AMS of natural 236U and 239Pu produced in uranium ores
Introduction
The radionuclides 236U and 239Pu are produced via neutron capture on 235U and 238U, respectively. Both are now widely dispersed in the environment as a result of atmospheric weapons testing and reprocessing operations of spent nuclear fuel. Because of the long half lives of 236U and 239Pu, 2.3 × 107 a and 2.4 × 104 a respectively, these isotopes persist in the environment for periods much longer than human lifetimes. This feature is being turned to advantage by using plutonium as an isotopic tracer for soil erosion and sediment accumulation rates [1]. Plutonium concentrations in sediments and water, together with the 240Pu/239Pu ratio, have also been used to identify sources of plutonium and to monitor its dispersal away from nuclear facilities [2].
Although the main sources of 236U and 239Pu in present-day environments are anthropogenic, small amounts are also produced naturally in uranium ores via the same neutron capture reactions as in nuclear weapons and reactors. Naturally occurring 239Pu was first discovered by Seaborg et al. in 1948 in pitchblende from the Great Bear Lakes region of Canada [3]. This discovery was soon confirmed by Peppard et al. who measured 239Pu from Belgian Congo pitchblende [4]. The occurrence of natural 239Pu was subsequently demonstrated in a range of uranium ores as early as 1951 by Levine and Seaborg [5].
Natural 236U/238U ratios have been measured by several groups [6], [7], [8], [9], [10], either by more conventional mass spectroscopic methods or by accelerator mass spectrometry (AMS). However, detection limits with conventional mass spectroscopy for 236U/238U are ∼10−10, leading to errors larger than 50% for uranium ore samples. Therefore the measurement of natural 236U is difficult or even impossible without AMS [6], [8]. AMS offers significant advantages over more conventional mass spectroscopic methods, by making it possible to work with smaller samples, and achieving better discrimination against molecular interferences with higher sensitivity and shorter measurement times. However, even with AMS, measurements of natural 236U/238U are challenging because of the interferences from the abundant isotopes 235U and 238U. Consequently, time-of-flight detection and a combination of mass and energy analyzers are required [11].
Applications of natural 236U and 239Pu have been explored by a number of authors. The use of 236U as a neutron flux integrator has been proposed by Purser et al. [12], and was extended a few years later by Valenta [13]. Natural 239Pu has been studied as a natural analogue of the behaviour of plutonium in waste repositories by Fabryka-Martin and Curtis [14]. Furthermore, isotopic “fingerprints” (i.e. characteristic 236U/238U ratios for particular orebodies) of uranium ores were examined by Richter et al. [8]. In addition to these applications there might be the possibility of using 236U to study the environmental impact of uranium mining by monitoring the levels of 236U in drainage water from the mine. This feature can potentially also be used for uranium exploration by monitoring the levels of 236U in ground water.
In this paper, measurements of both 236U and 239Pu from the same uranium ore samples are reported for the first time. It was necessary to develop a sample preparation protocol that ensured both efficient extraction of plutonium and uranium from the ore and isotopic equilibrium between the 239Pu in the ore and a 242Pu spike. The results obtained in the present work are compared with previous measurements on uranium ores and possible discrepancies are explored.
Section snippets
Chemistry and sample preparation
To make precise measurements of the 236U/238U ratio and 239Pu content in an ore, a sample preparation protocol was developed for the extraction and purification of each element. For efficiency and to ensure that the 239Pu and 236U were products of an identical neutron flux, we separate 236U and 239Pu from the same uranium ore sample. Because of the sensitivity of AMS, a sufficient amount of 239Pu for a measurement can be extracted from less than 1 g of uranium ore. Therefore the following
Measurements of 239Pu
AMS measurements of 239Pu were carried out using the 14 MV accelerator in the Department of Nuclear Physics at the Australian National University. Plutonium was injected into the accelerator as the PuO− molecular ion and charge state 5+ was selected after the second accelerator stage following gas stripping in the high-voltage terminal. The accelerator was operated at ∼4 MV. In broad outline, the methodology is similar to that described by Fifield et al. [21], but performance has been improved
Results
The first measurements of natural 239Pu and 236U from the same uranium ores are presented in Table 1. The samples, which cover a range of high-grade uranium ores, were selected because they were expected to contain the highest concentrations of 236U and 239Pu and would therefore be relatively easy to measure.
The samples ANU-0102 (collected from Boemi (nowadays part of Czech Republic)) and ANU-099 (collected from Val Redena, Italy) are both pitchblende. Sample ANU-267 came from our radioactive
Discussion and conclusions
The production of 236U and 239Pu in uranium ores via neutron capture on 235U and 238U, respectively, is determined by both the neutron flux in the ore, and the probabilities of neutron capture by uranium isotopes. The factors that influence these processes are as follows:
- (1)
The dominant contributions to neutron production in the ore are the spontaneous fission of 238U, and (α,n)-reactions on light elements such as Li, Be, B, Na, Mg and Al. Rocks with high concentrations of these elements will
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2017, Journal of Environmental RadioactivityCitation Excerpt :Rarely 236U was detected by ICP-MS in soils and sediment collected in the vicinity of known contaminated sites (Pourcelot et al., 2011; Lloyd et al., 2009; Ketterer et al., 2003; Tortorello et al., 2013). Measurement of 236U in environmental samples requires advanced analytical techniques such as TIMS (Thermal Ionization Mass Spectrometry) and AMS (Accelerator Mass Spectrometry), which enable to detect low level of 236U by providing (236U/238U) isotopic ratio (Ketterer et al., 2003; Wilcken et al., 2007; Steier et al., 2008; Srncik et al., 2010). According to Castrillejo et al. (2017), the liquid input of 236U discharged into the Mediterranean Sea by the Rhone River may be equivalent to the contribution from the global fallout.