  The effect of atomic disorder induced by neutron irradiation on the crystal
structure and electronic states near $E_F$ of the lightly overdoped
LaFeAsO$_{0.85}$F$_{0.15}$ ($T_c=21K$) was studied by X-ray diffraction and
$^{75}$As NMR. The irradiation of the polycrystalline sample by "moderate"
neutron fluence of $\Phi=(0;0.5;1.6)\cdot10^{19}cm^{-2}$ at $T=50^{\circ}C$
leads to the suppression of superconductivity. It is shown that neutron
irradiation produces an anisotropic expansion of the tetragonal lattice almost
due to an increase of the Fe-As distance. A partial loss of the 2D character of
the FeAs layer is accompanied with a suppression of the gap-like feature in
temperature dependence of the spin susceptibility. In the most disordered state
the $^{75}$As spin-lattice relaxation rate follows the Korringa law
$^{75}T_1^{-1} \sim T$, the thermal behavior being typical for an isotropic
motion of the conducting electrons.
