Analysis of Naturally Produced Technetium and Plutonium in Geologic Materials

Abstract

In uncontaminated natural materials, plutonium and technetium exist exclusively as products (daughters) of nuclear reactions in which uranium is the principal reactant (parent). Under conditions of chemical stability over geologic periods of time, the relative abundances of daughter and parent elements are fixed by the rates of nuclear reactions and the decay of the daughter radionuclide. The state of this nuclear secular equilibrium condition is the primary basis of the geochemical study of these elements in nature. Thus, it is critical that nuclear parent and daughter abundances are measured in the same sample. We have developed a quantitative procedure for measuring subpicogram quantities of plutonium and technetium in gram quantities of geologic matrices such as uranium ores. The procedure takes advantage of the aggressive properties of sodium peroxide/hydroxide fusion to ensure complete dissolution and homogenization of complex materials, the precision provided by isotope dilution techniques, and the extreme sensitivity offered by thermal ionization mass spectrometry. Using this technique, a quantitative aliquot can be removed for uranium analysis by isotope dilution thermal ionization mass spectrometry or α spectrometry. Although the application of the procedure is unique, the analytical concepts may find more general application in studies of environmental contamination by nuclear materials. To assess the precision and accuracy of the analytical results, blanks and standards were analyzed routinely for a 1-year period to ensure quality control of our sample analyses. The average technetium blank is 5 ± 4 fg (n = 8), and that for plutonium is 0.17 ± 0.15 pg (n = 7). Thus, the detection limit for technetium (defined as 3 times the standard deviation of the average blank) is 11 fg, and that for plutonium is 0.44 pg. To assess the procedural precision, Canadian Reference Material BL-5 was analyzed routinely with samples. The results of seven replicate analyses for technetium in this standard reference material yield a technetium concentration of 59.0 fg/g, with a remarkably small standard deviation of 0.6 fg, 1.0% of the average value. The results of six replicate analyses for the concentration of plutonium in BL-5 give 1.012 pg/g, with an equally small standard deviation of 0.016, 1.6% of the average value. No direct measure of accuracy can be done on the technetium or plutonium analyses, because no standard reference material exists for these elements. To help constrain the accuracy of our measurements, equilibrium technetium/uranium and plutonium/uranium abundances were calculated using the nuclear reaction code MCNP. For technetium, such calculations are relatively insensitive to variations in model parameters, and measurements fall within a 21% high/low bias. For plutonium, the calculations are very sensitive to model parameters and hence inherently less precise. Indirectly, spike and isotope mix calibrations made from weighted quantities of certified isotopes (both technetium and plutonium) can be used t...

Critical comparison of radiometric and mass spectrometric methods for the determination of radionuclides in environmental, biological and nuclear waste samples

“…In TIMS, the separated TcO 4 -is transferred to filament and reduced to TcO 2 using HI, the iodine is removed by pre-heating to 930 °C and the Tc is then measured using magnetic sector thermal ionization mass spectrometers equipped with ion counter. A detection limit of 0.02 mBq (or 11 fg) has been reported for geological samples [196].…”

“…The long half-life of 99 Tc (0.211 My) and low specific activity (6.3 × 10 8 Bq/g) makes mass spectrometry a sensitive method for the determination of 99 Tc, AMS [192][193][194], TIMS [195][196] and ICP-MS [46][47][48][49][50][51][52][53]197] have been used for the determination of 99 Tc. In AMS, the separated TcO 4 -is mixed with Al or Nb to prepare a target, the sputtered TcO -ions is then stripped to TcO 14+ , separated in mass spectrometry, and measured by a gas ionization chamber.…”

Determination of technetium-99 in environmental samples: A review

“…Up to now, a huge amount of 99 Tc has been produced during the operation of the nuclear power plants. It has been estimated that [16][17][18][19] PBq ((1.6∼1.9)×10 16 Bq) of 99 Tc had been produced worldwide up to the mid 1980's [29]. It was calculated that the production rate of 99 Tc in nuclear power plants is about 5.8 TBq (GW(th)y) -1 [30].…”

“…23 Before further chemical separation, 99 Tc should be released from the sample matrices into solution. Two common techniques for releasing 99 Tc from solid samples after ashing are acid leaching [8,11,12,82] and alkali fusion [17,[87][88][89]. Concentrated HNO 3 [6,90] 99 Tc and 2 h are necessary to release 99 Tc from sample matrices.…”

“…The long halflife, and therefore low specific activity, of 99 Tc (0.64 Bq ng -1 ) makes mass spectrometry (MS) a potentially sensitive method for measurement of 99 Tc. Accelerator mass spectrometry (AMS) [14][15][16], thermal ionization mass spectrometry (TIMS) [17] and inductively coupled plasma mass spectrometry (ICP-MS) [18][19][20][21] have been applied for 99 Tc measurement. Among these MS techniques, ICP-MS is the most widely and frequently used mass spectrometric method for measurement of 99 Tc in many types of environmental samples, due to its low cost and easy accessibility.…”