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Numerical analysis of grid plate melting after a severe accident in a Fast-Breeder Reactor (FBR)

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Abstract

Fast breeder reactors (FBRs) are provided with redundant and diverse plant protection systems with a very low failure probability (< 10 − 6/reactor year), making core disruptive accident (CDA), a beyond design basis event (BDBE). Nevertheless, safety analysis is carried out even for such events with a view to mitigate their consequences by providing engineered safeguards like the in-vessel core catcher. During a CDA, a significant fraction of the hot molten fuel moves downwards and gets relocated to the lower plate of grid plate. The ability of this plate to resist or delay relocation of core melt further has been investigated by developing appropriate mathematical models and translating them into a computer code HEATRAN-1. The core melt is a time dependent volumetric heat source because of the radioactive decay of the fission products which it contains. The code solves the nonlinear heat conduction equation including phase change. The analysis reveals that if the bottom of grid plate is considered to be adiabatic, melt-through of grid plate (i.e., melting of the entire thickness of the plate) occurs between 800 s and 1000 s depending upon the initial conditions. Knowledge of this time estimate is essential for defining the initial thermal load on the core catcher plate. If heat transfer from the bottom of grid plate to the underlying sodium is taken into account, then melt-through does not take place, but the temperature of grid plate is high enough to cause creep failure.

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Acknowledgements

The authors wish to express their sincere thanks to Dr P Chellapandi, Director, Nuclear Safety Engineering Group and Mr BK Nashine, Head, Safety Engineering Division for their support. Thanks are also due to the reviewers for the meticulous review of the manuscript and useful suggestions.

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Authors and Affiliations

  1. Nuclear and Safety Engineering Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, 603 102, India

    A JASMIN SUDHA & K VELUSAMY

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  1. A JASMIN SUDHA
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  2. K VELUSAMY
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Correspondence to A JASMIN SUDHA.

Nomenclature

Nomenclature

  • C   Specific heat capacity

  • H   Enthalpy

  • L   Latent heat of fusion

  • T   Temperature

  • Fo   Fourier number

  • GP   Grid Plate

  • h   Heat transfer coefficient

  • k   Thermal conductivity of plate/turbulent kinetic energy

  • L   Thickness of the fuel

  • M   Thickness of Grid Plate

  • t   Time

  • u   Radial velocity

  • v   Axial velocity

  • ΔT   Temperature difference

  • Δz   Axial mesh size

  • Δt   Time step

  • α   Thermal diffusivity

  • ε   Dissipation rate of turbulent kinetic energy

  • ρ   Density

  • μ   Dynamic viscosity of Sodium

  • τ   Time constant/shear stress

  •    Subscripts

  • f   fuel

  • m   melting point

  • o   initial value at t = 0

  • Na   Sodium

  • SS   Stainless Steel

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SUDHA, A.J., VELUSAMY, K. Numerical analysis of grid plate melting after a severe accident in a Fast-Breeder Reactor (FBR). Sadhana 38, 1241–1257 (2013). https://doi.org/10.1007/s12046-013-0189-2

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  • DOI: https://doi.org/10.1007/s12046-013-0189-2

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