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In summary, a scaling analysis of a water-cooled Reactor Cavity Cooling System (RCCS) system was performed based on generic information on the RCCS design of PBMR. The analysis demonstrates that the water-cooled RCCS can be simulated at the ANL NSTF facility at a prototypic scale in the lateral direction and about half scale in the vertical direction. Because, by necessity, the scaling is based on a number of approximations, and because no analytical information is available on the performance of a reference water-cooled RCCS, the scaling analysis presented here needs to be 'validated' by analysis of the steady state and transient performance of a reference water-cooled RCCS design. The analysis of the RCCS performance by CFD and system codes presents a number of challenges including: strong 3-D effects in the cavity and the RCCS tubes; simulation of turbulence in flows characterized by natural circulation, high Rayleigh numbers and low Reynolds numbers; validity of heat transfer correlations for system codes for heat transfer in the cavity and the annulus of the RCCS tubes; the potential of nucleate boiling in the tubes; water flashing in the upper section of the RCCS return line (during limiting transient); and two-phase flow phenomena in the water tanks. The limited simulation of heat transfer in cavities presented in Section 4.0, strongly underscores the need of experimental work to validate CFD codes, and heat transfer correlations for system codes, and to support the analysis and design of the RCCS. Based on the conclusions of the scaling analysis, a schematic that illustrates key attributes of the experiment system is shown in Fig. 4. This system contains the same physical elements as the PBMR RCCS, plus additional equipment to facilitate data gathering to support code validation. In particular, the prototype consists of a series of oval standpipes surrounding the reactor vessel to provide cooling of the reactor cavity during both normal and off-normal operating conditions. The standpipes are headered (in groups of four in the prototype) to water supply (header) tanks that are situated well above the reactor vessel to facilitate natural convection cooling during a loss of forced flow event. During normal operations, the water is pumped from a heat sink located outside the containment to the headered inlets to the standpipes. The water is then delivered to each standpipe through a centrally located downcomer that passes the coolant to the bottom of each pipe. The water then turns 180{sup o} and rises up through the annular gap while extracting heat from the reactor cavity due to a combination of natural convection and radiation across the gap between the reactor vessel and standpipes. The water exits the standpipes at the top where it is headered (again in groups of four) into a return line that passes the coolant to the top of the header tank. Coolant is drawn from each tank through a fitting located near the top of the tank where it flows to the heat rejection system located outside the containment. This completes the flow circuit for normal operations. During off-normal conditions, forced convection water cooling in the RCCS is presumed to be lost, as well as the ultimate heat sink outside the containment. In this case, water is passively drawn from an open line located at the bottom of the header tank. This line is orificed so that flow bypass during normal operations is small, yet the line is large enough to provide adequate flow during passive operations to remove decay heat while maintaining acceptable fuel temperatures. In the passive operating mode, water flows by natural convection from the bottom of the supply tank to the standpipes, and returns through the normal pathway to the top of the tanks. After the water reaches saturation and boiling commences, steam will pass through the top of the tanks and be vented to atmosphere. In the experiment system shown in Fig. 4, a steam condensation and collection system is included to quantify the boiling rate, thereby providing additional validation data. This system does not exist in the prototype. The first part of the report presents the non-dimensional conservation equations describing the response of the water-based RCCS during steady-state and transient operations. These equations are used to develop similarity relationships that define a scaling of the NSTF that minimizes distortions between scaled experiments and prototype operation. Then, some limited simulations of heat transfer in cavities are presented that strongly underscore the need of experimental work to validate CFD codes, and heat transfer correlations for system codes, and to support the analysis and design of the RCCS. Finally, a summary is provided of the NSTF modifications needed to conduct scaled simulations of the RCCS

Feasibility study for use of the natural convection shutdown heat removal test facility (NSTF) for VHTR water-cooled RCCS shutdown.