Core Thermal Analysis for AP1000 Nuclear Plant
For every computational cell (one fuel assembly wide and chosen axial node length):
Inputs:
Core Physical Characteristics:
# of fuel pins per fuel assembly, Pitch, Clad Outside Diameter, Clad inside Diameter, Fuel
rod diameter, Length of active core (fuel rods)
Thermal Characteristics:
Fission Heat Produced per one axial fuel assembly cell (input from nuclear model)
Physical properties:
Water : density, viscosity , thermal conductivity,
Fuel Pin: thermal conductivity, gap conductivity, clad
Flow properties:
Mass flow rate into the cell (constant), inlet pressure
Calculated values
Re, Pr, and Nu numbers and heat transfer coefficient in every cell (h),
Using the value of h and a const q’’’, calculate the temperature profile in the fuel rod and use
the heat flux at the surface to determine the temperature rise in water across the cell.
Pressure at the outlet of the core, calculated separately – not in every cell.
Outputs:
Average Temperature in every cell.
Pressure drop across the core.
Heat Conduction – Overall Resistance
Step 1.
For a given q’’’, calculate clad surface q’’ and q‘, and use heat balance to calculate cell exit
temperature (assuming steady state heat transfer).
Mdot * Cp * (Texit – Tin) = q’’ * total clad surface area ( n * pi * D* cell height)
Texit = Tin + q’’ * A / ( Mdot * Cp)
Therefore, Tav for the cell = (Tin + Texit )/ 2
Step 2. For every cell, calculate a value for heat transfer coefficient - “h”
With the given mass flow rate, density and viscosity(at Tave) for water,
Calculate Re as (mdot*Dh/A*Mu), Pr ( Mu * Cp/ K) water and use Dittus-Boelter correlation to
calculate a value for “h”
h = 0.023 * (Re) ^0.8 * (Pr) ^.333
For initial Tcoolant ( = Tin), calculate Fuel centerline Temperature( Tmax) as shown below
Tmax – Tcoolant = q’ [ 1/(4*pi*Kf) +
1/ ( 2*pi*Rg*hg) + ( 1/(2*pi*Kc))ln (Rco/Rci) + 1/(2*pi*Rco*h) ]