mit-crpg/openmc — pull request #279 by nhorelik (2014-06-02 23:12:13+00:00)
Title: Added additional check for negative values in URR probability tables

When running some test problems with the NNDC cross sections, I found several nuclides that have negative values in the URR probability tables:

WARNING: Negative value(s) found on probability table for nuclide 11022.71c
WARNING: Negative value(s) found on probability table for nuclide 18036.71c
WARNING: Negative value(s) found on probability table for nuclide 34079.71c
WARNING: Negative value(s) found on probability table for nuclide 41094.71c
WARNING: Negative value(s) found on probability table for nuclide 41095.71c
WARNING: Negative value(s) found on probability table for nuclide 42099.71c
WARNING: Negative value(s) found on probability table for nuclide 48106.71c
WARNING: Negative value(s) found on probability table for nuclide 50123.71c
WARNING: Negative value(s) found on probability table for nuclide 53131.71c
WARNING: Negative value(s) found on probability table for nuclide 55136.71c
WARNING: Negative value(s) found on probability table for nuclide 63156.71c

This led to NaNs and an un-handled segfault. This PR eliminates those NaNs at the expense of an additional comparison to make sure we're not taking the log of a negative number.