.. _section-13.1:

Introduction and Overview
-------------------------

.. _section-13.1.1:

Background and Description of the CLAP Physical Model
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The CLAP Model [13-1, 13-2] computes cladding relocation and phase
changes for accident situations in which the fuel geometry is still
essentially intact. Such a situation typically arises in undercooling
accidents during which coolant voiding and pin dryout occur, followed by
rapid heating and subsequent melting of the cladding. In the usual
accident scenario, the cladding motion proceeds as follows: (i) the
molten cladding may initially rise due to the pressure gradient and
viscous shear forces generated by the sodium coolant vapor flow in the
dried-out section of the core, (ii) the rising molten cladding then
freezes upon encountering cooler structure in the region immediately
above the active fuel (either the upper reflector or upper blanket
region, depending upon the reactor design), and (iii) the upper
frozen-cladding blockage throttles the vapor flow, thus allowing the
remaining molten cladding to drain under the influence of gravity. The
cladding motion and phase changes significantly affect the subsequent
course of events in the scenario both through the influences of
cladding-motion-related reactivity changes and frozen-cladding blockages
that possibly inhibit later fuel motions.

The available SAS3D cladding relocation model, CLAZAS [13-3], is
inflexible in calculation of the consequences for such diverse
mechanistic and/or postulated phenomena. This model suffers from several
shortcomings in the estimation of cladding accelerations. First, the use
of large cladding segments can lead to a significant change in results
following relatively minor input changes. Second, the model-dependent
Lagrangian mesh leads to difficulties in integrating consistently with
the voiding dynamics and fuel motion models. Third, the formalism does
not provide flexibility with respect to the mode of cladding refreezing
or with respect to variations in the time-dependent coupling to the
sodium-vapor dynamics. If treated correctly, the influence of
sodium-vapor dynamics could result in a possible display of oscillatory
effects.

The CLAP model uses and Eulerian numerical formulation coupling the
time-dependent continuity, momentum, and energy equations for a film of
moving cladding, the continuity and energy for sodium vapor, and the
SAS4A pin heat-transfer calculation. This is accomplished by
interconnecting an implicit solution for the low-density sodium vapor
with an explicit, modified upwind differencing procedure for the
high-density cladding. Axial resolution depends on the mesh spacing
selected by the user. The key phenomena influencing cladding motion in
CLAP are the degree of interfacial sodium vapor friction and the
cladding heat transfer. Input that controls these phenomena permits a
variety of experimental situations to be simulated.

As shown in the schematic in :numref:`figure-13.1-1`, the CLAP model divides the
core axially into two types of zones. In the central zone, the cladding
is molten and is in motion. In that region, heat is transferred directly
from the fuel surface to the molten cladding. In the zones at either end
of the fueled region is intact cladding. Due to cladding motion, the
intact cladding may be coated with refrozen cladding and/or molten
cladding.

.. _section-13.1.2:

CLAP Module Structure and Interaction with Other SAS4A Models
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The initiation of CLAP and the decision for adding axial segments to the
molten-cladding zone occurs in the fuel-pin model TSHTRV. During
cladding motion, the fuel-pin model TSHTRV continues to evaluate the
fuel temperatures and, outside of the molten zone, the temperatures of
the intact cladding. Coupling between TSHTRV and CLAP allows for heat
transfer between the fuel and molten cladding in the molten-cladding
zone and between the initial cladding and refrozen (or molten) cladding
outside the molten zone.

The CLAP subroutines are called every coolant time step and, as shown in
:numref:`figure-13.1-2`, are called from various locations in the sodium voiding
module. The functions of the CLAP subroutines are as follows:

TSCLD1 - (i) initiates CLAP variables, (ii) adds segments to molten
zone, (iii) calculates local heat transfer between molten steel and fuel
within the molten zone, (iv) calculates local heat transfer between the
initial cladding, refrozen cladding (if any), and molten cladding (if
any) outside of the molten zone, and (v) computes mass and areas of the
refrozen and molten cladding allowing for convection and phase change;

TSCLD2 - solves the momentum equation to obtain the velocity of the
molten cladding film and contains the CLAP output and debug print
statements;

SODFRC - computes the two-phase friction factor used to evaluate the
interfacial shear stress between the sodium vapor and molten steel;

DENSIT - evaluates the cladding melt fraction, temperature, and density
as a function of cladding specific internal energy.

The CLAP subroutine TSCLD1 computes the current (local) vapor flow areas
and hydraulic diameters, which are used by the sodium voiding model.
Also, TSCLD1 may reduce the current coolant time step, if necessary
(based on a Courant condition criterion), and require the coolant
dynamics model to reevaluate current flow parameters.

The CLAP subroutine TSCLD2 is called later and utilizes updated sodium
vapor velocities and pressures to compute the molten cladding velocity
for the beginning of the next time step. Local two-phase friction
factors (to evaluate the gas/liquid interfacial stress), which are
computed by TSCLD2 for the next time step, are passed to the sodium
voiding model.

Coupling of CLAP with the reactivity model consists of current cladding
mass distributions (axial) computed in TSCLD1 being utilized in the
subroutine FEEDBK to evaluate the reactivity change due to cladding
relocation.

.. _figure-13.1-1:

..  figure:: media/image2.png
	:align: center
	:figclass: align-center

	Schematic of CLAP Geometry Treating Cladding Relocation

.. _figure-13.1-2:

..  figure:: media/image3.png
	:align: center
	:figclass: align-center

	CLAP Flowchart