Technology Development of Core Catcher for Indian Advanced Nuclear Reactors

Considering the carbon footprint and demand for electricity for a growing population of India, nuclear energy has to play a major role in the energy mix. Currently, India produces just around 3% of electricity from nuclear in the energy mix, which has to multiply by several folds in near future. Of course, the safety of current fleet of nuclear reactors are excellent; however, the core melt accidents at TMI-2, Chernobyl and the recent Fukushima have raised concerns on the safety of nuclear reactors. Soon after the TMI-2 and chernobyl accidents, advanced nuclear reactors (Gen III and III+) were designed with innovative safety systems to practically eliminate core melt accidents, and some are being built with mitigatory features against core melt accidents such that the impact in public is minimum. In the Indian context, PHWRs are the main workhorse of the nuclear power at present. These reactors have several tons of cold water in calandria vault which can cool the corium debris by in-vessel retention in case of a very low probable severe accident. Passive safety systems have been designed in other advanced nuclear reactors which are robust enough against occurrence of core melt accidents. For further enhancement in safety in these advanced nuclear reactors, a core catcher is required to contain and cool the core melt for extended period and reduce the radioactivity release to public domain substantially, so that the public is not affected. The core melt is a complex mixture of nuclear fuel, clad material, structural material, control rod materials, etc. (also known as corium) and the corium forms at very elevated temperature (more than 3000 K). Retention and cooling of several tons of this aggressive material to very low temperature is technologically challenging and scientifically complex. The objective of this paper is to present technology of an innovative core catcher with the sacrificial material which can cool and absorb the enthalpy of high temperature corium and facilitate density inversion to cause low density oxide material to move to the top and high density metallic components to remain at bottom of core catcher, known as “density inversion”. The density inversion is very important from undesirable hydrogen generation point of view. In addition, the melt forms a stable crust enveloping the heat generating high temperature melt like a “capsule”, so that when water is added to the top of melt to cool it, the stable crust prevents water ingression into the bottom of core catcher and eliminates metal water interaction to cause hydrogen generation and create eruptions for aerosols to leave. The phenomenology of crust formation, growth and its stability by cooling the melt from top and side of core catcher, and elimination of water ingression is difficult to predict and scientifically unresolved. The geometry of the core catcher and cooling strategy directly affect the performance of core catcher. To address it, several simulated tests have been conducted to understand the physics of corium coolability, water ingression, and melt inversion. These tests helped to optimize the design of the core catcher to accomplish the objectives of (1) containing and localizing 100% core melt inside the core catcher, (2) to prevent the re-criticality of molten core, (3) to quench the melt within stipulated time, and (4) to stabilize the melt inside core catcher for sufficiently long time (several months). This paper provides a review of the series of tests done for core catcher design optimization and validation.