FACTS ABOUT NUCLEAR WEAPONS

6. Nuclear tests

• Why are nuclear tests necessary ?
• What are the different types of tests ?
• How can underground nuclear tests be monitored ?

Why are nuclear tests necessary ?

In order to design a nuclear weapon, it is necessary to mathematically model the processes that lead to the explosion. The device is then simulated on a computer. In fact, a large amount of computing is required. Several assumptions go into the modelling and one cannot be absolutely sure how accurate the model is without actually testing it.

The more complicated the design, the less reliable the modelling and the more the testing required. Thus the simple ``gun assembly'' design (used in the Hiroshima bomb) does not require testing. It is generally felt that an implosion device needs to be tested (as was done for the Nagasaki bomb).

A thermonuclear weapon is significantly more complicated and requires more testing than a fission device.

What are the different types of tests ?

Apart from a full scale test of the weapon, several other tests are done to check different aspects of the device and the mathematical modelling.

1.

Hydrodynamic tests : These are tests which do not use fissile material (an inert material with similar properties is substituted). These tests could, for example, be used to determine how well the conventional explosives in an implosion design work. Such tests are not banned by the CTBT (Section 8).

2.

Hydronuclear tests : In these tests actual fissile material is used, but the nuclear yield is kept low. The yield could vary from a few kg to a few tons. These tests are banned by the CTBT.

3.

Sub-critical tests : These are tests where actual fissile material is used, but the nuclear yield is zero (the material never attains criticality, see Section 9). These tests are also not banned by the CTBT.

How can underground nuclear tests be monitored ?

An underground nuclear explosion shakes the ground as much as an earthquake does. Thus the shock produced by the explosions causes vibrations of the earth which can be detected far away by seismic stations which monitor earthquakes. The signal produced by a nuclear explosion can be clearly distinguished from that produced by earthquakes.

There is now a network of seismic stations all over the world which are constantly monitoring the vibrations of the ground. Thus any underground explosion of a significant strength will get detected. The yield of the explosion can also be estimated from the signal but this measurement is not very accurate.