Repères - N°48 / March 2021 - The IRSN magazine - 12

SAFETY
Tricastin nuclear power plant

Dike reinforcements follow assessment
monitoring plan will ensure that the
dike remains stable in the event of a safe
shutdown earthquake (SSE). The monitoring
plan developed by EDF is based on
piezometric 2 level measurements.
Nonetheless, assessment experts highlighted
" the absence of a design margin for an earthquake exceeding the SSE baseline. " They
recommended overall reinforcement of the
dike. It must now withstand a " hardened
safety core design basis earthquake " 3. IRSN
also stated that the specific features of the

site must be taken into account, primarily
because of the soil characteristics.
In 2020, EDF drew up a file detailing its
planned measures and works, which will
be examined by IRSN. n
1. Électricité de France/French national
power company.
2. Measurements taken of the water level
in the dikes.
3. Extreme earthquake used for
the hardened safety core design baseline
for facilities, as specified following
the Fukushima Daiichi accident.

FLOODING
SOFIA simulator

Simulating flooding

" T

aking seismic risk into account as
it has been up to now is not
enough anymore, " explained
Olivier Loiseau, Safety expert. He goes on
to say that " the accident at Fukushima
Daiichi showed us that nature can be much
more powerful than the hazard levels used
by design engineers in facility design. "
As a result of IRSN's recommendations,
EDF1 has had to meet higher safety requirements at all its facilities. This applies to a
precise list of existing protective equipment
and systems, which are part of the hardened safety core, since they are necessary
for carrying out safety functions (see p. 8).
These new requirements apply to the dike
that protects the Tricastin nuclear power
plant. Here, we look back at the timeline
of events and the assessment conducted.
On September 27, 2017, ASN ordered EDF
to temporarily shut down all four reactors
at Tricastin, in order to reinforce a section
of gravel dike measuring a few hundred
meters. In the event of an earthquake, it
could collapse. EDF conducted additional
geotechnical investigations and reinforced
the section.
IRSN analyzed the data submitted by EDF
and drew up its own design calculations.
IRSN believed that the works and the

Page 12 - Repères No. 48 - March 2021

Robin Dorel, a specialist in charge of the SOFIA simulator, presents the schematic control pannel
of the main feedwater system of the steam generator for the 900 MWe reactors.

s safety assured at the Tricastin nuclear power
plant (Drôme department) while work
to reinforce the Donzère-Mondragon canal
dike is ongoing? To find out, IRSN safety experts
used the SOFIA1 simulation tool.
The following critical situation is considered,
in which an earthquake has destroyed the
weakened part of the dike, the site is flooded
and must deal with the total loss of power supply
and cooling water. SOFIA was used to model
the consequences of such an event with
the aim of facilitating analysis of the measures
implemented by EDF to manage it2.
Starting from the initial state of the facility
as selected by EDF, which considered residual heat
equivalent to that of a reactor in shutdown state
for fourteen days, the tool calculated the physical
parameters of the reactor in real time. It simulated
changes in pressure, core outlet temperature,
and the flowrate in the reactor coolant system, etc.
These calculations gave an estimate of 37 hours
for the steam generators to drain, and the time
to core uncovery as 62 hours. That means

Since the accident at
Fukushima Daiichi, nuclear
facility design had to be
improved to ensure that it
withstand exceptional events.
This led to the " hardened
safety core " , which includes
assessing the strength
of infrastructure built
to protect sites, such as
the Donzère-Mondragon
canal dike that borders
the Tricastin nuclear power
plant (Drôme department).
IRSN recommended
reinforcing it.

that the Nuclear Rapid Response Force (FARN),
which can reach the site in 12 hours, has enough
time to respond.
More recently 3, SOFIA has been used to validate
operating a 900 MWe reactor following an extreme
hazard. Robin Dorel, who manages the simulator,
pointed out that " the new 'hardened safety core'
equipment and systems must be taken into account.
Simulation demonstrated that the facility was
strong enough to resist. "
1. Simulator for Observation of Functioning
during Incident and Accident.
2. EDF memo dated October 9, 2017 -
" Measuring resilience with regard
to the conditions considered for shutdown
of the four reactors in the event of flooding
on the NPP platform. "
3. Supporting the assessment of operating
strategies in the event of extreme loads
for reactors in service.
For more information:
The SOFIA simulator
irsn.fr/sofia-EN

https://www.irsn.fr/sofia-EN http://www.irsn.fr/sofia

Repères - N°48 / March 2021 - The IRSN magazine

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