diff --git a/dependencies/reactor_class.js b/dependencies/reactor_class.js
index 9e02fca..adf744f 100644
--- a/dependencies/reactor_class.js
+++ b/dependencies/reactor_class.js
@@ -179,20 +179,20 @@ class Reactor_PFR {
     }
 
     makeDoperator() { // create the upwind scheme gradient operator
-        const I = math.diag(Array(this.n_x).fill(-1), 0);
-        const A = math.resize(math.diag(Array(this.n_x).fill(1), 1), [this.n_x, this.n_x]);
+        const I = math.diag(Array(this.n_x).fill(1), 0);
+        const A = math.resize(math.diag(Array(this.n_x).fill(-1), -1), [this.n_x, this.n_x]);
         I[0][0] = 0;
         I[0][1] = -1;
-        I[this.n_x-1][this.n_x-1] = 0; // Neumann boundary condition at x=L
         return math.add(I, A);
     }
 
-    makeD2operator() { // create the upwind scheme second derivative operator
+    makeD2operator() { // create the central second derivative operator
         const I = math.diag(Array(this.n_x).fill(-2), 0);
         const A = math.resize(math.diag(Array(this.n_x).fill(1), 1), [this.n_x, this.n_x]);
         const B = math.resize(math.diag(Array(this.n_x).fill(1), -1), [this.n_x, this.n_x]);
         I[0][0] = 0;
         I[0][1] = -1;
+        I[0][0] = -1; // Dichelet boundary condition at outlet
         return math.add(I, A, B);
     }
 }