def fuelinput(wp):
    """
    function to calculate the weight percentage of U235 and U238 based on initial enrichment
    
    formulae from http://holbert.faculty.asu.edu/eee460/NumberDensity.pdf
    
    Parameters
    ----------
    wp : float
        Initial enrichment in percentage
        
    Returns
    -------
    fuelstr : str
        Serpent formatted material composition
        
    Notes
    -----
    1, Right now the temperature is hard coded (ie ZAID ends with '.15c'), this can be modified.
    2, Right now the density of fuel is hard coded, this can be modified.
    """
    
    u=1000*1.660539040e-27  #g
    NA=6.0221409e23   ##/mol
    MU235=235.0439299*u*NA    #g/mol
    MU238=238.05078826*u*NA   #g/mol
    MO16= 15.99491461956*u*NA
    rhoUO2=10.5     #g/cm3
    wp=wp/100
    MU=1/(wp/MU235 + (1-wp)/MU238)
    MUO2=MU+2*MO16
    
    rhoU=rhoUO2*(MU/MUO2)
    rhoO=rhoUO2*(MO16/MUO2)
    
    NU235=(wp*rhoU*NA)/MU235
    NU238=((1-wp)*rhoU*NA)/MU238
    NO16=(rhoO*2*NA)/MO16
    NTOT=NU235+NU238+NO16

    M_U235=(wp*rhoU)
    M_U238=((1-wp)*rhoU)
    M_O16=(rhoO*2)
    M_TOT=M_U235+M_U238+M_O16
    fuelstr='mat UOX -10.5 burn 1'
    fuelstr=fuelstr+'\n 92235.15c  -%.8f'%(M_U235/M_TOT)
    fuelstr=fuelstr+'\n 92238.15c  -%.8f'%(M_U238/M_TOT)
    fuelstr=fuelstr+'\n 8016.15c   -%.8f'%(M_O16/M_TOT)
    fuelstr=fuelstr+'\n 98249.15c  -0.0'
    fuelstr=fuelstr+'\n 98250.15c  -0.0'
    fuelstr=fuelstr+'\n 98251.15c  -0.0'
    fuelstr=fuelstr+'\n 98252.15c  -0.0'
    fuelstr=fuelstr+'\n 98253.15c  -0.0'
    fuelstr=fuelstr+'\n 98254.15c  -0.0'
    return fuelstr