DETERMINATION OF I-129 IN LOW-LEVEL RADIOACTIVE-WASTE BY RADIOCHEMICAL INSTRUMENTAL NEUTRON-ACTIVATION ANALYSIS

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 b elow ground vault to be constructed at CRL. One of the most restrictiv e radionuclides for IRUS is I-129, which has been assigned a maximum a ctivity concentration in waste of 10(6) Bq/m(3). The limit of detectio n for radionuclides in waste has been set st 1% of the approximate max imum activity concentration, or 10(4) Bq/m(3) for I-129. A radiochemic al instrumental neutron activation analysis method has been developed to determine I-129 in two waste streams, incinerator ash and liquid fe ed to a bituminizer. Solid samples are spiked with I-125 tracer, fused at 960 degrees C with Li2B4O7 in a platinum boat in a flowing oxygen stream inside a three zone tube furnace, and the volatilized I-2 is tr apped on in-line charcoal filters. The charcoal filters are irradiated together with a filter containing a spiked I-125/I-129 standard, in t he NRU reactor, and then subjected to post-irradiation chemistry to re move Br-82 interference. The I-129 concentration in the sample is dete rmined by comparing the activity of the activated I-130 in the sample with that of the standard, and the chemical recovery for I-129 is dete rmined from the activity of I-125 tracer. Limits of detection for I-12 9 in solids are typically 0.005 Bq/g, based on a 4 hour counting perio d on a 10% efficient HPGe gamma-spectrometer at a source to detector d istance of approximately 12 cm. This paper presents a summary of the m ethod and the results from analysis of two waste streams.