The control rods are as long as the reactor is high, but the active region in the middle of a control rod is only 16.4 feet long. The rest is just hollow tubes, at the top and bottom of the boron neutron-absorber section. At the bottom of each control rod is a rounded tip made of pure graphite, designed to act as a lubricant that will ensure smooth running of the rod in its metal tube under conditions that would ruin ordinary grease. If a control rod is withdrawn all the way, then the first thing that enters the core during an emergency shutdown is a big chunk of graphite. Putting additional graphite into the core increases the activity instead of decreasing it, so scramming from a condition of all-out rods does the opposite of what is desired for a long five seconds. If the reactor is critical and a shutdown is needed, the reactor goes supercritical until the active section of the rod is able to overcome the positive reactivity introduced by the graphite tip. If the reactor is supercritical when the rod is inserted, then the power rise proceeds at increased speed. Push the rods in one at a time, and this is only an irritation. There are 211 control rods in an RBMK. Put all of them in at once, and the reactivity will increase explosively. Under any normal operating condition, all rods are never out all the way.
Another bad characteristic of the RBMK reactors is the positive void coefficient. The neutrons are moderated down to optimum fission speed by the graphite, which has a very low tendency to scavenge neutrons out of the process. The coolant is water, which does scavenge neutrons, and the water runs through metal tubes perforating the reactor core. There is enough graphite to overcome the negative effects of the water and maintain criticality. If water is lost out of a tube or if it boils dry into steam, then the overall reactivity of the core is improved. The reactor goes supercritical, and the power level starts to rise exponentially. This is not good. In a PWR or a BWR, which use water for both coolant and moderator, if water is lost or turned to steam, the reactor shuts down instantly. In an RBMK, the power goes up with similar enthusiasm.
The first 3.23 feet of each control rod is a hollow metal tube, and it displaces the water out of the guide pipe as it is pushed in.[246] From an all-out position, the control rod would first introduce graphite to the core, and then empty the cooling water out of the guide.
The water inlet tubes for the reactor all come up from the bottom, which is the logical way to do it, but fear of leakage from these tubes caused the engineers to design a large gallery under the reactor, sealed tightly and filled with water. The tubes run through the water tank, and this will dilute any fission products that happen to escape the fuel, get into the cooling system, and leak out through cracks or failed welds. This seemed like a good idea, but if the core ever melts it will fall directly into the water underneath and flash it into steam. Being tightly sealed with no way out makes rapidly derived steam into a bomb, sitting right under the core. The force of such a blast would be directed upward, making short work of the thin walls and ceiling above the reactor and pushing the big traveling crane above the refueling machine skyward. The mechanical engineers who designed the plant were good at anticipating bad welds but gave insufficient thought to what happens when a reactor runs away.
Chernobyl is an ancient town in the Byelorussian-Ukrainian woodlands on the banks of the Pripyat River, where the land is a featureless steppe. It is at least 1,000 years old, and was most memorable for providing Prince Svyatoslav the First with a particularly spirited, almost feral bride around the year 963. In the 1970s, work began on a cluster of six RBMK-1000 reactors, built on a flat spot 11 miles northwest of the town. A new, modern village, Pripyat, rose up 1.9 miles west of the sprawling plant, just outside the safety zone. Its population grew quickly to 50,000 people. Most employment was at the power plants or in jobs supporting the families living in the area. There were schools, multi-story apartments, a park with a Ferris wheel, a bookstore, recreation center, library, and every trapping of civilization. Reactor No. 1 was completed and came online in 1977, followed by No. 2 in 1978, No. 3 in 1981, and No. 4 in 1983. No. 5 and 6 were still under construction in 1986. Central Planning back in Moscow was dreaming of a 20-reactor complex.
In April 1986, reactor No. 4 still had 75 percent of its original fuel load and was looking at a refueling in the near future, meaning that its core was saturated with a nearly full load of radioactive fission products in its 200 tons of uranium oxide. Nikolai Maksimovich Fomin was Chief Engineer at the station, Viktor Petrovich Bryukhanov was the Plant Director, Taras Grigoryevich Plokhy was Chief of the Turbine Unit, Anatoly Stepanovich Dyatlov was Deputy Chief Engineer in charge of operations for No. 3 and No. 4 reactors, and Leonid Toptunov was the Senior Reactor Control Engineer.[247] The foreman in charge of the reactor section of the plant was Valery Ivanovich Perevozchenko and Aleksandr Fyodorovich Akimov was the Shift Foreman. None of these men and nobody in the entire power plant had a clear understanding of the nuclear end of the power plant. They were experts in turbines, wiring, and mechanical engineering specialties, but had no training or experience that would lead to a comprehension of graphite reactor dynamics. Dyatlov, a physicist, was the most unusually slow-witted and argumentative of the lot, and as one who had an inkling of nuclear experience, he was in charge. He had worked briefly on very small experimental naval reactors, and this may have given him a distorted view of how a power reactor should behave.
Reactor No. 4 was scheduled to run a safety experiment on April 25, 1986. The Nuclear Safety Committee had long been concerned that if an RBMK reactor were to shut down for emergency reasons while producing power, there would be a delay between having lost power from the generator and starting the diesel engines that ran the backup generators. In this time lapse, the coolant pumps would not be running, and this could cause damage to the plant from a sudden heat buildup. There were theories that one of the eight heavy turbine rotors would have enough momentum to keep its generator turning for a few minutes, supplying just enough power to keep the pumps running, but a previous test at Chernobyl No. 4 had been disappointing. The generator fields had been modified to reduce the drag, and now the committee wanted the test to be run again. The reactor and all the plant systems were to be shut down suddenly, as if a major breakdown had occurred, after which the performance of one turbo-generator would be observed over several minutes.
Gennady Petrovich Metlenko was in charge of the electrical aspects of the trial, and on April 11 he had a special control-panel switch installed, called the “MPA.” The letters in Russian stood for “Maximum Design-Basis Accident,” or the worst thing that could possibly happen. Actuate the MPA switch, and the worst happens instantly. It shuts off the turbine, disables the ECCS, turns off all the pumps, blocks the diesel generators from starting, and basically kills everything that keeps the reactor running smoothly, bypassing the automatic controls. This was a monumentally bad idea.
The experimental sequence started at 1:00 P.M. on April 25, right on time, when Dyatlov ordered the reactor power reduced. No. 4 had been running at maximum power. Five minutes later, the No. 7 turbine was kicked off the power grid and the station’s power needs were switched to turbine No. 8. The reactor was now running at a little over half power. At 2:00 P.M., the ECCS was disconnected. One thing they thought they did not want during the experiment was 12,360 cubic feet of cold water gushing into a red-hot reactor from the ECCS, thinking that it would warp something. Just then, a call came in from the electrical load dispatcher in Kiev, requesting that they delay the experiment a while longer. Demand for electricity in the area seemed to be peaking that afternoon.
246
This is no longer the case. After the smoke had cleared from the Chernobyl disaster, the control rods in all RBMK reactors were ripped out and replaced with an improved design.
247
Fomin rose rapidly through the leadership posts and was Deputy Chief Engineer for Assembly and Operations when he knocked Plokhy out of the position as Chief Engineer. Although the Ministry of Energy did not support the decision to promote Fomin to this post, he had the solid endorsement of the Central Committee of the Communist Party, Nuclear Division, and that was all that was needed.