CHORNOBYL 40 YEARS ON | SAFETY & SECURITY
invention. However, for their time, they possessed a number of significant advantages. These included the ability to reload fuel without a shutdown, reduced primary circuit parameters, the absence of expensive and structurally complex reactor vessels and steam generators, no restrictions on the core size or shape, high maintainability, a less severe accident response caused by leaks in the circulation circuit, low uranium enrichment, and others. Of course, the reactor’s main deficiencies were well known
before 1986. The RBMK reactor operates on a direct cycle scheme which led to radioactivity in the turbine hall. Most of the personnel therefore worked in ‘especially harmful conditions’ which in the Soviet Union meant higher wages, free meals during work hours, extended vacations, and a retirement age of 50. An additional problem with aging RBMK reactors is
graphite stack distortion, caused by irradiation-induced swelling and subsequent cracking of the graphite. Technological solutions reduced the distortion and allowed for continued operation. However, due to the accumulation of defects, this labour-intensive maintenance had become an annual procedure. Other issues included the enormous number of channels, pipelines, and valves, and therefore personnel, the challenge of flow rate regulation in each channel, the large volume of radioactive structural materials, graphite swelling and the lack of a technology for its disposal. But the USSR faced numerous other problems and the RBMK reactor’s deficiencies were not particularly noticeable. Moreover, these nuclear power plants operated successfully with normal capacity factors and even paid off within a few years. One could compare a Ford model A and a modern car with the same methodological success.
A day of disaster The RBMK reactor’s critical deficiency fully manifested itself in April 1986, in Ukraine, but in some respects this was not a stand-alone event. In 1975, unit 1 of the Leningrad NPP experienced conditions very similar to those which would ultimately lead to the destruction of unit 4 at Chornobyl. At low power, with the steam turbine shutdown, and with unacceptable withdrawal of the control rods from the core, the reactor saw a thermal power surge of 100 MW. This resulted in the rupture of one channel and a release of radioactive substances into the environment. The unit was in an outage for several months. This mini-Chornobyl, while serving as a pre-cursor to the future catastrophic events in Ukraine, did not lead to the meaningful responses from the authorities that could have prevented the subsequent disaster. There were also other accidents caused by the destruction of reactor channels at other RBMK plants, but that were unrelated to the physics of the reactor. It subsequently emerged that in certain modes – and with the number of control rods in the core below the permissible limit – the reactor could reach prompt criticality and explode. The designers seemed to be aware of this deficiency and ‘solved’ the problem by prohibiting such modes in the operating procedures. The operating personnel were unaware that such a mode could have irreparable consequences and thus sometimes, when it was necessary to raise or decrease power, they would operate the reactor in unstable and potentially dangerous configurations. For the most part nothing happened – until 26 April 1986. The beginning of a new sad chapter in the history of nuclear power took place
A minute to disaster ● 01:23:04: Turbine emergency power test begins with turbine stop valve closure
● 01:23:40: Reactor SCRAM button pressed ● 01:23:43: The power excursion rate emergency protection system signals come on
● 01:24:00: Reactor Control Engineer log reads: “Severe shocks; the RCPS rods stopped moving before they reached the lower stop switches; power switch of clutch mechanisms is off.”
Two explosions were reported, the first a steam explosion, followed two or three seconds later by a second, possibly from a hydrogen build-up.
when the accident at the Chornobyl NPP destroyed the core of a brand new unit that had only been in operation for two years and contaminated a vast area.
The immediate aftermath In the first days following the explosion, the primary focus was on reducing radioactive emissions from the destroyed reactor. The satellite town of Pripyat and residents of settlements within a 10 km zone were evacuated one day after the accident. In the following days, settlements within a 30 km zone were evacuated. But in the typical Soviet atmosphere of secrecy at the time, most people had no idea of the scale of the disaster. TV news reported that “an accident had occurred, a government commission was working, and the consequences were being successfully dealt with”. The main objective in the months that followed was to enclose the damaged reactor with a reinforced concrete structure – a ‘sarcophagus’. This was achieved by November, just six months later. Units 1 and 2 at the plant were reconnected to the grid in October and November of the same year. In December 1987, unit 3, separated from its damaged twin unit by a mounted wall, was also restarted. In the autumn of 1986, construction of a new town for workers at the nuclear plant began. This town, Slavutych, some 45 km from the NPP in a straight line, saw staff begin to move into the new housing a year and a half later. Approximately 600,000 people participated in the ‘liquidation’ of the accident. A huge sum was spent – approximately 20 billion Soviet roubles. This sum is difficult to estimate reliably today due to the rouble’s inconvertibility and the distribution of costs among various ministries. Nevertheless, it represents approximately 5% of the Soviet Union’s annual budget. The main expenses were social payments, resettlement, and land decontamination. The total damage, including long-term economic and health consequences, is estimated at hundreds of billions of dollars. Today about 50,000 km2 of land has been withdrawn
from agricultural use, a 30 km exclusion zone was created around the plant and remains in place and dozens of small settlements have been destroyed and buried. Approximately 200,000 people have been evacuated and are displaced to this day. While living in the surrounding areas was unacceptable, it was possible to work there. Despite the tragedy, it seemed life at the Chornobyl NPP continued. But in October 1991, while reducing the speed of turbine generator #4 in unit 2 for shutdown and repair of turbine hall equipment, a false signal in the control circuits caused the generator, which had been practically stopped, to be re-connected to the grid. Significant vibration, resulted
www.neimagazine.com | April 2026 | 35
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112
