Based on this evidence the most plausible interpretation of the 11 May tests is that the fission bomb was successful and yielded 12 kt. The straightforward nature of this technology and the lengthy time India had to master it makes doubts about its performance difficult to support. The yield of the thermonuclear device was apparently 22-25 kt. This is at or above the expected yield range of a thermonuclear primary, so it appears that the fusion boosted device used for the primary was successful. The plausible yield for the secondary stage is from 10 to 15 kt, well below the 30 kt or so apparently expected by India. The 0.3 kt yield for the third device is appropriate for the unboosted yield of a boosted fission design - that is, a device that is fired without the deuterium-tritium boost gas. It suggests that India now as the ability to employ full yield fusion boosted weapons using less-than-weapon grade plutonium. This could be fuel grade plutonium as is produced by India's power reactors or perhaps an intermediate grade. In the first case all of India's considerable holdings of plutonium could be used for weapon manufacture, in the second case the stock of weapon grade material could be extended by mixing it with fuel grade plutonium.
The reasons given for the second group of tests are entirely credible and are consistent with the objectives of other nuclear weapons nations in recent testing activities (either nuclear tests prior to the CTBT signing, or sub-critical tests afterward). India's stated interest in gathering high quality physical data for use in simulations is quite consistent with a well planned weapon development effort.
The use of simultaneous detonations of multiple nuclear devices has been a routine practice by other nuclear weapons states. The principal purposes of this technique are to minimize the ability of other nations to collect intelligence data about the tests (ironically too successful, given India's subsequent and not entirely convincing efforts to publicize its test results), and to reduce the cost and difficulty of test preparations. In India's case the desire to minimize political repercussions and outside pressure also motivated a compressed schedule of simultaneous tests.
Corroboration of a sort for India's explanations for the different tests on 11 May was given immediately after they were fired, and four days before the official AEC/DRDO announcements, by commentary on their military significance offered by P.K. Iyengar, the former chairman of India's AEC. On 12 May he told Reuters in Mumbai (Bombay) that their differing sizes corresponded to three ways in which nuclear bombs might be used.
According to Iyengar, the smallest was the size that might be fired as an artillery shell or dropped from a combat support aircraft. The mid-size blast was from a standard fission device equivalent to about 12 kilotons -- the size that might be dropped from a bomber plane. The largest of the three warheads tests on Monday was not a full hydrogen bomb. Most of its 50 to 100 kiloton explosive force came from the primary, a fission device which serves as a trigger for the H-bomb's big fusion explosion. Iyengar said the device contained only a token amount of the hydrogen variant tritium. It showed that India's thermonuclear technology worked, but did not produce the megaton explosion typical of a full H-bomb.
"We need not go for a megaton explosion while testing an H-bomb," said Iyengar, one of the scientists involved in India's only other nuclear test, in 1974. "Such tests are required only if we are planning for a total destruction of the opposite side. They don't have relevance in our strategy."
One technical issue that affects all three of the low yield tests, with yields of 0.2, 0.3, and 0.5 kilotons, is that the yields of low yield fission tests are very sensitive to physical parameters like the amount of fissile material present, the degree and uniformity of compression, and the nuclear properties of the materials. Even if the Indian designers have achieved very accurate control over the implosion process, without test data in this yield range to calibrate their models they would have difficulty predicting the exact yield of their devices. This of course is one reason for conducting these tests in the first place, but it also means that there is a good chance of overshooting or undershooting the target yields. Thus the intended yields of these devices may be significantly different than the yields produced, on the other hand if the advertised yields are the expected ones, then the actual ones may be significantly different.