The nuclear fallout at Hiroshima and Nagasaki

Apart from ²³⁵U and ²³⁸U, several other uranium isotopes exist that have low abundance, yet can be of value in understanding what did or did not happen at Hiroshima. Sakaguchi et al. [56] examined the abundance of ²³⁶U, which forms from ²³⁵U by neutron capture without fission. A complicating factor, however, is that ²³⁶U also arises through radioactive decay of ²⁴⁰Pu, the second most abundant plutonium isotope. Since ²³⁶U decays very slowly and therefore has low specific activity, the method used in this study was mass spectrometry.

Starting from conventional estimates of bomb size, degree of ²³⁵U enrichment, and fission yield, the authors estimate that 69 g ²³⁶U should have been generated in the detonation, and they set out to look for it in the area affected by the black rain (see Figure 3.1).41 At this point, you might not be surprised to learn that they do not find it; or more accurately, they do find some ²³⁶U, but after comparison with plutonium levels and with samples from a control area in Japan taken to be unaffected by ‘Little Boy’, they conclude that all of it must be attributed to the global fallout. To explain the lack of a discernible local contribution, they assume that the black rain transported only a very small fraction of the radioactive matter generated in the blast.42

Figure 3.1: Area affected by black rain in and near Hiroshima. The areas of heavy and light black rainfall extend in NWN direction from the hypocenter. Concentric rings indicate distances of 10, 20, and 30 km from the hypocenter. Map adapted from [57]. The studies cited in this chapter mostly used soil samples from within the heavy black rain area.

The major component of natural uranium, ²³⁸U, undergoes α-decay, which is followed rapidly by two successive β-decays; this yields ²³⁴U. The half-life of ²³⁸U is very long (4.47 billion years), whereas that of ²³⁴U is comparatively short (246,000 years). At steady state, ²³⁴U will decay exactly as fast as it is formed through decay of ²³⁸U (see Section 2.3.3). Therefore, if we stick a sample of natural uranium into a radiation counter, we should measure equal activities for these two isotopes. The relation should be different, however, with enriched uranium, as was supposedly used in the Hiroshima bomb. Because ²³⁴U is close to ²³⁵U in atomic weight, both isotopes should have been enriched together relative to ²³⁸U. Assuming that in the Hiroshima bomb ²³⁴U, like ²³⁵U, was enriched by a factor of about 100 over its natural abundance, whereas ²³⁸U was reduced by a factor of 5, the activity (but not the abundance) of ²³⁴U in the bomb material should exceed that of ²³⁸U by some 500 times. Therefore, the ²³⁴U /²³⁸U activity ratio should be a very sensitive probe for the detection of residual bomb uranium.

Figure 3.2: α-Ray spectra of uranium extracted from soil samples using 0.1 N nitric acid (taken from Takada et al. [57]). The α-particles emitted by the various uranium isotopes are distinguished by their characteristic energies, which correspond to ‘channels’ along the x-axis; the abundance of each isotope is represented by the area under its peak (rather than the peak height). See text for details.

A very careful study that employed this probe was carried out by Takada et al. [57]. The samples consisted again of soil from the black rain area. What makes this study particularly interesting is the attempt to chemically separate bomb-derived uranium from that which constitutes the natural background. The bomb fallout should only adhere to the surface of the soil mineral particles, whereas the natural uranium should mostly reside within them. Thus, to extract the fallout, the soil samples were gently leached with dilute acid, which should strip only a shallow, superficial layer from the particles; the background was then recovered by dissolving the residue with concentrated acid.

In the fraction recovered with dilute acid, ²³⁴U activity indeed exceeded that of ²³⁸U—but only by a factor of approximately 1.15; compare this to the factor of about 500 expected for pure, highly enriched bomb uranium. This slight excess was observed only with samples from the black rain area, but not with those from a control area outside it.43 The activity of ²³⁵U, which in pure bomb-uranium should exceed that of ²³⁸U some 25 times, remained very low in all samples (see Figure 3.2).

As with the study by Shizuma et al. [6] cited before (Section 1.2), we have evidence of a small yet distinct deviation from the natural uranium isotope distribution; and the magnitude is similar between the two studies. There are two explanations in principle—namely, either that a minuscule amount of highly enriched bomb uranium was diluted to near nothingness by natural background, or that the degree of ²³⁵U enrichment in the dispersed artificial material was much lower than announced. Takada’s failure to detect a higher degree of enrichment even when taking steps to concentrate the bomb uranium clearly militates in favor of the second alternative.

Considering this evidence, as well as the state of technology which then prevailed (see Section 3.6 below), I feel certain that no highly enriched ²³⁵U was released at Hiroshima. However, here is how to prove me wrong: obtain a sample of pristine glacier ice, and analyze it for ²³⁵U and ²³⁸U. This has been done for both cesium and plutonium on a sample from Ellesmere Island in the Canadian arctic, and it is claimed that the imprint of the Nagasaki bomb is detectable in the layer of ice that was deposited in the year 1945 [58]. Such a sample should be largely free from terrestrial background, and using the exquisite sensitivity of modern mass spectrometry, the isotopic signature of ‘Little Boy’ should be unmistakable.44

Since global fallout is rich in plutonium and in radioactive fission products such as ¹³⁷Cs, soil samples that were protected from it should have great value for examining the fallout from the Hiroshima event alone. Two studies on soil, rock, and roof tile samples that were preserved in 1945 in Hiroshima itself, and which were retrieved from storage several decades later, exhibited distinct yet very low ¹³⁷Cs activity [59,60]. The latter study actually reexamined a series of samples which were reportedly collected by famed nuclear physicist Yoshio Nishina on his visit to Hiroshima only three days after the bombing. Among these samples, the spread in activity is very large. The two samples that had been collected the closest to the hypocenter gave no detectable ¹³⁷Cs activity. A single sample—obtained from the Koi area, which is located approximately 2 km from the hypocenter and is considered the zone most affected by fallout within the city limits—gave a value of 10.6 mBq/g; all other samples contained less than 1 mBq/g.

Figure 3.3 shows the γ-ray spectrum of one of the samples; the ¹³⁷Cs peak is indicated. Since the measurement was reported in 1996, approximately two thirds of the ¹³⁷Cs had decayed since the bombing. Most other peaks in the spectrum, particularly ⁴⁰K, are caused by natural background radioactivity. Concerning this background, Shizuma et al. [60] note:

In 1950, soil samples were repacked in air-tight glass vials. … In the present measurement, soil samples were repacked in plastic containers … to eliminate the ⁴⁰K gamma-ray background from the vial itself.

Let that sink in for a moment—the radioactivity of fallout from ‘Little Boy’, collected in the city three days after the bombing, is obscured by that of the glass vials used to preserve it.

Nishina’s samples have also been analyzed for uranium isotopes [61]. In this study, the isotope ratio ²³⁴U /²³⁸U was somewhat variable but always close to 1, whereas the abundance of ²³⁵U was consistent with natural background. Therefore, these soil samples, which are untainted by global fallout and very likely were not exposed to rain other than the black rain which transported the fallout,45 fit into the general pattern of detectable but very low levels of ¹³⁷Cs, and negligible or absent bomb-derived ²³⁵U.

Figure 3.3: γ-Ray spectrum measured by Shizuma et al. [60] on one of the samples collected on August 9^th 1945 by Yoshio Nishina. The ¹³⁷Cs peak is due to fallout, whereas the ⁴⁰K peak is part of the natural background.

Yamamoto et al. [63] collected samples from soil underneath houses that had been erected throughout the black rain area after the Hiroshima bombing, but before 1950, and thus before most of the global fallout struck. All samples contained some ¹³⁷Cs. The levels scattered by almost two orders of magnitude; however, even the highest values, which were observed in samples from two houses built as early as 1946, remained well below those which are caused in unprotected soil near Hiroshima by the subsequent global fallout. Thus, even in the black rain area, the ¹³⁷Cs fallout from the Hiroshima bombing was small.

To explain the variability of their observed ¹³⁷Cs levels, the authors quite plausibly invoke the excavation that may have occurred in preparation for construction in some of the buildings; however, they also state that

according to carpenters we interviewed, most of the wooden houses built around this time were built without causing major disturbance of the surface soil,

which suggests that the fallout was indeed quite inhomogeneously distributed within what is considered the fallout area.

We will revisit the question how the fallout may have come to be distributed so unevenly in Section 13.1.5. Here, we only need to note the following crucial point: whether or not the soil was disturbed before construction, it should have been protected from any fallout once the houses had been completed. It is therefore remarkable that, in all of Yamamoto’s presumably protected sub-floor samples, plutonium is also found.

Since the Hiroshima bomb is supposed to have consisted of enriched uranium, but not plutonium, its fallout should have contained at most minuscule amounts of plutonium.46 The observed activity of plutonium (²³⁹Pu  +²⁴⁰Pu) activity was indeed only about 4% of that of ¹³⁷Cs (see Figure 3.4B). However, after accounting for the much longer half-lives of both plutonium isotopes, its molar amount—that is, the total number of its atoms—exceeds that of ¹³⁷Cs about 20-25 times on average.

A further consideration is the time of measurement. Plutonium has not decayed significantly since the bombing, but ¹³⁷Cs decays much faster and would have been reduced to about one fifth of the original amount between the event and the publication of Yamamoto’s study; therefore, the ratio of abundance (Pu/Cs) at the time of the bombing would have been close to 4.

The authors, starting from the pious assumption that the official story of the bomb is true, stipulate that essentially no plutonium should have been present in pristine samples, and they ascribe that which they find to contamination by the global fallout. Since this completely voids the very premise of their study—namely, that their samples should be free of such pollution—one would expect some effort on their part to explain this unexpected outcome. However, no such explanation is forthcoming. More importantly, the authors do not test their assumption that such contamination was possible, which they could have easily done by obtaining soil samples from underneath houses built in the same area before August 1945. If the original premise of the study held, such samples should have been protected from any fallout; on the other hand, according to the authors’ revised hypothesis, fallout radioactivity should be present in all of these samples as well.

Figure 3.4: Cesium and plutonium activities in soil samples from Hiroshima. A: Activity vs. depth profiles of ¹³⁷Cs in soil samples retrieved from underneath buildings constructed in the Hiroshima black rain area in 1945-1949. All four individual samples shown by Sakaguchi et al. [64] are replotted here. B: Activity ratio (A.R., Pu/Cs) in similar samples, grouped by Pu activity. This graph contains all data points from Table 1 in Yamamoto et al. [63]. The equation and R² apply to the regression line.

The only carrier I can think of that might transport some global fallout from soil outside a house to underneath it would be percolating rainwater. Note, however, that according to a preliminary report by the same authors [64] most of the radioactivity was found in a very shallow layer at the very top within the soil (Figure 3.4A). It is difficult to see how percolating water from outside the house would have produced such a distribution. Moreover, plutonium and cesium are not equally mobile within the soil; the aforementioned study by Sakaguchi et al. [56] shows that plutonium is carried downward faster than is cesium, and thus is more mobile. Hence, if indeed global fallout had been carried by percolating rainwater from soil outside to that underneath the house, the Pu/Cs ratio in the latter place should have been considerably increased. In this case, those among Yamamoto’s samples which contain the highest plutonium activity, that is, presumably the highest contamination, should also have the highest ratio of plutonium to cesium activity. However, if we plot the ratio of plutonium activity to cesium activity against plutonium activity, then no such trend is apparent, but the scatter is very large (Figure 3.4B). Thus, percolating rainwater can be dismissed as a mechanism for the presumed contamination.

There is, of course, another explanation for the plutonium in samples that should not have been touched by global fallout—namely, that they were indeed not touched by it, and the plutonium was really contained in the fallout of the Hiroshima bombing. This hypothesis has the dual advantage of simplicity and physical plausibility; its only difficulty is that it runs counter to the official narrative.

Figure 3.4B showed that the ratio of ¹³⁷Cs to plutonium (²³⁹Pu  + ²⁴⁰Pu) in the fallout of the Hiroshima bombing is subject to large variation. A very considerable variation is also reported by Shizuma et al. [6] in the ratio between ¹³⁷Cs and bomb-derived ²³⁵U among the black rain samples taken from a single piece of plasterboard (see Figure 1.2). This isotope ratio should be proportional to the bomb’s fission yield, which the authors peg at 1.2%; and according to their own calculations, the observed values of this ratio span a range of 0.62 to 8.1 times that yield. Similarly, the soil samples studied by Takada et al. [57], which were discussed in Section 3.1, show no clear correlation between the degree of ²³⁴U enrichment to ¹³⁷Cs levels (Figure 3.5A).

Figure 3.5: Variability of isotope ratios in studies on fallout from Hiroshima. A: Activity ratio of ²³⁴U to ²³⁸U vs. ¹³⁷Cs activity in fallout samples from within and outside the black rain area in Hiroshima. Replotted from Figure 5 in [57]. B: Molar ratio of ²⁴⁰Pu to ²³⁹Pu vs. molar ratio of fissioned nuclei to total plutonium [63]. Molar ratios are estimated from the activity ratios reported in the reference. The trend line is fitted without using errors; it would descend more steeply if errors were used.

In order to explain the marked variability in their observed isotope ratios, Shizuma et al. suggest that cesium and uranium were separated while being suspended in the air through “condensation,” but they do not provide any details on this proposed mechanism. They also do not discuss the possibility that heterogeneous isotope abundance ratios would result from the detonation directly and persist until after the expansion stage.

In this context, it is noteworthy that the samples studied by Yamamoto et al. [63] show a substantial variation in the ratio of ²⁴⁰Pu to ²³⁹Pu (Figure 3.5B). If we suspend disbelief for a moment and assume that both plutonium isotopes indeed originated from a nuclear detonation, their variable ratio could not possibly be due to differential condensation during transport, since any such effect would have to be based on different chemical properties of the elements in question; thus, we could not expect it to separate different isotopes of the same element (and Shizuma et al. do not suggest that it does). We would therefore have to ascribe the observed variability of the ²⁴⁰Pu /²³⁹Pu ratio to inhomogeneity of the detonation itself.

We had seen earlier that ²³⁹Pu arises from ²³⁸U through the capture of a single neutron, whereas the formation of ²⁴⁰Pu involves the successive capture of two neutrons, which must derive from separate fission events. We should therefore expect the proportion of ²⁴⁰Pu to show a positive correlation with the ratio of fission events to total plutonium, but this is not observed (Figure 3.5B). Thus, the hypothetical condensation mechanism proposed by Shizuma et al. would have to account for the loss of a correlation not only between ²³⁵U and ¹³⁷Cs, but also between ²⁴⁰Pu and ¹³⁷Cs (which represents the number of fissioned nuclei).

How plausible is this hypothetical separation mechanism anyway? I have not seen this question addressed in the scientific literature; therefore, I will give my own reasoning. I assume that immediately after a nuclear detonation each of the resulting nuclides will be present in multiple states of ionization. It is the net charge of each ion which should dominate its interactions with other particles, rather than the chemical reactivity in the neutral state of the chemical element to which the ion belongs. This applies in particular to its association with water molecules, which will begin once the temperature has dropped sufficiently.

As soon as some of the ions have managed to attract and retain a hydration shell, the resulting aerosol particles will scavenge additional ions in their path, and they will ultimately coalesce into larger droplets. Both of these processes will tend to mix different nuclides, not to separate them. Overall, differential condensation seems ill-suited to explain the very pronounced variations in isotope ratios between the individual large black rain droplets whose residues were studied by Shizuma et al. [6].

Since the Nagasaki bomb (‘Fat Man’) used ²³⁹Pu, as did most nuclear bombs tested in the subsequent decades, isotopic signatures are less suitable for distinguishing local from global fallout in this case. However, there is one circumstance that makes up for it: at Nagasaki, the heaviest fallout reportedly occurred in and around the Nishiyama reservoir, a small body of water located some 3 km from the hypocenter. The timeline of fallout deposition was examined by Saito-Kokubu et al. [65], who analyzed the sediments at the bottom of this reservoir. The lowermost peaks of plutonium and cesium were found at a depth of 435-440 cm (Figure 3.6A); these must represent the earliest fallout.

The entire sediment core contains only a single layer of macroscopic charcoal particles, which the authors quite plausibly ascribe to the deposition of soot from the burning city. Intriguingly, however, this layer is found at approximately 450 cm. Since the study was published 63 years after the bombings, sedimentation had occurred with an average rate of close to 7 cm per year; assuming this rate to have been fairly uniform, a separation by 10-15 cm corresponds to a time interval of approximately two years.

Figure 3.6: Radioactive fallout in sediments from Nishiyama reservoir near Nagasaki. A: Plutonium and cesium activities and charcoal particles vs. sediment depth. B: Plutonium and estimated fission yield vs. sediment depth. Data from Table 1 and Figure 2 in Saito-Kokubu et al. [65]. See text for details.

The authors of the study acknowledge that the peaks are separated, but nevertheless ascribe the radioactivity to the Nagasaki bomb fallout. They do, however, not provide an explanation for the mechanism of separation beyond stating that it requires ‘further study’. Considering the (macroscopic) size of the charcoal particles, we can assume that they are immobile within the sediment; thus, any separation would have to come about through upward migration of the radioactive isotopes. Such a migration, however, is very unlikely to have happened, for the following reasons:

1. It lacks a driving force. On dry land, isotopes may slowly be transported downward through the soil by percolating water; however, considering that the reservoir is already water-filled, there will be no upward movement of more water into it from the ground underneath.
2. The plutonium and cesium peaks are close to the charcoal layer, but they have practically no overlap with it. If the radioactivity had slowly leached out of the charcoal layer, then the radioactive peaks should be broader and exhibit more overlap with the charcoal layer.
3. The findings reported by Sakaguchi et al. [56] show that plutonium is carried by percolating water more rapidly than is cesium; therefore, in the reservoir, the plutonium peak should have moved upward further than the cesium peak. However, the peaks of the two isotopes coincide.

Another incongruity emerges if we examine the ratio of plutonium to cesium in the sediments. Using the half-lives of the three isotopes (²³⁹Pu, ²⁴⁰Pu, and ¹³⁷Cs), the age of a given layer of sediment, which can be estimated by interpolation from its depth, and the yield of ¹³⁷Cs per fission event (approximately 6%), we can calculate the fission yield of the bombs whose fallout is contained in that layer. In Figure 3.6B, this calculated fission yield is plotted vs. sediment depth, along with the plutonium content. We see a low plateau of plutonium activity between 360 and 390 cm; in this region, which most likely contains the fallout from nuclear bomb tests conducted after the war, we see fission yields in the range of 20-40%. As we go deeper and reach the large peak of the supposed Nagasaki bomb, however, the fission yield drops to 5% and below.

According to standard lore [4], the Nagasaki bomb (‘Fat Man’) contained 6.2 kg of plutonium, of which 1 kg is said to have fissioned; this amounts to a fission yield of 16%. Thus, the fission yield of at most 5%, which is evident from the isotope ratio observed in the sediment layers said to contain the ‘Fat Man’s’ fallout, disagrees with the official narrative.47

As before, there is a politically incorrect but physically straightforward explanation for the observed discrepancies: charcoal and radioactivity are found in distinct layers of the sediment because they entered the reservoir at different times. The radioactivity was therefore not delivered by the ‘Fat Man’; this also accounts for the discordant isotope ratio, which is at odds with the bomb’s purported fission yield.

We have seen above that no highly enriched ²³⁵U can be demonstrated in the local fallout at Hiroshima, even though the bomb is said to have contained some 50 kg of it. We might therefore wonder if the technology for producing bomb-grade uranium even existed in 1945.

According to the official narrative, the “Trinity” test carried out on July 16, 1945 at Alamogordo (see Section 13.6.4) employed a plutonium bomb of the same type as the “Fat Man” bomb used on Nagasaki [40, p. 288]. However, this story is contradicted by some contemporaneous documents.

Studies from neither Hiroshima nor Nagasaki furnish any clear evidence of radioactive fallout commensurate with the purported nuclear detonations. Levels of plutonium and ¹³⁷Cs near Nagasaki are suitably high, but they do not agree with the bomb’s stated fission yield. Moreover, they were apparently deposited approximately two years afterward, which corresponds well with Compton’s estimated time of plutonium availability. The studies on the fallout of the Hiroshima bomb can be summed up as follows:

1. No evidence exists of highly enriched ²³⁵U in the fallout. The measurements on soil samples indicate a very low degree of isotopic enrichment only, and those on black rain drops dried in situ suggest the same. A high degree of enrichment is only ever stipulated, and the calculations based on this premise result in vanishingly small absolute amounts of bomb uranium.
2. ¹³⁷Cs attributable to the Hiroshima bomb is readily detected. Its level remains well below the global fallout that arose from later bomb tests, but in most of the samples described in sufficient detail it nevertheless exceeds the amount we should expect from ²³⁵U measurements in conjunction with the key tenets of the official story of the bomb.
3. Samples protected from global fallout also contain plutonium, in amounts and isotopic compositions that are incompatible with its formation by a detonating ²³⁵U bomb.

While none of these observations fit the ‘Little Boy’ narrative, all of them are consistent with the dispersal of reactor waste, for example by means of a ‘dirty bomb’. We also note that measured isotope ratios are highly variable, suggesting the use of several different batches of radioactive waste, within which the weakly enriched ²³⁵U had undergone fission to different degrees.

Overall, the findings and writings reviewed in this chapter consistently indicate that neither uranium nor plutonium were available in the required amounts and purities at the time of the alleged bombings, and that no atomic bombs were detonated. They also demonstrate inadequate, but determined efforts to forge the fallout of true nuclear detonations in both cities. With that in mind, let us now consider some of the physical studies adduced to prove that those nuclear bombings did indeed occur.