diff --git a/include/openmc/photon.h b/include/openmc/photon.h
index 989d6dfa9c..e8ec82d107 100644
--- a/include/openmc/photon.h
+++ b/include/openmc/photon.h
@@ -23,10 +23,10 @@ namespace openmc {
 class ElectronSubshell {
 public:
   struct Transition {
-    int primary_subshell;
-    int secondary_subshell;
-    double energy;
-    double probability;
+    int primary_subshell;   //!< Index in shells_ of originating subshell
+    int secondary_subshell; //!< Index in shells_ of Auger electron subshell
+    double energy;          //!< Energy of transition
+    double probability;     //!< Probability of transition between subshells
   };
 
   // Constructors
@@ -105,6 +105,12 @@ private:
   void compton_doppler(
     double alpha, double mu, double* E_out, int* i_shell, uint64_t* seed) const;
 
+  //! Calculate the maximum size of the vacancy stack in atomic relaxation
+  //
+  //! These helper functions use the subshell transition data to calculate the
+  //! maximum size the stack of unprocessed subshell vacancies can grow to for
+  //! the given element while simulating the cascade of photons and electrons
+  //! in atomic relaxation.
   int calc_max_stack_size() const;
   int calc_helper(std::unordered_map<int, int>& visited, int i_shell) const;
 };
diff --git a/src/photon.cpp b/src/photon.cpp
index 385583ee33..a9b9cdbf50 100644
--- a/src/photon.cpp
+++ b/src/photon.cpp
@@ -18,6 +18,7 @@
 #include "xtensor/xview.hpp"
 
 #include <cmath>
+#include <fmt/core.h>
 #include <tuple> // for tie
 
 namespace openmc {
@@ -191,9 +192,10 @@ PhotonInteraction::PhotonInteraction(hid_t group)
   // Check the maximum size of the atomic relaxation stack
   auto max_size = this->calc_max_stack_size();
   if (max_size > MAX_STACK_SIZE && mpi::master) {
-    warning("The subshell vacancy stack in atomic relaxation can grow up to " +
-            std::to_string(max_size) + ", but the stack size limit is set to " +
-            std::to_string(MAX_STACK_SIZE) + ".");
+    warning(fmt::format(
+      "The subshell vacancy stack in atomic relaxation can grow up to {}, but "
+      "the stack size limit is set to {}.",
+      max_size, MAX_STACK_SIZE));
   }
 
   // Determine number of electron shells