diff --git a/src/enrichment_calculator.cc b/src/enrichment_calculator.cc
index 81efa78..45b98f2 100644
--- a/src/enrichment_calculator.cc
+++ b/src/enrichment_calculator.cc
@@ -39,7 +39,7 @@ EnrichmentCalculator::EnrichmentCalculator(
       feed_qty(0.), product_qty(0.),
       max_swu(max_swu),
       use_downblending(use_downblending),
-      isotopes(IsotopesNucID_vector()) {
+      isotopes(IsotopesNucID()) {
   if (feed_qty==1e299 && product_qty==1e299 && max_swu==1e299) {
     // TODO think about whether one or two of these variables have to be 
     // defined. Additionally, add an exception that should be thrown.
@@ -59,7 +59,7 @@ EnrichmentCalculator::EnrichmentCalculator(const EnrichmentCalculator& e) :
     target_tails_assay(e.target_tails_assay),
     target_feed_qty(e.target_feed_qty),
     target_product_qty(e.target_product_qty), max_swu(e.max_swu),
-    use_downblending(e.use_downblending), isotopes(IsotopesNucID_vector()),
+    use_downblending(e.use_downblending), isotopes(IsotopesNucID()),
     gamma_235(e.gamma_235) {
   CalculateGammaAlphaStar_();
   BuildMatchedAbundanceRatioCascade();
@@ -91,7 +91,6 @@ EnrichmentCalculator& EnrichmentCalculator::operator= (
 
   use_downblending = e.use_downblending;
   
-  //IsotopesNucID(isotopes);
   gamma_235 = e.gamma_235;
   CalculateGammaAlphaStar_();
   
diff --git a/src/miso_enrich.h b/src/miso_enrich.h
index e0542fa..da8ae54 100644
--- a/src/miso_enrich.h
+++ b/src/miso_enrich.h
@@ -79,8 +79,7 @@ class FeedConverter : public cyclus::Converter<cyclus::Material> {
     double feed_used = e.FeedUsed();
     
     cyclus::toolkit::MatQuery mq(m);
-    std::vector<int> isotopes;
-    IsotopesNucID(isotopes);
+    std::vector<int> isotopes(IsotopesNucID());
     std::set<int> nucs(isotopes.begin(), isotopes.end());
     double feed_uranium_frac = mq.atom_frac(nucs);
 
diff --git a/src/miso_helper.cc b/src/miso_helper.cc
index 7933764..412a7a2 100644
--- a/src/miso_helper.cc
+++ b/src/miso_helper.cc
@@ -13,8 +13,7 @@ namespace misotest {
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 bool CompareCompMap(cyclus::CompMap cm1, cyclus::CompMap cm2) {
-  std::vector<int> isotopes;
-  IsotopesNucID(isotopes);
+  std::vector<int> isotopes(IsotopesNucID());
   std::vector<int>::iterator it;
   // The following for-loop has been added to ensure that the all of the
   // uranium keys are present in both compmaps, else the comparison fails.
@@ -97,15 +96,7 @@ cyclus::Material::Ptr mat_natU() {
 }  // namespace misotest
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-void IsotopesNucID(std::vector<int>& isotopes) {
-  isotopes = {232, 233, 234, 235, 236, 238};
-  for (int i = 0; i < isotopes.size(); i++) {
-    isotopes[i] = (92*1000 + isotopes[i]) * 10000;
-  }
-}
-
-// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-std::vector<int> IsotopesNucID_vector() {
+const std::vector<int> IsotopesNucID() {
   int iso[6] = {232, 233, 234, 235, 236, 238};
   std::vector<int> isotopes(iso, iso + sizeof(iso)/sizeof(int));
   for (int& i : isotopes) {
@@ -128,8 +119,7 @@ int IsotopeToNucID(int isotope) {
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 int NucIDToIsotope(int nuc_id) {
-  std::vector<int> isotopes;
-  IsotopesNucID(isotopes);
+  std::vector<int> isotopes(IsotopesNucID());
   std::vector<int>::iterator it;
   
   it = std::find(isotopes.begin(), isotopes.end(), nuc_id);
@@ -207,8 +197,7 @@ double MIsoAssay(cyclus::CompMap compmap) {
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 double MIsoFrac(cyclus::CompMap compmap, int isotope) {
-  std::vector<int> isotopes;
-  IsotopesNucID(isotopes);
+  std::vector<int> isotopes(IsotopesNucID());
   
   double isotope_assay = 0;
   double uranium_atom_frac = 0;
@@ -234,10 +223,9 @@ double MIsoFrac(cyclus::CompMap compmap, int isotope) {
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 std::map<int,double> CalculateSeparationFactor(double gamma_235) {
-  std::vector<int> isotopes;
-  IsotopesNucID(isotopes);
+  std::vector<int> isotopes(IsotopesNucID());
   std::map<int,double> separation_factors;
-  
+
   // We consider U-238 to be the key component hence the mass differences
   // are calculated with respect to this isotope.
   for (int i : isotopes) {
diff --git a/src/miso_helper.h b/src/miso_helper.h
index 40b20fc..cc9c262 100644
--- a/src/miso_helper.h
+++ b/src/miso_helper.h
@@ -24,8 +24,7 @@ const double kEpsDouble = 1e-5;
 const double kEpsCompMap = 1e-5;
 const int kIterMax = 200;
 
-std::vector<int> IsotopesNucID_vector();
-void IsotopesNucID(std::vector<int>& isotopes);
+const std::vector<int> IsotopesNucID();
 int IsotopeToNucID(int isotope);
 int NucIDToIsotope(int nuc_id);
 int ResBufIdx(
diff --git a/src/miso_helper_tests.cc b/src/miso_helper_tests.cc
index 0060aad..4d5ae69 100644
--- a/src/miso_helper_tests.cc
+++ b/src/miso_helper_tests.cc
@@ -25,8 +25,7 @@ TEST(MIsoHelperTest, ChooseCorrectResBuf) {
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 TEST(MIsoHelperTest, NucIDConversion) {
-  std::vector<int> isotopes;
-  IsotopesNucID(isotopes);
+  std::vector<int> isotopes(IsotopesNucID());
   
   for (int i : isotopes) {
     int isotope = NucIDToIsotope(i);
@@ -72,8 +71,7 @@ TEST(MIsoHelperTest, SeparationFactor) {
   std::map<int,double> separation_factor = CalculateSeparationFactor(
                                                               gamma_235);
 
-  std::vector<int> isotopes;
-  IsotopesNucID(isotopes);
+  std::vector<int> isotopes(IsotopesNucID());
   std::map<int,double> expected;
   expected[922320000] = 2.2;
   expected[922330000] = 2.0;