SAFPWR home page Overview of t&h model

Overview of thermal-hydraulic model

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Before digging into the detailed analysis of the program mathematical model, we will start with a qualitative description and illustrate its actual implementation on a simple application case (003.dat) involving the complete PWR system

General description of the balance equations solution method

Despite of the major simplifying assumptions set up at the genesis of the project, the exact resolution of the system of tightly coupled, non-linear, balance equations remains a difficult task.

We will have recourse to a decomposition method: it consists in splitting the composite physical transformation undergone by the fluid in the course of the time-step, into an iterated sequence of partial transformations in each of which a part only of the physical variables is allowed to vary.

As explained earlier, thanks to the implicit solution scheme, the severe restrictions imposed by the Courant numerical stability criterion are avoided, which makes it feasible to accept large time-steps and volumes without jeopardizing stability and also allows evaluating solution accuracy by simply decreasing node volumes.

Furthermore, as the resolution method does not resort to any preliminary linearization (through Jacobian calculation) and as the converged solution strictly verifies the original field balances, it is feasible to check the solution by directly substituting it into the original balance equations.

At last, as we are not faced with having to solve large linear equation systems, the calculation time increases only linearly (as opposed to a 2.5 power) with the number of nodes.

Primary system

More explicitly, as far as the primary side is concerned, the system of mass, enthalpy and boron balance equations (in short "m,h,b-balances") is firstly solved under the assumption that the (global) primary pressure p3 and the primary pumps volumetric flows wvpl are available.
The m,h,b-balances route starts from the core outlet o, which plays the important role of primary system cross-road point.
Starting with the current values of (w,h,b)el at loop_1 inlet, implicit Euler (m,h,b) balances are firstly carried out on the successive nodes li(1:i9l(l)) of the current loop (loop_l), following the fluid displacement.
Next, through dowcomer a, core bottom b, the successive nodes of the core c and the dome d.
Back to core outlet, the enthalpy balance becomes necessarily explicit there, insofar as the enthalpy hel of the fluid outgoing o and feeding the loops was already assumed at initiation of loops balances.
Actually O processing is the only exception to implicitness in balance calculations.
In order to avoid the possible numerical instability which could result thereof, an under-relaxation factor omo [OMega;Outlet] has been provided.
As already mentioned, it is possible that the primary balances need to be repeated, in case of large time-step, if instability is observed for the o-balance.
In such a situation, the detailed balances processing (do, component-name) are not repeated, but can be replaced by a simplified node balance (redo, component-name) making use of the isobaric h-derivatives vmh of vm saved along the do, component_name route.
It should be pointed up that the local field conservation is not affected by incomplete convergence.

Balances for the oxp subsystem

"oxp" stands for: core Outlet + eXpansion branch of the pressu + compressible vapor-water mixture of the Pressu.

The (m,h,b)balance on o generates the flow wso escaping o towards x. wso actually represents the accumulation rate of the mass dilatation/contraction of the successive nodes along the m,h,b-balances route, including o itself.
If wso > 0 , hso = h2o and bso=b2o according to the implicit scheme.
On the contrary, if wso < 0 a "special balance of o must be performed wherein hso cannot result from the o-balance but is taken as the current h value at the foot of x, and the balance solved for wso in place of hso.
Next, the balances are carried out though the successive nodes xm=1,i9m of the expansion line
The balances calculations are, at last, completed by a m,h,b,v-balance on p .
As no fluid can escape from p, which is the dead end of the balances ox route, we must accept that the p-volume varies freely (v2p /= v1p) /= [means "not equal to"] and this will result in a volume error v2p-vp (v2p: eos volume, vp: geometrical volume).
Primary pressure updating.:
p3 is now updated in such a manner as to nullify the volume error by means of an isentropic transformation of the whole primary system, by making use of the nodal fluid compression coefficients collected along the balances routes.

Primary flow updating.

At last, the mass flow at loops entry (wel), pressurizer spray (wspr) and dome by-pass (wed) are updated by solving the fluid kinetic momentum balance equations, making use of the dilatation rates calculated along the nodal balances.

Steam-generator model

The SG deserves a reasonably realistic modeling in view of the important potential reactivity release, caused by a cooling accident, at of cycle, when the temperature coefficient is large.

The simplified model implemented for the SG is inspired from that of the primary model. It features modelling water natural recirculation, level regulation and vapor-water separation in the dome. This should provide a fair prediction of mass and enthalpy inventories and heat-exchange through the SG tubes.