The 13th International Conference on Environmental ... - Events

Abstracts
decommissioning program. It can help establishing guidelines for the waste classification, dismantling and demolishing
activities. ORIGEN2 code has long been in use for computing radionuclide concentrations in reactor cores and near core
materials for various burn-up-decay cycles, using one-group collapsed cross sections. Since ORIGEN2 assumes a
constant flux and nuclide capture cross-sections in all regions of the core, uncertainty in its results could increase as
region of interest goes away from the core. This uncertainty can be removed by using a Monte Carlo Code, like MCNP,
for the correct calculations of flux and capture cross-sections inside the reactor core and in far core regions. MCNP has
greater capability to model the reactor problems in much realistic way that is to incorporate geometrical, compositional
and spectrum information. In this paper the classification of radioactive waste from the side structural components of a
CANDU reactor is presented. MCNP model of full core was established because of asymmetric structure of the reactor.
Side structural components of total length 240 cm and radius 16.122 cm were modeled as twelve (12) homogenized cells
of 20 cm length each along the axial direction. <strong>The</strong> neutron flux and one-group collapsed cross-sections were calculated
by MCNP simulation for each cell, and then those results were applied to ORIGEN2 simulation to estimate nuclide
inventory in the wastes. After retrieving the radiation level of side structural components of in- and ex-core, the
radioactive wastes were classified according to the international standards of waste classification. <strong>The</strong> wastes from first
and second cell of the side structural components were found to exhibit characteristics of class C and Class B wastes
respectively. However, the rest of the waste was found to have activity levels as that of Class A radio-active waste. <strong>The</strong>
waste is therefore suitable for land disposal in accordance with the international standards of waste classification and
disposal.
4) 40068 – Uranium refining and conversion plant decommissioning project
Naoki Zaima, Yasuyuki Morimoto, Noritake Sugitsue, Kazumi Kado, JAEA (Japan)
<strong>The</strong> uranium refining and conversion plant at Ningyo-toge (URCP) was constructed in 1981 for the purpose of
demonstration on refining and conversion processes from yellow cakes to UF6 via UF4, and then as modified to develop
the conversion of reprocessed uranium production of natural UF6 and purification of reprocessed UF6. Through 20 years,
385 tons of natural uranium of UF6 and 336 tons of reprocessed uranium of UF6 had been conducted. <strong>The</strong>re are two
refining processes in the URCP facilities. One is the wet-type process for the natural uranium and the other is the
dry-type process for the reprocessed uranium. It was found the large amount of uranium residuals such as wet slurry and
dried powder inside the vessels and pipings. Careful consideration had always been required against the diffusion of
contamination. <strong>The</strong> basic policies concerning plant decommissioning are the optimization of the labor costs and the
minimization of the radioactive wastes. <strong>The</strong> procedures are followings; i)measuring doserate by high sensitivity
surveymeters and identificating nuclide by gamma ray spectrometry, ii)estimating uranium mass inventory, iii) planning
workers distributions including of radiation control staffs, iv)deciding dismantling methods and decontaminating
schematically if required, v)measuring and classifying doserate and contamination level, vi) managing for radioactive
waste container, vii)control for personal exposures. Through two years and half, almost all equipment had been
dismantled except building decontamination. Several hundreds tons of dismantled wastes had accumulated in 200 litter
drums approximately. In addition, the secondary wastes had also been generated. Several thousands day of working time
had spent totally. <strong>The</strong> radiation monitoring of working places had been performed during dismantling, the results were
generally less than 20?Sv/h under the doserate limitation. However, followed by the trace of the reprocessed uranium,
U-232 progenies nuclides such as Th-228 and Tl-208 were observed. <strong>The</strong> expected exposures are apprehensive by
accumulation of the high energy gamma emission nuclides. For example, the fluidization media storage tank in which
Th-228 progenies originated from U-232 accumulated. <strong>The</strong>re arises the case of remote controlled tools for decreasing
personal exposure if required. As for the waste disposal, the determination of uranium content must be necessary. We are
now developing for measuring systems with better accuracy. <strong>The</strong> further tasks imposed us summarized the followings;
i)dismantling method for high doserate area, ii)reduction of radioactive wastes volume, iii)decontamination for the
buildings, iv)waste disposal.
5) 40161 – Radiochemical Characterization of Concrete Samples from the Environment of Accelerator Facilities
Dorothea Schumann, Paul Scherrer Institute (Switzerland)
<strong>The</strong> Paul Scherrer Institute (PSI) in Switzerland operates the most powerful proton accelerator facility in Europe
and also world-wide, with a beam current of 2 mA and a final proton energy of 590 MeV. It consists of two injector
cyclotrons with beam energies of 72 MeV each and a ring cyclotron accelerating the protons to the final energy. Besides
the targets (mainly carbon wheels for the muon production and solid lead as the target for the neutron production) also
the closer surroundings of the facility are highly activated. One of the components there is concrete which is used in huge
amounts as construction material for walls and floor and acts as an effective and cheap shielding. Activation of these
components could be neglected in the past because of the relatively low dose rates, but this situation changed with the
upgrade of the accelerator during the last years. <strong>The</strong>refore, detailed declaration of the radionuclide content is also
requested for concrete from the Swiss authorities (NAGRA). In the present contribution, we report on the radiochemical
analysis of two different kinds of shielding concrete. <strong>The</strong> first samples stem from the floor of a former graphite target
station, which was dismounted in 1989. Here we expect activation by secondary neutrons produced by the primary proton
beam of 590 MeV from the ring cyclotron. <strong>The</strong> second series was taken from the environment of a copper target hit by 72
MeV protons from one of the injector cyclotrons. <strong>The</strong>se samples were taken in 1998. <strong>The</strong> main activation is also in this
case caused by the produced secondary neutrons. Besides conventional gamma-spectrometry, identifying mainly
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