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This paper reports on two large-scale decay heat removal experiments analyzed to support the validation of the thermal-hydraulic code COMMIX and the design of advanced liquid metal reactors (ALMRs). The experiments were performed in the reactor vessel auxiliary cooling system (RVACS) test facility, which provides a scaled simulation of the passive decay heat removal paths of a pool ALMR with the core simulated by electrically heated rods. The first experiment simulates a transient where decay heat is removed by the direct reactor auxiliary cooling system (DRACS) only. In the second experiment, heat is removed by both the DRACS and RVACS. These experiments are characterized by (a) three-dimensional in-pool sodium flows of very low velocity, driven by sodium density differences, (b) a significant pool thermal stratification, and (c) a complex heat sink.

This paper reports on the effects of design parameters on the performance of the reactor vessel auxiliary cooling system (RVACS) of a pool liquid-metal reactor (LMR). These parameters include stack height, size of the airflow gap, system pressure loss, fins on the guard vessel or the baffle wall, and repeated ribs on the airflow channel walls. As a measure of performance , the peak sodium pool temperature during transient following a reactor scram from full power was used. Horizontal ribs with a 0.003-m height and a 0.015-m pitch gave the best performance, i.e., the lowest peak sodium pool temperature during the scram transient. For a 3500-MW(thermal) LMR, they gave peak hot pool and peak cladding temperatures that were 52{degrees}C lower than those obtained with a reference RVACS having smooth airflow channel walls.

This paper characterizes the areas of Integral Fast Reactor (IFR) safety research in terms of their importance in addressing the risk of core disruption sequences for innovative designs. Such sequences have traditionally been determined to constitute the primary risk to public health and safety. All core disruption sequences are folded into four fault categories: classic unprotected (unscrammed) events; loss of decay heat; local fault propagation; and failure of critical reactor structures. Event trees are used to describe these sequences and the areas in the IFR Safety and related Base Technology research programs are discussed with respect to their relevance in addressing the key issues in preventing or delimiting core disruptive sequences. Thus a measure of potential for risk reduction is obtained for guidance in establishing research priorites.

Two RVACS tests, A and B, were analyzed to support the validation of COMMIX and the design of Advanced Liquid Metal Reactors (ALMRs). The RVACS test facility provides a scaled simulation of the decay heat removal paths of a pool ALMR during the operation of the passive decay heat removal systems with the reactor simulated with electrically heated rods. Test A was characterized by a power input of 50 kW, natural convection in the sodium pool, forced RVACS air circulation, sodium pool overflow, and a heat up period of 8 h. In test B the power was 64 kW and the sodium pool heatup led to overflow at ∼ h from the beginning of the transient. In test A, after 7.5 h, the system reached near steady state with a temperature difference between the bottom and top of the pool of 96°C. In both tests the vertical temperature gradient in the hot pool remained small throughout the transient while the cold pool exhibited a significant stratification. The sodium temperature variations in the radial and azimuthal directions were insignificant. The COMMIX predictions for the sodium pool temperatures and the air outlet temperatures were in good agreement with measurements.

A review of existing modeling concepts and studies of sodium-concrete reactions is presented. Consistent with experimental observations, the current modeling study being conducted at Hanford Engineering Development Laboratory assumes for hydrated concretes the presence of liquid layer of reaction products intervening between sodium pool and concrete surface. Primary liquid component in this layer is NaOH which has a low melting point. This liquid component dissolves the reaction products such as silicates, aluminates and forms a very viscous liquid more dense than sodium. As this layer assumes a significant thickness, the only mechanism available for transport of sodium to fresh concrete surface is the motion and agitation induced by gas bubbles consisting of hydrogen, water vapor, CO2 and sodium vapor. However, to date there exists no satisfactory model that describes this transport mechanism. To rectify this shortcoming, we propose a mass “iffusion” model for sodium transport. The model reduces the sodium transport process by bubble motion to a single unknown parameter which has the appearance of a diffusion coefficient and consequently can be determined by solving an inverse problem in conjunction with measured “concentration” distributions in simulant material experiments.