**RSICC CODE PACKAGE PSR-417**

**1. NAME AND TITLE**

FLODIS: Code System to Calculate Thermal Response of FSV HTGR Core.

**2. CONTRIBUTORS**

Oak Ridge National Laboratory, Oak Ridge, Tennessee through the Energy Science and Technology Software Center, Oak Ridge, Tennessee.

**3. CODING LANGUAGE AND COMPUTER**

FORTRAN IV; IBM360 (P00417I036000).

**4. NATURE OF PROBLEM SOLVED**

FLODIS was developed to analyze shutdown transients for the Fort St. Vrain high-temperature gas-cooled reactor (HTGR) core. The program is a lumped node representation of the 37 refueling regions in the active core, the side reflector blocks, the gas annulus between the core barrel and the prestressed concrete reactor vessel (PCRV) liner, and the PCRV cooling system. Heat conduction in all three coordinate directions and to the coolant is modeled. The calculation uses the specified operating conditions for the reactor at power to determine appropriate loss coefficients for the variable orifices in each refueling region. Iterative techniques are used to determine the distribution of coolant flow as a function of time during the transient. Both forced and natural convection flows can be calculated. FLODIS can be adapted to other HTGR systems with minor modifications.

**5. METHOD OF SOLUTION**

The mathematical model consists of two parts, the flow distribution and the temperature distribution. Navier-Stokes equations are applied to the flow problem. The temperature distribution differential equations are integrated using a Crank-Nicholson technique. The resulting algebraic equations are solved with a Gauss-Seidel iterative procedure. A rectangular grid is superimposed on the face of the reactor to define the mesh spaces. There are 142 mesh spaces in the active core, 146 in the side reflector, and 52 in the gas annulus between the core barrel and PCRV liner. Axially, there are 20 mesh spaces for a total of 6800 representing the entire system.

**6. RESTRICTIONS OR LIMITATIONS**

The orifice loss coefficients are assumed to remain constant throughout the transient. The effect of conduction between refueling regions is not included.

**7. TYPICAL RUNNING TIME**

21.73 seconds of CPU time were required on an IBM 3090 to run the sample problem.

**8. COMPUTER HARDWARE REQUIREMENTS**

540K bytes of memory are required on an IBM mainframe.

**9. COMPUTER SOFTWARE REQUIREMENTS**

FLODIS was developed under OS/360. A Fortran IV compiler is required.

**10. REFERENCE**

D. D. Paul, "FLODIS: A Computer Model to Determine the Flow Distribution and Thermal Response of the Fort St. Vrain Reactor," ORNL/TM-5365 (June 1976).

**11. CONTENTS OF CODE PACKAGE**

Included in the package are the referenced document and one DOS formatted diskette which contains a source file, input and output file, and a jcl file. Executables are not included with the package.

**12. DATE OF ABSTRACT**

July 1999.

** KEYWORDS:** FLUID DYNAMICS; HEAT TRANSFER