell stress is restricted within a specified range.3) An experimental facility was established to study the bubble rising velocity in the different kinds of liquid and to analyze the correlation between bubble rising velocity and fluid physical properties. The tests confirmed that the ratio of surface tension to density for liquid phase had great influence on rising velocity for bubbles, and the larger the ratio, the bigger the rising velocity.4) In order to find the main factors of liquid physical properties affecting void fraction and to provide the theoretical basis for the mockup tests working fluid selection, a cold model moderator cell was made of Lucite with compressed air contained to simulate the evaporation in the moderator cell. The tests are carried out with different working fluid. It proved that the ratio of surface tension to density for liquid phase is the main factor that determines the physical properties on void fraction, the larger the ratio, the smaller the void fraction. The ratio of surface tension to density of Freon 113 is lower than that of liquid hydrogen. Therefore, Freon 113 can be used as working fluid to study the void fraction in the hydrogen two-phase thermo-siphon loop in the CNS of CARR, and the results are conservative.5) Two mockup test systems were established to investigate changes of liquid level in moderator cell, the self-regulating characteristics of loop, and the data of void fraction in the liquid of the moderator cell. One moderator cell was cylindrical annulus without an auxiliary cooling channel, and others were crescent-cylindrical annulus with an auxiliary cooling channel.6) Theoretical analysis and computation are fulfilled to the above two hydrogen two-phase thermal-siphon loop. It is verified that the designing preconceive, in which the inner shell only hydrogen vapors and the outer shell two-phase hydrogen, can be realized if some parameters be controlled to a reasonable value. The collapsed liquid levels in moderator cell during startup of the test loop indirectly shows that the moderator cell in the mockup test is able to establish above state under the operating conditions.