Discerning Melting and Volatilization Process of Fukushima's Fuel After the Tōhoku Tsunami.
The paper I'll discuss in this post is this one: Fukushima Daiichi Nuclear Power Plant Accident: Understanding Formation Mechanism of Radioactive Particles through Sr and Pu Quantities Junya Igarashi, Kazuhiko Ninomiya, Jian Zheng, Zijian Zhang, Miho Fukuda, Tatsuo Aono, Haruka Minowa, Hideki Yoshikawa, Keisuke Sueki, Yukihiko Satou, and Atsushi Shinohara Environmental Science & Technology 2024 58 (33), 14823-14830.
The tsunami associated with the Tōhoku Earthquake on March 11, 2011 killed about 20,000 people. The deaths were more or less instantaneous, the results of collapsing buildings, drownings, trauma's associated with being caught in energetic flows of seawater, etc. This demonstrated that living in coastal cities can be, and sometimes is, dangerous, something that we can expect to only get worse since we are doing effectively zero to address extreme global heating.
The deaths resulting from living in a coastal city of course, receive little attention, falling into the "ho hum" area concern. There is no movement to phase out coastal cities as a result of this disaster.
Of course, the tsunami associated with the Tōhoku Earthquake also destroyed three nuclear reactors, and unlike the attention paid to the deaths resulting from living in a city inundated by seawater, there is a lot of attention paid to the possibility that someone may have their lifespan shortened by exposure to leaked radioactivity associated with the failure of the fuel in the nuclear reactors. There is, at this point, little or no evidence that anyone was immediately killed by radioactivity associated the failure of nuclear fuel, and more than 13 years later, it's not clear that a large number of people have had their lives shortened by this release of radioactive material, although it's possible, if not clearly discernable, that there will be or have been some such shortened lives, but the numbers are clearly not huge.
The paper cited at the outset is not about health risks, but rather is an interesting account of the nature and types of radioactive materials that were ejected from the reactor as the result of the hydrogen explosion associated with the oxidation of the zirconium based cladding with the concomitant reduction of water to hydrogen gas. At the temperatures in the fuel when cooling was interrupted by the failure of pumps, some elements were volatilized, that is boiled away, and deposited in the environment when they cooled.
I am interested in the elements in nuclear fuel that are volatile, since this has potentially, to my mind at least, many practical implications, in particular with respect to the radiation based destruction of the fluoride gases, refrigerants, insulation gases, etc., that along with CO2, are driving extreme global heating.
From the introductory text:
...Most radionuclides released during the FDNPP accident have undergone decay over the past decade, making their identification challenging. However, long-lived radionuclides persist in the environment. Cesium-137 is one of the most notable radionuclides in nuclear accidents due to its large inventory of nuclear fuel and long half-life property (30.2 y). Other important long half-life radionuclides include 90Sr (T1/2 = 28.7 y) and Pu isotopes such as 239Pu (T1/2 = 24100 y) and 240Pu (T1/2 = 6560 y). Due to their low volatility, the release amounts of these radionuclides were limited during the FDNPP accident. For instance, the concentration of 90Sr in the soil and dust sample was 3–4 orders of magnitude lower than that of 137Cs. (7,12,13) Zheng et al. (8) estimated the release amount of 239+240Pu to be 1.0–2.4 × 109 Bq through Pu isotope measurements in litter samples. Subsequent investigations aimed to elucidate the accident mechanism, such as the release amounts and timing of the radionuclides from the reactor. However, the complexity arises from environmental samples accumulating deposition at various emission events from reactor Units 1, 2, and 3, posing challenges in discerning the release processes from each reactor...
The paper demonstrates that the amount of plutonium released from the reactor was very small, which is unsurprising, and difficult to distinguish from fallout from open air nuclear testing in the 20th century. However distinction is possible, because the ratio of two isotopes of plutonium 240Pu to 239Pu is much higher in reactor plutonium as opposed to that released in a nuclear weapon. The authors report that for the small amounts ejected from the Fukushima reactors as a result of the hydrogen explosion was roughly 33% 240Pu on an atomic ratio basis, from which one can show that roughly 36% of the Pu radioactivity results from the decay of 239Pu and 64% from 240Pu, owing to the shorter half-life of the latter. The data for Pu is reported as a sum, as above, of 239+240Pu. The figure for the amount of plutonium ejected into the environment, 239+240Pu to be 1.0–2.4 × 109 Bq suggests, using the upper limit, that the amount of plutonium that escaped into the environment was less than a gram, around 370 mg of 239Pu and around 180 mg of 240Pu.
It appears that in the cases examined here, radioactive particles, the isotopes studies are insoluble, encased in SiO2, essentially glass, the source of which is believed to have been concrete in the reactor floor and walls. Thus they are not really biologically accessible unless deposited as particles and located in close proximity to living tissue, for example lungs.
There is a great deal of interesting stuff in this paper, and unfortunately I will not have an opportunity to discuss it at length owing to time constraints. One of the most interesting pieces of information actually comes from a reference, reference 39 in the paper, Estimation of fuel compositions in Fukushima-Daiichi Nuclear Power Plant which gives a full (Origen-2) estimate of all of the nuclides that were present in the reactors at the time of the natural disaster in tabular form. It is, understandably, written in Japanese, although the tables themselves are in Western symbols and numbers and are easy to follow. Google Translator does an excellent job translating the Japanese into coherent English.
From the ejecta, the authors are able to describe an estimated process by which the reactor melted.
Cs forms oxide precipitation, and Sr dissolves as oxides in the fuel matrix. (48) At the accident, these elements were volatilized from these oxide forms and were incorporated into the SiO2 matrix under a temperature of 2000–2500 K, which was estimated in the previous section using the MELCOR code. The fuel fragment, which is the main source of Pu injection into the radioactive particle, was formed by core degradation under high-temperature conditions of the nuclear fuel. The XR2-1 test is one of the representative simulation studies that investigated the collapsing behavior at core damage in a boiling water reactor. (49) The meltdown of nuclear fuel in the FDNPP accident may occur in the same manner as this experiment’s results. First, the Zr covering the fuel rods melted and dropped under high-temperature conditions. Then, the UO2 pellets, which are the nuclear fuel, were infiltrated by the molten Zr and collapsed, producing many fuel fragments, and at the same time, fuel debris containing Pu was produced. Pu injected into the radioactive particles was derived from the Pu-containing fragments. Some of these particles were released into the environment by hydrogen explosion of the Unit 1 reactor. The source of SiO2, which is a matrix of radioactive particles, is still unclear. However, Satou (50) and Martin et al. (51,52) pointed out that the heat-insulating material presented in the PCV and the operation floor wall were the only source of SiO2, while the other ones mentioned the possibility that the source of SiO2 was concrete in the reactor. (25,28−30)
The amounts of Cs, Sr, and Pu contained in nine radioactive particles categorized into the type B from Unit 1 were investigated. The injection processes of these elements into the radioactive particles were discussed from the viewpoint of the volatilized behavior of these elements. We concluded that Cs and Sr were introduced into the particles by the volatilization process, whereas the fine particles of the nuclear fuel injected Pu into the radioactive particles during meltdown. While the emission of radioactive particles was only a portion of the overall release event from the reactors in the FDNPP accident, the emission behavior of Sr and Pu exhibited stark differences: the former occurred in a gaseous form, whereas the latter involved fine fuel fragments. The detailed analysis of Pu released into the environment by the FDNPP accident has been challenging due to the relatively high concentration of Pu contamination from global fallout. Investigating the Pu emission inventory from the accident can be accomplished through the determination of 239+240Pu/137Cs activity ratio, as analyzed in the radioactive particles. This study predicts the presence of numerous fine fragments containing Pu, offering valuable insights for the future decommissioning process of the FDNPP reactors.
I have argued that expending large amounts of money to "decommission" the Fukushima reactors is not justified beyond perhaps a sarcophagus for each reactor, since few lives are actually at risk from declining to do much more. The money would be better spent building more reactors to address to the maximal extent possible, extreme global heating, the effects of which dwarf any events or risk of events associated with nuclear power, media frenzies and public stupidity notwithstanding. As the paper noted, many of the nuclides that were highly radioactive have decayed to stable elements. This process will only continue. For example, as of this writing, 4930 days after the tsunami struck, less than 1% of the 134Cs, an induced radionuclide that does not form directly in fission because of the stability of 135Xe, but is formed by neutron capture in nonradioactive 133Cs that does form from nuclear fission, remains as of today. (This isotope 134Cs was involved in the very stupid media brouhaha over the Fukushima Tuna Fish).
Anyway, it's an interesting paper.
I trust you're having a pleasant weekend.