Steam generator tube rupture study

This report describes our investigation of steam generator behavior during a postulated tube rupture accident. Our study was performed using the steam generator, thermal-hydraulic analysis code THERMIT-UTSG. The purpose of our work was to provide an independent assessment of the Los Alamos National Laboratory system code TRAC-PF1 with respect to steam generator tube rupture analysis. Results of our work are presented and compared with previous TRAC-PF1 results. There are substantial differences in the results of the two codes. These discrepancies are discussed and deficiencies in both codes are noted. Our results lead us to believe that further investigation and code development are necessary to gain more than a basic understanding of steam generator behavior during such accidents and to provide a simulation capability that is acceptable

The numerical treatment of condensation.

The simulation of complete condensation continues to challenge the numerical methods currently used for multi-phase flow modeling; especially at low pressures, the change of phase process from a two-phase mixture to liquid leads to severe pressure field perturbations and often failure of the calculations. During condensation, the local void fraction and pressure decrease rapidly; at the time of complete condensation, the strong nonlinearities of the equations at the phase-change point lead to convergence difficulties and/or unacceptably large mass or energy errors.Various ad-hoc "fixes" for this phenomenon - often referred to as "water packing" - have been proposed and/or implemented over the last few years. However, they have failed to clarify the core of the problem and are still unsatisfactory. Indeed these solutions cast doubt on the numerical predictions and occasionally are unable to prevent the breakdown of the calculations.The present investigations have focused on the roots of these difficulties, particularly on the nonlinear effects involved. A time-step control strategy was developed which removes or at least, greatly mitigates the aforementioned computational problems. Numerical experiments as well as a mathematical analysis have both demonstrated the existence of a critical time-step size beyond which larger time-steps shall accommodate the liquid flow field to any perturbations; smaller time-steps shall cause the pressure to bounce, going out of range as it is indeed witnessed for condensation simulations where the time-steps are drastically reduced when the two phases are still coexisting.Similar studies have been conduced on variety of numerical methods yielding some unexpected results in terms of time-step limit.Sponsored by Northeast Utilities Co., Foxboro Co

A four-equation two-phase flow model for sodium boiling simulation of LMFBR fuel assemblies

A three-dimensional numerical model for the simulation of sodium boiling transients has been developed. The model uses mixture mass and energy equations, while employing a separate momentum equation for each phase. Thermal equilibrium on the saturation line between coexisting phases is assumed.The four governing equations are supplemented by a number of constitutive relations, addressing the interphase and intraphase exchanges, as well as the fluid-solid interactions. It should be noted that this four-equation two-phase flow model requires only one interfacial relation, i.e., the momentum exchange, compared to the six-equation model which needs two additional relations, describing the mass and energy exchanges. Consequently, the relatively high degree of uncertainty currently associated with the interfacial exchange phenomena is considerably reduced.From a numerical point of view, the basic approach in this work is a semi-implicit method, in which pressure pulse propagation and local effects characterized by short characteristic times are treated implicitly, while convective transport and diffusion heat transfer phenomena, associated with longer time constants, are handled explicitly. The method remains tractable and efficient in multidimensional applications.Simulation of a number of experiments has yielded very encouraging results. The numerical method and the constitutive relations have performed well, especially so in light of the extreme severity of the conditions involving sodium boiling

Advanced information processing system for advanced launch system: Avionics architecture synthesis

The Advanced Information Processing System (AIPS) is a fault-tolerant distributed computer system architecture that was developed to meet the real time computational needs of advanced aerospace vehicles. One such vehicle is the Advanced Launch System (ALS) being developed jointly by NASA and the Department of Defense to launch heavy payloads into low earth orbit at one tenth the cost (per pound of payload) of the current launch vehicles. An avionics architecture that utilizes the AIPS hardware and software building blocks was synthesized for ALS. The AIPS for ALS architecture synthesis process starting with the ALS mission requirements and ending with an analysis of the candidate ALS avionics architecture is described

Sodium-cooled systems and loop simulation capability.

A one-dimensional loop simulation capability has been implemented in the thermal-hydraulic analysis code, THERMIT-4E. This code had been used to simulate and investigate flow in test sections of experimental sodium loops and of LMFBR fuel assemblies. Such analyses had required the use of boundary conditions specified at the inlet and outlet. The new code, THERMIT-4E/L simulates the entire primary coolant loop and therefore eliminates the need to specify such boundary conditions. The additions and modifications to the THERMIT-4E code include: constant temperature heat sinks, implicit heat transfer to environment and generalized body force field specification. To date, applications have been focused on natural circulation.A series of experiments performed in the Sodium Boiling Test Facility (SBTF) at the Oak Ridge National Laboratory have been simulated with the loop code. The results of single-phase calculations are generally in good agreement with the experimental data. However, we have not as yet been able to obtain a stabilized flow configuration when a significant amount of boiling takes place in the heated section. It appears that the extremely violent condensation n the plena loads to the .noted calculational difficulty.An analytical treatment approximating the single-phase loop behavior has also been developed. The results are quite general and can be applied to other loop systems. Approximate expressions have been obtained for the frequency and damping coefficient of a flow oscillation in a loop. The analysis has also yielded a criterion for stability, dependent on the input power, difference between the upper and lower plena temperatures, and a modified Stanton number

Relationships between the use of pharmacomechanical catheter-directed thrombolysis, sonographic findings, and clinical outcomes in patients with acute proximal DVT: Results from the ATTRACT Multicenter Randomized Trial.

Few studies have documented relationships between endovascular therapy, duplex ultrasonography (DUS), post-thrombotic syndrome (PTS), and quality of life (QOL). The Acute Venous Thrombosis: Thrombus Removal with Adjunctive Catheter-Directed Thrombolysis (ATTRACT) trial randomized 692 patients with acute proximal deep vein thrombosis (DVT) to receive anticoagulation or anticoagulation plus pharmacomechanical catheter-directed thrombolysis (PCDT). Compression DUS was obtained at baseline, 1 month and 12 months. Reflux DUS was obtained at 12 months in a subset of 126 patients. Clinical outcomes were collected over 24 months. At 1 month, patients who received PCDT had less residual thrombus compared to Control patients, evidenced by non-compressible common femoral vein (CFV) (21% vs 35%