Over Estimation of Uranium Releases into the Sea from the UK Nuclear Fuel Reprocessing Plant at Sellafield.

[NNadir]

The rather long paper I will not have as much time as I would like to discuss but to which I'll refer anyway, is this one: Forty-Year Time Series of Anthropogenic Uranium in Fucus vesiculosus from Kattegat: Validating 236U Releases from European Nuclear Reprocessing Plants Jixin Qiao, Yiyao Cao, Mu Lin, Yanchun He, Peter Steier, and Karin Hain Environmental Science & Technology 2025 59 (5), 2675-2685.

The oceans, ever since oxygen appeared in the atmosphere, have participated in a uranium cycle between the Earth's mantle, its crust (both marine and terrestrial) and waters, (again both marine and terrestrial). The seas contain about 4.5 billion tons of uranium, and a great deal of research has been conducted around the idea of using seawater as a uranium source that will not be subject to depletion ever. A review of the topic can be found here: Carter W. Abney,* Richard T. Mayes, Tomonori Saito, and Sheng Dai, Materials for the Recovery of Uranium from Seawater Chem. Rev. 2017, 117, 23, 13935–14013. An account of the riparian flows of uranium, from the weathering of terrestrial rocks can be found in the following reference in Table 1 on page 199 of Chapter 6 (Brent McKee) in S. Krishnaswami, J. Kirk Cochran (eds) U-Th Series Nuclides in Aquatic Systems Other literature in my files also has data along these lines.

Natural uranium contains three isotopes, two of which are primordial, ²³⁸U and ²³⁵U as well as ²³⁴U which is in secular equilibrium with the ²³⁸U as it is a member of its decay series.

Two other isotopes are now present on Earth, ²³⁶U, now extinct in natural sources but is a product of neutron capture in nuclear reactors (and also is formed in nuclear weapons tests) and ²³³U which could be a major nuclear fuel if obtained from thorium, but appears in the environment from nuclear testing.

The paper uses the measurement of ²³⁶U, released into the sea at the nuclear fuel reprocessing sites at Sellafield in the UK and La Hague in France, to trace water flows in the North Sea.

From the paper:

The long-lived ²³⁶U (t_1/2 = 2.34 × 10⁷ y) has been increasingly applied as a promising oceanic tracer to investigate water circulation transport pathways and time scales. (1−6) Besides marginal natural production (35 kg), the majority of ²³⁶U in the environment is from anthropogenic sources, including (1) global fallout (GF) of the atmospheric nuclear weapons testing (ca. 900-1400 kg), (4,7) (2) discharges (estimates at 115-250 kg) from the two European nuclear reprocessing plants (RPs) at Sellafield (SF), UK, and La Hague (LH), France, (8,9) and (3) releases from nuclear accidents and other nuclear installations.

In recent years, the accessible measurement of another uranium isotope, ²³³U, at environmental levels by ultrasensitive accelerator mass spectrometry (AMS) opens up the possibility to identify the origin for ²³⁶U. (10,11) Anthropogenic ²³³U (t1/2 = 1.59 × 105 y) is assumed to have been mostly produced during nuclear weapons testing via 235U (n, 3n) ²³³U reaction by fast neutrons or directly by ²³³U-fueled devices, whereas negligible amounts of ²³³U are produced in thermal power reactors or reprocessing plants. (10,12,13) The representative ²³³U/²³⁶U atomic ratios were suggested to be (1.40 ± 0.12) × 10^-2 for GF (10) and 1 × 10^–7 - 1 × 10^–6 for sources associated with civil nuclear reactors. (13,14) Application of these different and characteristic ²³³U/²³⁶U ratios in a two-end-member model makes it possible to quantify ²³⁶U input from GF and nuclear reactor/reprocessing.

A good understanding of the input function of ²³⁶U is critical for its reliable oceanic tracer application. In the Atlantic–Arctic Ocean, the two European RPs are considered as the dominating sources for many radionuclides, including ²³⁶U. (15,16) Given this fact, reprocessing-derived ²³⁶U has been used as a point-source tracer, in coupling with other chemical tracers, to track Atlantic water transport pathways and time scales in the sub-Arctic and Arctic Ocean. (17−19) However, we must acknowledge that the ²³⁶U input function from LH and SF into the marine environment remains unclear due to incomplete records of ²³⁶U emission, especially from SF.

A good understanding of the input function of ²³⁶U is critical for its reliable oceanic tracer application. In the Atlantic–Arctic Ocean, the two European RPs are considered as the dominating sources for many radionuclides, including ²³⁶U. (15,16) Given this fact, reprocessing-derived ²³⁶U has been used as a point-source tracer, in coupling with other chemical tracers, to track Atlantic water transport pathways and time scales in the sub-Arctic and Arctic Ocean. (17−19) However, we must acknowledge that the ²³⁶U input function from LH and SF into the marine environment remains unclear due to incomplete records of ²³⁶U emission, especially from SF.

The currently available LH’s ²³⁶U discharge data sets include official records available for the time span of 1966–1996 (20) and reconstructed data from shell records during 1967–2017. (8) For SF, only reconstructed data from shell records during 1971–2018 are available. (8) Historical discharges of ²³⁶U from LF and SF have been reconstructed based on modeling (9,21) and revised very recently based on the observation data of ²³⁶U/238U in shells in the English Channel, with the sampling location downstream of LH, and in the Irish Sea, with the sample location upstream of SF, respectively (see Figure 1). (8) The shell data revealed that there might be unreported significant ²³⁶U release from SF in 1970s. (8) Nevertheless, the reconstruction for SF release in the period of 1977-1984 was calculated based on linear interpolation of ²³⁶U/238U atomic ratios.

The work follows uranium absorption, both naturally occurring uranium in seawater and the anthropogenic uranium in a species of seaweed in an area off Denmark.

Unfortunately I can't go into details, but the paper estimates that accounts of the discharge of reactor processed uranium is higher than has been previously estimated.

From the conclusion:

This work presents a 40-year time series dataset of ²³⁶U and ²³⁸U concentrations and ²³⁶U/²³⁸U and ²³³U/²³⁶U atomic ratios, based on a rare and unique archive of seaweed collected at Klint, a coastal area of Kattegat, during 1983–2021. These measurements are compared with estimates based on available discharge data from two European RPs at SF and LH. We observed a distinct temporal trend in the reactor-²³⁶U concentration compared to that derived from SF and LF discharges. Our results reveal that there is a 7-fold overestimation in the recently reconstructed SF discharge. This work demonstrates that seaweed serves as a valuable natural archive of a decadal-long record of ²³⁶U, which is useful for validating and improving the reconstructed ²³⁶U discharge history by other proxies for the European RPs. The improved estimate of discharge data provides the essential input functions for both dynamical ocean general circulation models and statistical models for estimating the advection and mixing of these anthropogenic radiotracers in the ocean. This work is expected to significantly improve the applications of radioisotope tracers in oceanic studies such as the spreading of marine pollutants and ocean mixing and circulation, especially in the regions of the North Atlantic and Arctic Oceans.

Interesting I think.