At 11:10 P.M. they were able to resume the power-down. At midnight was the shift-change, and the Shift Foreman Yuri Tregub was replaced by Akimov. Toptunov replaced the Senior Reactor Control Engineer. The goal was to level out at 1,500 megawatts, but they had disabled the local automatic control system (LAL) for the test, and Toptunov was having trouble keeping the flux profile balanced as the power dropped. There were too many neutrons on one side of the reactor and too few on the other, and things were getting out of hand as the operators juggled the controls.
A graphite pile is a ponderous beast, and controlling it with no automatic assistance is like driving a concrete truck on the Monte Carlo racing circuit. All actions must be performed slowly, or it will turn over in a curve. The reactor power slid through 1,500 megawatts and kept going, down to 30 megawatts, very quickly. Toptunov had now steered Chernobyl No. 4 reactor into the dreaded “iodine valley,” from which there is no easy return.
What is an “iodine valley”? When a nuclear reactor produces power by fission, one of the many fission products is iodine-135. It is radioactive, and does a beta-minus decay into xenon-135 with a half-life of 9.10 hours. This is perfectly natural. The energy-releasing beta decay of iodine-135 is a small component of the delayed energy from fission. The product of this decay, Xe-135, is unique in that it has a monstrous thermal neutron-absorption cross section of 2.6 million barns. That is a reaction killer. If the Xe-135 builds up from I-135 decay, then it will snatch so many neutrons out of the normal fission transactions, it will shut down the reactor. Fortunately, one Xe-135 atom can only capture one neutron, activating into Xe-136, which is stable with a very low, 0.26 barn, capture cross section. Xe-135 also undergoes a beta-minus decay, becoming stable cesium-135, also with a low tendency to capture neutrons. A reactor running at high power both makes Xe-135, indirectly, and destroys it by providing excess neutrons to be captured. The neutron population reaches an equilibrium of I-135 production and Xe-135 destruction, and the reactor is able to remain in a critical condition and produce power at a steady rate.
If the power is reduced, then the equilibrium is disturbed. At the lower power level, there are fewer fissions per second and fewer neutrons produced per second, but the level of Xe-135 from the previous, higher power level remains. Recall, the production of Xe-135 from an I-135 breakdown is slow. It takes 6.57 hours for half of it to turn into xenon, so the production rate of Xe-135 continues on at the previously established level. There is now more Xe-135 than the reactor can knock down with surplus neutrons, because its power level has been reduced. More neutrons are captured than are produced, and the power level plummets. If the reactor has enough reactivity in reserve, held in check by the control rods, then the power can be stabilized at the desired level, and the now-excessive level of Xe-135 presence can be “burned off.” After a few hours, the control rods are slowly returned to their previous positions, holding the excess reactivity in reserve. If there is insufficient reserve reactivity, then the power level drops to zero. The reactor is stuck in the iodine valley, and it will take about 45 hours for enough Xe-135 to have beta-decayed away to allow the reactor to be restarted.
Dyatlov was enraged. He paced up and down the control panel, berating the operators, cursing, spitting, threatening, and waving his arms. He demanded that the power be brought back up to 1,500 megawatts, where it was supposed to be for the test. The operators, Toptunov and Akimov, refused on grounds that it was against the rules to do so, even if they were not sure why.
Dyatlov turned on Toptunov. “You lying idiot! If you don’t increase power, Tregub will!”
Tregub, the Shift Foreman from the previous shift, was officially off the clock, but he had stayed around just to see the test. He tried to stay out of it.
Toptunov, in fear of losing his job, started pulling rods. By the time he had wrestled it back to 200 megawatts, 205 of the 211 control rods were all the way out. In this unusual condition, there was danger of an emergency shutdown causing prompt supercriticality and a resulting steam explosion. At 1:22:30 a.m., a read-out from the operations computer advised that the reserve reactivity was too low for controlling the reactor, and it should be shut down immediately. Dyatlov was not worried. “Another two or three minutes, and it will be all over. Get moving, boys!”
At 1:23:04 a.m., Igor Kershenbaum, the Senior Turbine Control Engineer, closed the throttle on the No. 8 turbine to begin the test, just as an operator pushed the MPA button in the control room. With the power demand from the turbine stopped, the water in the still-hot reactor began to boil with fury, and the power level shot up as the water left the cooling channels dry. The reactor was now supercritical. The operators watched in horror as the power level rose rapidly, out of control. After observing it for an agonizing 36 seconds, Akimov shouted, “I’m activating the emergency power reduction system!” and he punched the red button for AZ-5, throwing everything in at once.
The reactor was now prompt supercritical at 1:23:40 a.m. The controls jammed after moving only 6.5 feet, versus their usual range of 23 feet. The guide-pipes had twisted and warped as the reactor began to melt. In seven seconds, the reactor jumped to a power level of 30 billion watts and started to disintegrate.[248]
At that moment, Valery Ivanovich Perevozchenko, foreman in charge of the reactor section, happened to be standing on a balcony looking down at the lid on the reactor, 45 feet below. This lid was affectionately called the pyatachok, or the “five-kopek piece.”[249] It was 49 feet in diameter, and on top of it were 2,000 fuel bundle covers. Each cover was a heavy cube, weighing 770 pounds. There was a deep rumble, and the entire building began to shake. The cubes started dancing and then leaping into the air. It looked and sounded like a popcorn popper heated with an oxyacetylene torch. The cracking and popping noises were so loud, Perevozchenko could not hear himself scream as he took the stairway down, burning the skin off his palms with friction on the handrails, taking four steps at a time, and descending with the equivalent speed of falling down a well 130 feet deep. He hit the ground at level “+10” and sprinted 229 feet down corridors and through the safety lock to the control room, where he burst through the door shouting the ultimate nuclear understatement, “There’s something wrong!”[250]
A loud, percussive noise rocked the building, and the operators bounced off the walls. The pressure-relief valves on the steam separators had all opened with a collective bang.[251] A second later, they all broke free, blasted through the roof, and whistled away into the calm night air. All four separators, each weighing 130 tons, tore out of the floor at level +24 and followed the relief valves, trying desperately to get away. The steam lines into the reactor came loose, and the big tanks of water underneath the reactor instantly boiled to steam under the blast of neutrons radiating from the center of the reactor core. The white-hot zirconium fuel cladding quickly scavenged the oxygen out of every molecule of water it could find. Where there had been water a second ago, there was now superheated steam mixed with hydrogen gas.
248
Four nuclear power plants were not the only high-tech equipment headquartered near Chernobyl. There was also the Chernobyl-2 over-the-horizon radar station, code-named DUGAR-3 and known to ham radio operators in the West as “the Russian Woodpecker.” It was a 10-megawatt radio transmitter feeding the world’s largest directional high-frequency antenna, and its signals, which sounded like a woodpecker hammering on a tree, disrupted amateur radio communications on the 20-meter band beginning in 1976. The antenna was 150 meters tall and 500 meters wide, and this masterpiece of mechanical engineering weighs about 14,000 tons. The NATO code name was “Steelyard.” It was looking for intercontinental ballistic missiles fired at Russia from the United States. Suddenly, at 1:23:40 a.m. local time, the woodpecker went silent and never jammed the 20-meter band again. When reactor No. 4 went wild, it disturbed the power feed to the transmitter, and the woodpecker went dark. The antenna is now in the radiation exclusion zone, and the station cannot be manned. See it on Google Earth by searching on the name “Russian Woodpecker.”
249
In the Old Days of Tsarist Russia, the five-kopek was a comically huge brass coin, made large so that one could use it to buy bread from a street vendor without removing a mitten. One kopek was 1/100 of a ruble, roughly the equivalent of a U.S. penny.
250
Level +10 is ten meters above ground. The turbine hall sump is at level -5.2 and the reactor building floor is at level +35.5.
251
The steam-relief valve was invented by the French physicist Denis Papin in 1679, making possible the subsequent invention of the pressure cooker, or “Papin’s digester.” The old valves used weights or steel springs to hold back the steam pressure, and these units would open gradually, with a groaning or wailing noise. By the time nuclear-powered steam plants were being built, the steam-relief valve had evolved into a mechanism that would open with digital precision amounting to a controlled explosion. There was something to be said for the old ways.