Patent Number: 
Section: claims

1. A fully passive decay heat removal system comprising: a hot pool having received therein hot sodium heated by a nuclear reactor core; an intermediate heat exchanger which heat-exchanges with the sodium of the hot pool; a cold pool having received therein cold sodium cooled by passage through the intermediate heat exchanger, the cold pool being isolated from the hot pool; a support barrel extending vertically through the boundary between the hot pool and the cold pool, in which the upper end of the support barrel is higher than the liquid level of the hot pool, and the lower end thereof penetrates into the cold pool; a sodium-sodium decay heat exchanger having a heat transfer tube, the sodium-sodium decay heat exchanger being received in the support barrel in order to remove decay heat from the inside of the nuclear reactor; a sodium-air heat exchanger which is provided at a position higher than the sodium-sodium decay heat exchanger; an intermediate sodium loop for heat removal which connects the sodium-sodium decay heat exchanger with the sodium-air heat exchanger; and a primary pump which pumps the sodium of the cold pool to the hot pool via the nuclear reactor core in a normal operating state to maintain a difference in liquid level between the hot pool and the cold pool, such that the liquid level of the hot pool is higher than that of the cold pool, wherein a portion of an effective heat transfer tube of the heat transfer tube is immersed in the cold pool in a normal operating state, and a surface of a lower end of a shroud for the sodium-sodium decay heat exchanger, the lower end being immersed in the sodium of the cold pool, has perforated holes. 2. The fully passive decay heat removal system of claim 1, wherein a length of the effective heat transfer tube of the sodium-sodium decay heat exchanger is immersed in the cold pool and is ⅓ of the total length of the effective heat transfer tube. 3. The fully passive decay heat removal system of claim 1, wherein the shroud of the sodium-sodium decay heat exchanger is formed so as to extend beyond a lower end of the sodium-sodium decay heat exchanger, and the extended portion of the shroud is formed in a baffle form. 4. The fully passive decay heat removal system of claim 3, wherein the lower end of the shroud formed so as to extend beyond the lower end of the sodium-sodium decay heat exchanger is located at a middle portion of a reactor separation plate in the region of the cold pool. 5. The fully passive decay heat removal system of claim 1, wherein a heat transfer tube distributor connects a sodium flow downcomer of the sodium-sodium decay heat exchanger with the heat transfer tube and is formed of a tube sheet of the same outer diameter as that of the sodium flow downcomer. 6. The fully passive decay heat removal system of claim 1, wherein the sodium of the hot pool overflows the support barrel upon loss of the normal heat removal function and a flow guide plate is provided immediately below a heat transfer tube inlet formed above the shroud of the sodium-sodium decay heat exchanger. 7. The fully passive decay heat removal system of claim 6, wherein the flow guide plate is provided in a space through which the sodium of the cold pool, which flows upward in the support barrel upon loss of the normal removal function, can pass and which is formed between the support barrel and the shroud. 8. The fully passive decay heat removal system of claim 7, wherein the flow guide plate is a perforated ring plate which is inserted into a ring-shaped space formed between the support barrel and the shroud. 9. A fully passive decay heat removal system comprising: a hot pool having received therein hot sodium heated by a nuclear reactor core; an intermediate heat exchanger which heat-exchanges with the sodium of the hot pool; a cold pool having received therein cold sodium cooled by passage through the intermediate heat exchanger, the cold pool being isolated from the hot pool; a support barrel extending vertically through the boundary between the hot pool and the cold pool, in which the upper end of the support barrel is higher than the liquid level of the hot pool, and the lower end thereof penetrates into the cold pool; a sodium-sodium decay heat exchanger having a heat transfer tube, the sodium-sodium decay heat exchanger being received in the support barrel in order to remove decay heat from inside of the nuclear reactor; a sodium-air heat exchanger which is provided at a position higher than the sodium-sodium decay heat exchanger; an intermediate sodium loop for heat removal which connects the sodium-sodium decay heat exchanger with the sodium-air heat exchanger; and a primary pump which pumps the sodium of the cold pool to the hot pool via the nuclear reactor core in a normal operating state to maintain a difference in liquid level between the hot pool and the cold pool, such that the liquid level of the hot pool is higher than that of the cold pool, wherein a portion of an effective heat transfer tube of the heat transfer tube is immersed in the cold pool in a normal operating state, the sodium of the hot pool overflows the support barrel upon loss of the normal heat removal function and a flow guide plate is provided immediately below a heat transfer tube inlet formed above a shroud of the sodium-sodium decay heat exchanger. 10. The fully passive decay heat removal system of claim 9, wherein the flow guide plate is provided in a space through which the sodium of the cold pool, which flows upward in the support barrel upon loss of the normal removal function, can pass and which is formed between the support barrel and the shroud. 11. The fully passive decay heat removal system of claim 10, wherein the flow guide plate is a perforated ring plate which is inserted into a ring-shaped space formed between the support barrel and the shroud.