**1. NAME AND TITLE**

AIRDIF: A Two-Dimensional Atmospheric Radiation Diffusion Code.

**2. CONTRIBUTOR**

U.S. Air Force Weapons Laboratory, Kirtland Air Force Base, New Mexico.

**3. CODING LANGUAGE AND COMPUTER**

Fortran IV; CDC 6600/7600.

**4. NATURE OF PROBLEM SOLVED**

AIRDIF computes free field radiation environments from atmospheric nuclear detonations. It may
be run in either homogeneous air or 2-D variable-density air. In addition to environments, AIRDIF
can also compute K-Factors (ratio of 2-D to 1-D doses) for a particular source, dose response function,
and at a number of mass ranges.

**5. METHOD OF SOLUTION**

AIRDIF employs a special form of diffusion theory which separates collided and uncollided
multigroup fluences. An analytical first-collided source is used so that diffusion theory is only applied
to the solution of the collided fluences. Solutions are thus obtained separately for the collided and
uncollided fluences over each energy group throughout the region of interest. The total group fluences
are then obtained by simple addition of the collided and uncollided particle fluences everywhere.
Doses are computed by summing over all groups the product of the fluence and the dose response
function in each group.

**6. RESTRICTIONS OR LIMITATIONS**

None noted.

**7. TYPICAL RUNNING TIME**

Execution times vary depending on the problem. The major factors causing variation in the time are the number of mesh points, the number of energy groups, and the rate of convergence of the iterative solution.

Typical times for execution on the CDC 7600 are approximately 60 seconds for 1500 mesh points
and 58 energy groups. Execution on the CDC 6600 typically takes 4 to 5 times longer than on the
CDC 7600.

**8. COMPUTER HARDWARE REQUIREMENTS**

AIRDIF is operable on the CDC 6600/7600 computers. The memory requirement for the CDC 6600 is approximately 200 K OCTAL. This provides adequate storage for 1500 mesh points and 61 particle energy groups. A maximum of 40 neutron groups and 21 gamma-ray groups can be used.

The CDC 7600 requires 220 K OCTAL LCM AND 33 K OCTAL SCM for 5000 mesh points and
61 particle energy groups.

**9. COMPUTER SOFTWARE REQUIREMENTS**

A Fortran IV compiler is required.

**10. REFERENCE**

R. A. Shulstad and E. L. Wolf, "AIRDIF: A Two-Dimensional Atmospheric Radiation Diffusion
Computer Code," AFWL-TR-77-29 (June 1977).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced document and one (1.2MB) DOS diskette which contains the source
codes.

**12. DATE OF ABSTRACT**

January 1982.

**KEYWORDS: ** ONE-DIMENSION; TWH-DIMENSIONS; AIR TRANSPORT; NEUTRON;
GAMMA-RAY; DIFFUSION THEORY; WEAPONS RADIATION;
RADIATION ENVIRONMENT; MULTIGROUP