Migration and Sources of Plutonium and Neptunium in the South China Sea and Indian Ocean

[NNadir]

The paper I will briefly discuss is this one: Modeling the Migration Characteristics and Identifying the Sources of 237Np and 239+240Pu in Seawater and Sediments in the South China Sea and the Indian Ocean Yongjing Guan, Jianhui Lai, Ruihan Zhang, Xiaomin Wei, Shenzhen Wang, Shuyi Wang, Lingyi Wang, Xibin Han, Gang Li, and Zhiyong Liu Environmental Science & Technology 2025 59 (16), 8231-8243.

I'll just start this post by quoting the introductory passages, which refer to the testing of nuclear weapons and then make a point:

Plutonium (Pu) and neptunium (Np) are artificial radionuclides primarily generated by human nuclear activities. The primary source of fallout in the northern hemisphere is the atmospheric nuclear weapons testing in the early 1960s. Another important source of pollution in the Pacific Ocean and adjacent seas is the Pacific Proving Ground (PPG, 1946–1958). Ocean currents can transport these artificial radionuclides from the PPG to other regions. (1−5) According to previous studies, PPG-Pu’s contributions to the South China Sea (6) and the Indian Ocean (7) are approx. 57% and approx. 34%, respectively. Therefore, the impact of PPG cannot be ignored when studying radionuclides in the Pacific Ocean and adjacent seas.

The plutonium isotope is a radionuclide with particle reactivity that easily adsorbs to particulate matter and eventually settles into sediments, becoming a persistent source of pollution. (8,9) The global fallout (GF) peaks of plutonium isotopes from the 1950s and early 1960s in sediments can often be used to construct a chronological record of oceanic deposition. The activities, inventories, and deposition rates of plutonium isotopes in various oceanic environments, including seawater and sediments, have been extensively documented by numerous researchers. (1,7,10−14) Studies on neptunium have been relatively limited due to the low concentrations of ²³⁷Np in the oceans, which are typically three to 4 orders of magnitude lower than plutonium isotopes when measuring both in sediments. (8,15,16) However, with the development of instrumentation and experimental methods, ²³⁷Np can be effectively measured. (17−19) In addition, ²³⁷Np is of significant concern due to its higher mobility in the environment compared to other alpha-emitting radionuclides and its potential for easy uptake by humans through the consumption of seafood. (20) The high mobility of neptunium in the marine environment depends mainly on its solubility, which is affected by the pH and Eh of the seawater. However, the pH (approx. 7.5) and Eh (mostly greater than 250 mV) in the deep sea environment are relatively stable, and neptunium has a high solubility within these ranges. (21,22) Therefore, neptunium always maintains a high solubility in the deep sea environment. At the same time, Np can also be adsorbed by environmental media. Under near-neutral conditions, the mechanism of adsorption is mainly surface complexation. (23,24) Clay and iron-bearing minerals have a relatively strong adsorption capacity for Np, especially under low-oxygen conditions. (25) This also explains why Np can still exist and migrate in seafloor sediments despite its very low concentration and high solubility in the marine environment. Although the concentration of neptunium in the marine environment is small, its high mobility and adsorption behavior make its impact on the marine environment a potentially significant safety risk. In studies by Huang et al. and López-Lora et al., Np was found to be more readily uptake by marine organisms relative to other actinides. (26,27) This highlights the urgent need for research on ²³⁷Np in oceanic environments.

The ²⁴⁰Pu/²³⁹Pu atom ratios from different sources differ, making the ratio an important fingerprint for identifying the source of plutonium. For example, the ²⁴⁰Pu/²³⁹Pu atomic ratio in weapons-grade plutonium typically ranges from 0.01–0.05, compared to 0.17–0.19 in global fallout, and 0.30–0.36 in the close-in (tropospheric) fallout from nuclear weapons testing at the Pacific Proving Grounds (PPG). In recent years, reports have indicated that the ²⁴⁰Pu/²³⁹Pu atomic ratio along the coast of China is higher than GF. This increase is attributed to the mixing of plutonium from GF (0.178 ± 0.019) and plutonium from PPG (0.33 ± 0.03). (1,5,28−33) Additionally, the ²³⁷Np/²³⁹Pu atomic ratio can serve as a reference fingerprint for identifying the sources of ²³⁷Np. However, due to differences in the spatial distribution of ²³⁷Np and ^239+240Pu, which arise from their distinct removal mechanisms, ²³⁷Np demonstrates high solubility in marine environments. In contrast, ^239+240Pu is particle-reactive and exhibits low solubility, readily binding to particulate matter in the ocean. (25,34) Therefore, the ²³⁷Np/²³⁹Pu atomic ratio is not a consistently reliable indicator for identifying ²³⁷Np sources due to the variability introduced by environmental factors and their differing behavior in marine systems. However, it can serve as a comparative measure to evaluate the differences in migration characteristics between ²³⁷Np and ^239+240Pu. (35,36)

For this study, surface sediment and sediment core samples were collected from the seabed of the South China Sea (SCS) and the Indian Ocean (INO) to understand the long-range transport pathways, migration characteristics, etc. of radioactive contaminants from the PPG. (The Indian Ocean sample includes the Arabian Sea sample.) We extracted and analyzed these samples to obtain the activity distributions of ^239+240Pu and ²³⁷Np, as well as the atomic ratio distributions of ²⁴⁰Pu/²³⁹Pu and ²³⁷Np/²³⁹Pu in the sediments...

Open air nuclear testing lasted from 1945 to 1963, and in terms of magnitude dwarfed the two open air nuclear weapon denotations over cities in the only nuclear war ever observed. The nuclear war started, so far as the war between the victims of the nuclear war and the nation that initiated the nuclear war, started as an oil war. The victims of the nuclear war decided to destroy the fleet of nation that initiated the nuclear war (although they didn't have nuclear weapons when the war started), because the initiators, then the world's largest oil exporter, cut off their oil supply, thus forcing the victims to attack oil fields in South East Asia and take them from other colonials. The fleet of the initiators of the nuclear war needed to be eliminated, in the minds of the victims of nuclear war, to keep it from interfering in this colonial substitution.

Anyway. It is said that sediments and water in the South China Sea and Indian Ocean are "polluted" with ²³⁷Np and ^239+240Pu.

One may ask, "how polluted?" I'll get to this shortly, but before doing so, I need to note, as I often do, that the oceans have always been radioactive owing to the presence of a radioactive isotope of the essential element potassium, ⁴⁰K, which has a half life of 1.227 X 10⁹ years. Some years back - I don't have time to give the references now - I calculated that the ocean contains, roughly 75 billion tons of this particular isotope, based on the fraction of this isotope present, 0.00017 or 0.017%.

The standard unit of radioactivity is the Becquerel, abbreviated B (more rarely Beq). One Becquerel is equal to one nuclear decay per second. It follows that the ocean contains, as radioactivity from potassium alone, roughly 1.95 X 10²² Bq. It can be shown that this translates into about 13 million Bq per cubic meter, 1.3 X 10⁶ Bq m^-3.

(When oxygen evolved on Earth, uranium and its daughters dissolved in the water, at low concentrations. It can be shown that the ocean contains about 4.5 billion tons of uranium more or less in secular equilibrium with its radioactive decay daughters.)

Now we can compare the amount of radioactivity left over from neptunium and plutonium deliberately released into the environment in the period between 1945 and 1963 that subsequently found in the ocean, thus answer the question "how polluted?"

From the text:

...Samples from three major marine regions were simulated and analyzed using the S-ADE model. Data on ^239+240Pu concentrations, ²⁴⁰Pu/²³⁹Pu atomic ratios, and inventories (I) in seawater from the SCS, the AS, and the Eastern INO were obtained by S-ADE model. From Tables S7 and S8, it can be seen that the concentration of ^239+240Pu in the surface seawater (5 m) with the SCS ranged from 0.142 mBq/m³ to 14.436 mBq/m³, with a mean value of 3.049 ± 2.781 mBq/m³ (n = 50), consistent with previous studies...

...Similarly, ²³⁷Np concentration and corresponding inventory data were listed in Tables S9 and S10. The concentration of ²³⁷Np in the surface seawater of the SCS ranges from 0.010 mBq/m³ to 0.719 mBq/m³, with a mean value of 0.128 ± 0.148 mBq/m³. This data is close to that observed in the surface seawater of Tokyo Bay, Japan, which were between 0.083 and 0.114 mBq/m³

Note that the units are mBq, 1000th of Bq. If one were to drink a cubic meter of seawater, roughly 1.02 to 1.03 tons of seawater, one would need, at 1/(3.049 X 10^-3Bq sec^-1) = 328 seconds, roughly 5 and a half minutes on average to observe 1 decay (Bq) from plutonium. The potassium will get you first. (Actually the ton of seawater would get you first.)

None of this is designed to excuse nuclear weapons testing, or the existence of nuclear weapons at all. I personally approve of neither. It is merely to point out that the "pollution" of seawater in the discussed area by nuclear weapons testing is trivial. I'm more concerned, speaking for myself, about carbon dioxide in seawater than I am about plutonium and/or neptunium.

Some time ago, I covered, in extensive detail, in a response I enjoyed writing, in an overkilled but fruitful response to a humorless "I'm not an antinuke" antinuke whining about the collapse of a tunnel at the Hanford Nuclear Weapons site, the legacy of plutonium resulting from underground nuclear weapons testing.

That post is here: 828 Underground Nuclear Tests, Plutonium Migration in Nevada, Dunning, Kruger, Strawmen, and Tunnels

Have a nice Sunday afternoon. If you're a mother, have the happiest of Mothers' Days.