Ten years ago today, India claims to have completed Operation Shakti by detonating two more nuclear devices. It said that device number four was a 0.5 kiloton low-yield device, and that device number five was a 0.3 kiloton experimental device. These tests are the most controversial in India's test cycle since they could not be detected by any seismic stations outside India. In 2001, Carey Sublette wrote that:
... no outside seismic station has reported any evidence of tests on this day. The closest CTBT monitoring station was in Nilore, Pakistan 740 km to the north of Pokhran. This station is estimated to have sufficient sensitivity to detect a test at least down to 25 tons in Pokhran, yet no signal was detected. Wallace estimated that with standard seismic coupling, the test would have had to total no more than 10-15 tons to escape all detection. Assuming very poor coupling (and India's description of one test being conducted in a dry sand dune lends some support to this) allows yields up to perhaps 100 tons to escape detection, still far smaller than those claimed. The seismic signals detected for Pokhran-I and Shakti I-III provide a benchmark for evaluating the detectability of small tests. Even if India's claims of 12 kt and 43 kt are accepted for the two tests (thus indicating that seismic signals from this region are weaker than expected), the seismic signal of 800 tons of yield would be easily detectable unless extreme decoupling of both devices is assumed. Given the reported haste in shaft construction, and the unsophisticated shaft construction techniques used by the India Army, the use of deliberate decoupling measures seems ruled out.
A defence was written by Rajagopala Chidambaram in an May 2002 article entitled 'The May 1998 Pokhran Tests: Scientific Aspects'. There, he presents a gamma spectrum of a sample allegedly taken from the Shakti V shaft. He claims that the inability of the monitoring station in Pakistan to record any seismic signature on 13 May is because 'the threshold limit for seismic detection is much higher in, say, a sand medium than in hard rock: the Pokhran geological medium comes somewhere in between'. Indeed, he is implicitly questioning the very feasibility of seismic detectability of low yield tests.
The confirmation would come through acquiring radionuclide data from the test. However, despite an initial report of 'radiation sickness' at the neighbouring town no fallout seems to have been picked up at India's borders. If airborne sampling was carried out (and one has to assume that India's borders were packed with whatever capability the superpowers could hurry there), it wasn't reported. It is also not known how distinguishable the later samples would be from the 11 May Shakti test series.
There are three possible explanations why India's 13 May 1998 test was not detected by seismometers abroad.
- There was no test. Shakti-IV and V never happened;
- The test was a fizzle. Both charges failed to detonate, or produced a very low yield (well below the announced 0.8 kilotons); or
- Shakti-IV and V was an successful attempt at achiving marginal criticality.
Most analysts seem to lean towards the second explanation, rather than seeing the 13 May 1998 incident as a failure for verification. Would the same event happen today, more CTBT monitoring stations would have been deployed (including noble gas monitoring stations). With an increasingly potent verification regime, violations are bound to be detected.
And to be fair, it did detect fairly low-yield detonations coming from left field, even in its first year of operation.
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