atw 2018-1

atw Vol. 63 (2018) | Issue 1 ı January
Clearance of Surface-contaminated
Objects from the Controlled Area
of a Nuclear Facility: Application of the
SUDOQU Methodology
F. Russo, C. Mommaert and T. van Dillen
1 Introduction During and after the Fukushima nuclear accident, the possibility existed that surface-contaminated
consumer goods, freight containers and conveyances would be imported from Japan, which revealed the need
for proper criteria and screening levels for surface contamination of these items, to insure protection of the public.
In this framework, it was concluded
that the then existing dose-calculation
models mostly addressed exposure
scenarios for occupationally exposed
workers, which were generally not
aimed at properly evaluating the
effective dose incurred by members of
the public exposed to surface-contaminated
objects. The main difference
between occupational and public
exposure scenarios is that, while
workers may frequently be exposed
to freshly contaminated objects,
members of the public are likely to
come in contact with only one (same)
object during a prolonged period of
time. Therefore, while the hypothesis
of a constant contamination level may
suffice for occupationally exposed
workers, it is less realistic for objects
handled by members of the public,
where the initial contamination
present on the object will be affected
by several removal mechanisms,
which need to be considered when
evaluating the annual effective dose.
Based on these findings, the Dutch
National Institute for Public Health
and the Environment (RIVM) developed
the SUDOQU (SUrface DOse
QUantification) methodology [1] for
the evaluation of the annual effective
dose for members of the public
resulting from exposure to surfacecontaminated
objects. It assumes
time-dependent surface- and air- contamination
levels, whose evolution is
governed by a system of coupled
differential equations, describing the
mass balance imposed by the involved
mechanisms. The surface-activity concentration
(Bq/cm 2 ) is considered to
decrease by radioactive decay,
resuspension and wipe-off (transfer
of activity to the hands). The resuspended
activity contributes to the
(increase in) air-activity concentration
(Bq/m 3 ) and can, in turn, partly re-deposit
onto the object surface. The air
activity concentration is further
affected by radioactive decay and
ventilation. Different exposure pathways
are considered: external-gammaradiation
exposure, inhalation, indirect
ingestion and skin contamination
through wipe-off. The effective dose
can then be calculated as the sum of
the contributions of the exposure
pathways. Based on these intrinsic
properties, the SUDOQU methodology
is particularly attractive for clearance
and exemption calculations, especially
when considering public reuse
scenarios, because they often involve
the prolonged use of the same object.
Therefore, in 2016, a collaboration
was started between Bel V and RIVM,
to extend the scope of the SUDOQU
model, and to test its suitability for the
derivation of surface-clearance levels
for objects released from the controlled
area of a nuclear facility.
2 Objectives and
methodology
The results presented in this paper
were obtained in the framework of a
pilot project, having as main objective
to investigate the applicability of the
SUDOQU methodology for clearance
calculations, and to gain a better
understanding of the interplay among
the involved mechanisms and how
this affects the resulting total effective
dose. This was achieved by performing
deterministic calculations
of the annual effective dose resulting
from exposure to a typical office
item, i.e. a bookcase, considering
different scenarios of use and different
nuclides.
2.1 Reference scenario
In the reference scenario (scenario 1),
a bookcase is considered that leaves
the controlled area of a nuclear facility
with a homogeneous surface contamination
of 1 Bq/cm 2 (different
radionuclides are considered, as
explained further in this Section).
Next, the bookcase is placed in an­
­office with a 50-m 2 area and a 2.5-m
height and is used by an “average”
­office worker, who will be exposed to
the contaminated surface. During
working hours (i.e. 8 h/d, 5 d/w, and
52 w/y, resulting in 2080 h/y, thus
in a duty factor f exp =0.24 [1]) the
worker is in the office at a distance of
3 m from the contaminated bookcase,
by which he incurs a certain exposure
by external (gamma) radiation. The
bookcase is assumed to be contaminated
only on its front panel,
characterised by a 6-m 2 surface.
For the calculation of the externalradiation
dose contribution, the
conversion factor from ambient
dose equivalent to effective dose
(E/H*(10)) is set equal to one, which
is conservative for any irradiation
geometry in the photon energy range
of the considered nuclides. During
­office hours, the worker is assumed
to occasionally touch the bookcase,
thereby wiping off some activity from
its surface, with a frequency of
approximately once every three
hours (ϕ = 0.31 h -1 during use) and
an ­efficiency of 20% (f oth = 0.2, corresponding
to the ratio of the contamination
level of the hands after a
wipe-off event and that of the bookcase).
Activity is also transferred
indirectly to the face after contact
with the hands. This transfer is
­modelled by an efficiency of f htf =0.2
(ratio of contamination levels of face
and hands). The individual will thus
incur a skin equivalent dose following
contamination of the skin area of
the hands (A hands =400 cm 2 ) and
of the face (A face =100 cm 2 ), which
eventually also contributes to the
effective dose. Furthermore, part of
the activity on the hands will be
transferred to the mouth (indirect
ingestion): this is assumed to occur
with a frequency equal to that of
wipe-off (0.31 h -1 ). The activity transferred
from the hands to the mouth
per ingestion event is set equal to
100 % (f htm =1) of the activity present
atw-Special „Eurosafe
2017“. In cooperation
with the EUROSAFE
2017 partners,
Bel V (Belgium),
CSN (Spain), CV REZ
(Czech Republic),
MTA EK (Hungary),
GRS (Germany), ANVS
(The Netherlands),
INRNE BAS (Bulgaria),
IRSN (France),
NRA (Japan),
JSI (Slovenia),
LEI (Lithuania),
PSI (Switzerland),
SSM (Sweden),
SEC NRS (Russia),
SSTC NRS (Ukraine),
VTT (Finland),
VUJE (Slovakia),
Wood (United
Kingdom).
Revised version
of a paper presented
at the Eurosafe,
Paris, France, 6 and
7 November 2017.
29
OPERATION AND NEW BUILD
Operation and New Build
Clearance of Surface-contaminated Objects from the Controlled Area of a Nuclear Facility: Application of the SUDOQU Methodology ı F. Russo, C. Mommaert and T. van Dillen