Development of two-dimensional hydrogen distribution model in small-scale spaces under severe accidents

Abstract

Hydrogen distribution and combustion are important problems in nuclear reactor safety analysis under severe accident conditions.IPWR (Integrated pressurized water reactor) are very different from conventional nuclear power plants in design, structure and operating power. In particular, the nuclear power plant containment has a large space volume, and there are multiple compartments inside, while the small reactor containment has a small volume and a simple and compact internal layout. This makes the hydrogen in the containment behave differently in a severe accident. Thus, the hydrogen flow distribution characteristics of IPWR under severe accidents need further study.In this paper, a two-dimensional continuum model of hydrogen distribution in small-scale under severe accident conditions is established by using finite volume method and staggered grid scheme based on hydrodynamics calculation method. SIMPER (Semi-Implicit Method for Pressure Linked Equations Revised) algorithm is used to construct the pressure correction equation, alternate direction implicit algorithm is used to solve the algebraic equations, and block correction algorithm is used to accelerate the iterative convergence. Compared with the experimental study of hydrogen flow distribution characteristics in local compartments, the model can predict the change of hydrogen volume fraction distribution well with time in the experiment. After that, the flow distribution characteristics of hydrogen in the RPV (Reactorpressurevessel) and containment under the low-pressure core melt accident were analyzed with IPWR-IP200 as the research object. The results show that the coolant level in the lower head of the RPV is still about 0.8 m during hydrogen generation, so there is no hydrogen in the lower head area. The highest hydrogen volume fraction in the RPV is at the outlet of the core, which is about 23 %. The highest volume fraction of hydrogen in the containment is the outlet above the pressurizer relief valve, which is up to 17 %. Because of the high volume fraction of water vapor in the RPV, there is no risk of hydrogen combustion in the RPV. During hydrogen generation, there is a risk of hydrogen combustion within the containment, and the risk area is located in the upper space of the containment. After hydrogen production ceases, as water vapor continues to enter the containment, the hydrogen volume fraction gradually decreases, and the risk of hydrogen combustion will also decrease.The two-dimensional continuum model established in this paper can predict the hydrogen distribution under severe accidents, which can provide a reference for the hydrogen mitigation strategy of IPWR.