Chernobyl

by Frederik Pohl

  "Who can say?" smiled the Chief Engineer. "A very bad storm? An earthquake? Or" — he frowned meaningfully at them—"a sudden nuclear attack from our enemies, perhaps."

  "Ah," said the shift chief, enlightened. "Of course. But there is still a question in my mind. Why don't we simply shut down the reactor instead of trying to lower the output?"

  "Because," said the Chief Engineer severely, "we must be quite sure. We will do this test a number of times, keeping careful record of the results each time. It is a matter of safety, after all — and we can't be too careful in a matter of the safety of the Chernobyl Power Plant!"

  Kalychenko groaned silently. A number of times! They would be at this all night! — and, likely enough, well into the Saturday morning shift, too, the way things were going. With resignation he bent to his work.

  The normal night shift in the control room was only half a dozen men, just a skeleton crew to keep things going. There was not much need for electrical power in the late night hours in the Soviet Union. Good Soviet citizens went to bed at night so they could rise, bright-eyed and refreshed, for the next morning's work.

  Tonight was different. Besides Kalychenko's own crew, there were four men left over from the late evening shift, looking oppressed at being kept on overtime for which they were not likely to be paid, plus the observers, the Chief Engineer, and the Personnel man, Khrenov.

  To lower the power on a reactor like the RBMK is not like turning down the gain on a radio set. To shut it off entirely is much easier. You simply thrust home all the boron rods, two hundred and eleven of them, piercing the graphite core from top and bottom and in all its parts. The element boron is poisonous to nuclear reactions. Boron soaks up neutrons; they cannot go on to make another atom fission, and so the reaction stops; that is the easy way.

  To slow the nuclear reactor down is another matter entirely. There are three separate ways to do it. First, for a rough approximation, you shove a few additional rods into the core. Not too many; you don't want the reaction to die. (Once the reactor stops waste products begin to accumulate — the element xenon is the worst of them, since it is a worse poison to nuclear reactions even than boron. Then it is impossible to start again until weeks have passed and the xenon has decayed away.)

  Then there is a certain measure of fine control that can be attained by varying the mixture of gases in the sealed space surrounding the core. Some of the gases soak up neutrons in the same way that boron does, though not as strongly; to slow the reaction a bit, you simply add more of those gases to the mix.

  Finally, there is water. The water that flows up through the core to turn to the steam that drives the turbines also has the neutron-absorbing characteristic — as long as it is water. Once it has turned into steam, which is less dense, it soaks up fewer neutrons, and thus the nuclear reaction picks up speed. This condition is called a "positive void coefficient," a technical term which means only that the more steam there is in the tubes the faster the reaction will go. This also means that the faster the reaction goes, the more steam will be generated— consequently adding to the "voids" — consequently adding to the speed of the reaction — consequently adding to the steam…. It is a delicate balance to keep a reactor, any reactor, poised between dying and running away, and so controlling a power reactor is a constant dance of rods and pumps.

  When things were going well, Kalychenko enjoyed his part in the dance. Most of it was automatic, anyway. There were heat sensors all through the reactor core. The optimum running temperature of the one hundred eighty tons of uranium fuel was hundreds of degrees hotter than the ignition temperature of the graphite slabs. Graphite is carbon. Carbon burns. But it couldn't burn without oxygen, and oxygen was carefully excluded from the mix of gases in the surrounding jacket. If the temperature of the reactor climbed too high or fell too low, there would be a signal from the expensive imported Western instruments that monitored it. Then the operator would engage the motors that thrust a few rods farther in or took them a bit less deep. If it climbed drastically high, the operator would not be involved at all; automatic pumps would rush floods of new cold water into the core to cool it down.

  That could not happen this night, because the automatic system had been turned off hours before, but then, no one ever wanted to let things get so far that the automatics were tripped anyway.

  Another thing no operator wanted — at least, Kalychenko certainly didn't want it! — was to try to lower the temperature slowly. That was a sweaty business, because at low power levels the RBMK was notoriously hard to control. The trouble was that it was so big. The temperature sensors could not be everywhere. One part of the core could be at exactly the temperature desired, while another, an arm's length away, could be soaring to dangerous levels without warning. So Kalychenko did sweat, and swore under his breath, because the bitch was obstinately rising and falling, down to ten percent power, then up to thirty, slowly down again as they inched a few rods back in — then almost dying on them, down to the range where xenon began to form, until they had withdrawn all but six of the rods entirely and were coaxing it back to life.

  When Kalychenko took his eyes off the board long enough to glance at a clock it was only one a.m.! He wasn't sleepy any more. He was simply exhausted. Only one, and he had worked harder than he usually did in a full shift. And everyone else was on edge too.

  Even the GehBeh, Khrenov, had lost his warm, hooded look. Just behind where Kalychenko sat at his board, Khrenov was quarreling softly with the Chief Engineer. "What is the matter, Varazin?" he demanded. "Can't you control this thing? Must I find Smin and bring him here?"

  Varazin flushed, glancing at the observers. "I am Chief Engineer, not Smin," he whispered fiercely.

  "And I am responsible for Personnel. Perhaps I have been deficient in my duties. It may be that I have not screened this plant's personnel with sufficient care."

  Varazin flinched, but said sturdily enough, "If you have complaints in that respect, Comrade Khrenov, there will certainly be time to discuss the matter. This is not the time. May I remind you that I am in charge here?"

  Khrenov looked at him thoughtfully for a moment, then gave a long sigh. He turned to the observers with the smile back on his face. "What a pity," he said genially, "that this operation should take so long. Since most of you are, after all, more interested in the turbines and the steam generation than in the nuclear aspects of the operation, perhaps we should take a look at some of the other systems?"

  "Can we take a look at something to drink?" one of the visitors grinned.

  "We can do our best. Let me see, it's one o'clock. If we come back, say, at two, I think things will be in order. Don't you think so, Comrade Varazin?"

  "I hope so," said Varazin.

  At least with Khrenov gone everyone breathed a little more freely, but the job didn't get easier. It got worse. With great difficulty they managed to stabilize the power output of Reactor No. 4 at 200 megawatts electric, a fifth of its normal capacity. Kalychenko called out the reading and reached for the switch that would maintain that level. "Shall I engage the automatic systems?" he asked, finger poised.

  "Certainly not," snapped Varazin, looking frayed. "It is far too high. Cool the reactor a bit."

  "There are six pumps already going," the shift chief reported.

  "Engage a seventh!"

  Kalychenko marked the time when the seventh pump was cut in, three minutes after one. And indeed the temperature of the core began to respond; it was not the cooling of the water that made it happen, but the added liquid water in the system absorbing a few more neutrons.

  The atmosphere in the control room was excited now, with the engineers and operators calling the numbers back and forth to each other, like spectators at a football game. Even old Varazin was shifting from one foot to another as he watched the readouts with them, and Kalychenko began to think about what all this meant. If this experiment succeeded, it could well be a model for every nuclear power plant in the Soviet Union. There would b
e commendations, perhaps cash awards — perhaps they would be written up in Literaturna Ukraina, even in Pravdal Well, no, he cautioned himself, that was not likely; this sort of thing one did not advertise in the open press, since the West had no business knowing what went on in critical Soviet industries. But it would be in the records! Even Khrenov would not fail to list all the people who had contributed to such a success somewhere in his file folders. .

  "It is still too high," Varazin announced. "Add another pump!"

  It was seven minutes after one. And all of a sudden, without transition, Kalychenko's bright mood vanished. He began to worry.

  The first indications of trouble were the pressure readings in the water system. "Pressure is dropping in the drying drum," reported one of the engineers.

  The shift chief glanced at Varazin, who said impatiently, "Yes, of course. Carry on." But he looked nervous too. With two extra pumps forcing water into the system, the steam generation had slowed; there was more water coming in than the core could boil into vapor at once, and so in the great drum, where the steam was extracted to feed into the turbines and the remaining water pumped back into the circulation system, pressure had begun to fall. Paradoxically, that meant more steam there, as the water that had been squeezed liquid found room to expand. Kalychenko listened and thought he could hear, in the distant throb of the pumps, a laboring sound as they tried to pump vapor instead of liquid water.

  Then the state printout computer flashed a warning: Reactor should be shut down at once.

  "Chief Engineer Varazin!" Kalychenko cried. The old man was looking strained now, but he said:

  "Yes, of course. We are operating under unusual conditions, which the program is not designed for."

  "Then shall I—"

  "Certainly not!" said Varazin, biting his lip. "Comrade Khrenov and our guests will be back at two, and I don't want to have a dead reactor for them." He glanced at the clock. It was twenty minutes after one. "Close the stop control valve," he ordered.

  Kalychenko looked at the shift chief for confirmation before he obeyed, but the man only nodded. His face was pale. Reluctantly Kalychenko switched the stop control valve off; it was the last of the automatic safety features. ..

  Then it all went sour.

  "Temperature's rising!" screamed the shift chief. And everyone stared at the thermal readings — from seven percent of normal power, already at fifteen.. twenty… in ten seconds it went to a full fifty percent of normal power. And in Kalychenko's mind, as he gazed awestruck at what was happening, there flashed a picture of the interior of the reactor core, with each of the 1,661 tubes filled with water. . only the pressure was dropping. . and the water turned prematurely into steam, steam that was not dense enough to soak up neutrons, that let the reaction pick up speed. .

  There was a distant thud.

  "What was that?" Varazin cried, and then in the same breath: "Insert rods! Fifty percent rods, immediately!"

  But the rod operator was reporting that the control rod motors were not responding; the rods would not penetrate the core. "Emergency shutdown then! At once!" Varazin shouted, and held his breath.

  But the rods would not go in. "Something is blocking them!" the rod controller shouted, his voice shaking. Kalychenko heard the words incredulously, for that was impossible! There was nothing to block the rods in their sockets — why, it would mean that the interior of the reactor itself had suddenly become warped, or shrunken, or broken—

  The next explosion was much louder. The walls shook. Dust sprang out from the walls, hanging like a sudden shimmer of ice fog in the air. The lights went out — all of them, even the lighted meters and dials on the full-wall instrument board.

  "Oh," moaned Varazin, "my God."

  "Emergency circuits!" cried the shift chief, and the man next to Kalychenko, muttering oaths, reached for the switch.

  At least then the instrument lights went on again, but what they said was insane. Temperature readings simply off the scale, radiation levels that could not be believed. And the noise did not stop with the explosion. There was a rumbling thunder of walls going down, a patter of something hard falling on the roof, a crackle that could only be flame.

  "Go and see what has happened to the reactor," ordered Varazin.

  It was at least an instruction to follow. Most of the men in the main control room jumped up to comply. Even Kalychenko rose from his useless board, but as he started through the door he caromed off one of the other hurrying men, who swore and thrust him out of the way. Kalychenko fell heavily. By the time he got up, most of the men had rushed out to peer down at the reactor chamber.

  Kalychenko's arm hurt where he had fallen on it. He hesitated, rubbing the arm, then turned and went the other way. It was unquestionably a cowardly act. It also saved his life.

  Chapter 6

  Saturday, April 26

  There is a difference between the nuclear reactions in a power plant — even a plant with a "positive void coefficient" — and an atomic bomb. The difference lies mainly in the fuel. Power-plant uranium is slightly enriched with the touchy isotope, U-235. Bomb uranium is very much so. This governs the speed of the reaction in which one fissioning atom releases a neutron, which strikes another atom and causes it to fission, and so on in the familiar "chain reaction." The links of this chain happen very fast in either case. In a bomb, there can be a hundred million successive links in a single second. In a power plant, only about ten thousand. For a human operator the difference doesn't really matter much, because he can't react quickly enough to intervene in either case. But within the core it is the difference between a nuclear accident and a bomb blast. If the core of Reactor No. 4 had been of weapons-grade uranium, the nuclear reaction would have gone on to involve far more of the fissionable material before the force of the explosion had time to blow it away. Since it was not, the nuclear explosion "blew itself out." Its kinetic force scattered its own fuel elements, and in the process destroyed only part of one building instead of an entire city. The later consequences, however, were of course another story.

  In that first moment the shift engineer, Bohdan Kalychenko, had saved his life by running away from the reactor. On the perimeter of the plant, the hydrologist-engineer, Leonid Sheranchuk, saved his by running toward it. When he saw the great fireworks display blossom terribly overhead, he stood transfixed. Flaming debris rained down on everything, on the ground, on the buildings, on the man with the bicycle, on the man on foot twenty meters away, even on the roof of the ambulance that was slowly turning around to return to the scene of the explosion. A huge chunk of something the size of a football fell only meters away; it blazed blue, and he could feel the heat of it. Graphite? Could it be graphite? From the core of the reactor itself? He couldn't tell; really, if that were the case, he didn't want to know. But none of the debris fell on Sheranchuk.

  At first he was shielded by the guard's cabin. Then he ran for the nearest entry to the plant — not because he reasoned out that that was the right thing to do, but because the plant was in mortal peril and he could not do anything else — and it happened to be the door to the section of the building that contained the main control room for Reactor No. 4, on the far side from the blazing, spitting inferno that had been the reactor itself, with the whole turbine hall between.

  Even as he entered he heard the clanging alarm that ordered evacuation. But that was wrong! Sheranchuk knew instantly that it was wrong; you didn't run away from a nuclear plant because there was an accident; you had to do whatever you could, whatever that might be, to keep the accident from becoming terribly worse.

  "Stop!" he yelled, trying to bar the door with his body, but someone roughly pushed him aside and someone else stumbled past to the red-lit outside. "No, wait!" he cried. "What are you doing? Go back to your stations! You can't leave the plant untended!"

  Some swore at him, some did not hear. Some he seized by the shoulders and turned around by brute force. There were too many for him — shift operators, maintenance wo
rkers, radiation monitors, two older men he thought were observers from another plant — he even caught a glimpse of two men, wrangling as they trotted away along another corridor, that looked like Khrenov and Chief Engineer Varazin.

  Then the alarm bell stopped in mid-clang. From outside, almost drowned in the hideous crackle and crash of the burning reactor building, Sheranchuk could hear the lesser sirens of the plant's fire brigade racing to the disaster point. "Do you hear?" he yelled. "The firemen are coming! Help them, get back to your work, make sure the other reactors are safe!" And then, abandoning the effort, he pushed past the dazed ones and hurried through choking smoke and alarming sounds of crash and rumble to the stairs. He was hardly aware of the long climb, and when he reached the control room for Reactor No. 4, he could not believe his eyes. Below the window, the entire turbine room was in flames. The top of the reactor building was simply gone. He could not see the burning core itself— that saved his eyes, as well as his life — but there were fires everywhere, everywhere, and the world had without warning come to an end.

  What went wrong at 1:23 a.m. on that Saturday morning in Chernobyl occurred in four separate stages, but they followed so closely on each other that they were only seconds from beginning to end.

  First, there was the power surge in one little corner of the vast graphite and uranium core. Although the reactor had been throttled back almost to extinction, a small section went critical; that was the atomic explosion.

  The second stage was steam. The nuclear blast blew the caps off the 1,661 steam tubes. All of them blew out at once, and the broken tubes of water were exposed to naked, violently hot fuel material. The water squeezed under sixty-five atmospheres of pressure was suddenly under no pressure at all. It flashed into steam, and the steam explosion shattered the containment vessel. At that point the disaster was completely out of control and everything that followed was inevitable.

  The next explosion was chemical. The terrible heat and pressure caused the steam from the ruptured pipes to break down into its gaseous elements, hydrogen and oxygen; the zirconium in which the steel pipes were clad helped the process along as a catalyst. That produced a hydrogen-oxygen explosion, the powerful reaction that drives spacecraft into orbit. The wreckage of the immense steel and concrete containment box was hurled into the air. The refueling floor, just above the reactor, was tossed aside, along with the forty-ton crane that transported the fuel rods. Fiercely radioactive material was thrown in all directions. Anything nearby that could burn was ignited. Major fires began on the tarred roofs of the building complex, and that was the third stage.