  We explore the theoretical observation that within the leading twist
approximation, the nuclear effects of shadowing and antishadowing in
non-perturbative nuclear parton distribution functions (nPDFs) at the input QCD
evolution scale involve diffraction on nucleons of a nuclear target and
originate from merging of two parton ladders belonging to two different
nucleons, which are close in the rapidity space. It allows us to propose that
for a given momentum fraction $x_P$ carried by the diffractive exchange,
nuclear shadowing and antishadowing should compensate each other in the
momentum sum rule for nPDFs locally on the interval $\ln (x/x_P) \le 1$. We
realize this by constructing an explicit model of nuclear gluon antishadowing,
which has a wide support in $x$, $10^{-4} < x < 0.2$, peaks at $x=0.05-0.1$ at
the level of $\approx 15$\% for $^{208}$Pb at $Q_0^2=4$ GeV$^2$ and rather
insignificantly depends on details of the model. We also studied the impact
parameter $b$ dependence of antishadowing and found it to be slow.
