Shaurya - the curious fact is that if you look at all the reports of all previous nuclear tests before the 1998 tests - nobody gave a damn about crater size. In the days of atmospheric tests yields were measured by fireball size and by atmospheric sampling. When they went underground yields were measured by seismology after setting a standard for the rocks of the area, confirmed by radiochemistry and also CORRTEX to a lesser extent.
In fact the use of seismology to detect global tests became a whole new field after India's 1974 test and the CTBT issues were brought in to discourage new entrants.
It is only after the Indian 1998 tests that a great hoohah was made about crater size and yield. It has never been used to seriously estimate yields until the Indian tests were done. he only other time it was studies was when the US did a series of tests precisely to test for cratering. I think this was th plowshare series of tests. If the USSR has done some studies - there is no public info on the net in English.
Crater size and yield is mostly mumbo jumbo because if you bury a bomb deep enough you get no crater. If it is too shallow it blows out and produces a enormous crater. At depths between these two extremes what happens is that a cavity is formed underground and that cavity starts caving in from above. Sometimes the caving in stops after a bit, but if the caving in continues with collapse right up to the surface you get a crater that is approximately the size of your original cavity. The size of your original cavity depends on yield and the strength/quality of the rock surrounding the device. There are reports (that I need to find) that state that the 1974 POK 1 test was nearly a blowout. I think PK Iyengar said "We were lucky" that it did not blow out. I will check and post here
After the US Baneberry test leaked radioactivity they made sure that all their tests were so deep that nothing leaked out. The leakage of material was avoided not just because of radioactivity - but because leaked material tells you about the bomb design. Why do you think a U2 was flying over Chagai in 1998? A lot of studies were done after the leaked Baneberry test about depth of burial and the BARC team have mentioned all that in their claim that they buried the device deep enough to avoid all venting.
Remember that if you have a design for a 200 kt thermonuclear weapon, can you really scale it down to something less?
Apparently you can, in the following ways (as per internet sources)
1) Use a non fissile tamper for the secondary (no fission after fusion)
2) remove the secondary altogether (no fusion)
3) remove the boosting of the primary (fission alone)
A fourth method would be to fiddle with the amount of LiD - but that is my guesswork.
I think the BARC team designed a 45 kt device out of pre existing components and this was done at short notice to suit a pre existing, hastily deepened shaft. Nobody in the world deliberately designs 45 kt TN devices and what was done was probably a fiddling with the components of a 200 kt or 300 kt design (figures as mentioned by Bharat Karnad). Perhaps they removed some LiD and replaced it with sand or Lonavla chikki
Fiddling with designs is a formula for failure especially if the design is not already tested. I am willing to believe that the exact yield of the TN device may not have been 45 kt as stated, but there is absolutely no information to prove that fusion did not occur and the 100 meter deep information is not credible because a 100 meter depth is too shallow for 45 kt. You have to use the "
BARC people are fundamentally stupid" argument to say a 45 kt device was tested at 100 meters. I will try and come up with the calculations for this using available data despite my mathematical disability.
But, am not sure it tells us anything, as some other numbers are way off.
