The ANSYS program provides a capability of irradiation induced swelling (accessed with TB,SWELL). Swelling is defined as a material enlarging volumetrically in the presence of neutron flux. The amount of swelling may also be a function of temperature. The material is assumed to be isotropic and the basic solution technique used is the initial stress method. Swelling calculations are available only through the user swelling subroutine. See User Routines and Non-Standard Uses of the Advanced Analysis Guide and the Guide to ANSYS User Programmable Features for more details. Input must have C72 set to 10. Constants C67 through C71 are used together with fluence and temperature, as well as possibly strain, stress and time, to develop an expression for swelling rate.
Any of the following three conditions cause the swelling calculations to be bypassed:
If C67
0. and C68
0.If (input temperature + Toff) U
0, where Toff = offset temperature (input on TOFFST command).If Fluencen
Fluencen-1 (n refers
to current time step).
The total swelling strain is computed in subroutine USERSW as:
 | (4–335) |
where:
 = swelling strain at end of substep n |
| Δεsw = rΔf = swelling strain increment |
| r = swelling rate |
| Δf = fn - fn-1 = change of fluence |
| fn = fluence at end of substep n (input as VAL1, etc. on the BFE,,FLUE command) |
For a solid element, the swelling strain vector is simply:
 | (4–336) |
It is seen that the swelling strains are handled in a manner totally analogous to temperature strains in an isotropic medium and that shearing strains are not used.