The two-phase instability analysis in natural circulation loops of the china advanced research reactor

Full text of publication follows: The thermal-hydraulic instability of a two-phase natural circulation loop is a very complex multivariable and high non-linearity system. In natural circulation the mass flow rate is decided by the heat power and the construction of the loop. The energy and momentum equations are coupling compact. The instability of natural circulation is different from that of forced flows. So nonlinear stability analysis for a natural circulation two-phase flow system has been a very active field of research in the past century. However, many researchers paid attention to the experiments. Few of them built the math and physics models to describe and explain the details of the phenomenon. Several simple models are based on the uniform heating and ignoring the subcooling. Most of the experiments and models are used for the high pressure system and the instabilities of them often occur in high quality region. Therefore the characters of instability of low pressure natural circulation system should be studied more. In this paper, two-phase flow instability in natural circulation loops of China Advanced research reactor (CARR) has been investigated. CARR is a low pressure and low power density research reactor. A natural circulation instability analysis model is built for the natural circulation loop of CARR. The homogeneous flow model is used to establish the system control equations. The non-uniform heating and subcooled boiling heat transfer is included. The accumulation of heat of the wall is also included. An analysis code is programmed. Numerical method Gear is employed to solve the system equations documented in form of ordinary differential equations. According to the calculation results, a stability map of natural circulation loops, which confirms the presence of an instability region under the condition of low exit quality and low pressure, was obtained. The instability region on the map is a very narrow region that is different with that of forced flow. It is a kind of density wave instability that occurs in very low quality region (The I type Density Wave Oscillation). The difference of density between the single-phase fluid and two-phase fluid is very big because of the low pressure. So the little variation of quality in low quality region can induce the large change of pressure drop of the heat section and rising section. Then the mass flow rate is changed by the variation of pressure drop. The calculation results show such oscillation course clearly. The variation of mass flow rate, pressure difference, wall temperature, fluid temperature, inlet loss coefficient and the boiling boundary are analyzed separately. Especially, the phase-space trajectory of boiling boundary and mass flow rate is discussed which can explain the oscillation course easily. At last the oscillation frequency is discussed. The calculated results have important significance for the safety operation and accident analysis. (authors)