Grid spacer

To account for the mixing effect of the grid spacer, the mixing vane grid spacer factor, Fs, is used ... 

Zirconium alloys are chiefly used as the cladding (structural) metal in nuclear reactors. The alloys are also used as grid spacers, guide tubes, pressure tubes, calandria tubes and other minor components. 

Upper support. umI grid spacer castings Upper end box FSel plntes Lover end box... 

In the analysis of flow in nuclear reactor cores, the geometry usually consists of plates with narrow rectangular channels, rods having square or triangular subchannels with rods located at the vertices. The plates and rods have wire or grid spacers to maintain separation between plates and rods. For these geometries various expressions for friction factors are available in the literature that enables one to calculate the pressure drop in the reactor core (Rust, 1979 Todreas and Kazimi, 1990). 

In the fuel subassembly, fuel pins are assembled with grid spacers and a top shield is installed to prevent activation of the EM pumps and the secondary sodium in the MX. Coolant inlet modules located beneath the fuel subassembly provide a lower shielding for the reactor internal structures including the core support plate and air in the RVACS. 

Enhancement of core radial expansion relative to grid spacer thermal expansion 0.1 0.1 0.1... 

FIG. XXIV-3. Open sidedfuel assembly with local grid spacer, mounted on a central heat pipe support spline. 

The coupled neutronics/thermo-hydraulic/thermo-structural reactivity feedback design approach for the STAR-H2 reactor has achieved the proper ratio between that reactivity which is vested in the coolant temperature rise relative to inlet temperature vis-a-vis that reactivity which is vested in the fuel temperature rise above the coolant, and at the same time in having designed an overall coolant flow circuit pressure drop tailored to cause coolant flow rate to adjust properly to changes in pressure driving head caused by source/sink temperature difference. A non-conventional open-pitch ductless fuel assembly structural design coupled with a non-conventional core support approach (the assemblies tend to neutral buoyancy in the dense Pb coolant) has been proposed to simultaneously provide low pressure drop, structural reliability of grid spacers, and an appropriate value for coolant power/flow reactivity temperature coefficient. 

The fuel elements are separated by a spiral wrapping wire (alternatively, grid spacers can be used). The sodium coolant flows through the spaces between the fuel elements. The fuel elements are placed in a tight triangular lattice arrangement to maximize the fuel volume fraction for core neutron performances and to minimize the core size for the plant capital cost reduction. 

The melt temperature of zircaloy is 3350 F (1843°C = 2116 K) however, the onset of Stage 4 may occur at lower temperatures if the core contains significant quantities of other metals with low melt temperatures. At TMI-2, a Ni-Zr eutectic was probably the first liquid formed as a result of interactions between the Inconel grid spacers and zircaloy cladding near the center of the core. The TMI-2 stainless steel... 

The axial extent of material relocation is typically within the distance between grid spacers, i.e., <0.5 m. The relocation phenomenon is heterogeneous with the sources of molten material originating from different components within the oxidation burn front and freezing at different locations lower in the bundle elevations. Grid spacers act as both sources and sinks of molten metal. 

Figure 6. Influence of grid spacers in selected CORA experiments.

Area of fuel rod surface wetted by liquefied grid spacer. 

L. J. Siefken and M. V. Olsen, "Effect of Inconel Grid Spacers on Progression of Damage in Reactor Core", Fifth International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Salt Lake City, September 21-24, 1992. 

Startup. The critical heat flux correlations should be improved, including the effect of grid-spacers on the boiling transition. 

Watts-Chou correlation was employed as the heat transfer correlation for the core design of the Super LWR and Super FR, because of the accuracy and applicability to both upward and downward flow cooling. For safety analysis, the Oka-Koshizuka correlation is used. CFD analysis of the effect of grid spacers on heat transfer correlation is foimd in ref. [124]. 

Heat transfer experiments and material research studies have been carried out at Kyushu University, IAEA, the University of Tokyo and elsewhere. Comparison of heat transfer coefficients predicted by different correlations is shown in Fig. 1.60. Accuracy above 500°C is important for the calculation of MCST. Calculated MCSTs with different correlations are compared in Table 1.16 [122]. The largest difference is 44°C. Current heat transfer correlations were developed based on experiments using smooth circular tubes. Experiments on fuel bundle geometry are necessary. The effect of grid spacers on the heat transfer correlation should be included in the prediction of MCST. The correlation of downward flow is necessary for the design. Downward flow is adopted in the low temperature region below the pseudo-critical temperature. 

Y. Ishiwatari, I. Kongo, Y. Oka, et al., Numerical Analysis of Heat Transfer Enhancement by Grid Spacers in Supercritical Water, Proc. NURETH-13, Kanazawa, Japan, September 27-October 2, 2009, N13P1436 (2009)... 

I. Hongo, Numerical Analysis of Heat Transfer Enhanccanent by Grid Spacers on High Temperature Supercritical Fluid , Master s thesis, the Univtasity of Tokyo (2009) (in Japanese)... 

The fuel is enriched uranium dioxide with 95% T.D. The fuel rods are arranged in the tight triangular lattice with grid spacers (Fig. 2.33 [18]). The fuel rod... 

The pressure loss by the grid spacer is evaluated with (2.31) ... 

The Oka-Koshizuka heat transfer correlation [7] and Watts-Chou correlation [25] are used to evaluate the cladding surface temperature for upward and downward flow regions, respectively, which is consistent with those in thermal-hydraulic coupled nuclear calculations. Heat transfer improvement by the grid spacer is not considered for conservatism. 

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