Patent Number: 
Section: claims

1. A nuclear fuel assembly for a thermal neutron reactor comprising:multiple fuel rods;multiple guide thimbles arranged between the fuel rods; anda passive safety device inserted in at least one guide thimbles,wherein the passive safety device comprises:a vertically extending passive safety device guide tube coaxially aligned with the guide tube in which it is inserted and having one or more coolant inflow/outflow holes providing coolant flow between an interior and exterior of the passive safety device guide tube; anda buoyancy providing part including a neutron absorber in the interior space of the passive safety device guide tube so as to be movable up and down therein. 2. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein a plurality of passive safety devices are provided in a plurality of the multiple guide tubes and the passive safety device guide tubes of the plurality of passive safety devices are connected to one another via passive safety device guide tube connection parts. 3. The nuclear fuel assembly for a thermal neutron reactor according to claim 2, wherein the passive safety device guide tube connection parts are formed on the multiple passive safety device guide tubes above the guide thimbles in which the passive safety devices are inserted. 4. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein a density of the buoyancy providing part containing the neutron absorber is less than coolant density during a nominal state operation of the thermal neutron reactor and is higher than coolant density during a transient state operation of the thermal neutron reactor. 5. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein the buoyancy providing part includes a neutron absorber-containing part and a buoyancy providing material-containing part. 6. The nuclear fuel assembly for a thermal neutron reactor according to claim 5, wherein the buoyancy providing material includes at least one material selected from the group consisting of air, helium, and argon. 7. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein the neutron absorber includes one of or both of boron and gadolinium. 8. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein the neutron absorber is porous. 9. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein water (H2O) is included as a coolant. 10. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, wherein the passive safety device guide tube includes an upper part and a lower part, wherein the coolant inflow/outflow holes are formed at either or both of the upper part and the lower part of the passive safety device guide tube. 11. The nuclear fuel assembly for a thermal neutron reactor according to claim 1, further comprising control rods inserted any of the multiple guide thimbles not having a passive safety device inserted therein.