Technical considerations in repowering a nuclear plant for fossil fueled operation Page: 2 of 6
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conditions in the vicinity of 540 F and 950 psig. Fossil
fueled steam sources for electrical power generation
usually operate at significantly higher pressures and
temperatures that are in the superheat region. This
improves the heat rate (efficiency) of the power cycle,
which is important for the more expensive fossil fuel. The
second challenge, is to utilize as much of the existing
turbine plant equipment as possible. While doing this, the
designer must strive to obtain the best turbine cycle heat
rate possible, consistent with the additional investment to
repower.
The variations cover a wide range of possibilities. They
can range from various simple approaches to complex
ones which alter the nuclear plant feedwater cycle or
replace the high pressure turbine of the nuclear plant with
a new steam turbine that provides a better match
between the higher pressure and temperature of the new
steam source and the low pressure sections of the nuclear
turbine. While intricate approaches improve the turbine
cycle heat rate, they add to the capital cost of
repowering. For the purpose of illustration of technical
considerations, this paper is limited to less complicated
approaches.
Gas Turbine Combined Cycle (GTCC)
In this approach multiple gas turbines are used in
combination with HRSGs that utilize waste heat from the
gas turbines to generate steam. The steam drives the
existing nuclear cycle steam turbine-generator. The gas
turbines also drive electrical generators. Consequently if
the full output of the original steam turbine is developed,
the electrical output of the repowered plant will be greater
than that of the original plant.
A range of variations on the basic approach are possible.
Some possibilities are:
" Supplementary firing of the HRSGs. This reduces the
number of gas turbines required to produce the steam
flow to match the flow of the nuclear cycle turbine.
" Topping steam turbine. This turbine is located
between the HRSGs and the existing nuclear cycle
turbine. It provides better utilization of the energy in
the steam from the HRSG.
" Low pressure auxiliary turbine. This turbine utilizes low
pressure steam available from the HRSG. Alternatively,.
this turbine can be shut down during the winter and
low pressure steam can be used for district heating.
Various gas turbine options are combined in a single
schematic diagram -in Figure 1. Four possible options,
based on this schematic are identified by the alternatives
tabulated in Table 1.
The cases considered in this table do not include use of
low pressure steam for district heating or process
purposes. If this were done, the output of the low
pressure turbines would be reduced by an amount
corresponding to the steam that is diverted. In the
extreme case none of the steam would be used in thetopping turbine. The first two cases do not have a topping
turbine. Hence, the gross power generated is somewhat
lower than the cases with a topping turbine. In these
cases, the heat rate is higher (poorer) than in the cases
with a topping turbine. This is to be expected since a
cycle with a topping turbine utilizes more of the energy in
the steam. The cases with. supplemental firing do not
require as many gas turbines, because some of the steam
for the nuclear cycle turbine is produced by the fuel fired
in the HRSG. Thus, they have the lowest gross power
output of the cases considered. On the other hand, the
cases with supplemental firing have the poorest heat rate,
because the fuel is fired in the steam generator and its
energy does not contribute to the gas turbine cycle.
In order to match the steam flow of the nuclear cycle
turbine, a large number of gas turbines are typically
required, even with relatively large (222 MWe), gas
turbines. For this reason, Table 1 shows that the electrical
output of the repowered unit is from 2.9 to 4.6 times the
output of the original nuclear unit. This requires that there
be a demand for the increased output. Otherwise, fossil
power conversion is not a viable option.
Gas turbines, such as those considered in this
application are standard components in electrical
generating systems. The modular features of these units,
together with integration of auxiliaries and controls,
simplifies installation and permits a relatively short
construction schedule. If needed to meet power demand,
the gas turbines can be brought on line prior to connecting
the steam lines to the nuclear cycle turbine.
Fossil Fueled Boiler
Fossil fueled boilers could either be gas fired or coal
fired. For gas firing the constraints on supply previously
discussed are applicable. However, fossil fueled boilers
are most frequently thought of in terms of coal firing.
They will be considered in this context in this paper.
Design of a saturated steam fossil fueled boiler is not
current conventional boiler practice for boilers of the size
that can provide the required steam flow for a large
nuclear turbine. From the standpoint of steam produced
by the boiler, there are two viable options in terms of
current boiler practice. The first is a subcritical boiler
producing 1000 F steam at 2400 psi. The other alternate
is a supercritical boiler producing steam at about 3500
psi. The latter alternative has a significantly better heat
rate than the first and is considered further. In the size
range under consideration, the steam can be produced by
two boilers and possibly one. Steam leaving the
supercritical boiler is at a significantly higher pressure and
temperature than the conditions acceptable at the throttle
of the nuclear turbine. Therefore, a high pressure topping
turbine is located between the boiler and the nuclear cycle
turbine. This turbine drives its own generator which
provides electrical output in addition to that obtained from
the nuclear turbine- generator.2
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Patti, F. J. Technical considerations in repowering a nuclear plant for fossil fueled operation, article, March 1, 1996; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc667391/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.