Page 158 - Dynamics and Control of Nuclear Reactors

12.8 Steady-state programs for PWRs 155

12.8.6 Steady-state program for PWRs with once-through steam
generators (OTSG)
The situation for PWRs with once through steam generators is quite different. The
significant difference between U-tube and once-through steam generators is the dif-
ference in heat transfer coefficients in the steam generator.
We have seen that the heat transfer coefficient in U-tube steam generators is
nearly constant at different power levels. In once-through steam generators, there
are regions with very different heat transfer coefficients. The overall heat transfer
coefficient has three components: the primary side coolant to metal, the internal
component in the metal tube, and the surface or film component. The primary side
and internal component heat transfer coefficients do not change when secondary
fluid conditions change, but the film component depends strongly on secondary fluid
conditions. Feedwater enters the heated region at close to the boiling point. So,
the heat transfer near the bottom of the steam generator is boiling heat transfer with
2
a large film heat transfer coefficient (typically 6000 to 7000 BTU/(hr-ft -°F). All of
the feedwater boils at some point and the remainder of the heated region causes
superheat. The film heat transfer coefficient on the secondary side in the super-
heating region is much lower than in the boiling region (typically 200 to 300
2
BTU/(hr-ft -°F)).
Since more heat transfer is required to boil the feedwater at higher power levels,
more of the heat transfer surface is used to boil secondary water at higher power
levels. We can see the effect on steam temperature of increasing heat transfer using
a simple and approximate assumption. That is, assume that the heat transfer coeffi-
cient increases linearly with power as indicated below.
ð
U SG A SG ¼ U SG A SG0 1+ bPÞ (12.10)
where
b¼slope of U SG A SG vs. power.
U SG A SG0 ¼value when P¼0.
Substituting Eq. (12.10) into Eq. (12.1) gives
P
T S ¼ θ avg (12.11)
ð U SG A SG0 1+ bPð ÞÞ
Since all quantities in the second term on the right-hand side are positive, the
steam temperature change is smaller when b is non-zero than when it is zero
(as with a U-tube steam generator). Therefore, steam temperature in a steady
state program (with constant average temperature) decreases relative to the steam
temperature variation in a system with a constant steam generator heat transfer
coefficient.
In reactors with once through steam generators, it is possible to implement a
steady state program with constant average coolant temperature over most of the
range of power levels. Eq. (12.1) is satisfied by increases in the effective heat transfer
coefficient and by decreases in steam temperature. Since there is no fixed relation
between temperature and pressure for superheated steam, the pressure can be