Natural-circulation loops

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. 

By cutting out a section of heated length of a Freon loop at the inlet and restoring the original flow rate, Crowley et al. (1967) found that the reduction of the heated length increased the flow stability in forced circulation with a constant power density. A similar effect was found in a natural-circulation loop (Mathisen, 1967). Crowley et al. (1967) further noticed that the change of heated length did not affect the period of oscillation, since the flow rate was kept constant. 

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. 

Holman, J. P., and J. H. Boggs Heat Transfer to Freon 12 near the Critical State in a Natural Circulation Loop, J. Heat Transfer, vol. 80, p. 221, 1960. 

A High Pressure Natural Circulation Loop (CAPCN) was constructed and operated to produce data in order to verify the thermal hydraulic tools used to design CAREM reactor. The CAPCN reproduces the dynamics phenomena of the CAREM primary cooling system, except for the three-dimensional effects. Dynamical experiment data are being used to test our numerical procedures and codes [1]. 

The reaaor residual heat is removed by a passive residual heat removal stem which connects to the intermediate circuit. There are two independent trains of theRHRS which composed of three natural circulation loops. (See Fig. 4)... 

Alternatively, primary water may be diverted to an auxiliary heat exchanger. An example of this arrangement is in AP600 where there is a natural circulation loop to a heat exchanger in the in-containment refuelling water storage tank which is brought into operation automatically. The tank has capacity for 72 hours of residual heat removal. 

Passive core cooling system X Natural circulation loop from pool, through core nser, to pool... 

Long-term passive RHR system X 8 coolers submerged m reactor pool, 8 natural circulation loops to natural-draft cooling towers... 

The molten salt coolant natural circulation loop includes the reactor core filled with spherical fuel elements and absorber and graphite elements the top, bottom and annular side reflectors the draught section (chimney) and salt-air heat exchangers. The side reflector material consists of the circulating molten salt coolant. 

Decay heat removal. Several types of passive decay heat removal systems have been used in liquid-metal reactors. The AHTR, like S-PRISM, uses RVACs. There are other options such as DRACs, a secondary natural circulation loop to remove heat from the reactor vessel to the environment. This provides multiple longer-term cooling options including the options that may ultimately allow larger power outputs. 

In static tests, tantalum showed good resistance to mercury to temperatures of 600°C. Refluxing capsule tests showed no attack up to 760°C. The excellent corrosion resistance of tantalum to mercury was further verified in a two-phase natural circulation loop test that ran for 19,975 h with a boiling temperature of 649°C and a superheat temperature of 704°C. Posttest evaluation of the loop showed no corrosion. 

The PMCS serves as an additional barrier to core damage. In an accident scenario, decay heat generated in the fuel within the fuel channel is transferred through radiation from the cladding to the inner liner of the insulator, flows through the channel insulator and pressure tube, and is deposited into the moderator. The PMCS uses a flashing-driven natural circulation loop to remove heat from the moderator, and it deposits the heat into the reserve water pool. 

The high-temperature salt molten salt test loop and the nitrate natural circulation loop operate with the liquid nitrate (Han et al., 2013). 

Figure 15.8 Schematic of lead-bismuth eutectic natural circulation loop (operating at 1000°C).

An IHTR is a pebble-bed molten salt-cooled reactor. Pebbles consist of TRISO-coated particle fuel, and the coolant is driven through natural circulation. The reactor core is a long right circular cylinder with an annular core that consists of fuel pebbles and molten salt coolant. Fig. 15.13 shows a schematic of a 600-MWth IHTR. There are graphite neutron reflectors in the center and on the top, bottom, and outside of this fuel annulus. Vertical bores in the central and outer reflectors are provided for the reactivity control elements. R D activities being pursued include a molten salt natural circulation loop, as shown in Fig. 15.14, which has been set up to perform thermal... 

Figure 15.13 Molten salt natural-circulation loop.

Provide natural-circulation loops for systems with inherently large heat capacity coolants or... 

Natural-circulation loop and parallel channel thermal-hydraulics... 

Natural-circulation loop (NCL) thermal-hydrauhcs are an essential aspect in the design, operation, and safety of all Gen IV concepts. Some concepts rely on natural... 

Supercritical fluid states and natural-circulation loops... 

Figure 16.4 Two vertical coupled natural-circulation loops.

Angelo, G., Andrade, D.A., Angelo, E., Torres, W.M., Sabundjian, G., Macedo, L.A., Silva, A.F., 2012. A numerical and three-dimensional analysis of steady state rectangular natural circulation loop. Nuclear Engineering and Design 244, 61—72. 

Basu, D.N., Bhattacharyya, S., Das, P.K., 2014. A review of modem advances in analyses and applications of single-phase natural circulation loop in nuclear thermal hydrauUcs. Nuclear Engineering and Design 280, 326—348. 

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. 

CreveUng, H.F., Schoenhals, R.J., 1966. Steady Flow Characteristics of a Single-Phase Natural Circulation Loop. Technical Report No 15. Purdue Research Foundation. COO-1177-15. 

Debrah, S.K., Ambrosini, W., Chen, Y., 2013. Discussion on the stability of natural circulation loops with supercritical pressure fluids. Annals of Nuclear Energy 54, 47—57. 

Garlid, K., Amimdson, N.R., Isbin, H.S., 1961. A Theoretical Study of the Transient Operation and Stability of Two-Phase Natural Circulation Loops. Argonne National Laboratory Report, ANL-6381. 

Greif, R., 1988. Natural circulation loops. Journal of Heat Transfer 110, 1243—1258. 

Harden, D.G., 1963. Transient Behavior of a Natural-circulation Loop Operating Near the Thermodynamic Critical Point. Argonne National Laboratory Report, ANL-6710. 

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

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