atw 2017-06

atw Vol. 62 (2017) | Issue 6 ı June
cooled, irradiated fuel assemblies. Per
the initial cooling time defined in
plant procedures (typically 4.5 years
or three fuel cycles), the most recently
cooled, irradiated fuel assemblies are
transferred from the Region 1 racks to
the Region 2 racks.
6 Experimental determination
of spray nozzle
coverage and distribution
A specific flow density and a uniform
coverage of cooling water over a large
rectangular area are required to cool
fuel assemblies in the unlikely event
that they become uncovered during a
beyond design basis accident. The
experimental testing used to determine
the optimal coverage area, spray
flow density and distribution of the
spent fuel pool spray nozzles for cooling
uncovered fuel assemblies in case
of Design Condition Extensions A and
B include testing different types of
nozzles with different volume flow
rates as the decay heat decreases with
cooling time (see Region 1 and Region
2 in Fig. 4). The measurement setup
of the experimental testing at the
Lechler GmbH Technology Center
Metzingen is shown in Figure 5.
• Volume Flow:
variable adjustable, depending on
spray nozzle type and pressure
The coverage area measurements
were performed by using adequate
collecting canisters with the same
dimensions as the rack cell canisters
at specified positions (see Fig. 5).
Pictures and video recording documented
the testing process and resulting
water distribution. An example of
the measurement setup is shown in
Figure 6.
| | Fig. 6.
Example of water distribution from a spray
nozzle [4].
7 Managed challenges
Retrofitting the AP1000® spent fuel
pool spray system to Krško Nuclear
Power Plant’s existing systems and
structures posed a few challenges.
These included:
• Planning the pipe routing and
meeting the space requirements
for pipe supports (see Figure 7),
• Assuring pipe routing had minimal
impact to equipment already existing
inside of the Fuel Handling
Building,
• Planning the installation process
for the system to meet the space
requirements in the Fuel Handling
Building,
• Assuring the number of fuel assembly
racks that cannot be used for
spent fuel storage due to nozzles
and/or supports which protrude
into the pool are as low as possible,
• Designing the retrofit system with
consideration of the fuel assembly
loading pattern at Krško Nuclear
Power Plant to assure that the
spray nozzles provide the required
water spray amount, coverage and
distribution to cool the spent
fuel pool during Design Extension
Conditions A and B.
Each of these challenges was successfully
resolved.
8 Summary
Due to requirements for nuclear
power plants to withstand beyond
design basis accidents, including
events such as happened in 2011 in
the Fukushima Daiichi Nuclear Power
Plant in Japan, alternative cooling of
spent fuel is needed. Alternative spent
fuel cooling can be provided by a
retrofitted spent fuel pool spray
system based on the AP1000® plant
design. As part of Krško Nuclear Power
Plant’s Safety Upgrade Program,
Krško Nuclear Power Plant decided
on, and Westinghouse successfully
designed a retrofit of the AP1000®
plant spent fuel pool spray system to
provide alternative spent fuel cooling.
The spent fuel pool spray system
will be installed inside and outside the
Fuel Handling Building. For diverse
water supply, sources such as the fire
protection system and river water
were considered and chosen by Krško
Nuclear Power Plant. The system has a
robust design that employs local
measurements and indicators and
ENVIRONMENT AND SAFETY 395
| | Fig. 5.
Exemplary measurement setup for experimental
testing of spray nozzle coverage and
distribution [4].
The following conditions were
applied to determine the flow density
and the coverage area:
• Spray height:
variable adjustable, depending on
local conditions around the spent
fuel pool
• Setting angle horizontal:
variable adjustable, depending on
local conditions around spent fuel
pool and spray nozzle type
• Pressure:
variable adjustable, depending
on spray nozzle type and hydraulic
design of spent fuel pool spray
system
| | Fig. 7.
Depiction of the local conditions around the Krško nuclear power plant spent fuel pool.
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