This paper describes computational and experimental investi-gations of statioc characteristics of a natural circulation system at nuclear heating reactor onditions. Characteristic curves of a natureal circulation system hae been obtatined by using a and flashing have been taken into account. Using characteristic curves to analyze the behavior of the natural circulation is a new method.As an example, flow excursion and heat siphon in a natural circulation with low steam quality and low pres-sure have been describe by means of using the method of characteristic curves. The results show, that, under certain con-ditions, flow excurves. The resuts show, that, under certain con-ditions, flow excursion wil occur in a natural circulation sys-tem. The value predicted by the computing model is very close to the experimental value.

The gravity-driven boron injection system (GDBIS), designed by the Institute of Nuclear Energy Technology (INET) of the Tsinghua University, PR China, is a new type of passive system to be applied in the 200 MW nuclear heating reactor (NHR-200), also designed by INET The function of this system iS to shut down the reactor in a13 emergency. in case con-trol rods do not operate properly. A borate water tank is 10 cated 10m above the top of the pressure vessel When the pressure of the reactor and the boron tank balances, the borate water will be driven by gravity to flow Into the reactor. and thus shut down the reactor The thermal hydraulic perforrnances of the system for cold froom temperature nitrogenl and hot (mixture of hot steam and nitrogen) operating conditions, especially the response time of pressure and water injection. have been researched under difierent initial conditions Firstlv several factors e.g. orifice on steam lines and the volume ratio of the gas steam spaces ofthe reactor and the boron tank, have effects on the pressure and water injection response time and other thermal hydraulic performance of the system Secondly the steam and fiquid comFllll. nication modes, namely the acting time and sequence ofthe action ofvalves connecting steam and liquid lines，have great influences oil the performance of the system Thirdly the limited pressure balance time(about 1.0s) can be achieved under the cold condition. This investiga-tion shows that GDBIS can be properly used in the 200 MW nuclear heating reactor.

In order to understand the influences of various factors on the depressurization rate under small break LOCA occurring at the upper plenum in a natural circulation loop, a basic analysis has been carried out. It is shown that the depressurization rate is influenced by system pressure, back pressure, break area, water mass in the system, heating power, operational state of the second loop, and so on. Experimental results of flow and heat transfer are also presented in this paper. The experiment was carried out at the 5MW unclear heating reactor test loop HRTL-5. The transient processes are described and heat transfer coefficients are estimated. The results show that flashing improves the natural circulation and the heat transfer, so the wall temperature of the heated rod can be kept at a lower level and the reactor is safe. The flow oscillation, which leads to cyclical variations of important parameters, is described and its development mechanisms are studied. It is pointed out that the interaction of pressure decreasing and bubbles producing caused by flashing is the main reason of the flow oscillation.

An experimental simulation study on the start-uD of a low temperature natural circulation nuclear heating reactor (5 MW developed by the Institute ol Nuclear Energy of Tsingbua University, Beijing) is presented The experiment was performed on the test loop (HRTL-5) which simulates the geometry and system design ol the 5 MW reactor The manifestation of difierenl kinds ot two-phase flow instability namely geysering flashing instability and low steam quality density wave instability on the start-uD are described The mechanism of flashing instabiIit)which has never been well studied In this field 1s especially interpreted Based Oll the study of these lnstabilitieg lt Is suggested that the start-uD process from Initial condition to boiling operation condition. should consist of three steps: increasing of Initial pressure by means of a noncondensabk gas (N,) start-up of the reactor at this pressurized condition (single-phase regime operation), and transition to a lower pressure boiling operation Three transition methods are +N34 discussed As a result of these studies the method of transition with low heat flux and low lnlet subcooling is proposed A stable start-up process of the 5 MW reactor is achieved by carefuI selection of the thermoh~draulic parameters.

Based on a physical model of the test loop HRTL-5 simulating the 5MW nuclear heating reactor, a drift-flux model of two-phase flow has been derived, in which the sub-cooled boiling and saturated boiling in the heated section, condensation and void flashing in the adiabatic riser etc, are taken into account. In is suitable to ivestigate the natural circulation two-phase flow, expecially at heating reactor conditions, By introducing the concept of condensation boundary layer, a condensation formula has been derived and complemented into the drift-flux model, which was used to investigate the beharior of the HRTL-5 in our previous research works. Based on the mathematical model, a computer program has been developed. At first, through comparing the calculations with the experimental results, the parameters in the two-phase drift-flux model have been determined. Then calculations on the flow characteristics of HRTL-5 have been performed, including the simulating of the mass flow rate and the distribution of various physical variables. At last, the characteristic curves of natural circulation have been simulated; thereby, also usful experiences have been acquired.