Repères - N°48 / March 2021 - The IRSN magazine - 15

EMERGENCY

Optimized models

Fukushima Daiichi required the C3X platform's modeling capabilities to be expanded.
" This accident shook the scientific and operational community. We have learned from
this, " said Denis Quélo. " We realized that
our atmospheric dispersion models were
insufficient for an initial approach, " he
explained. In 2011, they did not include
dispersion calculations over long distances;
priority was given to the first ten, twenty,
or thirty kilometers. However, for this
accident, it was crucial to be able to cover
greater distances.
Developed in research and development,
ldX software overcomes this shortcoming
and has been included in the C3X platform.
Calculation tools are constantly improved.
" We now take better account of the phenomenon of scavenging 4 of aerosol particles
by rain to predict soil contamination. Our
models also integrate fogwater deposition, "
described modeler Arnaud Quérel.

Measuring more quickly
and accurately

In metrology, too, practices are adapting
and mobile resources being developed.
The Ulysse airborne measurement system,
comprising detection devices on board
helicopters or aircraft, was launched in
late 2011. It provides an initial overall
assessment of radioactive fallout in the
event of an emergency.
Faster protocols are also emerging in laboratories to allow the analysis of the hazardous nature of the samples taken from the
environment to guide decisions. " After
Fukushima Daiichi, we realized that we were
too slow for the needs of emergency manage-

TECHNICAL TOOLS
Method

Inverse modeling improves
understanding of releases

" I

nverse modeling [IM] combines
measurements made in the environment
with atmospheric dispersion models. It helps
to accurately deduce a source term [ST], namely
the composition of a radioactive release from
a facility and its chronology, " summarized Olivier
Saunier, specialist in atmospheric dispersion.
IRSN was the first to implement an IM method
based on gamma dose rate measurements
to estimate the Fukushima Daiichi's source
term in 2013*.
How was IM developed? In 2011, this was
a research project. When the accident
in Japan occurred, the atmospheric dispersion
model used failed to accurately reconstruct
the contamination. " At the time, this model
required input data on the weather, to simulate
March 15, 2011 6 a.m.

Activity
[Bq/m3]

the cloud's path, and the ST, " Saunier explained.
These were not sufficiently accurate, as
the results did not correspond to the field surveys
that poured in during the accident.
To improve knowledge of releases and help
assess environmental and health consequences,
experts use dose rate measurements based
on networks of beacons. These are dense
and provide fine temporal resolution-
10 to 60 minutes in the case of Fukushima Daiichi.
Validated during the accident in Japan,
IM is now useful in the event of abnormal
detection of radionuclides.
In 2020, it helped assess releases from forest fires
in the Chernobyl power plant exclusion zone.
*O. Saunier et al. Atmos. Chem. Phys., 2013.

March 20, 2011 8 p.m.

to be received by humans, " explained Denis
Quélo, specialist in atmospheric dispersion models.
Zoning 3 contours are also changing.
In 2012, there were three zones: a relocation
perimeter, a population protection zone
and an enhanced territorial surveillance
zone.
The doctrine now defines two perimeters:
a relocation zone defined according to
the ambient radioactivity and a dietary
restriction zone which connotes the nonconsumption of fresh produce as well as
monitoring and management of agricultural
production, livestock farming, and
consumer goods. " Rather than defining an
overall zone, the plan works by sector, "
Dubiau added.

Simulated cesium-137 plume, obtained from the source term reconstructed by inverse modeling,
during the two main contamination events in March 2011.

ment, " said Béatrice Boulet, Radiochemist.
" You can't wait six weeks before determining
the presence of strontium 90, plutonium,
or something else. We have developed protocols that produce an answer in a day, " she
added. IRSN is also working on other topics
(see p. 16).

Special Intervention Plans

Following interministerial discussions on
feedback from Fukushima Daiichi, the
French Prime Minister adopted the new
doctrine relating to Special Intervention
Plans (PPI) in 2016. By 2022, it should
include the French national response plan
for major nuclear or radiological accidents.
This large-scale plan emerged in 2014, after
the accident in Japan, because the public
authorities wished to improve the safety

of the population in the event of accidents
of exceptional magnitude. " PPIs define
an organization that allows to be ready to
act in the event of an accident, " summarized
Erik Leclerc, specialist in the management
of emergency situations. Coordinated by
the prefects at the French department level,
they provide for three actions to protect
the population: sheltering in place and
counseling, stable iodine administration,
and evacuation. The latter is triggered when
exposure forecasts for the population
exceed 50 mSv in effective dose for the
whole body. " Fukushima Daiichi has
changed these measures. "
Among the accident scenarios considered,
the possibility of a rapid release was taken
into account in the 2000s with the establishment of an automatic shelter-in-place radius,
Page 15 - Repères No. 48 - March 2021

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

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