DEBATE

The question of nuclear yield

BARC scientists present the proof.

In the wake of the five nuclear tests at Pokhran on May 11 and 13, 1998, some questions were raised about their yield levels. Some Western seismologists said that the total yield of the three explosions on May 11 was between 10 and 15 kilotonnes, while t hat of the two tests on May 13 was 100 to 150 tonnes. Then doubts were raised about whether one of the three devices exploded on May 11 was really a thermonuclear device, or a hydrogen bomb as it is called. In response, Dr. R. Chidambaram, Chairman, Atom ic Energy Commission (AEC), and Dr. Anil Kakodkar, Director, Bhabha Atomic Research Centre (BARC), asserted that the total yield of the five explosions was around 60 kilotonnes. (Frontline, June 9, 1998 and January 15, 1999). Dr. Chidambaram and B ARC scientists also asserted that it was a thermonuclear device that India had exploded. In October 1999, questions were raised about whether the two sub-kilotonne tests on May 13 had fizzled out, that is, whether the explosion had occurred at all. The New York Times wrote in October 1999: "Had India faked explosive tests? Were they f lops? Or had small blasts eluded the eavesdroppers?... Though opponents of the treaty point to the Indian claim as a test ban embarrassment, the emerging consensus among nuclear experts is that what failed that day was not global monitoring but the pair of explosive devices." In January 1999, in an article titled "Nuclear Energy in India," Dr. Chidambaram stated that the sub-kilotonne devices, whose yield is less than one kilotonne (1,000 tonnes), were also fission devices. But designing a sub-kilotonne device was more diffic ult than designing a standard fission device. "In a sub-kilotonne device... where one goes marginally super-critical, one cannot afford to make a mistake. In the case of a mistake, one may have a fizzle. In the case of the May 1998 tests, all the three s ub-kilotonne tests gave a perfect match between the calculated and the measured yields, which is important. In case one signs the CTBT, one cannot carry out tests, which release any nuclear yield. If one can predict accurately the yield of a device whose yield is only a few hundred tonnes, one can also guarantee the design of an experiment where the fissile material in its optimum configuration will go close to criticality and still stay sub-critical. Thus, our sub-kilotonnes tests have also given us a capability to carry out sub-critical tests, if we consider them necessary. We have, however, no plans at the moment to carry out sub-critical tests..." In order to sustain an informed debate on the subject, Frontline reproduces here an article written by scientists of the Bhabha Atomic Research Centre, Trombay, in BARC Newsletter, September 1999. The authors of the article are R. B. Attarde, V.K. Shukla, D.A.R. Babu, V.V. Kulkarni and Anil Kakodkar.

THE five nuclear tests conducted during May 11-13, 1998 at Pokhran included three sub-kilotonne devices, in addition to a thermonuclear device and a standard fission device. One sub-kilotonne device was tested on May 11, while on May 13, two sub-kilotonn e devices were tested. This report gives some of the results of gamma radiation logging measurements in bore holes at the sites of the sub-kilotonne tests as well as the post-shot radioactivity measurements on the samples extracted from these sites.

Gamma radiation logging

Figure 1. The gamma dose rate at the test site of the 0.5 kilotonne device on May 13, 1998.

Gamma radiation logging was carried out in the bore holes drilled at each of the test sites. The equipment used for this purpose was developed at the Radiation Safety Systems Division, BARC. The equipment consisted of a measuring unit and a detector prob e unit that were coupled by a long cable wound on a cable winch. The main detector probe consisted of two energy-compensated Geiger-Mueller (GM) tubes. It covered six ranges, from 0-2 microGray to 0-200 milliGray. (The actual ranges were 0-2 microGray, 0 -20 microGray, 0-200 microGray, 0-2 milliGray, 0-20 milliGray, 0-200 milliGray.)

Figure 2. The gamma spectrum of a sample from the test of the 0.3 kilotonne device.

The measuring unit incorporates necessary high voltage supply for detectors, a count rate meter, scaling circuits and an audio circuit. All the readings were displayed by the linear 50 division meter. The audio facility was quite useful in monitoring the health of the instrument during logging. Before and after each logging, the instrument was checked with a test source. The instrument was initially calibrated at the BARC for all the ranges - at different points in each range - using various source stre ngths.

These measurements have shown the presence of gamma emitters at all the test sites. Figure 1 gives the variation of gamma dose along depth at the test site of the 0.5 kilotonne device.

Identification of radioactive species

Figure 3 shows the variation of ¹³⁷Cs activity with depth for the 0.2 kilotonne device.

The samples extracted from bore holes at all the test sites were assayed for radioactivity content at the Environmental Assessment Division of the BARC. The samples were powdered and dried. The homogenised samples were stored in plastic containers of 8 c m height and 7 cm diameter each. For the assessment of radionuclides, two high resolution gamma-ray detectors with efficiency levels of 20 per cent and 30 per cent with respect to a 7.5 cm x 7.5 cm thallium-activated sodium iodide detector (a versatile d etector for radiation) coupled with a multi-channel spectrometer consisting of 8,000 channels, were used. This detector is capable of giving a high-resolution spectrum. Europium-152 sources (as gamma radiation) were employed as standards for efficiency c alibrations of the detectors in the specified geometry.

Figure 2 gives a typical gamma spectre of a sample collected from the test site of the 0.3 kilotonne device. The spectra clearly show the presence of fission products such as Cesium-137, Zirconium-95 and Niobium-95. These isotopes are otherwise not pre sent in nature. Their presence in samples is a signature of nuclear fission.

Figure 3 shows the variation of ¹³⁷Cs activity with height, from the lowest point at the test site of the 0.2 kilotonne device.