Production of Plutonium in Reactors

The production of plutonium is accomplished most effectively if the reactor has a high conversion coefficient. This means the use of heavy water or graphite as the moderator. These are better moderators than light water for plutonium production, because their cross sections for neutron capture are low and their relatively slow moderation rates gives more time for neutron capture in 238U (see Section 8.3.2). Most weapons programs have obtained their plutonium from graphite-moderated reactors; the U.S. program used both graphite-moderated reactors (at Hanford in Washington) and heavy water moderated reactors (at Savannah River in South Carolina).

However, plutonium production is unavoidable in any reactor that uses 238U as a fertile fuel. Thus, any reactor is a potential source of plutonium for weapons, even if ostensibly being used for other purposes. For production of weapons-grade plutonium in a commercial reactor, standard fuel would have to be removed after a burnup period that is much shorter than normal. But even plutonium from fuel with the normal high burnup can be used for weapons, as discussed earlier. Alternatively, special uranium "targets" can be placed at selected locations in or near the reactor where they are irradiated by the neutron flux, producing weapons-grade plutonium.

A summary of plutonium production capabilities of different reactors (without special uranium targets) is given in Table 17.3, based on data from Ref. [16]. The indicated annual production for commercial operation corresponds to a 1-GWe reactor operating at a capacity factor of 100%. At a more attainable capacity factor of 80%, in the neighborhood of 300-600 kg of plutonium would be produced annually, depending on the type of reactor, corresponding to a production rate of about 0.3-0.6 kg of plutonium per GWd(t).23 The lower numbers pertain to LWRs, the dominant reactors in the world today. Of course, the product of commercial operation is normally reactor-grade plutonium, which is not ideally suited to bomb production. The critical mass for an explosive device using reactor-grade plutonium is under 10 kg (see Section 17.4.1). Thus, each year of normal operation of a 1-GWe LWR produces enough plutonium for 30 or more small nuclear bombs.

For reliable weapons, it is desirable to have a low 240Pu fraction (i.e., weapons-grade plutonium). In principle, this could be accomplished by iso-topic enrichment of plutonium that has been chemically extracted from the spent fuel. A simpler approach is to reduce the burnup of the fuel.24 A light

23 In Table 17.3, the graphite reactor is fueled with natural uranium and the LWR with enriched uranium, giving the former a lower burnup, less destruction of plutonium, and a higher plutonium output for a given energy output. (The HWR fuel was not specified in the data used for Table 17.3, but probably also was natural uranium.)

24 In addition to giving a higher percentage of 239 Pu in the plutonium, low burnup gives a higher plutonium output per unit energy generated than in a normal fuel cycle, because there is less destruction of 239Pu by fission or neutron capture.

Table 17.3. Plutonium production rates for reactors operating in a commercial mode and for production of weapons-grade plutonium.

Commercial Operation

Production of WG Plutonium

Moderator

Burnup

Prod. Rate

Annual Prod."

Burnup

Prod. Rate

Annual Prod."

Material

(GWd/t)

[kg/GWd(t)]

(kg/GWe)

(GWd/t)

[kg/GWd(t)]

(kg/MWt)

Light water6

30

0.29

330

1

0.5

0.18

Heavy water

7.5

0.50

630

1

0.9

0.33

Graphite

4

0.63

815

0.2

0.95

0.35

Graphite

1

0.86

0.31

"Calculated for capacities of 1 G We (commercial) and 1 MWt (plutonium production), with an assumed 100% capacity factor.

bThese numbers are approximate weighted averages of values reported separately in Ref. [16] for commercial PWRs and BWRs.

Source: Derived from data in Ref. [16, pp. 462-3 and 473], with burnup rates and thermal efficiencies used there.

water reactor at low burnup could be used for this purpose, but it is more efficient to use a graphite or heavy water reactor, as seen in Table 17.3.25

The precise amount and isotopic purity of the plutonium obtained from a dedicated reactor depends on the capacity factor achieved, the average bur-nup, and the location of the fuel in the reactor.26 For a graphite-moderated reactor, the output approaches 1 kg of plutonium per gigawatt-day thermal [GWd(t)] or 1 g of plutonium per MWd(t). Thus, even a small graphite-moderated reactor can in a few years produce enough weapons-grade plutonium for a modest nuclear arsenal, at a rate of about 1 bomb per 5000 MWd(t) of operation. To achieve "super" and "weapons" grades of plutonium, as defined in Table 17.2, the burnups should be about 0.3 and 0.9 GWd/t, respectively [16, p. 463].

Some rough rules-of-thumb are useful in evaluating the potential of dedicated graphite- or heavy water-moderated reactors:

♦ About 1 g of plutonium is produced per megawatt-day (thermal).

♦ About 0.25 kg of plutonium is produced annually per megawatt of thermal capacity (at an 80% capacity factor). (Thus, a 20-MWt reactor has a production potential of about 1 bomb per year.)

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  • ilmari
    How is plutonium produced in nuclear reactors?
    7 months ago
  • Christian
    What is the best reactor for producing plutonium?
    6 months ago
  • neftalem
    Why heavy water reactors are best for nuclear bombs?
    5 months ago
  • diamond
    Does a nueclear reactor with enriched uranium produce plutonium?
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