atw 2017-06

atw Vol. 62 (2017) | Issue 6 ı June
ENVIRONMENT AND SAFETY 394
| | Fig. 3.
Spent fuel pool spray system simplified process flow diagram.
5 Spent fuel pool spray
system function and
design
The spent fuel pool spray system will
be used in in the event of scenarios
described in Design Extension Conditions
A and B, which postulate a
highly unlikely drainage of the spent
fuel pool. In such scenarios, cooling
water can be provided by a spray
header with fixed spray nozzles
installed along the spent fuel pool
walls. Westinghouse conducted
experi mental testing to determine the
exact number of nozzles required and
their optimal positions. Water can be
supplied by two diverse sources: the
fire protection system and the Sava
River.
The fire protection system can be
flexibly connected using fire hoses.
The Sava River water can be pumped
using a mobile pump unit and fire hoses.
The mobile pump unit is powered
directly by a diesel motor. Either water
source is connected to the spent fuel
pool spray system with hose connections
that are installed inside and
outside of the Fuel Handling Building.
A simplified process flow diagram of
the spent fuel pool spray system is
shown in Figure 3.
The Krško Nuclear Power Plant’s
spent fuel pool spray system is
designed to perform its designated
safety function under Design Extension
Conditions. This includes being
designed to meet the seismic performance
requirements for operation
and mitigation during and after a
design extension condition earthquake,
which is equal to twice the
design requirements for a safe shutdown
earthquake for the existing
systems, structures and components
of the Krško Nuclear Power Plant.
The spent fuel pool spray system’s
permanently installed equipment will
be protected against flood events with
additional margin, even in the case
of a Sava River bank failure. The
system’s permanently installed equipment
is also designed to withstand
extreme winds and tornados. In order
to keep the design of the system as
simple as possible only local indicators
are used and there are no electrically
driven components.
As in the AP1000® spent fuel pool
spray system, the Krško Nuclear Power
Plant’s spent fuel pool spray system:
• Will provide a sufficient amount of
cooling water to maintain the
spent fuel cladding temperature at
lower than 400 °C for a long term
during the loss of ultimate heat
sink,
• Is sized to provide an adequate
amount of spray to the hottest fuel
assembly that will enter the spent
fuel pool during a postulated lossof-large-area
event,
• Has an analytical basis for determining
the minimum amount
of spray needed to cool a fuel
assembly adapted from the calculation
used in Section 3.3 of the
Sandia Letter Report “Mitigation of
Spent Fuel Pool Loss-of-Coolant
Inventory Accidents And Extension
of Reference Plant Analyses to
Other Spent Fuel Pools” [3].
An example of the water spray coverage
and distribution through the
nozzles throughout the spent fuel
pool is shown in Figure 4. The red
circles show the area inside of the
spent fuel pool covered by water
through the spray nozzles, which are
installed at the side walls of the spent
fuel pool. Darker red areas show
possible overlap, whereas water that
does not fit the spent fuel pool geometry
hits the spent fuel pool walls.
Westinghouse conducted experi mental
testing to optimize the spray configuration
around the spent fuel pool edge
and to verify the spray water distribution
and overlap.
Inside the spent fuel pool is a two
region rack design. Recently offloaded
fuel, which is comprised of the hottest
fuel assemblies, is placed in the
Region 1 racks. The Region 2 racks
provide high density storage for
| | Fig. 4.
Example of the spent fuel pool spray system’s nozzle coverage and distribution of water spray.
Environment and Safety
Retrofitting a Spent Fuel Pool Spray System for Alternative Cooling as a Strategy for Beyond Design Basis Events ı Christoph Hartmann and Zoran Vujic