Natural circulation instability

Natural circulation systems may undergo thermal-hydraulic instabilities under low-power and low-pressure conditions, which occur during start-up. The void reactivity feedback and void fraction fluctuations in the reactor core would create power oscillations during start-up. Three kinds of thermal-hydraulic instabilities may occur during start-up in natural circulation BWRs, which are as follows (1) geysering induced by condensation, (2) natural circulation instability induced by hydrostatic head fluctuation in steam separators, and (3) density wave instabilities. 

As the heat input is increased, geysering is suppressed and another instability called natural circulation instability is induced due to hydrostatic head fluctuation (caused by PDO), which varies the natural circulation force. As the heat flux is further increased, density wave instabilities appear. The period of natural circulahon oscillations is much longer than that of density wave instabilities, and reduces with an increase in heat flux and with a decrease in inlet subcooling. 

B14. Becker, K. M., Jahnberg, S., Haga, I., Hansson, P. T. and Mathisen, R. P., Hydro-dynamic instability and dynamic burn-out in natural circulation two-phase flow. An experimental and theoretical study, Nukleonik 6, 224 (1964). 

Pressure drop oscillations (Maulbetsch and Griffith, 1965) is the name given the instability mode in which Ledinegg-type stability and a compressible volume in the boiling system interact to produce a fairly low-frequency (0.1 Hz) oscillation. Although this instability is normally not a problem in modern BWRs, care frequently must be exercised to avoid its occurrence in natural-circulation loops or in downflow channels. 

Blumenkrantz and Taborek (1971) applied the density effect model of Boure to predict instability in natural-circulation systems in thermosiphon reboil-ers used in the petrochemical industry. An important conclusion of their work was that similarity analysis in terms of the model s dimensionless groups can be used to extrapolate threshold stability data from one fluid to another. 

Check the system (or loop) instability by using the Ledinegg criterion with an average lumped channel pressure drop. If it does not satisfy the Ledinegg stability criterion, one or more of the three remedies can be taken orifice the inlet, increase the steepness of the pump head-versus-flow curve or increase the resistance of the downcomer of a natural-circulation loop. 

Liquid level in the vapor body is an important variable affecting operation of natural circulation calandrias. Normally units are operated with the evaporator liquid level at the top tubesheet of the calandria. For non-fouling fluids, the liquid level can be lowered to the optimum value in order to minimize heat transfer surface or maximize performance. The optimum value is approximately half the distance between the top and bottom tubesheets of the calandria, and will vary with each system. The liquid level should not be appreciably above the top tubesheet and certainly should not be maintained above the caiandria outlet nozzle. Liquid levels above the vapor return will limit the performance of the calandria and may result in damage to the evaporator. Flow instabilities may also be experienced. 

For the boiling channels this programme Included, for Instance, very extensive burn-out and instability tests which culminated with tests of full scale 36-rod assemblies in a 6 to 8 MW rig where natural circulation powers for an excess of the hot channel power to the Marviken reactor was achieved. Fig. 5< In-plle tests were also performed in the Agesta and Halden reactors, whilst the physics -including the vital void reactivity coefficients - were established by extensive tests In the critical RO and hot exponential facilities at Studsvlk,... 

Basu et al. (2014) have reviewed applications of single-phase NCLs for nuclear power applications, and Misale (2014) presented a summary of the status of singlephase NCLs. Sarkar et al. (2014) present a review of supercritical NCLs. Previous reviews that supplement these include Prasad et al. (2007), Nayak and Vijayan (2008), and Vijayan and Nayak (2010), who reviewed instabilities for boiling two-phase NCLs, including natural-circulation boiling water reactors (BWRs). The latter reference has a list of instability events that have occurred in operating machines. 

These idealizations are usually applied to analyses of natural convection and natural-circulation flows. Note that use of these assumptions introduces the likelihood that important aspects of the onset of instabilities will not be correct. Each of these assumptions must be validated because potential effects on the onset of instability are subtle. In general, all assumptions and idealizations require in-depth justification in order that false positives and false negatives are eliminated. 

Becker, KM., Mafhisen, R.P., Eklind, O., Norman, B., 1964. Measurements of Hydrodynamic Instabilities, Flow Oscillations and Burnout in a Natural Circulation Loop. AB Atomenergi Report AE-131. 

Jain, P.K., Rizwan-uddin, 2008. Numerical analysis of supercritical flow instabilities in a natural circulation loop. Nuclear Engineering and Design 238, 1947—1957. 

Kozmenkov, Y., Rohde, U., Manera, A., 2012. Validation of the RELAP5 code for the modeling of flashing-induced instabilities under natural-circulation conditions using experimental data from the CIRUS test facility. Nuclear Engineering and Design 243, 168-175. 

Nayak, A.K., Vijayan, P.K., 2008. Flow instabilities in boiling two-phase natural circulation systems, a review. Science and Technology of Nuclear Installations 2008. http //dx.doi.org/ 10.1155/2008/573192. Article ID 573192, 15 p. 

Prasad, G.V.D., Pandey, M., Kaha, M.S., 2007. Review of research on flow instabilities in natural circulation boihng systems. Progress in Nuclear Energy 49 (6), 429—451. 

Vijayan, P.K., Nayak, A.K., 2010. Introduction to instabilities in natural circulation systems. In IAEA Training Course on Natural Circulation Phenomena and Passive Safety Systems in Advanced Water-cooled Reactors, ICTP, Trieste, Italy, 17—21 May 2010. 

In generally, a two-phase natural circulation system can be designed to avoid all the different types of instability discussed above. This is done by carrying out a stability analysis, which is usually phenomena-specific and therefore cannot be described in detail in a brief note as the present one. However, the objectives of most stability analyses can be listed as ... 

At low pressures the CIRCUS (at Delft University of Technology) and the PANDA (at Paul Scherrer Institute) facilities will be used to study the natural circulation and stability characteristics. It is known that at low-pressure, low-power operating conditions (e g. start-up) natural circulation flow is susceptible to gravity-driven instabilities. Because of the high sensitivity of the flow to perturbations, these conditions are very challenging for computer codes to predict. CIRCUS will be used for parametric studies [1], whereas specific tests will be performed in the large-scale PANDA test facility. PANDA has already been used to study passive decay heat removal in the ESBWR [3]. 

D AURIA F., GALASSI G.M., Characterisation of instabilities during two-phase natural circulation in PWR typical conditions, J. Experimental Thermal and Fluid Science 3 90 (1990). 

In natural convection plants, there are opportunities for the limits to be set by the absolute power output available from naturally convective flow, and the onset of instability in that flow. We are interested in the ultimate or maximum power output in order to both minimize power generation costs (both capital and operating), and to decide or determine how far the natural circulation designs can be developed. We call this a hypothetical design, to indicate the conceptual nature of the analysis. 

We can derive a formulation for the natural circulation flow, which includes the subcooled length and the effect of flow on the loss coefficients is consistent with homogeneous stability analysis, and extends previous work. This result can then be coupled with the instability treatment to determine the natural-circulation limit on instability. 

It is well known that a two-phase natural circulation flow can exhibit instabilities over certain regimes (Gulshani et al, 1995) so we now examine that phenomenon and give some relevant analytical results. 

The intersection of the natural circulation flow with the unstable region for larger downcomer heights fZ 0.3) is at a value of Np/Ns of about 2, which implies the maximum stable power level for the system is close to the theoretical and experimental stability limit. For lower downcomer heights, L 0.5, the intersection is closer to the minimum value of the unstable region. Thus, we also have the analytical result which determines the instability boundary and the onset of CHF in a natural circulation system. 

The maximum power output in a natural circulation boiling system without a HX is not derived on the basis when the natural circulation driving head is equal to the two-phase losses. Instead, as we have noted above, the ultimate or maximum power output is set by the onset of flow instability and hence subsequent CHF. 

D AURIA, F., GALASSI, G.M. Characterisation of Instabilities during Two-Phase Natural Circulation in PWR typical conditions J. Experimental Thermal and Fluid Science, Vol. 3, 1990. 

---