Robert E. Henry

The Two-Phase Critical Discharge of Initially Saturated or Subcooled Liquid

Nonequilibrium models describing two phase critical discharge of initially saturated or subcooled liquid through sharp edged and smooth inlet geometries

Thermophoretic Deposition of Particles in Natural Convection Flow From a Vertical Plate

Analyse du transport par thermophorese de petites particules a travers la couche limite de convection naturelle adjacente a une surface verticale froide. Resolution numerique des equations de couche limite gaz-particule pour les ecoulements laminaire et turbulent. Proposition dune correlation simple entre la vitesse de transport des particules et le coefficient de transfert de chaleur pour les 2 regimes decoulement

External cooling of a reactor vessel under severe accident conditions

Abstract The TMI-2 accident demonstrated that a significant quantity of molten core debris could drain into the lower plenum during a severe accident. For such conditions, the Individual Plant Examinations (IPEs) and severe accident management evaluations, consider the possibility that water could not be injected to the RCS. However, depending on the plant specific configuration and the accident sequence, water may be accumulated within the containment sufficient to submerge the lower head and part of the reactor vessel cylinder. This could provide external cooling of the RPV to prevent failure of the lower head and discharge of core debris into the containment. This paper evaluates the heat removal capabilities for external cooling of an insulated RPV in terms of (a) the water inflow through the insulation, (b) the two-phase heat removal in the gap between the insulation and the vessel and (c) the flow of steam through the insulation. These results show no significant limitation to heat removal from the bottom of the reactor vessel other than thermal conduction through the reactor vessel wall. Hence, external cooling is a possible means of preventing core debris from failing the reactor, which if successful, would eliminate the considerations of ex-vessel steam explosions, debris coolability, etc. and their uncertainties. Therefore, external cooling should be a major consideration in accident management evaluations and decision-making for current plants, as well as a possible design consideration for future plants.

Debris interactions in reactor vessel lower plena during a severe accident II. Integral analysis

Abstract The integral physico-numerical model for the reactor vessel lower head response has been exercised for the TMI-2 accident and possible severe accident scenarios in PWR and BWR designs. The proposed inherent cooling mechanism of the reactor material creep and subsequent water ingression implemented in this predictive model provides a consistent representation of how the debris was finally cooled in the TMI-2 accident and how the reactor lower head integrity was maintained during the course of the incident. It should be recalled that in order for this strain to occur, the vessel lower head had to achieve temperatures in excess of 1000 °C. This is certainly in agreement with the temperatures determined by metallographic examinations during the TMI-2 Vessel Inspection Program. The integral model was also applied to typical PWR and BWR lower plena with and without structures under pressurized conditions spanning the first relocation of core material to the reactor vessel failure due to creep without recovery actions. The design application results are presented with particular attention being focused on water ingression into the debris bed through the gap formed between the debris and the vessel wall. As an illustration of the accident management application, the lower plenum with structures was recovered after an extensive amount of creep had damaged the vessel wall. The computed lower head temperatures were found to be significantly lower (by more than 300 K in this particular example) with recovery relative to the case without recovery. This clearly demonstrates the potential for in-vessel cooling of the reactor vessel without a need to externally submerge the lower head should such a severe accident occur as core melting and relocation.

Transient Freezing of a Flowing Ceramic Fuel in a Steel Channel

The extent of penetration of flowing molten ceramic fuel in steel channels before solidification is a problem that arises in the analysis of hypothetical core disruptive accidents. Considerations of fuel crust behavior indicate that fuel freezing in steel channels can occur in two distinct ways that can be identified as conduction-limited freezing (fuel crust growth) and bulk freezing (fuel crust removal). Fuel crust removal can arise from two sources: (a) mechanical breakup and (b) melting heat transfer. Explicit formulas providing rough estimates of critical fuel crust removal conditions are presented. If the conditions in the fuel flow are such to prevent fuel crust growth then the steel wall melting can become severe. It is proposed here that steel ablation rapidly leads to fuel freezing in a bulk manner via turbulent mixing between the relatively cold molten steel and hot molten fuel. This steel ablation-induced freezing concept is used to obtain a simple expression for molten fuel penetration into steel channels.

Debris interactions in reactor vessel lower plena during a severe accident. I. Predictive model

An integral predictive physico-numerical model has been developed to understand and interpret debris interactions in the reactor vessel plenum such as those which took place in the TMI-2 accident. The model represents the extent of debris jet disintegration by a jet-water entrainment model which can result in two types of debris configurations. One is particulated debris which eventually quenches in the water as a result of the entrainment process. The remainder of the debris penetrates to the bottom of the lower plenum and collects as a continuous layer. Each is treated as a separate region and has governing principles for its behavior. The potential for creating gap (contact) resistance and boiling heat removal is considered for heat transfer between the debris bed, the reactor vessel and steel structures and, most importantly, the vessel-to-crust gap water. The proposed in-vessel cooling mechanism due to material creep and water ingression into the expanding gap between the core debris and the vessel wall was found to explain the non-failure of the TMI-2 vessel in the course of the accident. The particulate debris bed is a mixture of metal and oxide, which is distributed as individual spherical particles of sizes determined at the time of entrainment. Energy is received from the continuum bed below by radiation and convection. The continuum debris bed is described by the crust behavior with the heat flux to the crust given by the natural convection correlations relating the Nusselt and Rayleigh numbers for the central region of debris. Using these governing principles, the rate laws for heat and mass transfer are formulated for each type of debris condition in the lower plenum. With the integration of the individual rates, the formation, growth and possible shrinkage of these regions are calculated. The potential reactor vessel breach is accounted for by considering the combined thermal and mechanical response of the vessel wall. The two-step failure model allows the vessel to fail at two different locations and at two different times.

Cooling of Core Debris Within the Reactor Vessel Lower Head

Under severe accident conditions, the most crucial action for recovery from the accident state is to cool the core debris and prevent or terminate attack on the remaining fission product barriers. One means of preventing attack on the containment structures is to retain the core debris within the reactor vessel. Some accident situations could result in the transport of molten core debris to the lower plenum, as occurred to some extent ([approximately]20 tonnes) during the TMI-2 accident, boiloff of water in the lower plenum, and an inability to add water to the reactor coolant system (RCS). In this extreme set of circumstances, sufficient external reactor pressure vessel (RPV) cooling may be available to prevent failure of the RPV lower head and, thereby, retain the core debris within the vessel. Containment configurations like Zion would result in substantial accumulation of water around the lower parts of the reactor vessel for most accident sequences. The experiments which were performed in support of the Commonwealth Edison individual plant examination and accident management programs, are heat transfer tests designed to demonstrate that nucleate boiling is the dominant heat removal process from the outer surface of a simulated RPV lower head surrounded by typical reflectivemorexa0» insulation used in nuclear power plants.«xa0less

Two-Phase Critical Flow at Low Qualities Part I: Experimental

Steam-water, two-phase critical flows were obtained in long pipes (L/D > 40) for mass flow rates ranging from 512 to 6460 lbm/(sec ft2), exit pressures from 40 to 150 psia, and thermodynamic equilibrium qualities from 0.0019 to 0.216. A comparison of the three test sections employed indicates that previous experimental data are in error for qualities less than 0.10 due to the influence of the downstream two-dimensional expansion on wall pressure taps located near the exit plane.Although simultaneous temperature and pressure measurements were not taken, the data exhibit trends that suggest the existence of a nonequlibrium phase change. Experimentally determined exit and axial void fractions indicate (a) that the velocity ratios are considerably less than the existing analytical predictions and (b) that previously dissolved gases existing from the liquid provide a source for vapor formation under adiabatic subcooled conditions.

Film boiling on a reactive surface

Abstract To help understand the rapid oxidation of high-temperature materials immersed in water, we treat here the flow of a liquid over a reactive body; the temperature of the body is such that the liquid undergoes film boiling at its surface. Contained within the film that envelopes the surface is the evaporated liquid which diffuses to the surface and reacts there to form product gas which diffuses away from the surface. The two-phase flow and heat and mass transfer problem which arises is formulated within the framework of steady-state stagnation flow theory. The theory is applied to the quasi-steady oxidation of molten zirconium spheres falling through water and predicts results which are consistent with available zirconium sphere oxidation data.

Integral analysis of debris material and heat transport in reactor vessel lower plenum

Abstract An integral, fast-running, two-region model has been developed to characterize the debris material and heat transport in the reactor lower plenum under severe accident conditions. The debris bed is segregated into the oxidic pool and an overlying metallic layer. Debris crusts can develop on three surfaces: the top of the molten pool, the RPV wall, and the internal structures. To account for the decay heat generation, the crust temperature profile is assumed to be parabolic. The oxidic debris pool is homogeneously mixed and has the same material composition, and hence the same thermophysical properties, as the crusts, while the metallic constituents are assumed to rise to the top of the debris pool. Steady-state relationships are used to describe the heat transfer rates, with the assessment of solid or liquid state, and the liquid superheat in the pool being based on the average debris temperature. Natural convection heat transfer from the molten debris pool to the upper, lower and embedded crusts is calculated based on the pool Rayleigh number with the conduction heat transfer from the crusts being determined by the crust temperature profile. The downward heat flux is transferred to the lowest part of the RPV lower head through a crust-to-RPV contact resistance. The sideward heat flux is transferred to the upper regions of the RPV lower head as well as to the internal structures. The upward heat flux goes to the metal layer, water, or available heat sink structures above. Quenching due to water ingression is modeled separately from the energy transfer through the crust. The RPV wall temperature distribution and the primary system pressure are utilized to estimate challenges to the RPV integrity. Should the RPV be submerged, the heat removal is enhanced by the ex-vessel cooling due to nucleate boiling. The convection heat transfer correlations for the molten debris pool were validated against available experimental data and theoretical predictions. Testing of the model for a range of conditions in a PWR lower plenum produced consistent results. In addition, a comparison of the integral approach to a more detailed, special purpose model showed good agreement.