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<strong>atw</strong> Vol. 62 (<strong>2017</strong>) | Issue 6 ı June<br />
| | Fig. 2.<br />
Pipe routing of spent fuel pool alternate cooling with mobile heat exchanger (MHX) and spray system.<br />
during refueling every 18 months and<br />
stored in the spent fuel pool. The<br />
AP1000® plant’s spent fuel pool has<br />
the capacity to cool up to 889 spent or<br />
used fuel assemblies, which are<br />
continuously submerged beneath<br />
approximately 7.6 m of water.<br />
The spent fuel assemblies continue<br />
to generate decay heat naturally even<br />
when they are removed from the<br />
reactor and are placed in the spent<br />
fuel pool. This decay heat will<br />
decrease significantly over time so<br />
that older spent fuel produces less<br />
heat than spent fuel that has recently<br />
been removed from the reactor.<br />
The spent fuel in the spent fuel<br />
pool is cooled by transferring the<br />
decay heat from the used fuel to the<br />
water in the spent fuel pool. The spent<br />
fuel pool water is, in turn, pumped<br />
through a loop with a heat exchanger<br />
where it is cooled and decay heat is<br />
transferred to a second water cooling<br />
system. The cooled water is then<br />
returned from the second water cooling<br />
system to the spent fuel pool and<br />
the decay heat is transferred to the<br />
environment. There are two identical<br />
spent fuel pool cooling trains, though<br />
only one pump and heat exchanger in<br />
one of the two trains are in operation<br />
in most circumstances.<br />
3 AP1000 plant spent fuel<br />
pool spray system<br />
The AP1000® spent fuel pool spray<br />
system is designed to cool the spent<br />
fuel during a beyond design basis<br />
event in accordance with the B.5.b<br />
guideline [2].<br />
The AP1000® plant spent fuel pool<br />
spray system has two redundant spray<br />
headers located on either side of the<br />
spent fuel pool. There are 16 spray<br />
nozzles on each header (Figure 1,<br />
left). One header receives water<br />
through either gravity-fed draining of<br />
the passive containment cooling water<br />
storage tank, which is located on top<br />
the Shield Building, or from a flanged<br />
connection located in the truck bay,<br />
which is used with an onsite portable<br />
pump. The other header receives<br />
water from the fire protection water<br />
tanks and the diesel-driven or electric<br />
motor-powered fire protection system<br />
water pumps. Spray nozzles distribute<br />
water spray in the form of a hollow<br />
spray cone over the fuel assemblies.<br />
Only one spray header is required<br />
to assure sufficient cooling of the<br />
exposed spent fuel due to sensible<br />
heat and latent heat from water spray<br />
vaporization (Fig. 1, right).<br />
The spray system used to cool the<br />
spent fuel pool during a postulated<br />
loss-of-large-area event is sized to<br />
provide an adequate amount of<br />
spray to the hottest fuel assembly that<br />
will enter the spent fuel pool. The<br />
analytical basis for determining the<br />
minimum amount of spray needed to<br />
cool a fuel assembly is adapted from<br />
the calculation used in Section 3.3<br />
of the Sandia report, “Mitigation of<br />
Spent Fuel Pool Loss-of-Coolant<br />
Inventory Accidents And Extension of<br />
Reference Plant Analyses to Other<br />
Spent Fuel Pools” [3]. Further, to<br />
prevent pressurization inside the<br />
Fuel Handling Building, the system<br />
includes a relief panel to release steam<br />
that is produced during the cooling<br />
process.<br />
4 Krško nuclear power<br />
plant safety upgrade<br />
program<br />
Krško Nuclear Power Plant was<br />
already in the process of making<br />
significant upgrades as a result of<br />
applying for a license extension in<br />
2009 to operate beyond 2023. The<br />
Krško Safety Upgrade Program<br />
was designed in response to the<br />
Slovenian Nuclear Safety Administration’s<br />
re gulations and interpretation<br />
of reference safety levels<br />
from the report, “ WENRA Reactor<br />
Safety Reference Levels” [1], concerning<br />
reasonable measures to prevent<br />
and mitigate severe accidents in<br />
preparation for the possibility of<br />
extending original plant operating<br />
licenses. The reference safety levels<br />
within the report were updated in<br />
2014 to incorporate lessons learned<br />
from the event at the Fukushima site.<br />
The measures defined in the frame of<br />
the Krško Safety Upgrade Program<br />
are in agreement with the nuclear<br />
industry’s response to the Fukushima<br />
accident and the resulting update of<br />
the safety reference levels proposed<br />
by WENRA. This includes plant upgrades<br />
and design changes to address<br />
design extension conditions defined<br />
in the report and beyond design basis<br />
accidents.<br />
Krško’s Safety Upgrade Program is<br />
divided into various projects being<br />
carried out during three phases. The<br />
Spent Fuel Pool Alternative Cooling<br />
Project is in the scope of Phase 2. The<br />
project is scheduled to be completed<br />
by the end of <strong>2017</strong>.<br />
The Spent Fuel Pool Alternate<br />
Cooling Project shall assure alternate<br />
cooling of used fuel assemblies<br />
by using a mobile heat exchanger<br />
or spray system (see Figure 2).<br />
Furthermore, it shall assure depressurization<br />
of the Fuel Handling<br />
Building by using relief panels to<br />
release steam produced during the<br />
cooling process.<br />
The systems of the Spent Fuel Pool<br />
Alternate Cooling Project are designed<br />
to assure that heat is removed from<br />
the spent fuel during Design Extension<br />
Conditions A and B and to mitigate<br />
spent fuel damage. The operational<br />
conditions for the systems of the Spent<br />
Fuel Pool Alternate Cooling Project<br />
are classified according to the plant’s<br />
severe accident scenarios, following<br />
the WENRA guidance document “ Issue<br />
F: Design Extension of Existing<br />
Reactors” [1].<br />
ENVIRONMENT AND SAFETY 393<br />
Environment and Safety<br />
Retrofitting a Spent Fuel Pool Spray System for Alternative Cooling as a Strategy for Beyond Design Basis Events ı Christoph Hartmann and Zoran Vujic