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NuclearPlantJournal.com Nuclear Plant Journal, July-August 2015
Optimum Power...
and eventually discharged which eases
pressure on spent fuel pool storage. In
the event that a full core discharge of the
reactor core is necessary, the optimum
power coastdown strategy supports the
station achieving this condition during a
refueling outage.
The optimum power coastdown
strategy is implemented after the core
achieves the “all rods out” condition (i.e.,
all control rods have been fully withdrawn
from the core). This has the additional
advantage of decreasing the probability
of fuel failures in the next cycle. When the
core is shuffled to a new configuration for
the next fuel cycle, there is no fuel which
had a control rod inserted next to it for at
least 3 to 4 weeks during the coastdown
period at the end of the previous cycle.
This precludes a situation where a large
local power increase may occur at the
beginning of the new fuel cycle in a fuel
assembly which operated at low power
due to being controlled at the end of the
previous cycle.
Cost Savings
The optimum power coastdown
strategy results in the reactor being
operated at a lower power level over
the last few weeks of the fuel cycle.
The reduced power level decreases the
dose rates during that time period and
during the subsequent refueling outage.
For those Exelon plants that have high
moisture carryover (MCO), empirical
data from Limerick and Peach Bottom
illustrates that MCO decreases during
the coastdown period which reduces
the amount of Cobalt-60 carryover to
the secondary side of the plant. MCO
is the weight percentage of entrained
moisture in the steam leaving the steam
dryer. Graphic 3 on page 60 illustrates
how MCO decreased with core power
during the coastdown for Limerick Unit 1
Cycle 13. The power coastdown strategy
therefore reduces outage dose and the
savings are ongoing.
The optimum power coastdown
strategy uses an automated method to
determine the optimum coastdown length
(measured in days) and maximum net
cost savings based on many economic
and plant-specific factors. Two of the
primary factors in determining the
optimum coastdown length are the
cost of the new fuel and the cost of the
replacement power during the coastdown
period. Optimum coastdown is ultimately
achieved by balancing the incremental
fuel cost associated with maintaining
full power against the generation cost
associated with lost generation during the
coastdown. A gradual reduction in power
over the course of three weeks may yield
a generation cost of $1 million. However,
the same gradual power reduction allows
for more efficient use of fuel, resulting
in decreased reload fuel costs of up to
$4 million. The net savings for each
fuel cycle are significant – on average
between $1 million to $3 million per
fuel cycle when multiple cycle effects
are considered in the calculation. On an
Exelon fleet basis, this conservatively
results in savings of at least $8 million
annually. This strategy maximizes the
earnings and cash flow from our assets.
Because the net savings curve (Graphic
2 on page 59) is relatively flat near the
optimum point, selection of the design
coastdown length may be hedged to
a longer or shorter coastdown length
depending upon projected market risks
and plant operational considerations.
On average, a 5% reduction in the
number of new fuel assemblies required is
realized each fuel cycle. This also results
in a long term cost savings in spent fuel
storage. For example, a dry cask at Peach
Bottom stores 68 spent fuel assemblies.
The application of the optimum power
coastdown strategy over 6 fuel cycles will
reduce dry cask storage requirements by
1 cask. Each dry storage cask costs on
the order of $1 million.
Innovation Response
The Exelon optimum power
coastdown strategy is innovative in
several ways:
1) Exelon’s optimum power coastdown
strategy is sophisticated, accounting
for the time cost of money (new fuel
is purchased two years in advance
of the power coastdown for a two
year fuel cycle). It also accounts
for multi-cycle effects related to the
fuel’s energy carryover impacts on
future cycles from the reload fuel
batch size reduction. Coastdown
rates are adjusted for each individual
plant based on actual data accounting
for factors such as fuel type, number
of fresh bundles, gadolinium
concentration, etc. Replacement
power costs are projected based upon
plant location and time of the year.
2) The Exelon strategy of optimum
coastdown is embodied in a software
quality-assured spreadsheet model,
Graphic 1: Fuel Cycle Profile Including Power Coastdown.
