*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