Nuclear Nuances

Credible Deterrent Through Testing

By P K IYENGAR, August 2000

(The author is former chairman of the Atomic Energy Commission)

AFTER a long period of indecision and ambivalence regarding its nuclear preparedness, India detonated five nuclear devices in May 1998. Consequently, it declared itself a nuclear weapons country. However, it is unlikely that we will be accepted as a weapons country under the Comprehensive Test Ban Treaty (CTBT), because there is no provision for threshold states maturing to become nuclear powers.

When Prime Minister Atal Bihari Vajpayee visits the US next month, the CTBT will certainly be one of the important issues raised by the Americans. There may be political arguments both for and against signing the CTBT, but if we have decided to follow a policy of nuclear deterrence, which will require weaponisation, then, scientifically, we have no option but to continue testing. It is the scientific case that I wish to make here.

If one goes by the numbers for the total nuclear yield put out by the Department of Atomic Energy, which I see no reason to dispute, the yield of the thermonuclear device detonated on May 11, 1998 was around 40 kilotons. This is a rather low yield. If the yield was deliberately kept low to restrict damage to the nearby villages, then surely it would have been more sensible to test the thermonuclear device separately, and not along with the 15 kt fission device. Now, the thermonuclear device itself consisted of two parts: the fission trigger and the fusion core.

The crucial question is not what the total yield of the device was, but what was the ratio of fission energy to fusion energy? Clearly, for a given total yield, the greater the fraction of the fusion energy, the more efficient is your thermonuclear device. In my opinion, that ratio musts have been around 1:1, and no one has so far, to my knowledge either publicly or privately, disputed that number. Therefore, by my estimate, the fusion yield could not have been more than 20 kt. Further, it seems likely that a fission `spark-plug' was used at the centre of the fusion core, in which case the actual fusion yield would have been even less.

Sticking to the larger number of a 20 kt fusion yield, one can easily calculate that the amount of LiD fusion material needed would be only around 400 grams or around 500 cc. This is a very small size for the fusion core, and the actual core used must certainly have been much larger. This suggests that the fusion core burnt only partially, perhaps less than 10 per cent. This can easily be checked; if the burn was only partial, there should have been a lot of tritium produced, which should have been detected after the explosions.

In such a complex system as a two-stage thermonuclear device, getting any burn at all is a credit to the abilities of the scientists and engineers of the Bhabha Atomic Research Centre (BARC). However, a thermonuclear device that only burns partially is certainly inefficient. Logically and scientifically, the next step would be to improve the design of the device to achieve greater efficiency. This is particularly important from the point of view of a weaponisation programme.

The government has declared a policy of maintaining a minimum nuclear deterrent. Nuclear deterrence means that we have a demonstrable nuclear capability that deters a potential adversary from attacking us. For us to have a nuclear deterrent we must weaponise. For this, we must have fusion weapons, because these are smaller, lighter, and more efficient than fission weapons. But for that deterrent to be viable, we must master all aspects of thermonuclear weapons, and demonstrate that expertise not just in one, but many thermonuclear designs, particularly those of greater efficiency.

Whether that should include a neutron bomb or not, is not of the essence. In a neutron bomb, one establishes a thermonuclear burn by igniting only a part of the core, and making the burn propagate. This is the crux of the matter in designing an efficient thermonuclear device. One may not have a neutron bomb in one's arsenal, but it would strengthen our abilities if we successfully tested one.

Some people argue that we have benchmarked our computer simulations using the data from the Pokhran tests, and, therefore, further weapons can be designed based on those computer simulations. We should note that we have conducted only one thermonuclear test, and that too of low yield. It is, as mentioned before, likely that this device burnt only partially. Devices that are more efficient will have to be built. In order to weaponise, we will need missile-mountable devices, which will have a different geometry. They will also have to be of higher yield. Then these will have to be made compact, and integrated with delivery as well as command and control systems. Can our nuclear deterrent be credible if we go through this long process of weaponisation without a single additional test? The bottom line is that we just cannot hand over to the army, or deter potential aggressors with, weapons based on computer simulations.

It is unscientific to embark on a long programme of weaponisation, and develop elaborate plans for maintaining a credible nuclear deterrent, all based on just one, low yield, thermonuclear test. When we do not do this for the Agni or Prithvi missiles, why would we want to take this risk for nuclear weapons? I am sure that the BARC scientists themselves, like their DRDO counterparts, would prefer to take a more conservative approach and test further to refine their designs and their capabilities. This is the scientific way. It would be wrong for the government to pressure the scientists to put a premature end to nuclear tests, for political expediency.

In principle, India accepts nuclear disarmament, and hopes its problems will be solved if all countries accept non-discriminatory, global, nuclear disarmament. Yet, this is unlikely to happen, from what we see around us as well as in the `N5' (five nuclear weapons) countries. In spite of long and friendly discussions with the US, we haven't come to any concrete decision relating to a new status under the CTBT. If we are to maintain our independence in today's world, it is essential for us to have a credible nuclear deterrent, and this requires us to continue testing.