Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
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III. RELAP5
The RELAP5Mod3.2.2 code has been developed for the analyses of light water reactor coolant systems during
transients and postulated accidents. Code RELAP5Mod3.2.2 code is based on a nonhomogeneous
and nonequilibrium model for the twophase system that is solved by a fast, partially implicit numerical scheme to
permit economical calculation of system transients. The code includes many generic component models from
which general systems can be simulated. The component models include pumps, valves, pipes, heat releasing or
absorbing structures, reactor point kinetics, electric heaters, jet pumps, turbines, separators, accumulators,
and control system components. In addition, special process models are included for effects such as form
loss, flow at an abrupt area change, branching, choked flow, boron tracking, and noncondensable gas transport.
Code RELAP5 also includes a model for calculating the thermal conductivity, based on the composition of gas in
the gap. The RELAP5 dynamic gap conductance model defines an effective gap conductivity based on a
simplified deformation model generated from FRAP- T6.4.11-1 The model employs three assumptions as
follows: a the fuel-to-cladding radiation heat transfer, which only contributes significantly to the gap
conductivity under the conditions of cladding ballooning, is neglected unless the cladding deformation model is
activated, b the minimum gap size is limited such that the maximum effective gap conductivity is about the
same order as that of metals; c the direct contact of the fuel pellet and the cladding is not explicitly considered
[1]. The gap conductance through the gas is inversely proportional to the size of the gap. Since the longitudinal
axis of the fuel pellets is usually offset from the longitudinal axis of the cladding, the width of the fuel-
cladding gap varies with circumferential position. This variation causes the conductance through the gas in the
fuel-cladding gap to vary with circumferential position. The circumferential variation of the conductance is taken
into account by dividing the gap into several equally long segments, as shown in Fig. 3. [1]. The conductance for
each segment is calculated and then an average conductance, hg, is computed in the FRAP-T6 model by
the equation. The RELAP5 code analyzes the heat transfer in the gap on the basis of the following
relationships and for the given position of fuel and cladding shown in Fig. 3. [1].
Heat transfer coefficient in the gap is solved by following relationship 6 for given gas composition in
the gap in RELAP5 code [1]:
1 2
1
1 3 2
N G
G n
F C
n
N t
, r
r g
g
λ α
=
= +
⋅ +
+ +
∑
6 The width of the fuel-cladding gap at any given
circumferential segment is calculated by the equation:
Fig. 3. Segmentation at the fuel-cladding gap
2 1
1
n G
o
n t
t t
N ⎡
− ⎤ ⎛
⎞ =
+ − +⎜ ⎟
⎢ ⎥
⎝ ⎠
⎣ ⎦
7 The value of t
n
in equation 7 is limited between zero and 2t
g
. The circumferential averaged width of the fuel cladding gap, t
g
, in equation 7 is determined by the expression:
G F
C
t t
u u
= − + 8 The radial displacements, u
F
and u
C
, are primarily due to thermal expansion. The radial displacement, of the
fuel pellet surface, u
F
, is calculated by the equation:
F TF
r S
u u
u u
= + + 9
The radial displacement of the inner surface of the cladding is calculated by the relationship:
C TC
CC e
u u
u u
= +
+ 10
IV. Computational Model
