Search for sterile neutrinos at reactors with a small core

The sensitivity to the sterile neutrino mixing at very short baseline reactor neutrino experiments is investigated. If the reactor core is relatively large as in the case of commercial reactors, then the sensitivity is lost for $\Delta m^2 \gtrsim$ 1 eV$^2$ due to smearing of the reactor core size. If the reactor core is small as in the case of the experimental fast neutron reactor Joyo, the ILL research reactor or the Osiris reactor, on the other hand, then sensitivity to $\sin^22\theta_{14}$ can be as good as 0.03 for $\Delta m^2 \sim$ several eV$^2$ because of its small size.