Repères - N°48 / March 2021 - The IRSN magazine - 11

SAFETY
Severe accidents

Ultimate heat sink performance
The containment heat removal system, as part of the " hardened safety core " ,
plays a key role in the event of a severe accident. IRSN has examined its performance
and made some recommendations for additional improvements.
connected to the heat sink by the FARN, the
Rapid Nuclear Response Force, " (see p. 9)
explained Estelle Dixneuf, an expert in the
field of safeguard systems design.
According to IRSN's calculations, that is
the potential source of any problems. Julien
Chambarel, an expert on severe accidents,
explained: " Thanks to our calculations,
performed using ASTEC code, we have identified a risk in the event of an accident in
which design-basis pressure in the containment is exceeded in the first 24 hours, requiring containment venting. That is precisely
the amount of time needed for the FARN to
connect the heat sink to the UCHRS heat
exchanger. "
This has led to an IRSN recommendation
in the event of a severe accident. Now, an
additional step must be added to the list
of necessary actions, which is to resupply
the external borated water tank and inject
its contents into the reactor building as
quickly as possible. n

The Tricastin plant heat exchanger shown here
is the centerpiece of an ultimate heat sink.
It measures 7 meters long and weighs around
9 metric tons.

ow effective is the ultimate containment heat removal system (UCHRS)?
That is the question put to the
experts at IRSN regarding this part of the
hardened safety core system. Its role is to
remove residual heat from the containment, including under severe accident
conditions. The ultimate containment heat
removal system is called upon when the
safety injection system and the containment spray system both fail.
It is required in order to prevent any opening of the containment venting and filtration system which would result in
significant releases to the environment.
Since 2015, IRSN experts have assessed the
design, reliability and performance of the
system proposed by EDF to mitigate the
consequences of an accident with core melt.
" The ultimate containment heat removal
system injects borated water 1 contained in
a tank into the reactor vessel and into the
sumps at the bottom of the containment. Once
this tank is empty, the system draws water
from the sumps to recirculate it and cool it.
It is cooled by a UCHRS heat exchanger

1. Boric acid is added to the water in the reactor
coolant system due to its neutron-absorbing
capabilities.

RESEARCH

Containment

Basemat

Creating a more robust basemat
uring the accident at Fukushima Daiichi,
molten fuel penetrated through the reactor
vessels and flowed down onto the basemat.
The basemat, made of reinforced concrete, acts as
the foundation supporting the reactor containment.
To maintain containment integrity, ablation
of basemat by the corium must remain limited.
How is this risk managed in France? In 2019,
IRSN published a notice1 that addresses this issue.
As part of the studies on extending the service life
of its 900 MWe reactors, EDF is developing a
strategy for managing the flow of corium following
melt-through, often referred to as " ex-vessel
corium " . The principle behind this entails allowing
the corium to spread at the base of the reactor
where it is then stabilized under water.
The experts at IRSN have thirty years of research
on which to base their analysis of this strategy.

Experiments are carried out to study how
depleted uranium, replicating the actual fuel,
interacts with the types of concrete (limestone
or siliceous) used in reactor units.
Computer simulation is then used to study
conditions at full scale. " When siliceous concrete
is used, as is the case in fourteen reactors in France- Corium
our experts recommend making the base of the
reactor thicker, to minimize the risk of melt-through, "
explained Gérard Cénérino, an expert on
severe accidents. This research complements
other international post-Fukushima projects 2
(see p. 13 and the Webmag).
Basemat
1. IRSN/Notice 2019-00051.
2. PreADES, ARC-F and TCOFF,
under the auspices of the OECD.

Illustration of a core melt accident with the
corium flowing down and starting to interact
with the basemat concrete.

Page 11 - Repères No. 48 - March 2021

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

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