Inventories of Iodine-129 and Cesium-137 in the Gaps and Grain Boundaries of LWR Spent Fuels
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(a) Although 135Cs is the isotope of interest for long-term geologic disposal because of its very long half-life (2,300,000 years), 137Cs, with a half-life of 30 years, would be of interest in the event of early breach of a waste package. Also, 13 7Cs is the isotope commonly measured in gap and grain-boundary inventory studies because it is also representative of the 135 Cs inventories and because it is much easier to measure due to its much higher activity in the spent fuels being tested. 487 Mat. Res. Soc. Symp. Proc. Vol. 556 c 1999 Materials Research Society
Until now, gap and grain-boundary inventories of 1291 have not been measured for LWR spent fuels because their very low concentrations in the fuels were difficult to measure. This paper is the first to report the results of a systematic study. Inventories of 137 Cs in the gaps and grain boundaries were remeasured at the same time. These results are combined with previous measurements of the 137 Cs gap and grain-boundary inventories [3, 4] and are reported along with the 1291 results. EXPERIMENTAL Following the lead of Canadian researchers [5], the following methods were used to measure the 1291 and 117 Cs inventories in the gaps and in the grain boundaries of several different spent fuels. First, the spent fuel was pressed from the cladding of a 25-mm-long rod segment. Then both the spent fuel and the cladding segment were placed in a beaker with 250 mL of borate buffer (pH = 8.5) containing 20 mg/L of KI as an iodine carrier (natural iodine is 100% 1271) and allowed to stand for 24 hours. A filtered (0.45-pm pore size) aliquot of the solution was then analyzed for both 1271 and 129, concentrations with an inductively coupled plasma/mass spectrometer (ICP/MS). Preliminary scoping tests had indicated that 24 hours was sufficient to dissolve all of the readily accessible 1291 within the cracks and on the surfaces of the spent-fuel fragments as well as any 1291 adhering to the surface of the cladding. Recovery efficiencies of the iodine carrier, which ranged from 60 to 100%, were determined by comparing the measured 127j concentration with its starting value in the buffer solution. These recovery efficiencies were used to adjust the measured 1291 concentrations. A second filtered (0.45-pm pore size) aliquot of the solution was analyzed for its U concentration using a Scintrex UA-3 analyzer and for its 137 Cs concentration using gamma-energy spectroscopy. Measurement of iodine concentrations in aqueous solutions can be troublesome, particularly at low concentrations, because of the tendency for the iodine to vaporize from the solution and/or to deposit on surfaces of the ICP/MS. Subsequent iodine measurements by the ICP/MS can then be biased by desorption of the previously deposited iodine. Use of the KI (1271) carrier allowed for the loss of iodine by vaporization and/or deposition anywhere within the dissolution and analysis procedures to be measured so that an adjustment could be applied to the 1291 results. Frequent use of blanks and standards during ICP/
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