There are reasons for the persistent suspicion that the four Chernobyl units could produce either electricity or plutonium (in the latter case producing some electricity inefficiently from the waste heat, too).
A (production) reactor producing plutonium works under very different conditions from a (power) reactor producing electricity. The fuel rods of both include uranium 238, which is not fissile, but can be bred into plutonium. Some plutonium does form even in the rods of a power reactor, though only in modest amounts and in a form that is practically useless for weapons material, as explained below. One of the essential points distinguishing a production reactor from a power reactor is this: to make weapons-grade plutonium (239), the U 238 must be irradiated by neutrons at high intensity for a short time, or other isotopes of plutonium will form and contaminate it to the point of uselessness, which is exactly what happens in a power reactor (as again explained below).
Moreover, maintaining the chain reaction of splitting U 235 in a power reactor requires a different optimum neutron velocity than the optimum velocity for converting U 238 to plutonium. The neutrons shooting out of the split nuclei are too fast to split further nuclei, and they must be slowed down ("moderated") first. This is done by making them collide with the atoms of a moderator, such as the hydrogen atoms of water in the case of a power reactor. But breeding plutonium needs a higher neutron velocity, and this can be obtained by using a moderator with heavier atoms
¾such as carbon in its graphite form. Clearly a tennis ball will bounce off a concrete wall faster than off another flying tennis ball (to which it will give up much of its energy, i.e. velocity).That is why in a mature nuclear industry water (actually, its hydrogen atoms) is used as a moderator in power reactors; graphite is used for breeding weapons-grade plutonium, as in the N-reactor in Hanford, Wash.
But of course the neutrons don't bounce off the moderator nuclei at one exact velocity; there is a whole spectrum of velocities (albeit with a maximum dependent on the moderator) and if one is not prissy about efficiency, that spectrum is wide enough to cover both power and breeding. The first reactor ever designed (by Fermi, Chicago, 1943) was a graphite reactor, and the early power reactors, descended from production reactors, were graphite-moderated, too. In fact, the Chernobyl reactor is a direct descendant of a Soviet production reactor (as described in more detail by B. G. Levy in the Amer. J. Phys., July 1986).
Was the Chernobyl reactor, then, a case of getting stuck with an old technology? Definitely not: The Soviets were producing, and even exporting, pressurized-water reactors by the time Chernobyl Unit 1 went on line (1978); the now crippled Unit 4 went on line only last year. The only other explanation is that it was a dual-purpose reactor, designed to produce either weapons-grade plutonium by short, high-intensity neutron irradiation, or electric power by steady, long-term burnup of the fuel.
Nor was Chernobyl, though a recently completed plant, on the list of power reactors which the USSR, with great fanfare, opened to international inspection by the IAEA last year
¾belatedly and rather absurdly (since the USSR is an undisputed weapons state, anyway). What did they have to hide at Chernobyl?|
Vol. 14, No. 3
Newsletter: Access to Energy Newsletter Archive Volume: Issues Issue/No.: Vol. 14, No. 3 Date: November 29, 2004 05:01 PM Title: The roots of their power
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