New Delhi, Sept. 4: Whether India needs to conduct more nuclear tests hinges on the kind of nuclear arsenal that the country would like to possess — more tests could help scientists bolster the reliability and sophistication of weapons.

But scientists believe a class of tests that they call sub-critical tests — pursued by both the US and Russia for more than a decade — can also help maintain the reliability of a nuclear weapons arsenal without nuclear explosions.

After the five nuclear tests in May 1998 had lifted the veil off India’s nuclear weapons programme, scientists involved in weapons design had indicated that India did not need any more such tests.

One senior defence scientist had said that the programme had even successfully integrated nuclear weapons with “vectors” — the vehicles that would deliver the bombs — presumably aircraft and missiles.

Three of the five explosions in 1998 had tested kiloton devices, two fission (atomic) bombs and one fusion (hydrogen) bomb. The two others were smaller sub-kiloton devices used in battlefield situations. Scientists had said that the tests successfully demonstrated Indian mastery over nuclear weapons and that they had developed computer software to simulate nuclear explosions.

However, some analysts point out, the software that drives computer simulations can itself be improved by testing it against the very reality that it is intended to simulate — in this case, nuclear explosions. Fresh tests might allow weapons designers to perfect yields of weapons and enhance reliability through iterations in design.

But nuclear weapons studies may also be conducted without nuclear explosions.

Sub-critical tests provide a strategy to ensure the safety and reliability of a nuclear weapons arsenal without actually conducting nuclear explosions. In sub-critical tests, conventional chemical explosives are detonated along with weapons-grade plutonium, but without the critical mass so that there is no self-sustaining chain reaction and there is no explosion.

The Los Alamos Laboratory and the Lawrence Livermore National Laboratory in the US have been involved in conducting sub-critical nuclear tests since 1997. Russia has also conducted sub-critical tests at a site near the Arctic Circle.