The nuclear tests by India and Pakistan last month focused attention on the accuracy and reliability of the means of detecting them, which are critical to the Comprehensive Test Ban Treaty (CTBT).
There are worries that the CTBT could be circumvented by countries performing tests which go undetected.
These arose because three of the Indian tests went undetected, while there remains doubt over the number of tests performed by Pakistan. And, because a number of tests were simultaneous, there were doubts and confusions over yields (the strength of the explosions).
The Indian government gave the go-ahead for its five explosions, code-named Shakti (‘power’), 30 days before the first three blasts on 11 May, and carried out two more on 13 May. Pakistan followed within a month with, it claimed, five tests on 28 May and one more on 30 May.
There was no question about the CTBT monitoring regime’s ability to detect the first three Indian tests, at India’s Pokhran military test range in Rajasthan, even though only one big seismic signal was detected as they were carried out simultaneously.
Indian nuclear scientists said Shakti 1 was a 12-kiloton (12,000 tonnes of TNT-equivalent) fission device possibly a modernised weapon based on the ‘peaceful nuclear device’ India tested in 1974.
Shakti 2 was a 43-kiloton ‘thermo- nuclear’ device. US analysts suggest this was in fact a boosted fission bomb, in which an atom bomb’s power is boosted by a light element such as tritium, which India now produces. Shakti 3 was a 0.2-kiloton low-yield device which was apparently a weapon test, of a yield suitable for a nuclear artillery shell or small nuclear bomb. But this was undetected because it was masked by the other two.
R. Chidambaram, chairman of India’s Department of Atomic Energy, said the tests involved ‘weaponising’ the 1974 device.
At first foreign observers believed India had carried out just one test with a yield in the 30 60 kiloton range. Once India’s claims were set against the known information and evidence the scepticism diminished.
But the seismic data for the second pair of shots on 13 May was more problematic: there was none. The closest CTBT monitoring station was in Nilore in Pakistan, some 700km to the north of Pokhran. This is estimated to have enough sensitivity to detect a 0.025-kiloton test at Pokhran, yet preliminary reports say no signal was detected.
The Indian government said Shakti 4 and 5 had yields of 0.5 and 0.3 kilotons, and were conducted to generate additional data for improved computer simulation of weapon designs. Yet their yields were too low to register on automatic seismic monitors.
Roger Clark, a geophysicist at Leeds University’s School of Earth Sciences, says the recorded data from India’s 13 May tests was not consistent with the claimed yields. Seismic readings of the 11 May tests had provided comparative data on which to base expectations of what the claimed yields would have registered.
This suggests the tests may have been no larger than 60 100 tons of TNT-equivalent, Clark says.
Why? One explanation is that the yields of such small fission tests depend on the amount of fissile material used, the strength and uniformity of the compression of the fissile material used by the device’s implosion arrangement, and the isotopic specifics of the fissile material.
Even if the Indian designers achieved accurate control over the implosion process, without test data in this yield range to calibrate their models they would have problems predicting exact yields. So the intended yields may have been significantly different from their actual yields.
It is also possible that one or more of these low-yield tests were experiments with deuterium-tritium gas boosting of fission bombs. This boosting technology is used by all established nuclear weapons states to provide light, small and efficient atom bomb first stages or ‘primaries’ for classic two-stage H-bombs. India might be developing a modern thermonuclear weapon using such boosted primaries.
For the CTBT, discussion of yields is of only academic, and military, interest. Trevor Findlay, director of the Verification Technology Information Centre (Vertic) in London, which monitors the observance of arms control treaties, says: ‘The key issue for the CTBT regime is not the yield of a test, but its detection. If an event other than the normal or otherwise explicable can be detected, this means the CTBT will have been violated.’
Of the intended 120 total of CTBT seismological monitoring stations, only about 40 are running, but enough were operating to be sure seismologically anomalous events took place at the time of India’s first tests.
In the case of Pakistan’s five claimed tests on 28 May, only a weak seismic signal led observers to believe that, at most, two or three devices were tested.
One explanation is that the smaller tests may have been subcritical ones in which the fissile material was replaced by inert material.
Findlay says it is wrong to say the CTBT was ‘violated’ by the Indian and Pakistani tests, as neither country has signed the treaty.
It is only recognised and observed by its signatories. Article 14 prevents the treaty’s entry into force until it has been ratified by the 44 nations with nuclear reactors, including India, which has said it will not sign the treaty. Under international law, until the treaty has entered into force, countries that violate it cannot be said to have broken its obligations.
Because India and Pakistan refused to sign, the Geneva-based Conference on Disarmament has convened a Preparatory Commission next year to discuss how the CTBT can enter into force. The US attitude may be constrained by internal political wrangles.
On 21 January, Senator Jesse Helms, the chairman of the Senate foreign relations committee which has the power to block US ratification of the CTBT, warned President Bill Clinton that trying to seek ratification was a ‘deliberate confrontation’ and ‘would be exceedingly unwise because the CTBT is very low on the committee’s list of priorities. The treaty has no chance of entering into force for a decade or more’.
Influential Republican senator Pete Domenici and others have warned that even if the US ratifies the CTBT there is a provision which allows nations to test nuclear weapons if they believe their ‘national security’ is threatened. Even though China says it will abide by its own moratorium, this is one provision which it has reminded the world of in recent days.
India’s tests have indeed opened a Pandora’s box.