Melt-coolant Interactions Research Articles

An experimental study on droplet quench and steam explosion in boric acid solutions

Boric acid (H3BO3) is widely adopted as an additive in the coolant of light water reactors for reactivity control, but its effect on fuel coolant interactions (FCI) during severe accidents (especially on steam explosion) was rarely investigated. To examine the effect of the boric acid additive in coolant on steam explosion, a series of molten droplet-coolant interaction tests using H3BO3 solutions (with concentration ranging from 0–3.2% by weight) is carried out in the present study. The characteristics of melt-coolant interactions are the occurrence probability of typical phenomena (no fragmentation, minor fragmentation, or spontaneous steam explosion), lateral deformation ratio, quench depth, pressure impulse and debris particle size distribution. The statistical data of such characteristics are obtained through repeating 20 runs of the same test category. The experimental results show that the H3BO3 addition in coolant has various impacts on the above-mentioned characteristics of melt-coolant interactions, depending on the H3BO3 concentration. In particular, the probability of steam explosion sightly decreases as the H3BO3 concentration increases from zero to 1.2 wt.%, but significantly increases as the H3BO3 concentration further increases to 3.2 wt.% trough 2.2 wt.%. Namely, the inhibiting effect of boric acid on steam explosion is diminishing with increasing H3BO3 concentration beyond 1.2 wt.%. It is also found that both melt and coolant temperatures are crucial parameters impacting the likelihood and energetics of steam explosion.

Experimental investigation on debris bed formation from metallic melt coolant interactions

Motivated by risk quantification of severe core meltdown accidents which may occur light water reactors, an extensive DEFOR (DEbris FORmation) test series with various molten oxidic materials was conducted previously at Royal Institute of Technology to investigate the characteristics of debris beds formed from oxidic melt-water interactions, which are important to the coolability of debris beds and thus the retention of core melt. Yet, little attention has been paid to metallic melt which exist in the core melt. The present study is intended to fill in the knowledge gap, i.e., to characterize metallic melt-water interactions when a melt jet falls into a deep water pool. Nine DEFOR-M tests are carried out to clarify the effects of melt jet diameter, free fall height and water pool depth on metallic debris formation characteristics. Molten tin of 20 kg is employed as the simulant of metallic melt. A melt sensor and a mass sensor are installed to detect the discharging period of coherent jet and the mass accumulation of debris bed, respectively. A high-speed camera is applied to record the process of jet breakup, debris fragmentation and solidification, and debris bed formation. The experimental results show that the water pool depth has a significant influence on steam explosion and debris bed formation characteristics. Moreover, the jet breakup length is sensitive to jet diameter and free fall height. Finally, the features of metallic debris beds are compared with those of oxidic ones in respects of debris bed's configuration and porosity, particles' morphology and size distribution.

Experimental investigation on ex-vessel debris bed formation using low melting-point melt of binary metals

During severe accidents in a light water reactor, the core melt (corium) may relocate to the lower head and fail the reactor pressure vessel (RPV). The corium will be ejected to the reactor cavity upon the RPV failure and undergo melt coolant interactions (FCI) if the cavity is flooded with water. The FCI process does not only determines the characteristics of the resulting debris bed which are important to coolability, but also induces a steam explosion risk which may threaten containment integrity. The present study is concerned with characterization of debris bed formed from FCI of metal-rich corium failing into a deep water pool in the reactor cavity. Low melting-point metals Tin and Tin–Bismuth (20 kg) were employed as the simulant materials of metal-rich corium melt. Ten tests were carried out on the DEFOR-M test facility at KTH to investigate the effects of various parameters on debris bed formation, such as melt superheat, coolant subcooling, material. The melt jet fragmentation and fragments movement in the water pool as well as debris deposition on the pool floor were recorded by high-speed cameras. Melt sensors and weight sensors were installed to detect the period of melt jet discharge and the mass of forming debris bed. The porosity of debris bed was obtained through the debris bed volume measured by a three-dimensional laser scanner and the pore volume measured by water absorption. The final configuration of debris bed was also reconstructed through the laser scanner data, and the debris particles were sieved for their size distribution. The experimental results revealed the FCI phenomena and debris bed characteristics including configuration and porosity of debris bed as well as morphology and size distribution of debris particles under different melt superheats, coolant subcooling, materials.

A scoping study on debris bed formation from metallic melt coolant interactions

During a hypothetical severe accident in a light water reactor, the core and internal structures melt down and the molten materials (corium) may fall into a water pool, forming a debris bed from so-called fuel coolant interactions (FCI), whose characteristics are important to corium coolability. The present study is concerned with the characterization of an ex-vessel debris bed forming form the FCI of a metal rich corium jet falling into a water pool in the reactor cavity. Five scoping tests were carried out on the DEFOR-M test facility using metallic Tin melt at different superheats. The molten tin was employed as the simulant of metal (Zr/Fe) rich corium melt. The processes of melt jet fragmentation and debris formation was recorded by high-speed cameras. The contour and volume of the resulting debris bed were measured by a three-dimensional laser scanning system, and the debris particles were sieved for their size distribution. The experimental results revealed the effects of melt superheat and water subcooling on the characteristics of a debris bed, including the bed’s configuration and porosity, the particles’ morphology and size distribution. A preliminary comparison of debris bed characteristics between metallic melt and oxidic melt was also provided.

Development and application of a surrogate model for quick estimation of ex-vessel debris bed coolability

During a hypothetical severe accident of a Nordic boiling water reactor (BWR), an ex-vessel particulate debris bed is expected to form in the flooded lower drywell due to melt-coolant interactions after vessel failure. The key parameter to evaluate debris bed coolability is the dryout heat flux (DHF) or dryout power density, representing the limit of heat removal capacity by the coolant. Several numerical codes such as COCOMO have been developed to simulate thermal hydraulics in multi-dimensional debris beds and predict the cooling limit, but they are computationally expensive and not suitable for probabilistic risk analysis. This paper aims to develop a surrogate model which can serve as a quick-estimate tool for the dryout power density of a heap-like debris bed in a saturated water pool. The dryout power density predicted from the COCOMO code is treated as the full model. A characteristic factor is introduced as the dryout power density ratio between the multi-dimensional debris bed (predicted by COCOMO code) and the corresponding one-dimensional debris bed (predicted by Lipinski 0-D model). The characteristic factor is correlated by the Kriging method with six parameters: bed porosity, particle diameter, debris mass, bed slope, cavity radius and containment pressure. After the surrogate model is trained and validated, it is employed to analyze the coolability of prototypical debris beds of a reference Nordic BWR, given the bed mass and containment pressure from MELCOR simulation. Coolability maps are produced as quick look-up diagrams for identification of coolable domain with the variation of porosity, particle diameter and slope angle. A preliminary uncertainty analysis is performed to demonstrate the effect of uncertain input parameters on non-coolable domain.

Analysis of the Effect of Vessel Failure and Melt Release on Risk of Containment Failure Due to Ex-Vessel Steam Explosion in Nordic Boiling Water Reactor Using ROAAM+ Framework

Abstract Nordic boiling water reactor (BWR) design employs ex-vessel debris coolability in a deep pool of water as a severe accident management (SAM) strategy. Depending on melt release conditions from the vessel and core–melt coolant interactions, containment integrity can be threatened by: (i) formation of noncoolable debris bed or (ii) energetic steam explosion. Melt is released from the vessel affect ex-vessel phenomena and is recognized as the major source of uncertainty. The risk-oriented accident analysis methodology (ROAAM+) is used for quantification of the risk of containment failure in Nordic BWR where melt ejection mode surrogate model (MEM SM) provides initial conditions for the analysis of debris agglomeration and ex-vessel steam explosion which determine the respective loads on the containment. Melt ejection SM is based on the system analysis code methods for estimation of leakages and consequences of releases (computer code) (MELCOR). Modeling of vessel failure and melt release from the vessel in MELCOR is based on parametric models, allowing a user to select different assumptions that effectively control lower head (LH) behavior and melt release. The work addresses the effect of epistemic uncertain parameters and modeling assumptions in MEM SM on the containment loads due to ex-vessel steam explosion in Nordic BWR. Sensitivity and uncertainty analysis performed to identify the most influential parameters and uncertainty in the risk of containment failure due to ex-vessel steam explosion. The results of the analysis provide valuable insights regarding the effect of MELCOR models, modeling parameters, and sensitivity coefficients on melt release conditions and predictions of ex-vessel steam explosion loads on the containment structures.

Studies on hydrodynamic deformation and fragmentation of melt jet injected into water pool using level set method

Abstract The molten core material (corium) that can be formed during severe accident scenarios in light water reactors (LWRs) can drop in the water pool and form a “melt jet” (hereafter referred to as “jet”) flow. We presented a numerical model to study the hydrodynamic deformation and fragmentation of a jet in the water pool using the Level Set (LS) method, despite that previous investigations had employed the widely used Volume-Of-Fluid (VOF) method. The effects of jet inlet speed, diameter and surface tension on the jet breakup length were studied and the results were in good agreement with previously performed experiments and with results obtained using the VOF method. The dimensionless jet breakup length was found to generally decrease with increasing jet inlet speed and increase with increasing jet diameter. In conclusion, the present model could successfully simulate the jet breakup in melt coolant interactions, and the results demonstrated a promising use of the LS method.

Analysis of core degradation and relocation phenomena and scenarios in a Nordic-type BWR

Severe Accident Management (SAM) in Nordic Boiling Water Reactors (BWR) employs ex-vessel cooling of core melt debris. The melt is released from the failed vessel and poured into a deep pool of water located under the reactor. The melt is expected to fragment, quench, and form a debris bed, coolable by a natural circulation and evaporation of water. Success of the strategy is contingent upon melt release conditions from the vessel and melt-coolant interaction that determine (i) properties of the debris bed and its coolability (ii) potential for energetic melt-coolant interactions (steam explosions). Risk Oriented Accident Analysis Methodology (ROAAM+) framework is currently under development for quantification of the risks associated with formation of non-coolable debris bed and occurrence of steam explosions, both presenting a credible threats to containment integrity. The ROAAM+ framework consist of loosely coupled models that describe each stage of the accident progression. Core relocation analysis framework provides initial conditions for melt vessel interaction, vessel failure and melt release frameworks. The properties of relocated debris and melt release conditions, including in-vessel and ex-vessel pressure, lower drywell pool depth and temperature, are sensitive to the accident scenarios and timing of safety systems recovery and operator actions. This paper illustrates a methodological approach and relevant data for establishing a connection between core relocation and vessel failure analysis in ROAAM+ approach. MELCOR code is used for analysis of core degradation and relocation phenomena. Properties of relocated debris are obtained as functions of the accident scenario parameters. Pattern analysis is employed in order to characterize typical behavior of core relocation transients. Clustering analysis is employed for grouping of different accident scenarios, which result in similar core relocation behavior and properties of the debris.

A numerical simulation of jet breakup in melt coolant interactions

During a hypothetical severe accident of a light water reactor (LWR), molten corium could fall in the form of jet into a water pool. The jet fragmentation is crucial process during fuel coolant interactions (FCI) which fragment into droplets and disperse in the coolant, and it may cause a steam explosion. This paper deals with a study of computational fluid dynamics on the melt jet falling into a water pool in order to get qualitative and quantitative understanding of initial premixing phase of FCI. The preliminary objectives to pursue are modeling of jet fragmentation and estimation of the jet breakup length. A commercial CFD code ANSYS FLUENT 14.0 is used for the 2D numerical analysis employing Volume of Fluid (VOF) method. The problem and simulation conditions are similar to the jet breakup tests carried out at KTH (Manickam et al., 2014). Initially, a fragmentation/breakup pattern of the jet is discussed. Further, the effect of jet diameter and the jet injection velocity on the jet breakup length is studied, with a wide range of ambient Weber number (Wea) from 1.25 to 1280. The numerical results compared with the experimental data are in a reasonable agreement. The impacts of physical properties of melt (density, viscosity and surface tension) on the jet breakup lengths are also investigated and presented. Finally the droplet size distributions are obtained based on the simulation results. These preliminary data may be helpful to substantiate the understanding of the phenomena during melt jet interactions with coolant.

An Extension of the Large-Cell Radiation Model for the Case of Semitransparent Nonisothermal Particles

The recently developed model for thermal radiation in multiphase flows typical of melt-coolant interactions is generalized to account for transient temperature profile in large semitransparent particles of solidifying melt. A modification of the large-cell radiation model (LCRM) is based on the approximate solution for coupled radiation and conduction in optically thick spherical particles of a refractive material. The simplicity of the suggested approximation enables one to implement the modified model in a multiphase computational fluid dynamics code. The LCRM extension makes possible the use of this approach not only for the core melt in nuclear fuel-coolant interactions but also for other melt substances, which are widely used in the laboratory experiments. The numerical data demonstrate an effect of absorption coefficient of the particle substance on the rate of particle cooling and solidification.

THERMAL RADIATION MODELING IN NUMERICAL SIMULATION OF MELT-COOLANT INTERACTION

This paper is concerned with radiation heat transfer modeling in multiphase disperse systems, which are formed in high-temperature melt-coolant interactions. This problem is important for complex interaction of the core melt with water in the case of a hypothetical severe accident in light-water nuclear reactors. The nonlocal effects of thermal radiation due to the semitrans- parency of water in the visible and near-infrared spectral ranges are taken into account by use of the recently developed large-cell radiation model (LCRM) based on the spectral radiation energy balance for single computational cells. In contrast to the local approach for radiative heating of water by particles (OMM—opaque medium model), the LCRM includes radiative heat transfer between the particles of different temperatures. The regular integrated code VAPEX-P, intended to model the premixing stage of FCI, was employed for verification of the LCRM in a realis- tic range of the problem parameters. A comparison with the OMM and the more accurate P 1 approximation showed that the LCRM can be recommended for the engineering problem un- der consideration. The effects of the temperature difference in solidifying particles are analyzed by use of the recently suggested approximation of transient temperature profile in the particles. It is shown that the effect of the temperature difference on heat transfer from corium particles to ambient water is considerable and should not be ignored in the calculations. An advanced computational model based on the LCRM for the radiation source function and subsequent inte- gration of radiative transfer equation along the rays is also discussed.

Phenomenological studies on melt-coolant interactions in the ALPHA program

Two series of experiments to investigate melt-coolant interactions have been performed as part of the ALPHA program at JAERI. In the melt drop steam explosion experiments, melt simulating a molten core was dropped into a pool of water. Volume fractions of the melt, water and steam in the mixing region prior to the occurrence of spontaneous steam explosions were quantified. Other characteristics of melt-coolant interactions were evaluated for settling velocity of the melt in water, propagation and expansion velocities, energy conversion ratio and debris size distribution. It was found that the probability of the occurrence of spontaneous steam explosions could be reduced by using a melt dispersion device. Measurement of void fraction in the mixing region clearly showed that the melt dispersion device enhanced steam generation. However, one experiment indicated that the use of the dispersion device could possibly result in a more energetic steam explosion. It was found that the mixing of non-condensable gas in the steam phase of the mixing region during melt dispersion played an important role for the suppression of the spontaneous steam explosion. Knowledge of the parametric effects of melt mass, ambient pressure and water temperature was extended. In the melt coolability experiments, water was poured onto the melt to investigate melt-coolant interactions in a stratified geometry where water overlies on a melt layer. Melt eruptions which could induce an explosive interaction were observed when the subcooled water was poured through a pipe nozzle. The eruption was not observed when the water was near the saturation temperature or supplied through a spray nozzle. The explosive interaction in the stratified geometry was found to be much smaller in magnitude than the steam explosion in the melt drop configuration.

Ex-vessel melt-coolant interactions in deep water pool: studies and accident management for Swedish BWRs

In Swedish BWRs having an annular suppression pool, the lower drywell beneath the reactor vessel is flooded with water to mitigate against the effects of melt release into the drywell during a severe accident. The Thirmal-1 code has been used to analyze the effectiveness of the water pool to protect lower drywell penetrations by fragmenting and quenching the melt as it relocates downward through the water. Experiments have also been performed to investigate the benefits of adding surfactants to the water to reduce the likelihood of fine-scale debris formation from steam explosions. This paper presents an overview of the accident management approach and surfactant investigations together with results from the Thirmal-1 analyses. A description of the modeling incorporated in Thirmal-1 is also provided.