Being a "leader" doesn't make you a nuclear expert.
I mean that they were educated since the early days (well depending on the family) to a certain role and had no time for fun and games like us. That is especially true for Tatsuya. Even if not all of them were educated to work as experts in radiaton, going back to the origin of magical technicians
they were actually
the experts in radiations. They started as those who were supposed to guard against the use of nuclear weapons and specialized often in removing or defending from radiation, thus they had necessary skills for example to make barriers like Juumonji family does. But also their new bodyguard girl who tries to copy his skill. Does that make her an expert in radiaton? No, of course not! But she doesn't have to be. It's enough for her to follow one's orders.
In reality it's not that difficult to become a specialist on radiation since all you need to do is work hard, have access to the necessary knowledge and study... study during all your free time and eventually you'll make it even as a kid.
Most of us can't do that, but Tatsuya... Tatsuya
and unlike John Wick he not only trained his brawns but also his brain. When story enters previous arc with the Thesis story you can see him working on
exactly this project. And he was at such a stage that he decided "yeah, sure, I can join your project senpai
(since I work on that anyway so might as well use your insight into building a prototype for myself).
So if I remember right, this arc will try to answer why he chose to do this, or at least will hint at why, so if you want me I can explain further but will probably have to put it in a spoiler since I have read LN long time ago and have no idea what was said before and what not.
I think what I said so far was without spoiling unless somebody skipped an arc.
How much red tape and inspections did they have to go through to build them?
Don't underestimate the capabilities of the top school of national education system that's said to produce a hundred or more important specialists a year from the most prominent aristocratic families of the nation. At least they do one thing right what we fail in real life: education gets proper care and funding... Even though they still skimp at not assigning teachers to half of the students, but if that is what's happening right now then will the principal just leave it be unfunded and wait a month before answering?
He's a smart guy and knows what has to be done and how to do it quickly. It's probably why he might have pretended to have no idea about the upcoming inspection while most likely being fully aware of the trap being set by the students (he still was absent to not discourage the inspection).
And let's be honest. What they did was
just get some a little bit of heavy water (yeah I know it may not be that easy to get this much
BUT!) aside from that everything else was normal. A container, something to fill in more water
(the machinery to move something from the workshop to where they are was already there since they often use that. Last time, in the previous Thesis arc, they used the same machinery to move the prototype and showcase it to everyone in the school since they can watch there more freely), few dozens of specialized CADs and that could be problematic, but since Tatsuya is connected to 7 Leaves technology (or how was called his company again?) then it's no stranger since in the past they made a prototype
in a single day just because he sent them blueprint as a simple hobby! No wonder they'd even work overnight and make it quickly if he told them there's no time to lose.
And also, as I said before: he already worked on it before so he had all what he wanted and just needed an excuse to start working on it, just like my project that I have all the puzzles but just need to connect it together and I watch anime and type comments on the internet instead 🙃 (jk, it's just a hobby too, lol).