**1. NAME AND TITLE**

REFLUX: Code System to Predict LWR Reflood Heat Transfer.

**2. CONTRIBUTOR**

Los Alamos National Laboratory, Los Alamos, New Mexico through the Energy Science and Technology Software Center, Oak Ridge, Tennessee

**3. CODING LANGUAGE AND COMPUTER**

FORTRAN IV: IBM360 or 3033 (P00403I303300).

**4. NATURE OF PROBLEM SOLVED**

REFLUX calculates the temperature-time history of a representative fuel rod during the reflood stage of a hypothetical loss-of-coolant accident (LOCA). The logic used fo selection of the appropriate flow regime for analysis of the cladding temperature transient is based on the axial position with regard to the continuous liquid level (based on a mass balance), a liquid carry-over criterion (derived from a force balance on a drop suspended in a vapor stream), and the local cladding surface temperature. A generalized boiling curve is constructed, and the local flow and clad conditions determine the applicable heat transfer coefficient.

**5. METHOD OF SOLUTION**

The coolant thermal-hydraulic calculation of REFLUX is based on a single subchannel, one-dimensional, separated two phase flow formulation solution of the conservation equations of mass, energy, and momentum. The equations to be solved are written in backward finite-difference form, in space and time. A marching solution from subchannel inlet to exit is performed at each time-step. The code used the fuel-clad temperature field from the preceding time-step to advance the coolant thermal-hydraulic solution to the present time-step. The void fraction in the flow boiling regimes is defined by the drift flux model. Provision is made for analyzing non-equilibrium situations encountered under reflood conditions, and a complete two-step dispersed flow film boiling model is included. The coupled fuel-clad temperature field is solved by an implicit finite difference technique for radial conduction, including decay heat generation and temperature-dependent material properties.

**6. RESTRICTIONS OR LIMITATIONS**

REFLUX is expected to provide reasonable results for flooding rates of 0.4 to 10 in/sec, pressures of 15 to 60 psia, peak powers of 0.5 to 1.5 kw/ft, inlet coolant temperatures of 50 degrees F to saturation, and initial temperatures of 250 to 2000 F.

**7. TYPICAL RUNNING TIME**

RSICC executed the sample problem in 3.92 seconds on an IBM 3033.

**8. COMPUTER HARDWARE REQUIREMENTS**

100K bytes of memory

**9. COMPUTER SOFTWARE REQUIREMENTS**

OS/360 and Fortran IV compiler.

**10. REFERENCE**

W. L. Kirchner, "Reflood Heat Transfer in an Light Water Reactor," NUREG-0106, Vols. I and II (August 1976).

**11. CONTENTS OF CODE PACKAGE**

Included in the package are the referenced document and output for test case and a diskette which contains a self-extracting DOS file with the source code, sample problem, input and jcl to compile and run the test case.

**12. DATE OF ABSTRACT**

April 1999.

**KEYWORDS: **LOCA; HEAT TRANSFER