From 771670f87303cb392b6f40fdced2e866005ede09 Mon Sep 17 00:00:00 2001
From: walshjon <walshjon@mit.edu>
Date: Mon, 23 Dec 2013 17:01:24 -0800
Subject: [PATCH] handle negative 0K elastic xs values by setting to ZERO

---
 src/ace.F90 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/ace.F90 b/src/ace.F90
index e902b516db..6aec1702dc 100644
--- a/src/ace.F90
+++ b/src/ace.F90
@@ -398,6 +398,7 @@ contains
     type(Nuclide), pointer :: nuc
 
     integer :: NE ! number of energy points for total and elastic cross sections
+    integer :: i  ! index in 0K elastic xs array for this nuclide
 
     ! determine number of energy points
     NE = NXS(3)
@@ -419,6 +420,12 @@ contains
       ! Continue reading elastic scattering and heating
       nuc % elastic_0K = get_real(NE)
 
+      ! Negative cross sections result in a CDF that is not monotonically
+      ! increasing. Set all negative xs values to ZERO.
+      do i = 1, nuc % n_grid_0K
+        if (nuc % elastic_0K(i) < ZERO) nuc % elastic_0K(i) = ZERO
+      end do
+
     else ! read in non-0K data
       nuc % n_grid = NE
       allocate(nuc % energy(NE))