NESC1093 TORAC.

TORAC.

TORAC models tornado-induced flows, pressures, and material transport within structures. Its use is directed toward nuclear fuel cycle facilities and their primary release pathway - the ventilation system. However, it is applicable to other structures and can model other airflow pathways within a facility. In a nuclear facility, this network system could include process cells, canyons, laboratory offices, corridors, and offgas systems. TORAC predicts flow through a network system that also includes ventilation system components such as filters, dampers, ducts, and blowers. These ventilation system components are connected to the rooms and corridors of the facility to form a complete network for moving air through the structure and, perhaps, maintaining pressure levels in certain areas. The material transport capability in TORAC is very basic and includes convection, depletion, entrainment, and filtration of material.

The lumped-parameter method is the basic formulation used to describe a ventilation system or any other air pathway. Using the lumped-parameter method, network theory includes a number of system elements called branches, joined at certain points, called nodes. Ventilation system components that exhibit resistance or potential, such as dampers, filters, and blowers are located within the branches. Components that have larger volumes, such as rooms and plenums, are located at nodal points; nodes may possess some volume or capacitance where fluid storage or compressibility may be taken into account.

The gas dynamics governing equations require that the continuity equation be satisfied at every node and that a pressure-flow equation be satisfied for each element or branch. Variations in the node equations depend on whether the node represents a finite volume. This variation also exists for branches, depending on whether the branch is simply a duct or contains a filter, blower, or damper.

Material concentrations and material mass flow rates can be calculated at any location in the network as a function of time for arbitrary user-specified pressure transients imposed on the facility boundary. The basic mechanisms considered are transport initiation, convective transport, and transport deletion. The user must identify the type (aerosol or gas), quantity, and location of material at risk. If the material is a solid or liquid aerosol, a characteristic size and density must be specified. TORAC gives the user two options for transport initiation: specification of mass injection rate versus time and calculated aerodynamic entrainment. For each time-step of a calculation, the gas dynamics problem is solved first for the entire network to yield pressures and flow rates independent of material transport. Then the gas dynamics module calls the convective transport module to solve the mass conservation equation and advance the material transport calculation by one time-step.

Two-phase flow is allowed in the sense that normal ventilation gas (usually air) is one phase and a pneumatically transportable contaminant material is the other phase. The calculation of aerosol depletion is based on quasi-steady-state settling with the terminal settling velocity corrected by the Cunningham slip factor. The aerosol may consist of solid particles or liquid droplets.

The transport deletion module is restricted to gravitational settling and filtration.

NESC executed the sample problem in 15 CP seconds on a CDC CYBER170/875.

TORAC is one in a family of codes designed to provide improved safety analysis methods for the nuclear industry. Other members include TVENT1P (NESC 809), FIRAC (NESC 1092), and EXPAC. TORAC is essentially an improved TVENT computer code, modified to include material transport, particularly transport of radioactive material.

Approximately 126,200 (octal) words are required to run the sample problem on a CDC CYBER170/875.

CTSS (Cray1), NOS 2.4 (CDC CYBER170).

TORAC assumes that memory has been preset to zero prior to execution. CRT plots can be generated using a LANL-specific auxiliary program based on the proprietary CA-DISSPLA graphics software (see http://www.gaeinc.com). This program uses the data written to logical output unit 23; it is not included.

R.W. Andrae, P.K. Tang, R.A. Martin and W.S. Gregory
Los Alamos National Laboratory

Los Alamos, NM, USA