Code Manual for CONTAIN 2.0 - Federation of American Scientists

The coolant pool layer is unique in that it is the only lower cell layer, outside of the models in
CORCON, in which phase changes are allowed. As discussed in Chapter 10, surface evaporation ~
and surface condensation of coolant vapor can occur much in the same manner as for heat transfer
structures. As discussed in Section 11.2.1, pool evaporation and condensation can also occur with
respect to gases vented into the pool under the pool surface. Boiling of the coolant can occur if the
BOIL keyword is specified in the lower cell input block. If boiling is not activated, the pool is
treated like other nodes in the conduction model, and the pool could reach unreasonable
temperatures. Therefore, boiling should always be activated. The BOIL keyword is also required for
proper equilibration of the coolant vapor between the pool and any gases vented under the pool
surface. In the absence of the BOIL keyword, any coolant vapor in the vented gases is entirely
condensed out in the pool.
If boiling is activated, any net energy flux to the coolant pool that would raise the pool above
saturation is assumed to contribute instead to the pool boiling rate. When such a saturated condition
is detected, the conduction model solution is carried out with the pool temperature fixed at the
saturation temperature, for the purpose of calculating the conduction flux to the pool and the boiling
rate.
Pool boiling is calculated fully implicitly with respect to pressure, if the implicit flow solver is
invoked. Heat and mass sources to the pool are accumulated every cell timestep. They are then used
to calculate a continuous boiling rate during the system timestep in which the sources are
accumulated. Because of the fully implicit treatment, the pressurization due to the coolant mass and
energy entering the atmosphere as a result of boiling is consistent with both the pool saturation
temperature and the mass and energy flows to and from other cells. If the explicit flow solution is =
invoked, pool boiling is evaluated explicitly outside of the flow and atmosphere thermodynamics
solution. The expressions for the pool boiling rate for both flow methods are given in Section 4.4.6.
5.5 Interlayer Heat Transfer
The heat transfer coefficients used in the lower cell conduction model between layers are discussed
in this section. The following, with one exception, applies to the heat transfer between the
uppermost lower cell layer, which could be the pool, and any additional layers below. The heat
transfer across the interfaces between the atmosphere and uppermost lower cell layer and between
the coolant pool layer and submerged heat transfer structures is discussed in Section 10.1.2. The one
exception mentioned above is an option, discussed in Section 5.5.2, for the user to specify the total
convective heat transfer coefficient for the atmosphere-pool interface.
This section consists of three parts: the fust describes the overall interlayer heat transfer coefllcient;
the second discusses user-specified overall coet%cients; and the third discusses the heat conduction
model.
5.5.1 Interlayer Heat Transfer Coefficients
The present section discusses the overall interlayer heat transfer coefficients that are defined for use
in the heat conduction model, which is used when CORCON is not active. These heat transfer
correlations are applied to the interface between layers; the heat transfer between nodes of a layer
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