Autoradiography in Localization of Beta contamination in Facilities under Dismantling Process
Anumaija Leskinen, Pascal Fichet, Florence Goutelard
CEA, DEN, DANS/DPC/SEARS/LASE, 91191 Gif Sur Yvette, France, [email protected]
INTRODUCTION
Nowadays, large numbers of facilities which are
potentially contaminated by radionuclides are currently or
will be soon under dismantling process. These facilities
include old research laboratories built around 1960s.
Before the facility can be dismantled, the potential
radioactive contamination has to be quantified to ensure
safe disposal. The activities in the wastes have to be under
regulation limits, which vary with different radionuclides,
type of waste, and destination of the waste
[www.andra.fr]. Detection of gamma emitters in solid
material is relatively easy because measurements can be
carried out using gamma cameras, for example. However,
detection of beta and alpha emitters is more complicated
and often requires sampling and ex situ measurements.
Mapping of beta contamination is the aim of this study.
Digital autoradiography has been traditionally used in
biological application, but increasing number of
investigations has been carried out in detection of
radioactivity in industrial applications. For example,
digital autoradiography technique has been successfully
applied to 3H analysis on the plasma-facing surfaces of
various plasma devices [1]. A significant advantage in
digital autoradiography compared to traditional sampling
measurements (cores, wipes etc.) is the easy and fast
coverage of large areas. In addition, the digital
autoradiography films can be installed even on unleveled
surfaces such as on pipes while respecting the maximum
curvature of the film and they can be reused several tens
of times. However, the remaining challenge in the digital
autoradiography measurements in industrial applications
is the transition from qualitative measurements to
quantitative measurements.
This study concentrates on the localization and
quantification of 3H and 14C contamination on cement
surfaces (floor and walls) in a laboratory under
dismantling process. The radioactivity contamination
levels in the facility have to be assessed quantitatively or
at least semi-quantitatively to ensure correct
categorization of the dismantling waste.
DESCRIPTION OF THE ACTUAL WORK
The laboratory facility (280 m2) under dismantling
investigations includes a main laboratory hall and a few
small rooms. Before the closure of the laboratory, labeling
of organic molecules with 3H and 14C was carried out in
there for over 3 decades.
The investigations were started by dividing the
facility into 57 zones. The size of each zone is up to 4 m2.
Higher level of survey coverage will be implemented on
the zones with higher contamination potential. For
example, the offices will receive lower level of interest
whereas high level of interest will be implemented on the
zones on which the radiolabeling was carried out and
stored.
The experimental work is carried out by placing at
least 20 digital autoradiography films (12.5 cm x 25.2 cm)
on each zone for 2-4 days after which the changes on the
films due to radioactivity are scanned with a Cyclone
scanner (Perkin Elmer company) to obtain an image.
Mainly two types of layouts in the placement of the films
are used (Fig. 1). The type of layout depends on the size
of the zone. More films are placed on the zone if
contamination is found. The background of each
measurement is determined by placing a digital
autoradiography film next to the other films but a
shielding cardboard layer is placed between the film and
the floor.
Fig. 1. The two main layouts (10x4 and 15x5) in the
placements of the films (marked in black) on the floor.
The interpretation of each film is carried out by
placing a grid of 5 mm x 5 mm squares (47 columns and
24 rows) on top of the scanned image and by using the
Cyclone software (OptiQuant) to determine the digital
light units (DLU/mm2) in each square. The results are
then transferred into Excel in which the intensities in each
square are divided into 4 categories: no measurable
radioactivity (below background), low contamination,
intermediate contamination and high contamination.
These categories were determined using self made
polymerized polymethylmetacrylate (PMMA) standards
labeled with 3H and 14C. The results are shown as
percentage of each category.
RESULTS
After three months investigation, approximately 45%
of the laboratory surface has been investigated (more than
100 m2). The activity levels in 60% of the investigated
zones are below background level. Significant
contamination (more than 10% of the zone surface
activity is above background level) has been found in 4
zones. Fig. 2 shows an example image of a film on a
contaminated zone. The strong contrast between the light
gray regions and the black regions indicates large
difference in their activities.
The advantages in using digital autoradiography in
dismantling investigations are evident: the technique is
easy to use, it does not require expensive equipment, and
contamination can be localized.
ENDNOTES
This research was funded by the DEMSAC project in
CEA Saclay France.
REFERENCES
1. S. ROSANVALLON, “Steel detritiation optimization
of a process” Fusion Eng Des, 51-52, 605-609, (2000)
Fig. 2. Scanning of a film on a contaminated zone.
The results represent contamination on the relatively
thin top layer of the cement surface due to the short
penetration ranges of beta emitters in solid material. The
migration of the activity deeper into the material is
essential in the estimations of the activity levels in the
bulk material. Therefore, some sampling was carried out
in regions in which higher contamination levels has been
measured by autoradiography to determine if the
contamination is located on the surface or if it is also
present in the bulk material. Fig 3 shows an example
image of two films before and after sampling exhibiting
clear difference between observed activities and therefore
top layer contamination.
Fig. 3. Images of two digital autoradiography films before
(above) and after (below) sampling. Sampling area is
circled.