Design concept of DVI+

A new design feature of DVI+, to mitigate an ECC bypass fraction and to prevent switching an ECC outlet to a break flow inlet during a DVI line break, is presented for an advanced DVI system. The injected ECC water into the downcomer is easily shifted to the broken cold leg by a high steam cross flow which is coming from the intact cold leg during the late reflood phase of a LBLOCA in the current DVI system. For a protective ECC flow down channel from a high-speed cross flow in a downcomer, a 4-duct which is called an ECC extension duct is installed at the outside of a core barrel cylinder. The ECC extension duct has a gap (height to the radial direction) of 3/25-7/25 of the downcomer annulus gap. The DVI nozzle and the ECC extension duct are only connected by the ECC water jet which is called a hydrodynamic water bridge during an ECC injection period. Otherwise these two components are disconnected from each other without any pipes inside the downcomer. The ECC extension duct is full separated ECC water down flow channels in the downcomer annulus during a LOCA event. The injected ECC water flows downward into the lower downcomer through the ECC extension duct without a strong entrainment to a steam cross flow. The outer downcomer annulus of the ECC extension duct is the major steam flow zone coming from the intact cold leg during a LBLOCA. During a DVI line break, the separated DVI nozzle and extension duct have an effect to preventing the level of the cooling water from being lowered in the downcomer due to an inlet-outlet reverse phenomenon at the lower position of the outlet of the DVI extension duct. The feasibility of DVI+ has been tested to evaluate the LOCA performance using the CFX code. The test results show that it predicts the ECC water flows to the lower downcomer well and the ECC bypass fraction is very low. (author)