Showing papers by "Hideo Nakamura published in 2002"

Visualization and Measurement of Subcooled Water Jet Injection into High-Temperature Melt by Using High-Frame-Rate Neutron Radiography

Abstract: The dynamic plunging behavior of a subcooled water jet into a pool of molten lead-bismuth alloy, visualized and measured using high-frame-rate neutron radiography, is described. It is shown that the interactions between water and heavy melt in this geometry differ in several aspects from those in melt injection into water investigated extensively in relation to fuel-coolant interactions. The maximum depth of jet penetration is limited by the buoyancy on and the onset of bulk boiling in the water which has collected in a 'cavity'. The bulk boiling starts as the subcooled water supply to the cavity becomes limited due to pinching instabilities in the upper region of the cavity. This leads to a transition to the final steady state.

A Simple Evaluation Method of the Molten Fuel Amount in a Premixing Region of Fuel-Coolant Interactions

Abstract: A simplified method to evaluate the premixing molten core mass is proposed, based on the molten jet breakup length and settling/cooling dynamics of the particles generated from the jet surface. The present method was applied to KROTOS experiments at JRC Ispra with alumina and corium melts, as well as to a prototypic geometry for an ex-vessel fuel-coolant interaction during a PWR severe accident. The calculation on the KROTOS showed that the corium melt makes a premixture containing much less fraction of molten droplets than alumina, giving an explanation on the experimental fact that corium is hard to make a steam explosion. The calculation with PWR ex-vessel conditions showed that the particle mass in the premixing region tends to increase much slower than the total molten core ejected to the water pool, and that the molten mass in the premixing region is 6.5 t when 100 t of the core was ejected at 20 s after the start of the melt ejection. Sensitivities on the assumed molten core particle size, the void...

Multi-Dimensional Thermal-Hydraulic Analysis for Horizontal Tube Type PCCS

Abstract: A passive containment cooling system (PCCS) using a shell-and-tube type heat exchanger has been developed as a decay heat removal system following a severe accident. A horizontal heat exchanger has been studied for the PCCS heat exchanger since it has several advantages over a vertical large diameter tube heat exchanger that was originally proposed for the SBWR. Based on the fundamental thermal-hydraulic test using a single horizontal U- tube, the feasibility of the horizontal tube type PCCS has been confirmed, and the analysis models for the steam condensation heat transfer with a non-condensable and the pressure loss with steam condensation have been established. In addition, the criterion for the film dryout type heat transfer deterioration in the boiling side has been clarified in the test. The heat exchanger performance is affected by the multi-dimensional thermal-hydraulic behavior in the cooling water pool and the interactions among the multiple heat transfer tubes. In order to clarify the multi-dimensional behavior, numerical analyses have been conducted employing a two-fluid model. From the analyses results, it has been confirmed that the horizontal PCCS heat exchanger meets the design requirements for both the heat removal and the pressure loss and there would be no film dryoutmore » type heat transfer deterioration occurred in the cooling pool. (authors)« less

Unit Sphere Concept for Macroscopic Triggering of Large-scale Vapor Explosions

Abstract: The unit sphere concept was developed to predict the triggering stage of vapor explosions for coarse mixtures composed of hot liquid droplets, cold liquid and its vapor. With an assumption that hot liquid droplets are arranged with a uniform spatial interval to construct a hexagonal cell structure, a unit sphere with thirteen droplets is formed in the coarse mixture. A droplet and adjacent twelve droplets were placed at the center and on the surface of the unit sphere, respectively. Two indices for triggering were introduced in the unit sphere concept. The first index is the ratio of mechanical energy generated at the center droplet to one required for the mechanical collapse of vapor film on an adjacent droplet. Another shows the probability that the mechanical energy at the center droplet impacts onto the minimum number of molten adjacent droplets. The present concept predicted that the triggering could occur at smaller water subcooling for a coarse mixture with alumina droplets and water than corium droplet case, and that vapor explosions were suppressed when the ambient pressure was elevated up to approximately 0.5 MPa in both cases. The evaluation of KROTOS experiments indicated that the latter triggering index was smaller for corium droplets than alumina case due to the increase in the fraction of solidified droplets in the coarse mixture, implying less triggerability for corium droplets. Those findings showed a consistency with the results of vapor explosion experiments using corium and alumina. It was qualitatively confirmed in the experiments where a molten tin jet penetrated into a water pool that the latter index is applicable to the evaluation of the triggerability.

Roll Wave Effects on Annular Condensing Heat Transfer in Horizontal PCCS Condenser Tube

Abstract: A horizontal in-tube condensation heat exchanger is under investigation to be used for a passive containment cooling system (PCCS) of a next generation-type BWR. The flow conditions in the horizontal condenser tube were observed both visually and by local void fraction fluctuation. The observed flow regimes at a rated condition were annular flow at the tube inlet, and turned gradually into wavy flow and smooth stratified flow along the length of the tube. It was found further that frequency of the roll waves that appear on the liquid film in the annular flow is closely related to the measured local condensation heat transfer coefficient. Based on the flow observation, the roll wave frequency and measured condensation heat transfer coefficient, a model is proposed which predicts the condensation heat transfer coefficient particularly for annular flows around the tube inlet region. The proposed heat transfer model predicts well the influences of pressure, local gas-phase velocity and film thickness. (authors)