CARR Cold Neutron Source Performance Optimization And Self-stabilizing Characteristics

Cold Neutrons, provided by Cold Neutron Source (CNS) on research reactor, have been used for conducting neutron scattering experiments, which has been getting wider and wider applications in many fundamental and applied science fields. Neutron scattering experiment, especially cold neutron scattering experiment, is the most important application on CARR.Neutronics is the uppermost criteria whether the CNS system is effective and advanced, and self-regulation is an important index whether the CNS system could operate stably and provide cold neutron beam steadily. Difficulty is brought to the design of CARR CNS, because of not only those characteristics of CARR such as multi-purpose applications, high nuclear power and low-enrichment uranium, and those requirements as the status that CNS is off while reactor is on, but also without direct reference design in the world. In order to resolve the technical difficulty of CARR CNS encountered, the newest cooling method in the world, two-phase thermo-siphon with auxiliary cooling by helium surrounding moderator cell, is adopted. This paper conducts the optimization study on the neutronics and self-regulations based on the characteristics of design and operation of CARR. The study is focused on following three parts.The first part is to introduce the CNS equipment and the current and future status of neutron scattering experiment with cold neutron on the basis of the review for history and tendency of neutron and neutron source, and to explain carefully the necessity and significance of this optimization study. At the end of this part, the design of CARR and CARR CNS is briefly introduced.The second part is to improve the cold neutron flux or gain factor by optimization of the configuration of moderator cell for CARR CNS. By analysis of cold neutron flux distribution in moderator cell, the configuration of new crescent moderator cell is brought forward, in which the horizontal cross-section of liquid hydrogen is crescent and outer container is cylinder. The optimal configuration of moderator cell is achieved by optimized the thickness of liquid hydrogen and size of the openings for two kinds of moderator cells, which are concentric and eccentric inner part of moderator cell. The neutron flux in moderator cell, the neutron temperature and the neutron spectrum are analyzed for the optimal configuration, and these could conclude that analysis method and calculation results are correct and reasonable. Furthermore, the brightness and gain factor of cold neutron are analyzed and compared with gain factor of ORPHEE and NBSR of NIST (data of previous unit, modified in 2001), and the result shows that the neutronics characteristics of CARR CNS is good.The third part is to conduct the optimization study of self-regulation for CNS of CARR. Firstly, the self-regulation equation is derived after the model of new cooling method is simplified, and it is proved that this system possesses characteristic of self-regulation. It is also analyzed whether the model is reasonable and conservative. Secondly, it is determinated for obtaining the objective of optimization, minimum changing in liquid hydrogen surface level without adjustment of helium parameters on inlet of moderator cell. Many factors, like heat-exchanger capability, operation pressure of hydrogen system, are studied to improve the self-regulation. Thirdly, the stability with simple adjustment of helium parameters on inlet of moderator cell is also discussed here. The optimization of minimizing the changing of liquid hydrogen level is estanblished and many factors, such as heat-exchanger capability of hydrogen condenser, operation pressure of hydrogen system, heat capacity of aluminium in condenser, and so on, are also optimized. Finally, the comparison among direct cooling, two-phase thermo-siphon cooling and this cooling method is conducted after deriving self-regulation equations of other cooling methods adopted successfully abroad. The theoretical requirements under that the self-regulation of new cooling method is better than that of others have been derived. Meanwhile, numerical analysis is carried out to verify the theoretical analysis. This conclusion has very meaningful engineering significance.By the optimization study of neutronics, the new configuration of cell is approached with good neutronics performance, which is better than the previous NBSR CNS, improved in fiscal 2001. The optimal configuration and size are: transverse figure of insert is eccentric ellipse and that of crust is circle; the thickness of liquid hydrogen is 26 mm and opening size is 180 mm. By the optimization study of self-regulation and on the basis of current design, the small changing of liquid level and good operation stability could be achieved with the following methods: changing pressure measuring to differential pressure measuring; increasing the aluminium quality of condenser to 62 kg; reducing the original operation pressure of hydrogen system to 0.17 MPa.Those results of neutronics study have been used in the design of moderator cell and that of self-regulation study could be used to guide the design of hydrogen system for CNS on CARR. These results are also meaningful to guide the design and construction of other CNS in domestic and foreign country.