A concept for a new reactor system is developed where weapons-grade plutonium can be made worthless for weapons use. It is a pebble bed-type high-temperature gas-cooled reactor that uses plutonium burner ball and thorium breeder ball fuels. The residual amount of 239Pu in spent plutonium balls becomes <1% of the initial loading. The power coefficient is made negative by reducing the parasitic neutron absorption reaction rate of l35Xe.