Estimating the size of India and Pakistan's inventory of separated weapon-grade plutonium and highly enriched uranium (HEU) has become more difficult following their nuclear tests in May, 1998. Both countries treat these numbers as highly classified, partly because such estimates provide a direct indication of the number of nuclear weapons they possess.
In the case of Pakistan, what can be surmised is that it has resumed full-scale production of HEU, following a declared moratorium on such production since 1991. Abdul Q. Khan, the father of Pakistan's uranium enrichment program, announced soon after the Pakistani tests that his country had never stopped making HEU. Although his comment has been greeted skeptically, it indicates that Pakistan may have resumed making weapon-grade uranium well before its tests in May, 1998.
Exactly how long before the tests Pakistan resumed making weapon-grade uranium is not too important in these calculations. During its moratorium, Pakistan produced low enriched uranium (LEU), which can be up-graded to weapon-grade uranium relatively rapidly. Given the length of the moratorium, this stock of LEU was relatively large and would have enabled a rapid increase in Pakistan's stock of weapon-grade uranium.
In addition, in April, 1998, Pakistan announced that the Khushab reactor had started operating. However, as of mid-1999, there were few reports whether the reactor was operating consistently or if its irradiated fuel had been processed into separated plutonium. This reactor, in theory, could make enough plutonium for a few nuclear weapons per year.
With regard to India, information suggests that it is trying to increase the size of its stock of weapon-grade plutonium and perhaps HEU. There has been some discussion of building a new plutonium production reactor. India may be considering using its civil power reactors to increase its stock of weapon-grade plutonium. In addition, the debate about whether one of the Indian tests used reactor-grade plutonium is unresolved, leaving unanswered whether India considers its civil unsafeguarded power reactors and plutonium stock, at least potentially, to be part of its nuclear weapons program.
Faced with a swirling mixture of solid and ambiguous information, a new analytical approach was used to estimate India's stock of weapon-grade plutonium and Pakistan's stock of weapon-grade uranium. This new approach specifically aims to capture the varying and conflicting information about key parameters affecting estimates of the size of the inventories. Rather than decide on a best estimate of a specific parameter, such as lifetime reactor operating capacity, a distribution of possible values is derived. In this way, various choices can be assigned a probability of being true.
US officials, for example, have recommended strongly that India's Cirus and Dhruva plutonium production reactors have a lifetime capacity factor of about 40 percent. Indian officials have stated that the average capacity factor is significantly greater, as large as 60 to 70 percent. In this estimate, the most likely choice is selected as 40 percent with values up to 60 to 70 percent having a diminished probability of occurring. On the other end, a lifetime capacity factor less than 30 percent is viewed as highly unlikely. In the case of Pakistan's total enrichment capacity at the Kahuta and the newer Gadwal facilities, a wide range of possible values is given equal probability of occurring.
Using Crystal BallTM software, these distributions are sampled using a "Monte Carlo" approach to derive a distribution of results. This method varies from previous approaches used by the author, where central or best estimates are derived and an uncertainty is attached by making a judgement about the overall data and information. Although judgements are still necessary in any uncertainty analysis, they can be applied more credibly in this more advanced method.
India's inventory of weapon-grade plutonium is derived by estimating total production in its reactors and by subtracting drawdowns from nuclear testing, processing losses, and civil uses of the weapon-grade plutonium. The median value, which is the value midway between the smallest and largest value, is about 290 kilograms of weapon-grade plutonium at the end of 1998. The range in the values can be understood by considering the set of all values, which in this case, vary between 160 kilograms and 460 kilograms. Because values in the tails of the range carry a very low probability of being true, often only the values that fall between the 10th and 90th percentile are considered, which in this case are 240 kilograms and 355 kilograms, respectively. To be more certain that the actual value lies in the range, the 5th and 95th percentiles can be selected, which are 225 kilograms and 370 kilograms, respectively. One way to interpret the results is that, in the latter case, there is 90 percent certainty that the true value lies between 225 and 370 kilograms of weapon-grade plutonium, where the median value is about 290 kilograms (see table 1).
Pakistan's inventory of weapon-grade uranium is derived by estimating several factors, including total enrichment capacity and the feedstock into the enrichment plant, and by subtracting drawdowns. An important factor in this calculation is that in 1998, LEU, widely reported to have been produced during the HEU moratorium from 1991 to 1998, was up-graded in the enrichment plants to weapon-grade uranium. This significantly increases Pakistan's inventory of weapon-grade uranium. However, this rate of growth in the inventory cannot be sustained, because Pakistan would have run out of LEU in 1999. Table 1 (above) shows the results for the end of 1998.
Each country has additional stocks of plutonium and HEU. As mentioned above, Pakistan has started a reactor that can make weapon-grade plutonium. The amount that is separated cannot be estimated, although it is relatively small as of the end of 1998. Similarly, India operates a small enrichment plant that can, in theory, make HEU, although the amount produced so far is unknown. In any case, this amount is believed to be relatively small.
Estimates can also be made of the amount of reactor-grade plutonium in India and Pakistan. These estimates, which are taken from ISIS's Plutonium Watch, are in table 2.Their uncertainty is judged to be about 20 percent. Although almost all of this civil plutonium is in spent fuel, and thus not suitable for use in nuclear weapons, a nuclear weapons equivalent is calculated to facilitate comparisons.
1 A version of this issue brief is published in Joseph Cirincione, ed. Repairing the Regime: Preventing the Spread of Weapons of Mass Destruction, (New York, NY: Routledge and the Carnegie Endowment for International Peace, 2000) (forthcoming).
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