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by Arkady BRISCH, Dr. Sc. (Technol), honorary scientific advisor, All-Russia Research Institute of Automation (named after N. Dukhov)
In August 1949 the Soviet Union detonated its first atomic bomb. Years and years of strenuous work of many scientists and technical experts preceded this epoch-making event. I happened to be involved in the job too. Here's what I remember.
The door to the atomic age was set ajar in 1938 as the German physicists Otto Hahn, Friedrich Strassman and Otto Frisch as well as their Austrian counterpart Lily Meisner discovered that uranium isotope nuclei split into two fragments of nearly identical mass and two neutrons. A large amount of energy was released thereby, with other atoms becoming involved in the fission process too. The chain reaction phenomenon! As to the chain reaction theory, it was evolved in 1939 and 1940 by Soviet scientists, future members of the Academy of Sciences Yakov Zeidovich and Yuli Khariton. In 1940 these physicists, together with Igor Kurchatov and Georgi Flyorov, filed a memo on the use of uranium energy in the chain reaction.
Before launching its atomic project in full swing, the Soviet Union had to found a complex of new industries, the uranium-mining industry among them. It had to build reactors and develop appropriate technologies. In short, it had to begin from scratch. Igor Kurchatov* was appointed scientific coordinator of this supertask, while Yuli Khariton was to concentrate on the designing of atomic bombs.
To some extent Soviet physicists made use of the information passed by the German scientist Klaus Fuchs in the mid-1940s on the first US bomb tested in July 1945. Yet, as many competent scientists believe, we gained only a year at the most in the manufacture of our first atomic bomb. Klaus Fuchs himself said later that his information helped avoid erroneous and deadline pathways in research. No more than that.
Meanwhile we started acting on this grandiose atomic project with much elan. In keeping with a government decision, a goal-oriented design office (KB-2) was organized at Sarov near the city of Nizhni Novgorod. It still exists today-now the Russian Federal Nuclear Research Center and All-Russia Research Institute of Experimental Physics. But first let me give you an outline of the first Soviet A-bomb, "the article", as we used to say.
This baby was pear-shaped and 127 cm long. Its maximum diameter was 325 cm, and total mass, 400 kg. The "working" plutonium core was within an uranium sphere the surface of which was boron- and aluminum-coated. Mounted above all that was a coating of explosive composed of 32 blocks with a total weight of 2,000 kg, each supplied with a detonator. Activated simultaneously (synchronization equal to a ten millionth fraction of a second), these units gave rise to a converging detonation wave that triggered a spherical shock wave. This one compressed the fissile material just within millionth fractions of a second to a density making an explosive nuclear reaction possible. But this was not enough for a real atomic blast. To have it, one should "inject" a beam of neutrons right into the core of the plutonium "ball".
Today these are quite trivial, routine problems. But we had to attack them from the word go then. It was all-important to us to know the dynamic compressibility of condensed substances at high pressure and temperature caused by powerful chock waves. In May 1947 an experimental range was commissioned at Sarov. Built on that ground were a few casemate, and in one of them an X-ray setup was installed for obtaining snapshots of a blast as it proceeds. Another casemate housed a photochronograph for light phenomena scanning and fixation.
We could build our first pulsed oscillograph for registering microsecond electric moment only in September 1947-we did it on a war trophy German tube. Actually none among our research workers had a record of work with explosives, let alone dealing with processes lasting but one millionth fraction of a second. We had to think up appropriate techniques and gadgets, and make explosive charges right in our laboratories with the use fume cupboards.
Another important problem we had to tackle was to design circuits for synchronous multipoint exploding of azide (lead/nitrogen compound) spark electrodetonators which, in turn, were to detonate spherical, hemispherical and cylindrical experimental charges.
Early in 1948 we began our experiments. There were a few cases of spontaneous, unplanned explosions. We found the cause-it was the very high sensitivity of conventional detonators. These were composed of two parts with an air gap in between and went off as an electric spark broke down the air gap. Soon after a bridge- circuit electrodetonator was designed. An air gap no longer separated its two parts, instead, there was a wire (bridge) which melted away when a current of desired characteristics was applied. The detonator thus became absolutely safe and made it possible to employ an exploding procedure of required synchronization.
Next, we had to obtain a converging detonation wave with desired characteristics. Here's what our computations showed: to compress plutonium to a required density, a pressure of 250 thousand atm. was needed, and this was possible only if the divergence rate of blast products from external charges was at least 2,000 m/s. The first stage of our work was not trouble-free either, and thus our proof-of-concept experiments did not come off.
Initially, experimental samples were made up of a homogeneous explosive, they were in the shape of a cylinder of definite length and diameter. The detonator was placed in the middle of one end face, and the sensor wherewith the divergence rate of blast products was measured was mounted on the other face. The detonation wave thus obtained was a diverging one, that is not what we wanted. It became possible to rem-
* See: N. Knyazkaya, "The Atom Project in the USSR: First Stage", Science in Russia, No. 5, 1997-Ed.
edy the situation after, on Yakov Zeidovich's suggestion, another charge (in the form of a lens) had been sealed into the primary charge. The added charge possessed different characteristics. Now we had to attain the required rate of propagation. A special measurement method was used to determine the propagation rate parameters. Mounted at definite distances from the sensor were two electric contacts connected to the registration instrument. As the charge went off, the shock wave caused the sensor to hurtle at the same rate of propagation, and it closed the first and then the second contact. This way we could determine the sought-for quantity.
Such measuring techniques, we hoped, should work. Yet data obtained in 1948 by another method at another laboratory put a damper on our hopes. So new experiments had to be carried out, with yours truly in charge. We decided to seal the sensor into the lens charge rather than place it on the end face. The whole contraption was put within a field of a powerful indestructible electromagnet. An electromagnetic field momentum within the sensor was cabled to an oscillograph cum register.
We had to upgrade this setup to have our experiments "clean" and get reliable results. A lighter metal, aluminum rather than copper, was employed for making the sensor; its dimensions, thickness in particular, had to be sturdy enough to resist a detonation wave. Besides, we applied X-ray monitoring to control the quality filling of the sensor with explosive.
New experiments showed that our brainchild behaved well. And in August 1949 the first Soviet-made atomic charge was tested with much success.
To gain time we manufactured the first A-bomb to the US design, though we had blueprints of our own too. The first Soviet A-bomb made to our design, from start to finish, was detonated in 1951. It was half the size of the American baby, but twice as powerful.
From that time on we used our designs only. In the course of our further work we zeroed in on upgrading the neutron generation source, on trigger. First it was composed of polonium and beryllium; placed into the center of the charge, the trigger was activated with the intermixing of Po and Be. That meant that every six months the bomb had to be taken apart and readjusted, because the Po half-life is 138 days. Yet another subtle point: an explosion attains its maximum yield when the plutonium core is compressed the greatest; and this occurs when the spherical shock wave, upon reaching the center of the core, rebounds to its periphery. It is at this instant that a neutron initiating burst (trigger) should be "injected", something that was
Quick overview of the charge: 1-sensor; 2-main charge; 3-lens charge; 4- azide detonator.
downright impossible with this particular design, for Po and Be intermixed only when the shock wave reached the center of the bomb. So, no maximum yield in this case!
The idea of an external trigger that could solve the above problems was suggested as early as 1948. Yet according to informed expert opinion, it was unrealistic to build such a neutron trigger adjusted to the dimensions and mass of an airborne bomb. All that notwithstanding, late in 1949 Yuli Khariton and Igor Kurchatov decided to fend for themselves in looking into this problem. They recruited a research team and invited me over as its head.
Our work proceeded apace, and already in 1950, using a new high-voltage initiating source, we obtained a neutron burst whose characteristics happened to be good for triggering an atomic blast. A year later we could manufacture an appropriate prototype (sealed tube). Besides, quite a new system of automation was needed both for initial detonation and for the activation of a neutron trigger, i.e. a miniature high- precision pulsed accelerator.
This merits special mention. Activation of electrodetonators involves very specific conditions. To generate electric pulses to trigger an explosive device, the following condition should be satisfied: the rate of current buildup should exceed a hundred billion A/s. Only a special system of automation can produce such kind of current. Among other things, this system should be capable of resisting significant mechanical loads, thousands of times as high as its proper weight; it should continue workable in a wide range of temperatures, have compact dimensions and mass, be safe against the penetrating action of a nuclear blast, and have a long period of guaranteed life expectancy. Last but not least, its security should be
Electromagnetic techniques for measuring the mass velocity of explosion products: 1-sensor; 2-charge; 3-powerful electromagnets.
ensured. Being always on the ready, such an automatic system should never issue an unsanctioned command and send explosion-initiating neutrons.
Enthusiasm works wonders indeed! In less than a year after our target setting in 1951, we defined an outline of the desired system and in yet another year we built an experimental specimen. After thorough, all-out checks, we carried out experiments imitating a detonation of an atomic charge. The first tests in field conditions took place in 1954 when two nuclear bombs were exploded. And the following year we conducted even more spectacular experiments by detonating three identical atomic charges with different neutron sources. The power yield obtained with the use of an external trigger (initiator) proved to be tenfold as high as that with an internal analog.
Seeing that we were on the right track, we started upgrading the system of detonation and neutron initiation. Ultimately this system came to satisfy even the most stringent operational requirements in weight and dimensions for new carriers, including artillery systems. And all that was thanks to an external source that supplied much more neutrons than an internal one and, what is most important, it did that at the right moment for obtaining a maximum explosion yield. This one is up to a significant increase if a tritium/deuterium gas is put into the center of the charge. Which we did. Let me stress that a weapon with such a trigger is far safer for the crews and simpler in employment.
Aside from the work with warheads, we also carried out large-scale nuclear tests. All told we exploded 715 atomic bombs, among them 146 group blasts (with as many as 400 different charges and devices employed). These tests were of much help for upgrading our nuclear weapons, for the identification of emergency regimes and hazards.
All in all we developed something like thirty types of detonating automatic systems adjusted to all atomic charges available at the time and to any test conditions at that. We developed various types of electrodetonators and initiators triggering neutron bursts of different intensity and duration.
Looking back, we see what a streak of good luck it was that Yuli Borisovich Khariton was put in charge of the atomic bomb project. An illustrious theoretician and experimentalist, he was well conversant with production technologies. It was thanks to him that an atomic bomb was developed actually from zilch within a record short time. More than that, there were no glitches and emergencies during the manufacture and testing of nuclear charges. And yet another sidelight: Yuli Khariton was highly remarkable for his creative and physical longevity.
Yuli Borisovich Khariton set much store by safety standards where nuclear weaponry was concerned. Nuclear tests should be safe for the servicing crews, he maintained. He was the first to formulate rigorous standards for nuclear charges maintenance so as to preclude disaster. Academician Khariton worked out security standards for the transportation of nuclear charges. He designed safety devices for what we call "weak" and "strong" elements (that is reliable and not so reliable ones), among the many others.
He thus contributed to our nuclear parity with the United States, a guarantee of world peace.
Interviewed by Arkady MALTSEV
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