Radioactive materials have been released to the environment from the damaged cores of units 1, 2, and 3. Grambow and Poinssot [6] considered the extent of core damage, suggesting from the available evidence that not only the UO2 in the fuel but also the zircaloy cladding and steel melted forming a quenched melt (also seen at Three Mile Island and Chernobyl) called corium. They believe substantial amounts of volatile fission products (FPs), such as Cs and I, were released during melting, but that the less vola­tile FPs and actinides were incorporated into the corium. This corium thus contains most of the most radiotoxic species and presents a very large and long-term challenge. The escape of radionuclides from the damaged cores occurred both through atmospheric release and through water leaks to the sea. It is considered that the major part of the atmospheric release was by the unexpected leak of the containment vessel, whilst deliberate venting through the suppression chamber to reduce gaseous pressure gave rather limited result. A controlled release of very low-level contaminated water from the central waste treatment facility (ca. 4 Bq/cm3) to the sea was done once, but it has little significance in terms of the total amount of contamina­tion. The major release to the sea was by (1) fall-out of the atmospheric release to the sea, (2) carry over by rainwater, and (3) leakage of highly contaminated water via underground cable (sub-drain) pits.

The extent of radionuclide release to the atmosphere has been evaluated several times by government officials and Tokyo Electric Power Company (TEPCO), but it is still very much an estimate [5-13] . This can be seen in Table 24.2 which compares the estimated release of cesium-134, 137 and iodine-131 in March 2011 by several estimation studies. The differences in the data arise from the different assumptions used about the status and progress of the accident, due to unreliability in instrumentation data as well as in the monitoring data under the severe conditions. The estimate by TEPCO, which showed larger values than government authorities, was obtained by comparing the result of dose monitoring as well as of the analysis of radioactivity deposition onto surfaces, and the results of envi­ronmental dispersion calculation code. The calculation is based on an assumed rate of evolution of radioactive particles, and thus still has some uncertainty.

Cesium-134, 137 and iodine-131 are the nuclides that mostly decide the radiological impact on the public, among which iodine-131 due to its short

JAEA: Japan Atomic Energy Agency; NISA: Nuclear and Industrial Safety Agency, Japan; NSC: Nuclear Safety Commission of Japan; IRSN: Institut de radioprotec­tion et de surete nucleaire.

* INES index: Radiologically equivalent value to iodine-131. The International Nuclear and Radiological Event Scale (INES) was developed in 1990 by the IAEA and the OECD Nuclear Energy Agency (OECD/NEA) with the aim of communicat­ing the safety significance of events at nuclear installations [ 14] and Section 1.4.6). If there is an atmospheric release from a nuclear facility then a radiological equivalence to iodine-131 is calculated using conversion factors. For example, the actual activity of the isotope released should be multiplied by some factors (given in [ 14]) and then compared with the values given in the definition of each level. An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to more than several tens of thousands of TBq of iodine-131 is rated to the highest level 7 according to the INES scale.

half-life is only significant for a couple of months after its release. The esti­mated amount of the other released radioactive nuclides can be seen in Table 24.3, which was evaluated in June 2011 and corrected in October 2011 by the Japan Nuclear and Industrial Safety Agency (NISA). In a general sense, the nuclides other than cesium-134, 137 and iodine-131 have less radiological impact because of their lower radioactivity, low specific radio­logical effect, as well as short half-lives. It should be noted that the amount of strontium-90 believed to have been released is about 1/100 of that of the sum of cesium-134 and 137, and that of plutonium-239 is about 10-7 of that.

The amount of radioactivity released to the sea via the sub-drain pit was estimated to be 4.7 PBq in total of cesium-134, 137 and iodine-131 [8], which is much less than the atmospheric release. Due to the leak of the water from the sub-drain pit, the radioactivity concentration in the seawater sampled at the discharging point showed as high as 105 Bq/L of cesium-137 at the

released radioactivity spread in other directions, such as to the south. The dispersion of radioactive materials was tracked by the Preparatory Com­mission for Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO) [8] which is a monitoring system designed to detect nuclear explosions. CTBTO reported the large-scale radiation leak resulting in the 20 km exclusion zone being set up around the power plant and people within the 20-30 km zone being advised to stay indoors.