Getting NaN for a write output in FORTRAN

[antony1103]

Hi all,

I'm writing a program in FORTRAN to calculate neutron flux in a reactor, but when I try to get a numerical output it simply says NaN. Anyone know what this means or how to fix it? Code is below, and this is happening on the flux2(N) output on the last write statement. Thanks!

      program slabflux

         implicit none



         ! VARIABLES

         integer, parameter :: N=30		! Number of slab increments

         real :: thickness         		! slab thickness in meters
         real :: diff_const        		! diffusion constant of the slab
         real :: macro_cross_sec   		! macroscopic cross section of the slab
         real :: source            		! neutron source rate
         real :: distance          		! distance of neutron flux calculation in the slab
         real, dimension(-1:N+1) :: flux1       ! flux calculation at a certain distance in the slab
         real, dimension(0:N) :: flux2		! used for iterative flux calculation
	 real, dimension(0:N) :: S		! source array
	 real :: del				! increment length
	 real :: a				! flux coefficient
	 real :: b				! flux coefficient
	 real, dimension(-1:N) :: prev_flux	! previous flux summation for loop
         real, dimension(0:N) :: prev_iter	! flux summation from previous iteration
         integer :: i				! do loop counter
	 integer :: j				! do loop counter
         integer :: k				! do loop counter
         real :: e				! iteration error

	 



         ! INPUT

         write (*,'(a,$)') "Slab thickness (cm)?: "
         read *, thickness

         write (*,'(a,$)') "Diffusion constant (cm)?: "
         read *, diff_const

         write (*,'(a,$)') "Macroscopic cross section (1/cm)?: "
         read *, macro_cross_sec

         write (*,'(a,$)') "Neutron source rate (neutrons/cm^3*s)?: "
         read *, source



	 ! CALCULATION

	 del=thickness/N					! increment length

	 S(N/2)=source				         	! source centered in slab
						

         flux1(1:29)=1						! initial flux
         prev_flux(-1:N)=0
         prev_flux(1:29)=0

	 a=-(diff_const/(del**2))				! flux coefficients

	 b=(diff_const/(del**2))+macro_cross_sec

         if (0.997<e .AND. e<1.003) then
            flux2(0:N)=flux1(0:N)
	    do i=0,N,1						! first iteration
	       prev_flux(i)=flux2(i-1)+prev_flux(i-1)
               do j=i,N,1
                  prev_iter(i)=prev_flux(i)+flux1(i+1)
               end do
	       flux2(i)=(1/b)*(S(j)-(a*prev_flux(i))-(a*prev_iter(i)))
	    end do
            e=abs(flux2(15)/flux1(15))
         else
            do k=0,N,1
               distance=del*N
               write (*,'(a,f6.2,3x,a,f6.2)') "Distance (cm): ",
     +                                       distance,
     +                                       "Flux (neutrons/cm^2*s): ",
     +                                       flux2(N)
            end do
         end if

      end program slabflux

 

[D H]

First off, please use the [ code ] ... [ /code ] tags, sans the spaces, when you post code. I edited your post for you.

As far as NaNs are concerned:
NaNs can arise from some computations such as 0/0. They can also result from the compiler. I suspect you are using a compiler that initializes uninitialized floating point variables with NaNs. In particular, you did not initialize the array flux2 or the variable e. Since e is not set, the then part of the if statement never executes, and flux2 keeps its (uninitialized) initial values.

The best way to find these kinds of errors is to use your debugger. Learn to use it.