Chernobyl Nuclear Power Plant Accident, Detection and Monitoring

views updated

Chernobyl Nuclear Power Plant Accident, Detection and Monitoring

█ LARRY GILMAN

On April 26, 1986, a nuclear reactor in the town of Chernobyl (in the Ukraine, then a member state of the Soviet Union) exploded, collapsing the building in which it was located and releasing a radioactive plume that deposited material over much of Europe and Scandinavia. Although the Soviet government was unwilling to release information, satellite photographs by military and civilian satellites, as well as direct radiation measurements downwind, confirmed the event.

The accident and its consequences. The town of Chernobyl, some 60 miles (96 km) north of the city of Kiev (population 2.5 million), is the site of a nuclear electricitygenerating station comprising four identical units of the Soviet-designed RBMK1000 type. Each of the four units is designed to produce 1,000 megawatts of electricity; one of the units is still in operation. On April 25, 1986, operators began an experiment at Unit No. 4 to take advantage a scheduled annual maintenance shutdown. The goal of the

experiment was to see if the station's turbine generator could deliver temporary power to certain cooling pumps after cutoff of its steam supply. As a first step, the unit's operators deliberately disconnected the reactor's emergency core cooling system; such a system is necessary because every large reactor core can generate millions or billions of watts of thermal power (heat); this energy must constantly be removed by a flow of coolant, or the core may cause a steam explosion, melt down, or even (in reactors using highly-enriched fuel) a relatively small nuclear explosion. The emergency core cooling system is supposed to keep the core cool when the usual systems have failed. Unit No. 4's operators had not left the emergency core cooling system disconnected, but had committed a series of further errors that allowed the reactor's power output to fall far below planned levels. In attempting to restore the reactor's power output, the operators caused it to go out of control. In a period of approximately 5 seconds, the core's heat output increased exponentially to the point where a steam explosion occurred. This blew a 1,000-ton concrete lid off the reactor and damaged the roof of the reactor hall.

A few seconds later, an even larger explosion occurred when hydrogen released by the breakdown of water exploded. Burning chunks of graphite (a form of carbon of which 1700 tons were present in the reactor core) flew through the air and landed on other parts of the complex, starting fires. The remaining graphite started to burn, releasing a plume of radioactive smoke that was carried by the wind first north, toward Scandinavia, and later west and south over much of the rest of Europe. The graphite fire burned for over a week, but was finally brought under control by firefighters, many of whom died of radiation burns. The reactor was eventually encased in a shell or "sarcophagus" of concrete. In the late 1990s, United States and Ukrainian engineers worked together to evaluate conditions inside the sarcophagus, which may be vulnerable to collapse in an earthquake. The sarcophagus may need to be strengthened to prevent future releases of radioactivity from the site.

The Chernobyl accident is one of the worst nuclear accidents to date, surpassed only by the explosion at the Chelyabinsk-65 plutonium-processing facility in the Ural Mountains in 1957, which was kept secret for decades by the Soviet Union. Over 20 million curies of radioactive material were lofted into the atmosphere by the Chernobyl explosion and ensuing fire. Some of this material sifted down over nearby towns and countryside, while the rest was spread over Europe by winds, exposing 10 to 20 million people to significant fallout. The number of deaths caused immediately by the accident was in the dozens, but the number of deaths caused in the long term by radiationinduced cancer and other health damage will never be precisely known. Although initially dismissed by experts in the West as exaggerations, reports of 30-to-100-fold increases in thyroid-cancer rates among children in Belarus, northern Ukraine, and parts of the Russian Federation have recently been confirmed.

Nevertheless, the accident could have been worse. Total "meltdown," in which the molten uranium of the ruined core would have coalesced into a single superheated mass and melted its way down to the groundwater below the plant, causing a violent steam explosion and dispersing even larger quantities of radioactive material, did not occur.

The role of satellite imagery. The Chernobyl accident occurred on April 26, 1986, but the Soviet government did not acknowledge the event until April 28 and denied the extent of the disaster for some days thereafter. However, the West quickly had definite knowledge of the accident's occurrence. Radiation was detected in Sweden the day after the explosion and was soon being monitored by aircraft equipped with radiation-detection devices, including the U.S. Air Force's 55th Weather Reconnaissance Squadron. Also, Soviet communications were monitored by a geostationary U.S. military satellite called the Vortex, and both military and civilian Earth-imaging satellites were soon in position to image the site. Because of Soviet reluctance to admit observers or release videos, photographs, or accurate announcements about the accident, and because downwind radiation measurements could give no specific information about what was happening at Chernobyl, much news attention in the West focused on the satellite photographs.

The United States' KH-11 spy satellite provided high-resolution images of Chernobyl to the U.S. government on the afternoon of Tuesday, April 29th, three days after the initial explosion. The KH-11, also known as the Keyhole satellite, was the latest in the KH series of spy satellites that the U.S. began launching in the 1960s, primarily to spy on military activity in the Soviet Union. The KH-11 (whose capabilities were still secret in 1986) could resolve details on the ground down to 4–6 inches (10–15 cm) across. (It has since been replaced by the KH-12 satellite, with a resolution of 2.45 inches [6 cm].) U.S. officials were, therefore, soon as well informed about the Chernobyl accident as vertical views could make them. These images were not, however, released to the public; instead, the U.S. government's knowledge was filtered to the media through announcements.

The first civilian satellite to image the accident site was the United States' LANDSAT, which follows a polar orbit 260 to 570 miles (420 to 912 km) high and takes telescopic pictures of the Earth as it passes beneath. The first LANDSAT (for land-sensing satellite) was launched by the U.S. in 1972, and a series of LANDSATs have been launched as technology has improved. (The seventh LANDSAT in the series is in orbit as of 2003.) LANDSAT images first became available to TV news media on Wednesday, May 3, 1986, only one day after KH-11 images became available to the government. The resolution of the LANDSAT images was comparatively poor, however, being on the order of tens of meters, rather than of centimeters. Nevertheless, they gave visual access to the layout of the reactor complex and cooling pond. Infrared LANDSAT imaging showed both the fire in Unit No. 4 and the chilling of the pond, which indicated that the three remaining reactors in the complex had been shut down.

Several days after the accident, a French satellite named SPOT (for System Probetoire d'Observation de la Terre) was able to provide higher-resolution images to news media. These images were also seen throughout Europe and the United States; on May 1, 1986, for example, ABC news broadcast SPOT infrared photos that showed a plume of hot air trailing from the reactor building.

However, it is doubtful that these nonmilitary satellite images were of any substantive benefit. Despite warnings from professional photo interpreters, announcers on the CBS and NBC television networks announced that the LANDSAT images revealed two reactors on fire, a claim that had to be retracted. Little actual news was derived from the LANDSAT or SPOT images. They served to lessen the sense of mystery surrounding the Chernobyl disaster, but did not supply any specific information that was not already available from other sources. They confirmed—but also, through misinterpretation by amateur analysts, confused—reports already received from official sources.

The KH-11 satellite data, on the other hand, were probably of some utility. They at least gave U.S. officials independent information about the scope of the disaster. However, there was little the West could do with this knowledge. The accident was inside Soviet territory and the response to it was entirely a Soviet affair. Complete cover-up of the event would have been impossible even without spy-satellite imagery, due to the detection of radiation downwind.

Nevertheless, the role of satellite imaging during the Chernobyl accident shows that large-scale disasters can no longer be denied, whether as a whole or in detail, by national governments, given the imaging capabilities of both military and nonmilitary satellites. The basic story of Chernobyl, unlike that of the blowup at Chelyabinsk-65, was public property from the beginning.