**RSICC CODE PACKAGE PSR-436**

**1. NAME AND TITLE**

FRAP-T6: Code System for Transient Analysis of Fuel Rods.

Note: See http://frapcon.labworks.org/ regarding newer versions of FRAPCON-2, FRAPT6/MOD1 and FRAPT6/V21.**2. CONTRIBUTORS**

EG&G Idaho, Idaho Falls, Idaho, through the Energy Science and Technology Software Center, Oak Ridge, Tennessee.

**3. CODING LANGUAGE AND COMPUTER**

FORTRAN (99%) and Assembly language (1%).; CDC CYBER176.

FRAPT6/V21 (P00436C017601);

FRAPT6/MOD1 (P00436C017600).

**4. NATURE OF PROBLEM SOLVED**

FRAP-T6 is the most recent in the FRAP-T (Fuel Rod Analysis Program - Transient) series of programs for calculating the transient behavior of light water reactor fuel rods during reactor transients and hypothetical accidents, such as loss-of-coolant and reactivity-initiated accidents. The program calculates the temperature and deformation histories of fuel rods as functions of time-dependent fuel rod power and coolant boundary conditions. FRAP-T6 can be used as a stand-alone code or, using steady state fuel rod conditions supplied by FRAPCON2 (NESC 694), can perform a transient analysis. In either case, the phenomena modeled by FRAP-T6 include: heat conduction, heat transfer from cladding to coolant, elastic-plastic fuel and cladding deformation, cladding oxidation, fission gas release, fuel rod gas pressure, and pellet cladding mechanical interaction. Licensing audit models have been added . The program includes a user's option that automatically provides a detailed uncertainty analysis of the calculated fuel rod variables due to uncertainties in fuel rod fabrication, material properties, power and cooling.

**5. METHOD OF SOLUTION**

The models in FRAP-T6 use finite difference techniques to calculate the variables which influence fuel rod performance. The variables are calculated at user-specified slices of the fuel rod. Each slice is at a different elevation and is defined to be an axial node. At each axial node, the variables are calculated at user-specified locations. Each location is at a different radius and is defined to be a radial node. The variables at any given axial node are assumed to be independent of the variables at all other axial nodes. The solution for the fuel rod variables begins with the calculation of the fuel and cladding temperatures. Then, the temperature of the gases in the plenum of the fuel rod is calculated. Next, the stresses and strains in the fuel and cladding and the pressure of the gas inside the rod are computed. This calculation sequence is repeated until essentially the same temperature distribution is calculated for two successive cycles. The cladding oxidation and fission gas release are then calculated, and the time is advanced, after which the complete sequence of calculation is repeated to obtain the fuel rod variables at the advanced time. The models interact in several ways. The fuel temperature calculated by the thermal model is dependent upon the size of the fuel-cladding gap calculated by the deformation model, and the diameter of the fuel pellet depends upon the temperature distribution in the pellet. Mechanical properties of the cladding vary significantly with temperature. The internal pressure varies with the temperature of the fuel rod gases and the strains of fuel and cladding. The stresses and strains in the cladding are dependent upon internal gas pressure. Variables calculated in one model are treated as independent variables by the other models. Two nested calculational loops are cycled until convergence occurs. Convergence is accelerated by the Newton method. The optional uncertainty analysis is based on the response surface method.

**6. RESTRICTIONS OR LIMITATIONS**

Distribution is restricted to the United States. Since FRAP-T6 is dynamically - dimensioned, the only constraint on the number of axial and radial nodes is the size of the available computer memory. The amount of memory required is a function of the number of axial and radial nodes and the selected models given by the equation: S = LB + 1710 + 11NR + 257NZ + 8NR*NZ + I2D (7NA*NZA*NR + 24NA*NZA) + IB (6NCH*NZCH + NCH + 2) + IF2 (4 + 117NR + 2NZ + 20NR*NZ) + IG (7 + 4NRF + 8NZ + 26NRF*NZ) + IBAL (10,000) where S is the required number of words of central memory; LB is the memory required to load FRAP-T6 exclusive of array storage (98,000 words); NR is the number of radial nodes and NZ, the number of axial nodes. I2D=1, if two-dimensional r-theta heat conduction is modeled, 0 otherwise; NA is the number of azimuthal sectors and NZA, the number of axial nodes at which two-dimensional r-theta heat conduction is modeled. IB=1, if the fuel rod is in contact with more than one coolant channel, 0 otherwise; NCH is the number of coolant channels surrounding the fuel rod, and NZCH, the number of vertically-stacked zones in a coolant channel. IF2=1, if the FRACAS2 subcode (deformable pellet deformation model) is used, 0 otherwise. IG=1, if the FASTGRASS subcode (fission gas production and release model) is used, 0 otherwise; NRF is the number of radial nodes in the fuel. IBAL=1, if the BALON2 subcode (cladding ballooning model) is used, 0 otherwise. The LB variable can be reduced to a value of about 65,000 by overlaying.

**7. TYPICAL RUNNING TIME**

The running time varies with the size of the time-step, the number of nodes specified, and the models chosen.

**8. COMPUTER HARDWARE REQUIREMENTS**

260,000 (octal) words of SCM and 63,000 (octal) words of LCM are required for FRAP-T6.

**9. COMPUTER SOFTWARE REQUIREMENTS**

This code system was developed under NOS/BE 1.4 and required a FORTRAN IV compiler and Assembler.

**10. REFERENCES**

a) Included in documentation:

P. Johnson, "FRAP-T6/MOD1, NESC No. 658, FRAP-T6/MOD1 Tape Description and Implementation Information," National Energy Software Center Note 83-87 (September 2, 1983).

P. Johnson, "FRAP-T6/Version 21, NESC No. 658, FRAP-T6/Version 21 Tape Description and Implementation Information," National Energy Software Center Note 90- (no date given).

L. J. Siefken, C. M. Allison, M. P. Bohn, and S. O. Peck, "FRAP-T6: A Computer Code for the Transient Analysis of Oxide Fuel Rods," NUREG/CR-2148, EGG-2104 (May 1981).

L. J. Siefken, V. N. Shaw, G. A. Berna, and J. K. Hohorst, "FRAP-T6: A Computer Code for the Transient Analysis of Oxide Fuel Rods," NUREG/CR-2148, EGG-NSMD-2104, (Addendum, June 1983).

b) Background Information:

D. L. Hagrman and G. A. Reymann, Ed., "Matpro-Version 11 (Revision 1): A Handbook of Materials Properties for Use in the Analysis of Light Water Reactor Fuel Rod Behavior," NUREG/CR-0497, TREE-1280 Rev. 1 (February 1980).

G. A. Berna, M. P. Bohn, W. N. Rausch, R. E. Williford, and D. D. Lanning, "FRAPCON-2: A Computer Code for the Calculation of Steady State Thermal-Mechanical Behavior of Oxide Fuel Rods," NUREG/CR-1845 (January 1981).

**11. CONTENTS OF CODE PACKAGE**

Included in the package are the referenced documents and one 3.5" DOS-formatted diskette containing a compressed self-extracting file with source codes and job control language.

**12. DATE OF ABSTRACT**

March 2000.

** KEYWORDS:** HEAT TRANSFER; REACTOR SAFETY