Unfortunately, yes, this is intentional. I actually wrote a program (see the source code folder titled 'recipe_calc') to compute the recipe costs for me since they were so complex. The reason LMRs take so much uranium to build is because LMRs in real life take so much fissile material to online. Once they are online they are far more efficient and this is modeled in game both in the refueling costs and the overall efficiency of building the things.
https://www.osti.gov/biblio/6198515 is where I got the core load numbers for a LMR. It was a research reactor, but still. Go to page 24 in the PDF and look at table 5. Add up all the numbers (~764Kg) {Take care to realize they are in scientific notation, so you need to factor in all of the exponents!}, then multiply by 6 (~4.68 metric tons). Math below:
Factor Exponent Mass Per 1/6th core
9.37 -4 0.000937
1.34 0 1.34
3.78 -3 0.00378
6.87 2 687
2.96 -1 0.296
3.65 -1 0.365
2.29 -6 2.29E-06
5.47 1 54.7
1.74 1 17.4
2 0 2
9.14 -1 0.914
3.81 -2 0.0381
0 0
0 0
1.07 -3 0.00107
4.78 -4 0.000478
2.08 -2 0.0208
4.49 -3 0.00449
1.64 -3 0.00164
2.49 -5 2.49E-05
4.51 -3 0.00451
4.22 -1 0.422
2.3 -7 2.3E-07
Sum 764.51283242
Total Core load 4587.07699452
That means a 900Mw Thermal (IE: heat output, not power output) needs a little over 4.5 metric tons of fissile material to turn on.
Ok, so how much does a LWR need? https://www.nuclear-power.net/nuclear-power-plant/nuclear-fuel/fuel-consumption-of-conventional-reactor/ shows that a 1000 MwE (electircal output!) LWR has around 100 tons of fissile material in the core. It takes 3000 Mw Thermal to get that 1000 MwE. So we divide 100 by 3 and get ~33.33 tons (not metric) of fissile material per core. Converting that to metric tons gives us 33.8 metric tons of fissile material per 1000 Mw Thermal. Around...hm. 10x the fuel? Why? Ohhh. I know why. I think...I think I made a compromise:
Making the player mine a full fuel load for a LWR was just too brutal. You normally need nuclear power by the time you unlock it, so I compromised and only made you mine a single fuel bundle, instead of the whole core. When it came to a LMR, I made you mine the whole core, since it was 1/10th as much as a real LWR would take, and because by that point, you'd have the infrastructure to do so.
All this said, note the MASSIVE energy in a single LMR fuel rod. It's 5.85 PJ. PetaJoules. As in 5000 Terrajoules, or 5 million gigajoules! Insanity!
And yea, in my own games, I just rush tier 3. MSRs are so far ahead of LMRs that I personally don't see the point in building LMRs. If you really need a 1 GWe reactor for some reason, or you have a use for a reactor with a fuel rod that will last around 1 year IRL, then it's great. Otherwise, just rush MSRs (tier 3).
