Sources for anthropogenic \(^{233}\) U

In general, the main emission sources for anthropogenic radionuclides are either atmospheric nuclear weapons tests or nuclear industry, i.e., reprocessing plants or reactor accidents. Since the vast majority of nuclear power plants which have been in operation until today have used a thermal neutron spectrum and U as fuel, the production of \(^{233}\)U in nuclear reactors is strongly suppressed compared with \(^{236}\)U25. Both, official sources, e.g.26,27,28 and unauthorized web sites29 on nuclear weapons design are naturally scarce or impossible to verify. Yet, even though there are information sources stating that at least one nuclear weapons test using a mixture of \(^{233}\)U and \(^{239}\)Pu as fuel has been conducted (“Teapot MET”, April 195529), to our best knowledge, all nuclear weapon programs were clearly dominated by \(^{235}\)U or \(^{238}\)U, \(^{239}\)Pu based weapons30. In short, the most relevant production path for \(^{233}\)U via the reaction \(^{235}\)U(n,3n)\(^{233}\)U requires fast neutrons with energies above 13 MeV31. A contribution from the thorium fuel cycle32 producing \(^{233}\)U by thermal neutron capture on \(^{232}\)Th can be considered as negligible. In contrast, \(^{236}\)U can be also produced in nuclear power plants and fission bombs via \(^{235}\)U(n,\(\gamma\))\(^{236}\)U using thermal neutrons, apart from the production by fast neutrons in thermonuclear weapons via the reaction \(^{238}\)U(n,3n)\(^{236}\)U. Therefore, a significant production can be expected in thermonuclear weapons containing uranium enriched in \(^{235}\)U (sometimes referred to as oralloy). Fallout from the low-yield device “Teapot MET” (22 kt) mentioned in29 can be assumed to be mainly locally restricted to the surrounding area of Nevada test-site (NTS)33,34. However, it is generally accepted that surface detonations of kilotons bombs cause tropospheric fallout, which is deposited in a band around the globe at the latitude of the test site (20\(^{\circ }\)–50\(^{\circ }\) N for NTS)35. Therefore, a contribution from the MET test to the total inventory of \(^{233}\)U at the latitude band of NTS is, in principle, possible but can be expected to decrease in an eastward direction36. A detailed discussion of the production mechanisms of \(^{233}\)U and \(^{236}\)U can be found in the Methods section.

Selection of sample materials

The \(^{233}\)U and \(^{236}\)U content of samples from five different locations, which are summarized in Table 1, were analyzed in this study. Samples comprises sea water and sediment as well as a peat and coral core. In four cases, chemically separated U in an iron oxide matrix was available from archived AMS sputter targets (the kind of sample holder suitable for the AMS ion source) in which \(^{236}\)U was previously determined. \(^{233}\)U is often added as a chemical yield tracer, however, only samples which have not been spiked with \(^{233}\)U during sample preparation were considered in the present work. If available, the \(^{236}\)U/\(^{238}\)U data obtained in the corresponding previous study are listed in Table 1 with one sigma uncertainty. A more detailed sample description can be found in the Methods section.

Table 1 Overview of the sample material used in the present study for \(^{233}\) U/ \(^{236}\) U analysis. Full size table

The global fallout signature of \(^{233}\) U/ \(^{236}\) U in a peat bog core

The \(^{236}\)U/\(^{238}\)U and the \(^{233}\)U/\(^{238}\)U atomic ratios measured in the peat core are plotted against the age of the respective peat layer in Fig. 1a, dated by using the unsupported \(^{210}\)Pb method37.Values for the \(^{236}\)U/\(^{238}\)U ratios range from 7.2 \(\cdot\) 10\(^{-7}\) to 9.2 \(\cdot\) 10\(^{-6}\) and for \(^{233}\)U/\(^{238}\)U from 1.1 \(\cdot\) 10\(^{-8}\) to 1.8 \(\cdot\) 10\(^{-7}\) (see also Supplementary Table 1). The observed \(^{233}\)U concentration in this environment, which is not directly influenced by any nuclear source except global fallout as shown by analyzing the Pu nuclide vector38, is almost two orders of magnitude lower than the \(^{236}\)U concentration. The \(^{236}\)U/\(^{238}\)U data obtained in the present study agree reasonably well with the previously published data for the peat core37 (compare Supplementary Fig. 1).

Fig. 1: \(^{233}\) U and \(^{236}\) U signal in the peat bog compared with weapons yields. a Measured \(^{236}\)U/\(^{238}\)U (black squares) and \(^{233}\)U/\(^{238}\)U (gray dots) atom ratio in the peat core depending on the age of the peat layer. The measurement uncertainties shown are ±1 \(\sigma\) (s.e.m.). The solid lines represent a Gaussian fit of the \(^{236}\)U (black line) and the \(^{233}\)U (gray line) data indicating a time shift between the \(^{233}\)U and the \(^{236}\)U release. The \(^{236}\)U peak can be disentangled by two Gaussian distributions (dashed black curves in a) corresponding to the two major testing phases of nuclear weapons with respect to the explosion yield (1952–1958 and 1961–1962) shown in b39. The vertical dashed gray line marks the year 1960 for a direct comparison of the time scales in a and b. Full size image

Both depth profiles of \(^{236}\)U/\(^{238}\)U and \(^{233}\)U/\(^{238}\)U ratios (Fig. 1a) show a pronounced peak with the maximum value in 1961.5 and in 1955.3, respectively. The explosion yield of atmospheric nuclear weapons tests is narrowly distributed with an expectation value of 1959.5 and a standard deviation of 3.1 years (see Fig. 1b). This means around 90% of the total explosion yield of all atmospheric weapons tests (440 Mt39) was released within only one decade and marks the most active phase of atmospheric nuclear weapons testing. Two main phases of atmospheric testing can be identified in Fig. 1b, i.e., 1952–1958 and 1961–1962, leading to the maximum global fallout in 1963, to which the \(^{236}\)U/\(^{238}\)U maximum in the peat core was attributed37,39. The \(^{236}\)U as well as the \(^{233}\)U bomb peak detected in the peat core is approximated by Gaussian fits (black and gray solid lines) with similar widths, i.e., 19 \(\pm\) 1 years and 18.6 \(\pm\) 0.9 years (FWHM). As both nuclides were deposited predominantly during a rather narrow time interval, the peak shape results from migration of U in the peat. In contrast to the \(^{233}\)U peak, the baseline of the \(^{236}\)U/\(^{238}\)U does not reach pre-nuclear levels for younger layers as additional releases might have occurred. The resulting \(^{233}\)U peak (peak center at AD 1953.5 \(\pm\) 0.5) is shifted by 6.8 years towards older ages with respect to the \(^{236}\)U bomb peak (peak center at AD 1960.3 \(\pm\) 0.4).

This indicates that the maximum release of \(^{233}\)U happened before the maximum deposition of global fallout and hence, can be attributed to the earlier testing phase, i.e., 1952–1958. Regarding the number of tests, the respective estimated yield and the altitude at which the tests were conducted, it can be deduced that atmospheric fallout from the earlier period was dominated by the U.S. program whereas the fallout maximum in 1963 was dominated by the USSR weapon tests39 (see Supplementary Table 2). Considering the sampling location, the detected \(^{233}\)U contamination can be therefore attributed either to some early thermonuclear explosions conducted by the US at the Pacific Proving Grounds (PPG) which are said to have used oralloy as tamper material29 or unfissioned \(^{233}\)U from the “Teapot MET” explosion in 1955.

The overall \(^{233}\)U/\(^{236}\)U ratio for nuclear weapons fallout was calculated from the peak area of the two Gaussian fits of the \(^{236}\)U/\(^{238}\)U and the \(^{233}\)U/\(^{238}\)U data. In both cases, the sample with the age AD 1920.7 and a \(^{236}\)U/\(^{238}\)U and \(^{233}\)U/\(^{238}\)U atom ratio of (7.5 \(\pm\) 1.5) \(\cdot\) 10\(^{-7}\) and (1.1 \(\pm\) 0.3) \(\cdot\) 10\(^{-9}\), respectively, serves as upper limit for the blank level which does not significantly affect the value of the overall \(^{233}\)U/\(^{236}\)U isotopic ratio.

Dividing the peak area yields an average \(^{233}\)U/\(^{236}\)U ratio of (1.40 \(\pm\) 0.15)\(\cdot 1{0}^{-2}\). This value can be considered representative for compartments of the environment which do not preserve a high time resolution, and are only affected by global fallout. If the \(^{236}\)U/\(^{238}\)U peak is disentangled according to the two phases of nuclear weapons testings (dashed black curves in Fig. 1a), a \(^{233}\)U/\(^{236}\)U ratio of (5.1 \(\pm\) 1.1)\(\cdot 1{0}^{-2}\) for the earlier phase is obtained (see Discussion section for details).

The close-in fallout signature of the PPG in a coral core

The \(^{233}\)U/\(^{238}\)U and \(^{236}\)U/\(^{238}\)U atom ratios determined in the corals from Kume Island are presented in Fig. 2 as a function of the age of the respective coral band. The corals were cut along layers with low density which correspond to the fast growth rates during summer time. Hence, the year 1951, e.g., refers to the time period from summer 1950 to summer 1951. The individual \(^{236}\)U/\(^{238}\)U atom ratios are in very good agreement with the results from the previous study40 (see also Supplementary Table 3). The stated 1 \(\sigma\) uncertainties of the \(^{233}\)U/\(^{236}\)U ratio are clearly dominated by the comparably low statistics in case of the \(^{233}\)U measurement due to the low abundance of \(^{233}\)U in the corals and the availability of material left in some AMS sputter targets from the previous study.

In general, the \(^{236}\)U/\(^{238}\)U and the \(^{233}\)U/\(^{238}\)U atom ratio with a maximum of (1.05 \(\pm\) 0.05)\(\cdot\)10\(^{-8}\) and (1.6 \(\pm\) 0.2)\(\cdot\)10\(^{-10}\), respectively, are almost three orders of magnitude lower than in the peat core. In the ocean water, fallout U is mixed with higher concentrations of natural U than in the peat bog so that the fallout signature is diluted before the U is concentrated in the corals. The level of the \(^{236}\)U/\(^{238}\)U ratio for pre-nuclear samples is (1.0 \(\pm\) 0.2)\(\cdot\)10\(^{-10}\) and \(<\)3.1\(\cdot\)10\(^{-12}\) for \(^{233}\)U/\(^{238}\)U, respectively. Whereas two peaks of the \(^{236}\)U/\(^{238}\)U data in 1954 and 1958 can be clearly identified in Fig. 2, there is only one maximum in the \(^{233}\)U/\(^{238}\)U measurement data which is statistically significant, that is in the year 1958. The uncertainty of the \(^{233}\)U/\(^{238}\)U ratio at 1955 unfortunately is too large to consider this data point as reliable. On the basis of the present data a maximum of the \(^{233}\)U/\(^{238}\)U atom ratio in 1955, therefore, cannot be unequivocally identified. The center of the maximum of the \(^{233}\)U/\(^{238}\)U ratio in 1958 coincides exactly with the maximum of the \(^{236}\)U/\(^{238}\)U ratio which shows that also the \(^{233}\)U/\(^{238}\)U ratio in the corals is strongly affected by the close-in fallout from the PPG. Following the argumentation given by Nomura et al.40 who attributed the second peak at 1958 to the operation Hardtack I, our results suggest a considerable use of oralloy during this test series. However, no information about the tamper material in operation Hardtack is available to us at present. While no good data was obtained for the year 1955, there is clearly no maximum in 1954 corresponding to the first peak in the \(^{236}\)U/\(^{238}\)U atom ratio. This finding indicates that large quantities of \(^{236}\)U, but not of \(^{233}\)U, have been produced by the devices tested before 1954. This is in good agreement with the claim that Castle Nectar in 1954 was the first thermonuclear explosion with an oralloy tamper29. It also agrees with the assumption that the Ivy King test in 1952 was a pure oralloy fission device41 and hence, did not generate enough fast neutrons required for the build-up of \(^{233}\)U. Nevertheless, the \(^{233}\)U abundance in the marine environment of the Pacific Ocean seems to gradually increase from 1953 on, suggesting that \(^{233}\)U has been produced from the very first thermonuclear weapons, even though to a much smaller extent.

Fig. 2: Measurement results from coral samples. \(^{236}\)U/\(^{238}\)U (black squares) and \(^{233}\)U/\(^{238}\)U atom ratio (gray dots) in the Kume coral core as a function of time. The measurement uncertainties shown are ±1 \(\sigma\) (s.e.m.). The data points are connected by solid lines (bold for \(^{236}\)U) to guide the eye. The maximum at 1955 is not statistically significant and thus, not linked to the neighboring points. Full size image

The weighted average of the \(^{233}\)U/\(^{236}\)U ratio (see Fig. 3) was calculated from the measured \(^{233}\)U/\(^{238}\)U and \(^{236}\)U/\(^{238}\)U ratios for three time periods (I–III) that are characterized by a different \(^{233}\)U/\(^{236}\)U ratio. The ratios for samples before 1949 are not shown in this figure, as in most cases only upper limits for the \(^{233}\)U/\(^{236}\)U ratio were obtained because of the low \(^{233}\)U concentrations. In period I with no significant \(^{233}\)U production, i.e., until 1956, the average \(^{233}\)U/\(^{236}\)U = (0.31 \(\pm\) 0.07)\(\cdot 1{0}^{-2}\) is much lower than for the period 1957–1962. Period II is characterized by an increased release of \(^{233}\)U probably caused by the close-in fallout of operation Hardtack I and an average \(^{233}\)U/\(^{236}\)U ratio of (1.81 \(\pm\) 0.15)\(\cdot 1{0}^{-2}\) was obtained. The “1950” sample and also the “1955” sample, which corresponds to a possible first maximum in Fig. 2, show an elevated ratio but do not considerably affect the weighted average due to the large uncertainties. Starting from the year 1963 coinciding with the maximum of global fallout (period III), the ratio levels out to an average of (1.44 \(\pm\) 0.12)\(\cdot 1{0}^{-2}\) which is consistent with the global fallout average determined from the Western Europe peat samples.

Fig. 3: \(^{233}\) U/ \(^{236}\) U ratio in coral samples. \(^{233}\)U/\(^{236}\)U (black squares) calculated from the measurement results for the Kume coral core with ±1 \(\sigma\) uncertainty. The solid blue lines indicate the weighted average for the respective time period (I–III) and the dashed lines the corresponding 1 \(\sigma\) uncertainty (s.e.m.). Full size image

The signature of nuclear power production in the Irish Sea

The \(^{233}\)U/\(^{238}\)U ratios detected in Irish Sea sediment range from 9.6\(\cdot\)10\(^{-9}\) to 5.9\(\cdot\)10\(^{-8}\) and, hence, are comparable to the ratios found in the peat core (see also Supplementary Table 4 for details). The \(^{233}\)U/\(^{236}\)U ratios of three samples, which have been diluted by a factor of 100, show a high uncertainty (Fig. 4) and therefore have a low significance for the interpretation of the \(^{233}\)U/\(^{236}\)U ratios in the sediment.

Fig. 4: \(^{233}\) U/ \(^{236}\) U ratio in the Irish Sea. Depth profile of the \(^{233}\)U/\(^{236}\)U atom ratio in the Irish Sea sediment core collected close to the Sellafield reprocessing plant and in Irish Sea water (IAEA-381) with ±1 \(\sigma\) uncertainty. Increased uncertainties of three samples at depths 11 cm, 23 cm and 47 cm are caused by low counting statistics on \(^{233}\)U due to preceding dilution (1:100) of the material. The horizontal blue line marks the weighted average for the \(^{233}\)U/\(^{236}\)U ratio in the Irish Sea with ±1 \(\sigma\) uncertainty (s.e.m.). Full size image

The \(^{233}\)U count rate from the undiluted samples was four orders of magnitude higher than from a U sample considered as instrumental blank for \(^{233}\)U. Consequently, a clear \(^{233}\)U signal was detected, but as shown by the depth profile in Fig. 4, the \(^{233}\)U/\(^{236}\)U ratios in the sediment core are significantly lower than in the peat and the coral core. The weighted average from the sediment samples (n = 7) results in \(^{233}\)U/\(^{236}\)U = (0.13 \(\pm\) 0.02)\(\cdot 1{0}^{-2}\), which is consistent with the ratio determined in Irish Sea water of (0.11 \(\pm\) 0.01)\(\cdot 1{0}^{-2}\). Hence, the weighted average of \(^{233}\)U/\(^{236}\)U = (0.12 \(\pm\) 0.01)\(\cdot 1{0}^{-2}\) in the Irish Sea, close to the reprocessing plant Sellafield, is one order of magnitude lower than in nuclear weapons fallout found in the peat and coral core. In accordance with the theoretical discussion of the \(^{233}\)U and \(^{236}\)U production mechanisms in the Methods section, we attribute this low ratio to the U releases from the reprocessing plant because it indicates the lack of neutrons with energies above the threshold for the \(^{235}\)U(n,3n)\(^{233}\)U reaction. The elevated ratio of the sample from 19 cm depth deviates significantly from the calculated average; nevertheless it also clearly shows the low ratio expected for reactor dominated anthropogenic input.

Mixing of different source terms in the Danish straits

The measured \(^{233}\)U/\(^{238}\)U, \(^{236}\)U/\(^{238}\)U and \(^{233}\)U/\(^{236}\)U ratios in two samples from the Danish straits (Kattegat) are given in Table 2. These two samples were collected in a similar region at a distance of only \(\sim\)40 km from each other and, as expected, show very similar values for the three atom ratios. The \(^{236}\)U/\(^{238}\)U ratio is clearly elevated with respect to the natural abundance, which confirms the mainly anthropogenic origin of \(^{236}\)U in the Danish straits. The \(^{233}\)U/\(^{238}\)U ratio is quite low and comparable to the ratios found in the modern layers of the coral core from the Pacific Ocean. Within the uncertainties the \(^{233}\)U/\(^{236}\)U ratios of the two samples are indistinguishable and the resulting average of (0.45 \(\pm\) 0.02)\(\cdot 1{0}^{-2}\) is situated between the value attributed to the reprocessing plant Sellafield (0.12 \(\pm\) 0.01)\(\cdot 1{0}^{-2}\) and to the global fallout (1.40 \(\pm\) 0.15)\(\cdot 1{0}^{-2}\). This is consistent with the picture of the Danish straits being a mixing zone of water masses carrying global fallout signature with waters containing uranium originating from the reprocessing plants as well as fallout from the Chernobyl accident42,43.

Table 2 \(^{236}\) U/ \(^{238}\) U, \(^{233}\) U/ \(^{238}\) U and \(^{233}\) U/ \(^{236}\) U results for two selected samples collected at the strait between Denmark and Sweden. Full size table

As discussed in the previous section, no difference in the \(^{233}\)U/\(^{236}\)U ratio between reprocessing plants and NPPs can be expected, and in this way no differentiation between a Chernobyl and a La Hague/Sellafield fraction is possible. However, the contribution of uranium from generic nuclear fuel and global fallout can be calculated by using a two end member linear mixing model, as commonly applied to Pu ratios, e.g., in21. The average \(^{233}\)U/\(^{236}\)U ratio of the two Kattegat water samples and the \(^{233}\)U/\(^{236}\)U ratio of global fallout (1.40 \(\pm\) 0.15)\(\cdot 1{0}^{-2}\) from the peat core and that of nuclear fuel (0.12 \(\pm\) 0.01)\(\cdot 1{0}^{-2}\) from the Irish Sea sediments yields a global fallout fraction of (25.8 \(\pm\) 3.4)% at the sampling location in the Danish straits. As expected, the larger contribution comes from the nuclear power industry which is most probably caused by the considerable releases from the reprocessing plants as discussed before in this paper and in previous publications5,18,42.