A Report prepared by
the Principal Working Group on
the Confinement of Accidental Radioactive Releases (P WG-4) of
the NEA Committee on the Safety of Nuclear Installations (CSNI)
1996 marks the tenth anniversary of the Chernobyl accident. The NEA Committee on Radiation
Protection and Public Health (CRPPH) has prepared for that occasion an authoritative collective statement on
the situation at Chernobyl¹, reviewing the current state of knowledge regarding the short-term and long-term
health impact of the accident, discussing remaining problems (e.g., land contamination, population exposure),
and presenting the lessons learned by the OECD Member counties in terms of radiation protection infrastructure
and emergency preparedness. In order to make a full assessment of health effects, the CRPPH needed to
consider dosimetric assessment. This implied, possibly, are-evaluation of the magnitude of the Chernobyl
accidental release. The CRPPH asked the Committee on the Safety of Nuclear Installations (CSNI) and the
Principal Working Group on the Confinement of Accidental Radioactive Releases (PWG-4) to discuss this matter
and come up, if necessary, with revised estimates for the release.
The worst accident ever experienced at a nuclear power plant occurred on 26 April 1986 at Unit 4 of the Chernobyl Atomic Energy Station. This accident captured worldwide attention because the radioactive material released from the damaged plant was detected in most countries in the Northern hemisphere. In August 1986, the scientists from the former Soviet Union provided the nuclear safety community at the IAEA Post-Accident Review Meeting in Vienna with a detailed account of the accident. This included preliminary assessments of the magnitudes, rates, and compositions of radionuclide releases (usually referred to as the "source term") during the 10 days following initiation of the accident.
During the last decade, a great deal more data concerning the events, phenomena, processes and environmental consequences have been gathered after several investigations conducted at the Chernobyl site, and also at other places both inside and outside the former Soviet Union. As a result of the continued activities, the initiating event, the extent of the damage, the characterisation of environmental release and its consequences have been, to an acceptable uncertainty, well understood. It is generally agreed that such an accident is highly unlikely in any Western-type power reactor, and could not possibly lead in these plants to consequences as dramatic as those resulting from the Chernobyl accident.
The purpose of the following sections is to put the Chernobyl accident in perspective,
very briefly, and to assemble and summarise the recent general findings and consensus concerning the
Chernobyl source term².
The magnitude and timing of the release of radionuclides during the Chernobyl accident has been widely discussed over the last decade. There was an initial, intense phase of radioactive material release during the rapid reactor core disruption. The core disruption was due to the fast overpower transient caused by poor reactor operation and the very unfavorable reactor physics characteristics of the RBMK reactor concept. This initial release included fragments of fuel as well as other radioactive materials in the form of aerosol particles, gases and vapours. The simultaneous high energy release heated the plume of radioactive substances and lifted it high into the atmosphere. Much of the released material and especially the fragmented fuel particles deposited within the borders of the former U.S.S.R. Nevertheless, large quantities of radionuclides, especially the more volatile (e.g., caesium, iodine and tellurium isotopes), were carried large distances by high altitude air currents and were deposited across vast areas outside the former Soviet Union.
After the initial peak release during the core disruption phase, the releases of
radionuclides continued but at a lower level, reaching a temporary minimum level around 29 April 1986.
Large-scale efforts had been undertaken at the reactor site to mitigate and manage the accident;
nevertheless, the release rate increased again. A second peak in the rate of radioactive material
releases occurred. Finally, on or about 5 May, some 9-10 days after the initial phase of the accident,
the radioactive material release rates dropped by about a factor of thousand and have remained very low.
The initial quantitative estimates of the released radionuclides during the initial and subsequent phases of the accident presented at the Post-Accident Review Meeting in August 1986 were mainly based on the integration of ground deposits within the territories of the former Soviet Union. These have been upgraded to incorporate the data gathered on the radioactive, material that fell out outside the former Soviet territories, and by closer examination of the destroyed reactor and analyses of materials from the core and from the reactor building. Clearly, the total release from the Chernobyl accident must have been greater than the material that fell within the former Soviet territories. Careful examination of the initial and currently known release estimates indicates that the total world-wide release of radiologically important nuclides (e.g. caesium and iodine) was two to three times more than what was initially estimated for the former Soviet Union alone.
More than half the initial core inventory of iodine is thought to have been released from the damaged reactor. As a result of integration of world-wide deposition and analyses of core debris within the reactor, one-third of the initial core inventories of caesium and tellurium are estimated to have been released.
Thus, for these materials, the total amount that was deposited over the whole world outside the former Soviet Union was of the same order as the amount that was deposited within the U.S.S.R. However, for other important materials, such as fuel fragments, relatively little was transported outside the former Soviet Union.
An exceptional feature of the radioactive material release during the Chernobyl accident was the release of a relatively substantial amount (3.5 per cent) of the total fuel inventory to the environment. Release of fuel fragments late in the accident might be the result of core debris oxidation. Low volatility elements such as cerium, zirconium and the actinides were retained in the fuel fragments. Significant quantities of these elements outside of fuel particles were not detected. This implies that there was little release of these elements by vaporisation. Certain radioisotopes within the fuel probably became liberated from the molten fuel or fuel fragments as a result of the high temperature and chemical environment in the days following the accident. This is probably the source of the particles composed almost totally of ruthenium isotopes. Air samples indicated that the releases of molybdenum and ruthenium were substantially higher in the late phase of the radionuclide emission period than the releases of fuel fragments. This suggests that the release mechanism of molybdenum and ruthenium was by oxidation to volatile forms.
Although fuel fragments themselves were not carried out very far, monitoring at many
locations showed that the fission products in gaseous or aerosol form were transported over large distances
from the site. Distance tends to dilute the concentration of fission products in the atmosphere. However,
climatic and local weather conditions often resulted in local concentration onto relatively small areas,
even at large distances.
At the request of CRPPH, CSNIS PWG-4 re-evaluated the Chernobyl reactor accident
release. The initial release estimates made by the Soviets in 1986 based on limited data did not cover
territories outside the U.S.S.R. Subsequent studies conducted over the last decade have considered the
additional results obtained from experts in Russia and other countries, The PWG-4 work has updated the
release estimates making use of the more recent results. However, its re-evaluation does not modify the
essence of earlier conclusions regarding the source term or health consequences; the latter will be
discussed separately in documents prepared by the CRPPH.