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Understanding how much fissile material — enriched uranium and separated plutonium — was produced, and at what time, by the Manhattan Project is one of those seemingly-obscure technical questions that comes with a lot of important historical implications. It is what determined the scheduling of the bombings, for one thing. For another, it is how you might answer questions about what would happen if the war had carried on. And for yet another, it is a way to debunk the various conspiracy theories that periodically emerge about either the United States having atomic bombs before July-August 1945, or being dependent on German uranium, or what have you.

Unfortunately for us, information that relates to the production rates of fissile material was considered among the most secret nuclear information in the postwar and Cold War period, because from that kind of information you can estimate stockpile production capabilities and limitations. So even in a secret project, this kind of data has historically been kept even more secret than the rest of it.

The Manhattan District History, General Leslie Groves’ secret, official history of the Manhattan Project, contained this information in two different “Top Secret Appendices.” I’ll handle them both separately since one is very straightforward and the other is not.

Enriched uranium

The one for uranium stockpiles is part of the book on the electromagnetic enrichment method, as the Y-12 Calutron plant was the final step in the enrichment process before the material was sent to Los Alamos. When the Department of Energy released a major declassification of the MDH in 2014, they failed to release a version of the appendix volume. The volume was, however, included as part of the 1977 microfilm release of the MDH. 

The graph above is a cleaned up version of the graph that is included in the relevant volume of the MDH (Book 5, Volume 6, “Electromagnetic Operation: Top Secret Appendix”), with the background grid reduced in intensity, in order  to make it easier to read the data.

It’s an interesting graph, but it doesn’t exactly read itself. Looking at it closely, and the numbers included in the relevant volume, it is clear that they are being very literal about “U-235” here: this only measures the actual content of U-235 in the shipments, not the total uranium. As the uranium shipped at different times had different levels of enrichment, the actual total amount of enriched uranium on-hand is a bit different than what the graph suggests. Which is somewhat clear when one notes that at the point marked “Hiroshima” on the map, they had only shipped a bit more than 50 kg, when the Hiroshima bomb itself contained 64 kg of uranium (~80% enriched), and had been sent from Los Alamos to Tinian some time before the operation took place. 

Here is a table I made which better indicates the details of the uranium production, based on the data in the volume and a little bit of math.

Date	U-235 (kg)	Enrichment	HEU (kg)	Rate (HEU g/day)	LB Bomb Equiv.
10/7/44	1.7	67.5%	2.6		0.04
10/21/44	2.3	68.9%	3.3	52.8	0.05
10/28/44	2.6	69.3%	3.7	58.2	0.06
11/19/44	3.8	70.6%	5.4	76.8	0.08
3/4/45	20.4	77.2%	26.4	199.9	0.41
3/25/45	24.9	78.1%	31.9	261.6	0.50
4/22/45	30.5	79.0%	38.7	241.2	0.60
5/20/45	35.6	79.9%	44.6	212.3	0.70
7/17/45	52.5	81.6%	64.3	340.4	1.00
8/12/45	66.7	82.6%	80.8	632.5	1.26
9/9/45	88.2	84.5%	104.4	842.4	1.63
12/2/45	182.1	88.9%	204.9	1,196.5	3.19
12/30/45	231.4	90.1%	256.8	1,853.3	4.00
12/29/46	1,041.0	95.0%	1,095.8	2,305.0	17.08

Above, “U-235” refers to the total content of the isotope U-235 in the uranium sent — the same as the graph above. “Enrichment” is the percentage of U-235 in the material sent. It is a little unclear in the source whether this is “the uranium sent in the last batch” versus “the total average enrichment of all uranium sent to Los Alamos up until this point,” but I suspect they are pretty similar numbers either way. “HEU” is a derived value from the U-235 and the enrichment — how much actual enriched uranium metal had been shipped, regardless of the enrichment level. So this is the number that lets you see that they definitely had enough metal on hand for the Little Boy bomb (64 kg) to be dropped on Hiroshima in early August. “Rate” refers to what the production rate of HEU was, in grams per day, and is a nice way to see how the rate of production improved over time (just as the “Enrichment” column lets one see how much better they got at enriching it), as they further debugged their plants and more parts came online. Lastly, “LB Bomb Equiv.” is just a simple measurement of how many Little Boy bomb cores the HEU number represents — it is just that number divided by 64 kg.

My estimate, based on the above data, is that you don’t have enough HEU for a second Little Boy bomb until mid-September 1945. Which is only slightly relevant, since, if the war had gone on, they likely would have not made more Little Boy bombs, but instead would have made composite (uranium-235 + plutonium) implosion bombs. But it does impact the question of “what would the atomic bombing schedule have been like if they only had gun-type bombs” (about two months between each bomb). 

Plutonium

For plutonium, it gets more difficult. The relevant volume (Book 4, Volume 6, “Pile Project – X-10 – Top Secret Appendix”) was not included in the 1977 release, but was partially included in the 2014 release. The relevant graph, though, was totally redacted:

So that’s the end of the story, right? This is a graph too spicy for people like you and me, right? Right? As people of my generation used to say: “As if.”

I have to admit, when I see something like this — data that I don’t feel has any legitimate reason to be classified, and would be interesting/useful for my historical study — I take it as a bit of a challenge. I take it personal, you could say. 

Now, I happen to know that parts of the MDH have been declassified and scattered across various databases and archives. Depending on who reviewed it, and when they did so, their interpretations of what could be released varied. So if one can locate multiple versions of its sections, one can sometimes reconstruct a “master” version with more content than any individual one. This is what the version of the MDH that I have uploaded to Archive.org (linked to in this post) is, and I have been doing this over the last ten years as I come across multiple copies. 

The little insight I had in this case was that there might be copies of the MDH in the Hanford DDRS database, which has been at a strange URL for a while now. And… I was right! There are actually two different copies of the relevant volume in there, and they both include the “plutonium production” graph, from a release of these volumes back in 1991. The only problem is that they are terrible microfilm scans, and so are barely legible. The best one (document D8719897) is this one:

One can, at least, see the relevant lines. But what are they of? What is the scale? Better than nothing, but it “reads itself” even less than the uranium graph. There’s no bit depth there; it’s essentially all black and all white, so there’s no hope of drawing out the missing data by playing with the contrast. 

Judging from other similar graphs in the file, the two lines are at different scales; one is “Scale x 1000” and the other is “Scale x 1,” and then when line “A” wraps around on the right, it becomes “Scale x 2000.”  So if the units for “B” are kilograms, that would make the units for “A” in tons. Or if “B” was grams, “A” would be kilograms. Other graphs in the file have pretty standard scales, with each “square” having a height of 10 (of whatever unit). In the above, the visible squares are really twice as high, so each horizontal line corresponds to 20 “units.” 

Here is the raw data, expressed just as “A” and “B”, with the scale differences applied, based on my pixel readings of the graph (so there is some error there). I am reading each “date” value as the rightmost value of the date range, as that seems to be where the person graphing them made changes to the slope:

Date	Curve A	Curve B
Dec-44	1,397.8	0.0
Jan-45	2,580.6	0.3
Feb-45	5,161.3	0.5
Mar-45	7,311.8	0.9
Apr-45	15,053.8	1.6
May-45	21,828.0	8.5
Jun-45	30,537.6	13.7
Jul-45	49,462.4	20.2
Aug-45	70,000.0	32.8
Sep-45	97,096.8	44.5
Oct-45	120,215.1	64.6
Nov-45	145,161.3	86.1
Dec-45	164,946.2	102.0
Jan-46	180,860.2	121.8
Feb-46	197,311.8	136.7
Mar-46	221,075.3	151.3
Apr-46	235,914.0	167.3
May-46	249,784.9	184.1
Jun-46	258,064.5	198.7
Jul-46	271,505.4	211.8
Aug-46	283,440.9	226.6
Sep-46	294,408.6	236.3
Oct-46	308,172.0	246.1
Nov-46	320,000.0	255.7
Dec-46	330,752.7	269.1
Jan-47	346,451.6	278.9
Feb-47	356,774.2	287.3
Mar-47	371,828.0	295.9
Apr-47	383,010.8	302.0

My pixel counts are based on an assumption of scale that translates to 1 pixel being about 1.2 of the units of value, which gives some sense of the error bars of this kind of eyeballing.

I have graphed the above data here (without the 1000X scale applied, but the 2000X area is plotted as 2X):

Which lets us think about what “A” and “B” most likely correspond to. Some observations:

• The percentage of “A” that “B” is changes over the course of the graph. Starting in May 1945, “B” is some 40% to 80% of “A,” not taking into account the scale differences (0.04% to 0.08% if you do take them into account).
• You could also read “B” as possibly being offset in time by “A”: whatever (unscaled) value of “A” repeats in “B” some several months later. The fact that “A” seems to be repeated (at 1000X less scale) in “B” after 30-50 days seems meaningful.
• Both numbers only go up, and never go down, which suggests to me that they must be cumulative, since the actual rates of plutonium production fluctuated a bit over this time period. 
• Curve A starts in December 1944, and Curve B starts potentially in January 1945 (but its value may be 0), definitely in February 1945. 

So my guess, based on other of plutonium production records kept at the time, is that Curve “A” is related to tons of irradiated uranium processed, and Curve “B” is kilograms of final “product” (separated plutonium) isolated.

We have some data points we can compare these assumptions to to check them. Another document in the Hanford database (says that in early 1947, Hanford shipped 9.2 kg of plutonium in January, 8.6 kg in February, 9.6 kg in March, and 8.6 kg in April.¹ Our numbers for Curve B above, when rendered into a “rate of change” per month for those months in 1947, come out with 9.8, 8.4, 8.6, and 6.2 — all close-enough look plausible for Curve B.

Los Alamos produced several inventories of their plutonium in 1945 that have been at least partially declassified. I have one of these; the Center for Disease Control released a graph of 20 datapoints of plutonium as part of their Los Alamos Historical Document Retrieval and Assessment (LAHDRA) Project in 2010:²

They don’t provide the raw data (boo!) but using my pixel-picking method I basically came up with this:

Date	Plutonium (kg)
4/18/45	0.7
4/25/45	0.7
5/2/45	1.1
5/9/45	1.6
5/16/45	3.1
5/23/45	4.5
5/30/45	6.4
6/6/45	7.4
6/13/45	8.8
6/20/45	10.1
6/27/45	12.5
7/4/45	13.8
7/11/45	16.2
7/18/45	16.2
7/25/45	17.1
8/1/45	18.7
8/8/45	22.1
8/15/45	24.3
8/22/45	27.6
8/29/45	27.6

Which seems about right. (The report I have, for 8/29/1945, has the total as 27.5 kg.)

If we put that graph up along with our graph for Curve B, we get this:

Which is interesting. They both have basically the same slopes, but Curve B is definitely higher than the LAHDRA curve. 

I have basically two hypotheses about this. One is that Curve B is the mass of plutonium in a different form, like plutonium nitrate, that might be heavier than the isolated pure plutonium metal that Los Alamos might have been using. I’ve poked around with this a bit, but I’m not really that convinced that is the issue.

The other, which I currently favor, is that we’re really just seeing the difference between plutonium separated at Hanford (Curve B) and the time it took to send it to Los Alamos. So Curve B has a value of about 1.6 on 4/30/45, and the LAHDRA data has 1.6 on 5/9/1945. There are other such “correspondences”; in general, there is what looks like ~10 day delay between the two curves. A section in the MDH’s “Top Secret Appendix” says that the normal schedule was for, once a week, Hanford to pack “all the cans available for shipment” into wooden boxes and ship them by truck to Los Alamos in a modest convoy. The convoy itself took 36 hours to reach New Mexico. So 8-9 days or so. At which point it would need to be processed and then added to the Los Alamos inventory. So those numbers more or less add up.

All of which makes me think that “Curve B” is indeed plutonium produced at Hanford — which is not necessarily the same thing as the amount of plutonium that was on hand and ready to use Los Alamos.

So if that is correct, we can generalize Curve B’s data for plutonium produced by Hanford:

Date	Plutonium (~kg)	Rate (kg/Pu/mo.)	FM equiv.
Dec-44	0.0		0.0
Jan-45	0.3	0.3	0.1
Feb-45	0.5	0.2	0.1
Mar-45	0.9	0.3	0.1
Apr-45	1.6	0.8	0.3
May-45	8.5	6.9	1.4
Jun-45	13.7	5.2	2.2
Jul-45	20.2	6.6	3.3
Aug-45	32.8	12.6	5.3
Sep-45	44.5	11.7	7.4
Oct-45	64.6	20.1	10.4
Nov-45	86.1	21.5	13.9
Dec-45	102.0	15.9	16.5
Jan-46	121.8	19.8	22.0
Feb-46	136.7	14.8	24.0
Mar-46	151.3	14.6	25.2
Apr-46	167.3	16.0	27.0
May-46	184.1	16.8	29.7
Jun-46	198.7	14.6	32.0
Jul-46	211.8	13.1	34.2
Aug-46	226.6	14.7	36.5
Sep-46	236.3	9.8	38.1
Oct-46	246.1	9.8	39.7
Nov-46	255.7	9.6	41.2
Dec-46	269.1	13.4	43.4
Jan-47	278.9	9.8	45.0
Feb-47	287.3	8.4	46.3
Mar-47	295.9	8.6	47.7
Apr-47	302.0	6.1	48.7

This data is much less accurate than the uranium graph, because I am eyeballing the graph values for each month. But it looks about right to me. The plutonium (kg) column is the eyeballed data — as you can see, it is not really very precise. The “rate” is the change in those values change per month. And “FM equiv.” is just assuming that each core for a Christy-type Fat Man bomb is 6.2 kg. It should be kept in mind for thinking about stockpiles that a) the US “expended” ~12 kg of plutonium in the July and August detonations, as well as another ~12 kg in the tests of Operation Crossroads in mid-1946, and b) that any stockpile assumptions are complicated by the fact that after World War II, the US was also developing composite cores, as mentioned before, which did not use the same amounts of plutonium as the original “Fat Man” bombs.

One thing that jumps out from all of the data above is how different the production rates were between the uranium and plutonium methods. The uranium method just got better and better, in terms of quantity and quality (enrichment level). But the plutonium production rates bounce around a lot more. This is unsurprising, knowing the history — they had severe problems at Hanford with the reactors in the postwar, and even shut down B Reactor in March 1946 so as to preserve its longevity. 

Putting them together

We can somewhat haphazardly combine our two datasets now into one master graph for fissile material production by the Manhattan Engineer District:

I have thrown out the plutonium production for early 1947, just because that was under the Atomic Energy Commission, and because we don’t have the enriched uranium data to go alongside it. We can see two things here. One is that when plotted against the same axis, the dramatic differences in production rates are really apparent. One can readily see why the postwar Manhattan District, and the early AEC, took the issue of plutonium scarcity and uranium abundance so seriously, and why they were very invested in developing composite cores that could allow them to “stretch” their much more constrained plutonium supply further. 

What a lot of effort to reconstruct a rather unsurprising (but still interesting) graph of data from some 80 years ago, we might observe. I cannot see how any of the above could have compromised national security in any way. But I will further note that all of the above was accomplished on the basis of records that the US government actually already judged, in the past, as being declassifiable. So if somehow it did compromise national security in some nebulous way, the fault is on the redactors in either 1977 or 1991 who decided — I think quite correctly — that it was innocuous data. 

1. “SF Materials Shipped or Received from 12/31/1946 thru 05/01/1947,” (May 12, 1947), Hanford DDRS access number D8265435. See also this cover letter for a little more context.
2. Center for Disease Control, Final report of the Los Alamos Historical Document Retrieval and Assessment (LAHDRA) Project (November 2010), chapter 4, page 5.

So basically you are shown a 3x3 window of chess pieces and have to guess where on the board they are in a chess game. Some friends and I came up with it recently and called it GeoChessr and I created it for real on geochessr.io submitted by /u/_dinkelhuber_ to r/chessvariants [link] [comments]

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