CFD Simulation of Fuel Dispersion and Fireball Formation Associated with Aircraft Crash on NPP Structures

ABSTRACT

The accidental or intentional crash upon structures of Nuclear Power Plant (NPP) by an aircraft cause the fuel spreading followed by a fireball formation. This fireball is large enough to engulf the entire NPP and radiates a large amount of heat. This engulfment may lead to a local rise in temperature, which causes the spallation of concrete structure and fatalities to the human being. This may affect the integrity of NPP structures and has safety implications. The building structures influence the spatial evolution of fireball due to the generation of large turbulent structures, which further increases the local temperature. The evolution of fireball following fuel dispersion due to the aircraft crash upon an NPP is presented in this article. Numerical simulation to study the effect of fireball on the target NPP structure and its surrounding has been performed using a three-dimensional computational fluid dynamics (CFD) code. This analysis is used to study the evolution of fireball and thermal hazard associated with the radiated heat. It is found that some parts of fireball energy go as heat input to containment and the remaining portion is dissipated to the atmosphere by convection and radiation.

Abbreviations

CCPS: Centre for Chemical Process Safety; CID: Controlled Impact Demonstration; CFD: Computation Fluid Dynamics; EDC: Eddy Dissipation Concept; FAA: Federal Aviation Administration; FDS: Fire Dynamic Simulator; FVDOM: Finite Volume Discrete Ordinate Method; GAMG: Geometric Algebraic Multi-Grid; HRR: Heat Release Rate; IAEA: International Atomic Energy Agency; NIST: National Institute of Standards and Technology; NPP: Nuclear Power Plant; NTSB: National Transport Safety Board; OpenFOAM: Open Field Operation And Manipulation; PBiCG: Preconditioned Bi-Conjugated Gradient; PISO: Pressure Implicit with Splitting of Operator; SMD: Sauter Mean Diameter; TNO: The Netherlands Organization of applied scientific research; VTT: Technical Research Centre of Finland; WTC: World Trade Centre

KEYWORDS:

• Aircraft crash
• droplet dispersion
• fireball
• NPP safety assessment

Nomenclature

Ad	=	surface area of droplet
Cd	=	drag coefficient
Cp	=	specific heat capacity (kJ kg−1 K−1)
D	=	diffusion coefficient
DFB	=	fireball diameter (m)
hFB	=	lifting height (m)
h	=	convection coefficient (w/m2K−1)
hs	=	sensible enthalpy (kJ/kg)
ΔHc	=	Heat of combustion (kJ mol−1)
H°(T)	=	absolute enthalpy (kJ/kg)
Hv	=	latent heat of vaporization
I	=	radiation intensity (W sr−1)
k	=	turbulent kinetic energy (m2 s−2)
L	=	distance from the target
M	=	mass of fuel (kg)
$\vec{r}$	=	radius vector
$\vec{S}$	=	direction vector
Srad	=	source term for thermal radiation
${\vec{S}}^{\prime }$	=	scattering vector
S	=	path length (m)
T	=	time (s)
Δt	=	integration time step
Φ	=	scattering phase function
${\mathrm{\Omega }}^{\prime }$	=	spatial angle (sr)
Xv	=	vapor mole fraction
Y	=	mass fraction

Subscript

a	=	ambient
d	=	Droplet
fuel	=	Fuel
FB	=	Fireball
h	=	Enthalpy
k	=	Species
n	=	Number
lift	=	Lifting
max	=	maximum
mix	=	Mixing
ox	=	Oxidizer
in	=	Inlet
t	=	turbulent

Greek Symbols

Ω	=	reaction rate (mol L−1 s−1)
Α	=	absorption coefficient
β	=	evaporation parameter
γ*	=	mass fraction
Ε	=	dissipation rate
µ	=	dynamic viscosity (N s m-2)
η	=	refractive index
τ	=	transmissivity
${\tau }_d^{st}$	=	droplet relaxation time
τmix	=	turbulent mixing
Ρ	=	density (kg m−3)
Σ	=	Stefan-Boltzmann constant (W m−2 K−4)
σs	=	scattering coefficient
χR	=	radiative heat fraction
χ	=	fraction of fine structure which may react

Non-Dimensional Numbers

Bi	=	Biot Number
BM	=	Splading Number
Nu	=	Nusselt Number
Pr	=	Prandtl Number
Re	=	Reynolds Number
Sc	=	Schmidt Number
Sh	=	Sherwood Number

Superscripts

“-”	=	Spatial filter
“~”	=	Favre filter
st	=	Stokes flow

Disclosure statement

No potential conflict of interest was reported by the authors.