Stopping the U-234: Hitler’s Atom Bomb Submarine

2100 tons submerged is correct. About 1700 tons surfaced. For comparison, the two boats that I was on, SSN 671 and SSBN 655, displaced about 5000 tons and 7250 tons respectively.
It doesn't seem to be mentioned here, but that boat is a Type IX-B, the so-called "Milchkuh" or "Milk Cow" that was used primarily as a resupply vessel due to their large size and unwieldiness in defensive maneuvers. The U-505, which can be seen at the Chicago Museum of Science (if you're brave enough to go there) was of the same type. However, don't expect to learn anything from the tour guides. I still vividly remember ours referring to Allied anti-submarine aircraft (in general) as "jet fighters." I corrected her a couple of times and then gave up.
The Monsun Gruppe (Monsoon Group) was the German U-boat effort in Asian waters.
A few ended up in Japanese hands either by transfer or being taken over when Germany fell.
The Japanese also seized some Italian subs in Singapore when Italy surrendered and subsequently turned them over to the Germans.These became part of a transport scheme code named "MerKator" which never came to fruition with the subs finally being taken over by Japan when Germany surrendered then ending up as war prizes and being sunk by the US Navy after the war.
Displacement was more like 2100 tons submerged not 20,000. That would have been one enormous u-boat!

Thanks for letting us know about this! I reached out the author, and he had this to say:

"As a total land-lubber the author apologizes for being too aggressive on the trigger with the "0" key! Displacement for the Type X Submarine Minelayer is listed as 1,735 tons (surfaced) and 2,143 tons (submerged). Based on my meat-bread-cheese diet, that is very close to my own displacement! "
Interesting. I have to admit I mis-identified this boat, having never seen a picture of the Type X. I also have to admit my relative ignorance of them. Probably because they weren't intended as anti-surface ship combat boats, but of course that's no excuse. Thanks for the clarification, Mike.
New member / submarine vet / reactor operator here. This was a very interesting story; I've long been interested in what Germany was trying to get out at the end of the war, and to where.

I can tell you that Germany did not place a high priority on its atomic developments. German scientists and engineers did in fact attempt to assemble a working reactor at Haigerloch right at the end of the war, but due to some basic physics assumptions on their part being wrong the reactor could not possibly have been made critical. All of the components were found after the war and their effort was judged years away from any success, be it a working reactor or (further on) a weapon. Since they did not know what a critical assembly looked like (physically and operationally) they could not have even begun to consider what a weapon core would look like.

(We had a working reactor in 1942, and I am fortunate to own a tiny slice of the graphite from it, encased in a lucite paperweight given to me as a gift some years ago.)

What the Japanese would have done with the blocks found on the submarine is questionable. I will add that the blocks from the Haigerloch reactor were dispersed far and wide to US laboratories and universities and they still surprise people by turning up now and again, after being forgotten in a locked storage box or something for years.