[en] The advanced thermal reactor (ATR) is a heavy-water-moderated boiling-water-cooled pressure-tube-type reactor, which possesses favorable characteristics in the utilization of plutonium. A demonstration ATR uses uranium-plutonium mixed-oxide (MOX) as a standard fuel material in order to take the advantage of ATR. The demonstration ATR has adopted a gadolinium-poisoned and axially multi-enriched fuel-assembly in order to flatten radial and axial power distributions. As a result, core performance improvements have been achieved, such as an increase in the average power per fuel assembly. The accuracy of the method applied to the nuclear design of the demonstration ATR has been confirmed by comparing calculated results with experimental data of the deuterium critical assembly (DCA) in Oarai Engineering Center of Power Reactor and Nuclear Fuel Development Corporation and operation data of FUGEN. (author)