T.C. Chawla

Thermophysical properties of mixed oxide fuel and stainless steel type 316 for use in transition phase analysis

Abstract For use in transition phase analysis, we have obtained thermodynamic and transport properties of mixed oxide fuel and stainless steel type 316, over a very wide range of temperatures. The majority of the thermodynamic properties for these materials in their solid and liquid states are based on either actual experimental data or extrapolation of low temperature data. However, a number of properties for these materials in the vapor state are based on very limited and scarce data. These properties include saturation vapor pressure, latent heat of vaporization and specific volume of saturated vapor. Due to complete lack of experimental data, a number of other properties such as specific heats, heat of vaporization (especially for stainless steel), transport properties, and the properties at the critical state had to be estimated by use of generally reliable empirical relations. It is recommended that efforts be initiated to obtain experimental data for these materials in the vapor state to verify both existing empirical relations and the relationships that are obtained here based on theoretical analyses.

A thermal stress mechanism for the fragmentation of molten UO2 upon contact with sodium coolant

Abstract An important aspect of fuel-coolant interaction problems relative to various hypothetical LMFBR accidents is the fragmentation of molten oxide fuel on contact with sodium coolant. In order to properly analyze the kinetics of such an event, an understanding of the breakup process and an estimate of the size and dispersion of such fragmented fuel must be known. A thermal stress initiated mechanism for fragmentation is presented using elastic stress theory for the cases of both temperature-dependent and independent mechanical properties. Included is a study of the effect of the choice of surface heat transfer boundary condition and the compressibility of the unsolidified inner core. Results of parametric calculations indicate that the thermal stresses induced in the thin outer shell and the pressurization of the inner molten core are potentially responsible for the fragmentation. For UO 2 in Na the calculated stresses are extremely high, while for aluminum in water they are much smaller and a strong function of the surface heat transfer boundary condition. Qualitatively, these results compare favorably with small scale dropping experiments, that is, molten UO 2 quenched in Na undergoes fragmentation while aluminum in water usually results in little breakup. The experimentally observed increase in breakup with decreasing coolant temperature is also in qualitative agreement with the thermal stress-induced mode of fragmentation.

A collocation method using B-splines for one-dimensional heat or mass-transfer-controlled moving boundary problems☆

Abstract A collocation method employing B-splines as approximating functions in the space variable and Gaussian quadrature points as the collocation points is developed for treating the heat or mass-transfer-controlled one-dimensional dissolution problems. It is demonstrated that this method provides an alternative technique which is considerably more accurate than the finite-difference method even at the very end of the dissolution process. The method presented here is first applied to the dissolution problem in the plane slab geometry where direct comparison has been made with the exact solution to assess the accuracy of the method. For spherical particle dissolution, the method is then compared with the existing finite-difference solution where it is shown that a more accurate solution can be obtained with considerably fewer equations.

A review of modeling concepts for sodium-concrete reactions and a model for liquid sodium transport to the unreacted concrete surface

Abstract 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, CO 2 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.

An evaluation of pin-to-pin failure propagation due to fission gas release in fuel subassemblies of liquid-metal-cooled fast breeder reactors

Abstract A survey is given of work performed to evaluate the potential for pin-to-pin failure propagation due to fission gas release in fuel subassemblies of LMFBRs. Reference is made to publications available in the open literature; recent experiments and analyses are dealt with in more detail. Two distinct failure propagation mechanisms are identified, namely: (1) thermal transients, and (2) mechanical loads. It is concluded that rapid and extensive pin-to-pin failure propagation due to fission gas release is not possible, even for the very high gas release rates considered.

Thermophysical properties of MgO, UO2, their eutectic solution and slurry of liquid-solid mixtures, concrete, sodium, stainless steel and debris beds for use in molten pool penetration of MgO substrate☆

Abstract We have prescribed various thermophysical and transport properties to describe various thermal states of the materials of interest such as MgO, UO 2 , stainless steel, sodium, and concrete undergo during post accident heat removal (PAHR) in an ex-vessel cavity lined with MgO bricks. A number of properties, especially of molten MgO, had no experimental determination and therefore, by necessity, these were prescribed through available “best” estimates. We have also included the equivalent properties of various “composite” materials such as debris beds with a prescribed composition, solutions, and slurries to describe their participation in various thermophysical phenomena of interest in PAHR.