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diff --git a/Trivac/src/NSS5TR.f b/Trivac/src/NSS5TR.f
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+*DECK NSS5TR
+      SUBROUTINE NSS5TR(NEL,NMIX,MAT,IQFR,QFR,SIGT,FD,A11)
+*
+*-----------------------------------------------------------------------
+*
+*Purpose:
+* Assembly of non-leakage system matrices for the coarse mesh finite
+* difference method.
+*
+*Copyright:
+* Copyright (C) 2021 Ecole Polytechnique de Montreal
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU Lesser General Public
+* License as published by the Free Software Foundation; either
+* version 2.1 of the License, or (at your option) any later version
+*
+*Author(s): A. Hebert
+*
+*Parameters: input
+* NEL     number of nodes
+* NMIX    number of mixtures
+* MAT     node mixtures
+* IQFR    boundary conditions
+* QFR     albedo functions
+* SIGT    macroscopic cross section.
+* FD      discontinuity factors
+*
+*Parameters: output
+* A11     assembly matrix.
+*
+*-----------------------------------------------------------------------
+*
+      INTEGER NEL,NMIX,MAT(NEL),IQFR(6,NEL)
+      REAL QFR(6,NEL),SIGT(NMIX),FD(NMIX,2),A11(3*NEL,3*NEL)
+*
+      A11(:3*NEL,:3*NEL)=0.0
+      NUM1=0
+      DO KEL=1,NEL
+        IBM=MAT(KEL)
+        SIGG=SIGT(IBM)
+        ! WEIGHT RESIDUAL EQUATIONS:
+        A11(NUM1+1,NUM1+1)=SIGG
+        A11(NUM1+2,NUM1+2)=SIGG/12.0
+        A11(NUM1+3,NUM1+3)=SIGG/180.0
+        NUM1=NUM1+3
+      ENDDO
+      ! continuity relations:
+      NUM1=0
+      DO KEL=1,NEL-1
+        IBM=MAT(KEL)
+        IBMP=MAT(KEL+1)
+        NUM2=NUM1+3
+        ! flux continuity:
+        FDP=FD(IBM,2)
+        FDM=FD(IBMP,1)
+        A11(NUM1+2,NUM1+1)=-FDP
+        A11(NUM1+2,NUM1+2)=-FDP/2.0
+        A11(NUM1+2,NUM1+3)=-FDP/6.0
+        A11(NUM1+2,NUM2+1)=FDM
+        A11(NUM1+2,NUM2+2)=-FDM/2.0
+        A11(NUM1+2,NUM2+3)=FDM/6.0
+        NUM1=NUM1+3
+      ENDDO
+      ! left boundary condition:
+      IF((IQFR(1,1) == -1).OR.(IQFR(1,1) > 0)) THEN
+        ! VOID
+        AFACTOR=QFR(1,1)
+        A11(NUM1+2,1)=-AFACTOR
+        A11(NUM1+2,2)=AFACTOR/2.0
+        A11(NUM1+2,3)=-AFACTOR/6.0
+      ENDIF
+      ! right boundary condition:
+      IF((IQFR(2,NEL) == -1).OR.(IQFR(2,NEL) > 0)) THEN
+        NUM2=3*(NEL-1)
+        ! VOID
+        AFACTOR=QFR(2,NEL)
+        A11(NUM1+3,NUM2+1)=-AFACTOR
+        A11(NUM1+3,NUM2+2)=-AFACTOR/2.0
+        A11(NUM1+3,NUM2+3)=-AFACTOR/6.0
+      ENDIF
+      RETURN
+      END SUBROUTINE NSS5TR