Analysis Of Natural Circulation Characteristics In Spent Fuel Storage Pool

Spent fuel pool is one of the important auxiliary equipments for nuclear reactor. All thespent fuel unloaded from reactor are required to be intermediately cooled in that pool, waitinguntil the radioactivity and decay heat reach a certain standard and then to be transferred anddisposed. Whether a spent fuel storage system is intact or the outer main cooling system isfailure, there will be a natural circulation in the whole spent fuel pool.，which is veryimportant for the security of entire system. Therefore, the study and analysis of the wholespent fuel pool characteristics on the premise of natural circulation have important practicalsignificance.In this paper, RELAP5code was selected as the research tool and Daya Bay spent fuelpool was chosen as the analysis object to carry out the calculation of many steady state andtransient cases. Firstly, the natural circulation state and the field distribution of flow andtemperature in the pool at steady-state with different locations of cooling system water returnpipe can be obtained. Meanwhile, a comparison between the results was discussed. Later, thepaper made a further discussion on the system when the main cooling system was failure,which include the temperature change of the pool water and the time spent fuel pool requiredto reach boiling from steady state and the time to make water level falling until the exposureof spent fuel. Finally, an establishment program of outside passive cooling system after maincooling system’s failure was proposed. And the project can take spent fuel decay heat out ofthe pool perfectly to improve the system security.The calculation results show that：there is a evident natural circulation in the pool. Andunder the influence of that natural circulation it would take a relative long time to make thepool water reach boiling even to uncover the spent fuel from steady state. During that time, ifsome certain remedial measures are adopted, it will not result in spent fuel pool seriousaccident. When cooling system water return pipe is located on top or in the middle of the pool,the natural circulation will include the water space under storage grids, and that is a wholepool natural circulation. While, for the bottom arrangement of water return pipe, the naturalcirculation in pool only exists in the water space above storage grids, and does not include thepool bottom. But in this way, compared with the other two water return pipe arrangement its average water temperature is the lowest, which can highlight the bottom arrangementadvantages from the aspect of reducing water temperature. In accident condition, the timewater required to reach boiling from steady state and to bare storage grids with differentlocations of water return pipe is not complete same. But the difference is quite small. They areall about16.7hours and5days respectively. Meanwhile during the entire transient process, anatural circulation still exists in the pool. And the existence of that circulation is veryfavorable for system’s security to bring the spent fuel decay heat into water spacecontinuously. Besides, by the results comparison it can be found that, different RELAP5nodalizations of the system have a certain influence on the final calculation results. So, inorder to obtain a more accurate and reasonable result when RELAP5is applied, it is necessaryto make some discussion on the different method of node distribution. Finally, by detailedcalculation and analysis, the paper confirmed that the proposed passive cooling system isquite reasonable, which can play an important role in guidance for spent fuel storage projectfuture development.This paper made a detailed analysis of the natural circulation characteristics in spent fuelpool under several different cases. At the same time, a detailed discussion on theestablishment of external passive cooling system after main cooling system’s failure wascarried out. The calculation results of this paper can be the certain reference for spent fuelpool engineering design.