The 12th International Conference on Environmental ... - Events

Session 36-37 Abstracts
8) CURRENT ACTIVITIES OF ALLDECO AT DECOMMISSIONING OF THE A1 NPP - 16333
Jan Rezbarik, Stanislav Sekely, Jaroslav Katrlik, Dusan Majersky, All Deco s.r.o. (Slovakia)
<strong>The</strong> operation of the A1 NPP terminated as a consequence of an accident accompanied by serious fuel damage and internal
contamination of technological equipment. <strong>The</strong> decommissioning of the NPP is currently going on and planned to be finished by
2049. <strong>The</strong> specific conditions require a specific approach to the realisation of the decontamination and decommissioning works.
Many advanced technologies and equipment has been developed and applied to perform partial works. A great emphasis is put on
the thorough preparation of the work, verifying of the actual state, monitoring of the radiation situation, non-active training, optimisation
of working procedures and employment of remotely operated devices. <strong>The</strong> common aim of all these activities is the minimisation
occupational doses at optimal costs.
<strong>The</strong> character of the current works is demonstrated on some partial projects: removal and on-site solidification of radioactive
sludge, pre-treatment (reduction of specific activity) of some specific liquid radioactive wastes (dowtherm), pre-dismantling decontamination
and dismantlement of technological equipment, post-dismantling decontamination of segments and free release of the
materials.
SESSION 37 - D&D RADIOLOGICAL CHARACTERIZATION AND MONITORING
1) CRITICAL EVALUATION ON THE USE OF NON-DESTRUCTIVE AASSAY OF NUCLEAR PACKAGES
THROUGH DESTRUCTIVE BREAKDOWN AND INVENTORY RECOVERY - 16081
Stephen Hepworth, Rob Way, Sellafield Ltd (UK); Johathan Sharpe, VT Nuclear Services (UK)
Two historic waste packages on the Sellafield Site with potentially high fissile content were destined for re-packaging. Prior
to relocation and subsequent breakdown, each item underwent a campaign of non-destructive assay. <strong>The</strong> aim of the assay was to
gather information that would assist with the production of a safety case. <strong>The</strong> assay work consisted of: conventional x-ray radiography
to determine the identity of the contents; gamma imaging and three-dimensional tomographic re-construction to determine
the location of the gamma emitting material; and neutron coincidence counting coupled with gamma spectrometry to assign a fissile
mass.
Most items on the Sellafield Site that undergo non-destructive assay normally remain intact or are re-packaged with minimal
interference of the content. However, in this instance each item was dismantled and the fissile material recovered. This paper provides
a comparison of the measured results with the actual results for each technique.
<strong>The</strong> x-ray radiographic information was used to construct a three dimensional representation of the contents of each item. This
information was useful in identifying the plant items contained within. <strong>The</strong> results were discussed with plant operators who were
familiar with the historic plant. <strong>The</strong> operators were able to identify areas of likely accumulations of fissile material.
<strong>The</strong> two-dimensional gamma survey and subsequent three-dimensional re-construction revealed the location of the gamma
emitting materials within the packages. It was assumed that areas of increased gamma activity indicated areas of increased fissile
mass.
2) DECOMMISSIONING OF A VITRIFICATION FACILITY: RINSING PHASE - 16231
Marielle Asou, CEA (France); Sebastien Leblanc, Fabrice Bouchet, Franck Martin, AREVA (France)
<strong>The</strong> Marcoule site was the host of the very first electricity-generating nuclear reactors (3 gas-graphite reactors operated
between 1956 and 1984) and spent fuel reprocessing plant (UP1 and various ancillaries operated between 1958 and 1997). Reprocessing
plant has been shut down ten years ago and since this time decommissioning, dismantling and waste retrieval activities is
under way.
An important part of this decommissioning program concerns the vitrification facility of Marcoule. This facility includes 20
tanks devoted to interim storage of highly active solutions, awaiting for vitrification.
<strong>The</strong> main objectives of the rinsing phase are to decrease activity in equipment before dismantling and to minimize the amount
of highly radioactive waste resulting from rinsing, which are to be vitrified.
3) THE LEVEL OF UNCERTAINTY IN MATERIALS CLEARANCE - 16090
Peter Burgess, Nuvia (UK)
Measurement of surface levels is essential in waste assessment and in clearing materials from nuclear licensed sites. Radiation
measurements in general are much less accurate than most forms of engineering measurement, even in relatively simple conditions,
such as radiochemical laboratories. Waste assessment during clearance is far more difficult. <strong>The</strong> areas of uncertainty include: (1)
<strong>The</strong> intrinsic limitations of the equipment employed. For surface activity measurement, the detector is often a large area alpha +
beta scintillation probe connected to a ratemeter. Any detector has an effective energy threshold below which it fails to register an
event. <strong>The</strong> detector response is rarely uniform over the window area. <strong>The</strong> detector is connected to a ratemeter. <strong>The</strong> design of a
ratemeter and the way it is set up will have a large influence on how easy the user finds it to classify correctly materials close to
the exempt limit.. (2)Calibration of the equipment. <strong>The</strong>re is only a limited set of surface contamination sources available. Prediction
of the response to other nuclides can be complicated. (3)Determination of the fingerprint. For many practical situations, the
potential contaminant is a mixture of nuclides emitting a mixture of alpha, beta, gamma and X radiation. Any detector will have a
response which depends on the radiation type and energy. Frequently, the response to the fingerprint of the most operationally
robust and convenient detectors will be dominated by only a small fraction of the total activity present. It is thus vital that that fraction
is well established and any area zoned so that the fraction remains reasonably stable. (4)<strong>The</strong> influence of natural activity in
materials and of gamma radiation from elsewhere. Many building materials have levels of natural activity in the Bq/g region. <strong>The</strong>se
often vary significantly from sample to sample and area to area, particularly where buildings and equipment have been extended
or modified. (5)Surface condition. For short range emissions such as alpha particles and low energy betas, the range for detection
in air is a few mm. Any material covering the activity, such as paint or grease, will reduce the emission rate.
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