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ST 4.3 –
Gestión de residuos radiactivos
WHY THE MANAGEMENT OF DESUSED IRIDIUM SOURCES
SHOULD BE CHANGED?
Dellamano, José Claudio*; Ferreira, Robson De Jesus; Hiromoto, Goro;
Potiens Jr., Ademar José; Vicente, Roberto
IPEN-CNEN/SP. Brasil
* Autor responsable, email: jcdellam@ipen.br
Iridium-192 sealed sources are extensively used in the industry, but have a relatively short
useful life, being replaced and discarded as radioactive waste after about one year of use,
because the half-life of the radioisotope is only 73.8 days. This short half-life is also the reason
why the usual recommended waste management strategy is storage for decay and release. In
fact, after about five years of decay, any 192Ir source with initial activity in the upper range of
activities of commercially available sources, should have been decayed to below the release
limit of 10 kBq of the Brazilian regulations. However, about 5,000 iridium disued sources stored
in the radioactive waste storage facility of the Nuclear and Energy Research Institute, some of
them after three decades of decay, still present dose rates much above the corresponding 0.16
microsievert per hour at 10 cm from the source. Cobalt-60 contamination was suspected as a
possible cause, but investigations on this unexpected fact revealed that the second meta-stable
state of the radioisotope, i.e. the 192m2Ir, with half-life of 241 years, which is also formed in the
neutron-gamma nuclear reaction with the 191Ir natural iridium isotope, was the main cause of
the observed radioactivity. As this isomer decays almost exclusively to the groundstate of the
isotope, secular equilibrium is achieved and the source decays with a half-life of 241 years. The
activation cross-section of the second meta-stable state with thermal neutrons is about 0.15
barn, very low when compared with the 954 barn of the first meta-stable state and the ground
state formation reactions, but enough activity results to explain the residual activity of the
sources. As a consequence, a new strategy should be recommended for the management of
this radioactive waste.