Effective energy management design of spent fuel dry storage based on hybrid control rod‐heat pipe

Summary Spent fuel dry storage cask, which stores the spent fuel from wet storage facility to final processing, has been developed and managed. Even though thermal management mechanisms of dry storage casks secure the thermal integrity of the cask, improvement of thermal management capacity of dry storage cask would enhance thermal margin against unforeseen off‐normal conditions with increasing specific storage capacity. Ulsan National Institute of Science and Technology (UNIST) CANister (UCAN) utilizing the new‐type thermosyphon heat pipe, is under development as an advanced design of spent fuel dry storage cask. Hybrid control rod‐heat pipe, which is an annular thermosyphon by containing neutron absorber, is a key component of UCAN. Heat removal performance of the hybrid control rod‐heat pipe was analyzed bya series of experiment withtest facility simulating single fuel assembly with full height. The UCAN fuel assembly showed the reduced structural and fuel temperatures compared to bare fuel assembly of conventional cask designs, exhibitingimproved thermal margin. According to experimentally observed heat removal rate of the hybrid control rod‐heat pipe, UCAN design could extend manageable thermal capacity from 13.3% to 33.8%. The removeddecay heat through hybrid control rod‐heat pipe could be reused in electricity generation with stirling engine and thermoelectric devices improving energy management efficiency. Intermittent boiling accompanying geyser phenomena was frequently occurred inside the test section, and the boiling behaviors according to fill ratios (FRs) and operating pressures dominated the heat removal performances. The hydrostatic pressure owing to large aspect ratio affected differentiated thermal‐hydraulic behaviors compared to conventional thermosyphons. To develop heat transfer and hydraulic models of the hybrid control rod‐heat pipe, geyser phenomenon inside the test section wasdiscussed physically. This workexhibited the feasibility of UCAN design and provides fundamental physics on intermittent boiling of thermosyphon operating at sub‐atmospheric pressures.