Abstract
A program was initiated at Chalk River Laboratories (CRL) to determine the physical, chemical and radiological properties of wastes intended for disposal in IRUS (Intrusion Resistant Underground Structure), a below ground vault to be constructed at CRL. One of the most restrictive radionuclides for IRUS is129I, which has been assigned a maximum activity concentration in waste of 106 Bq/m3. The limit of detection for radionuclides in waste has been set at 1% of the approximate maximum activity concentration, or 104 Bq/m3 for129I. A radiochemical instrumental neutron activation analysis method has been developed to determine129I in two waste streams, incinerator ash and liquid feed to a bituminizer. Solid samples are spiked with125I tracer, fused at 960°C with Li2B4O7 in a platinum boat in a flowing oxygen stream inside a three zone tube furnace, and the volatilized I2 is trapped on in-line charcoal filters. The charcoal filters are irradiated together with a filter containing a spiked125I/129I standard, in the NRU reactor, and then subjected to post-irradiation chemistry to remove82Br interference. The129I concentration in the sample is determined by comparing the activity of the activated130I in the sample with that of the standard, and the chemical recovery for129I is determined from the activity of125I tracer. Limits of detection for129I in solids are typically 0.005 Bq/g, based on a 4 hour counting period on a 10% efficient HPGe gamma-spectrometer at a source to detector distance of approximately 12 cm. This paper presents a summary of the method and the results from analysis of two waste streams.
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