**
1. NAME AND TITLE**

3DDT: Multigroup Diffusion Code System for Use in Fast Reactor Analysis.

**2. CONTRIBUTORS**

Los Alamos National Laboratory, Los Alamos, New Mexico under sponsorship of DOE-EH through the Energy Science & Technology Software Center, Oak Ridge, Tennessee.

**3. CODING LANGUAGE AND COMPUTER**

Fortran IV; CDC 6600.

**4. NATURE OF PROBLEM SOLVED**

3DDT is a three dimensional (x y z or r theta z) multigroup, diffusion theory code for use in fast reactor analysis. The code can be used to compute keff or to perform criticality searches on reaction composition, time absorption, and reactor dimensions by either the regular or the adjoint flux equations. Material burnup and fission product build up can be computed for specified time intervals, and criticality searches can be performed during burnup to compensate for fuel depletion and fission product growth.

**5. METHOD OF SOLUTION**

Standard source-iteration techniques are used to compute eigen values and flux profiles. Using an initial fission source distribution, new flux profiles in each group are sequentially computed, beginning in the highest energy group in regular problems and in the lowest energy group in adjoint problems. The group fluxes are computed by horizontal (r theta or x y) planes, beginning with the plane at the lowermost, axial position. Convergence is accelerated by group rebalancing, successive overrelaxation, and line inversion. Material burnup is computed using zone and group averaged cross sections which are recomputed after each time step. The burnup equation for each material in each zone provides for one decay source, two capture sources, seven fission sources, and losses by decay and absorption.

**6. RESTRICTIONS OR LIMITATIONS**

Upscattering is not treated in 3DDT.

**7. TYPICAL RUNNING TIME**

Execution times are on the order of .01 to .02 seconds of CP time per mesh point per energy group on the CDC 6600.

**8. COMPUTER HARDWARE REQUIREMENTS**

CDC 6600 computer with 65K words central memory, 500K ECS, 6,000K disk (four tape files are simulated on the disk), two magnetic tape units, and the usual input/output devices.

**9. COMPUTER SOFTWARE REQUIREMENTS**

SCOPE 3.2; Fortran IV. Variable dimensioning is used to make maximum use of fast core storage. Both extended core storage (ECS) and disk storage are utilized in 3DDT. 3DDT uses three LANL ECS system routines; ECRD, to fetch a vector from ECS, ECWR to store a vector in ECS, and ECFL to reduce the ECS block storage requirement.

**10. REFERENCES**

J. C. Vigil, "3DDT, A Three-Dimensional Multigroup Diffusion-Burnup Program," LA-4396 (September 1970).

"Random ECS I/O Note," (July 10, 1969).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced documentation and 1 diskette DS/HD including the source code and data file.

**
12. DATE OF ABSTRACT**

September 1992.

**KEYWORDS:** BURNUP; COMPLEX GEOMETRY; CRITICALITY
CALCULATIONS; DIFFUSION THEORY