Patent Number: 
Section: claims

1. A combination of a reactor vessel, a reactor core arranged in the reactor vessel, and a decay heat removal system for removing decay heat of the reactor core,wherein the reactor core includes a control rod insertion hole into which a control rod is allowed to be inserted,wherein the reactor vessel includes an insertion hole formed on a top thereof, andwherein the decay heat removal system comprises:a first heat pipe which is placed in an upper plenum of the reactor vessel and arranged vertically corresponding to a position of the insertion hole formed on the top of the reactor vessel and corresponding to a position of the control rod insertion hole;a control rod drive mechanism which is connected to an upper plenum of the first heat pipe and drives the first heat pipe to move up and down so that the first heat pipe is selectively inserted in the control rod insertion hole of the reactor core; anda second heat pipe which is coupled to and in close contact with a bottom surface of the reactor vessel and removes the decay heat generated in the reactor core,wherein the first heat pipe is a hybrid heat pipe which is internally provided with both a neutron absorber and a coolant, the first heat pipe not only absorbing a neutron generated in the reactor core by the neutron absorber but also removing the decay heat generated in the reactor core by the coolant. 2. The combination according to claim 1, wherein the first heat pipe receives the coolant from an upper plenum of the reactor vessel as a heat sink, and performs cooling by absorbing the decay heat generated in the reactor core, and transferring the absorbed heat to the coolant. 3. The combination according to claim 1, wherein a fin is provided in a condenser of the first heat pipe, which employs air, water, nanofluid, seawater, nitrogen or liquid metal as a coolant for exchanging heat with the first heat pipe. 4. The combination according to claim 1, wherein the second heat pipe is a hybrid heat pipe which is internally provided with both a neutron absorber and a coolant, the second heat pipe not only absorbing the neutron generated in the reactor core by the neutron absorber but also removing the decay heat by the coolant. 5. The combination according to claim 1, wherein the second heat pipe is made of a flexible material, is curved corresponding to the shape of the bottom surface of the reactor vessel, and closely contacts the bottom surface. 6. The combination according to claim 1, wherein the second heat pipe is curved corresponding to the shape of the bottom surface of the reactor vessel, and closely contacts the bottom surface. 7. The combination according to claim 4, wherein the coolant in the second heat pipe comprises one of water (H2O), a nanofluid, a refrigerant, mercury (Hg), lithium (Li) and FLiBe (LiF-BeF2). 8. The combination according to claim 1, wherein a wick placed in the first heat pipe comprises one of carbon fiber, copper, stainless steel, zirconium alloy, silicon carbide (SiC), and boron carbide (B4C). 9. The combination according to claim 1, wherein a case material forming an outer appearance of the first heat pipe comprises one of stainless steel, zirconium alloy, inconel alloy and molybdenum alloy. 10. The combination according to claim 1, wherein the coolant in the first heat pipe comprises one of water (H2O), a nanofluid, a refrigerant, mercury (Hg), lithium (Li) and FLiBe (LiF-BeF2). 11. The combination according to claim 1, wherein a wick placed in the second heat pipe comprises one of carbon fiber, copper, stainless steel, zirconium alloy, silicon carbide (SiC), and boron carbide (B4C). 12. The combination according to claim 1, wherein a case material forming an outer appearance of the second heat pipe comprises one of stainless steel, zirconium alloy, inconel alloy and molybdenum alloy.