The initial release

2.44. As noted above, the explosions in the reactor buildings, the venting, and direct release of contaminated water into the ocean at the time of the accident released radioactive material in the atmosphere, land, and ocean.

2.45. The amount of radionuclides released, also called the "source term", comprises radionuclides released from the cores and confining structures into the environment during and after the accident at the FDNPP. Source term analyses indicate that the major releases that contributed most to the radiological consequences on Japanese territory occurred on 15 March 2011. The releases were likely related to release of activity in Unit 2 due to core melting and subsequent loss of PCV integrity early in the morning, or to PCV venting at Unit 3. Other large peaks of activity release are thought to have occurred in the afternoon on 12 March 2011 (explosion at Unit 1), at noon on 14 March 2011 (explosion at Unit 3), and late at night on the same day (probably due to venting of Unit 3).¹²⁷

2.46. Based on estimates, approximately 17.5 PBq of Cs-134 and 15 Pbq of Cs-137 were released via the atmospheric fallout, 5 PBq of Cs-137 was directly discharged into the environment, 15-20 TBq/year of Cs-137 is released via ongoing groundwater discharge, and 10-12 TBq/year is released through ongoing river runoff.¹²⁸ The total atmospheric release of I-131 was estimated to be approximately 150-160 PBq.¹²⁹ In total, approximately 1.0-2.4 x 10⁹ Bq of Pu-239 and 240 was released into the environment from the FDNPP reactors.¹³⁰ Most of the Sr-90 released from the FDNPP was directly discharged into the North Pacific, with estimates of total inventories ranging

127 2015 IAEA DG Report, Technical Volume 1, (Exhibit JPN-7), p. 143.

128 K. Buesseler et al., "Fukushima Daiichi-Derived Radionuclides in the Ocean: Transport, Fate, and Impacts", 30 June 2016. First published online as a Review in Advance, (Buesseler et al. (2016),

(Exhibit KOR-134), p. 3.

129 P.P. Provinec, K. Hirose and M. Aoyama, Fukushima Accident: Radioactivity Impact on the Environment (2013), (Elsevier, 2013), (Exhibit KOR-97), p. ix.

130 Buesseler et al. (2016), (Exhibit KOR-134), p. 6.

from 0.04 to 1.0 PBq.¹³¹ Estimates to this day have varied and there is no definitive calculation of the amounts released.¹³²

2.5.1.1 Releases to the atmosphere

2.47. In the early phase of the accident, the noble gases krypton-85 (Kr-85) and xenon-133 (xe-133), with half-lives of 10.76 years and 5.25 days, respectively, contributed to external exposure from the plume of the atmospheric releases. Around 6,000–12,000 PBq of Xe-133 are estimated to have been released (or 500–15 000 PBq, if early estimates are included in the evaluation)¹³³. Iodine-131 (I-131, half-life of 8.02 days) and caesium-137 (Cs-137, half-life of 30.17 years were "the two most significant radionuclides from the perspective of exposures of people and the environment"¹³⁴ The estimated releases to the atmosphere ranged from 100 to 500 PBq for I-131 and from 6 to 20 PBq for Cs-137. The mean total activity of I-131 released was around 100-400 PBq, and that of Cs-137 was around 7–20 PBq (or 90–700 PBq and 7–50 PBq, if early estimates are included).¹³⁵ Other radionuclides were also released in amounts relative to their volatility.¹³⁶ Estimates for the release of other radionuclides, such as strontium and plutonium, have been more limited. The IRSN estimated the amount of Sr-90 released into the atmosphere to be 0.003 PBq. However, the IRSN added the qualification that "estimates of the radioactivity released by these radionuclides remain rough due to the lack of a sufficient body of measurements and information on the actual condition of the damaged reactors."¹³⁷ The Nuclear and Industrial Safety Agency of Japan estimated the amount of Sr-90 released into the atmosphere to be 0.14 PBq, and the amount of Pu-238, Pu-239, Pu-240 to be 0.025 TBq.¹³⁸

2.48. The fission products were released from the overheated reactor core, their vapours were transported by flows of gas or steam into cooler regions of the PCV where they condensed into aerosols. These aerosols either retained in the containment vessel or released into the environment through leaks. Aerosols formed and dispersed from the accident sooner or later deposit on surfaces. Three different release paths to the environment were distinguished at the FDNPP: (i) design leakage into the reactor building (where these aerosols could remain for a long time); (ii) containment venting where radioactive products (which had already been scrubbed within the water pool) were released unfiltered to the environment through vent stacks; and (iii) containment failure of three of the PCVs in three FDNPP units whereby significant amounts of radioactive airborne aerosols leaked into the reactor building and eventually into the environment.¹³⁹

2.49. UNSCEAR reported that, unlike the Chernobyl accident, where less volatile elements (such as strontium and plutonium) were released in relatively larger amounts directly into the atmosphere as a result of the initial explosion and physical destruction of parts of the core, such mechanisms did not occur in the FDNPP accident. According to UNSCEAR, the volatility of the elements, and the extent to which they were retained within the containment by other mechanisms (for example the suppression pool), were the principal determinants of the amounts released.¹⁴⁰ The IAEA confirms that the atmospheric release was dominated by the volatile isotopes of iodine and caesium because of their low vapour pressure, which resulted in their virtually complete release from the nuclear fuel during the core meltdown. The IAEA also indicates that the release of strontium was three to four orders of magnitude less than the release of caesium. Plutonium was released to the environment as a result of the FDNPP accident; however,

131 Buesseler et al. (2016), (Exhibit KOR-134), p. 6.

132 Buesseler et al. (2016), (Exhibit KOR-134), p. 4, Table 1.

133 2015 IAEA DG Report, (Exhibit JPN-2), p. 107.

134 2015 UNSCEAR White Paper, (Exhibit JPN-211), p. 4.

135 2015 IAEA DG Report, (Exhibit JPN-2), p. 107.

136 UNSCEAR 2013 Report Annex A, (Exhibit JPN-210), p. 49.

137 Institut de radioprotection et de sûreté nucléaire, "Fukushima, one year later: Initial analyses of the accident and its consequences" (12 March 2012), IRSN_Fukushima-1-year-later_2012-003.pdf, (Exhibit KOR-93), p. 48.

138 Japan, Nuclear and Industrial Safety Agency, Table 5: Estimates of the Amount (Bq) of Radioactive Materials Released into the Atmosphere During the Period Subject to the Analysis, (Exhibit KOR-94); J. Zheng, K. Tagami and S. Uchida, "Release of plutonium isotopes from the Fukushima Daiichi Nuclear Power Plant accident", Environmental Science & Technology, Vol. 47, No. 17 (2013), pp. 9584-9595, (Exhibit KOR-95).

139 2015 IAEA DG Report Technical Volume 1, (Exhibit JPN-7), pp. 142-143.

140 UNSCEAR 2013 Report Annex A, (Exhibit JPN-210).

the amounts released were more limited than the other radionuclides.¹⁴¹ Data indicate that plutonium release due to the core melts in the FDNPP did not notably increase the pre-existing environmental distribution of plutonium. The chemical composition of the radionuclides released had a direct consequence on the land contamination, which was dominated by iodine and caesium.¹⁴²

2.50. The release of lower volatility radionuclides such as strontium, barium and plutonium were much lower than those of iodine and caesium as confirmed by measurements of their levels in the environment.¹⁴³ Neutrons were also detected near the main gate of the plant (which is approximately 1 km away from Units 1–3). It is estimated that the neutrons came from the spontaneous nuclear fission of radionuclides that could have been released as a result of damage to the reactor core.¹⁴⁴ On a number of occasions, the meteorological conditions were such that radionuclides released to the atmosphere were dispersed over mainland Japan, and then were deposited on the ground by means of dry deposition and wet deposition with rain or snow.¹⁴⁵ The main deposition occurred to the north-west of the FDNPP site, but significant deposition also occurred to the north, south and west of the FDNPP site.¹⁴⁶ A significant amount of atmospheric release was also deposited in the ocean and on land, as discussed in the sections below.

2.5.1.2 Releases to the ocean

2.51. The ocean received two types of radionuclide deposits. First, atmospheric releases dispersed over the North Pacific Ocean and fell on the oceanic surface layer. Second, there were direct releases and discharges into the Pacific Ocean at the site, with the primary source being highly radioactive water from a trench at the FDNPP. The peak radioactive releases were observed at the beginning of April 2011. The direct releases and discharges of I-131 into the sea were estimated to be 10–20 PBq. The direct releases and discharges of Cs-137 were estimated by most analyses to be in the range of 1–6 PBq, but some assessments reported estimates of 2.3–26.9 PBq.¹⁴⁷ In addition to I-131 and Cs-137, other radionuclides were released to the ocean directly and indirectly. Radionuclides of low volatility such as strontium and plutonium were measured in seawater and sediments. Estimates of direct release of Sr-90 to the ocean range from 0.04 to 1 Pbq, while plutonium radioisotopes in seawater have generally been below limits of detection.¹⁴⁸ 2.52. There have been reports of additional spills of liquid radioactive waste from the FDNPP into the ocean causing Sr-90 activities to exceed those of Cs-137 in the ocean near the FDNPP for short periods of time. It is hypothesized that the decrease in the ratio of caesium to strontium is a result of continuing accidental spills of strontium or the higher mobility of strontium. While the ratio has been decreasing, caesium is still in greater quantities than strontium.¹⁴⁹

2.5.1.3 Dispersion

2.53. The effect of a release of radionuclides is not necessarily localized, but may be dispersed through the atmosphere and ocean currents. Extensive measurements of activity concentration of I-131, caesium-134 (Cs-134) and Cs-137 in the environment, including in air, soil, sea water, sediments and biota, were performed and have been used for estimating the dispersion of the releases.¹⁵⁰ The IAEA report includes a variety of theoretical models used to estimate the dispersion patterns of the radionuclides released during the accident at the FDNPP.

141 This conclusion finds support in the statement of the IAEA that the fact that concentration of plutonium isotopes found at the FDNPP site corresponded to the background level was an indication of the limited nature of the release of plutonium during the accident. See 2015 IAEA DG Report Technical Volume 1, (Exhibit JPN-7), p. 149.

142 2015 IAEA DG Report Technical Volume 1, (Exhibit JPN-7), pp. 148-149.

143 2013 UNSCEAR Report Annex A, (Exhibit JPN-210), pp. 40-41.

144 2015 IAEA DG Report, (Exhibit JPN-2), p. 107.

145 2015 UNSCEAR White Paper, (Exhibit JPN-211), p. 4.

146 2015 UNSCEAR White Paper, (Exhibit JPN-211), p. 4.

147 2015 IAEA DG Report, (Exhibit JPN-2), p. 107.

148 2013 UNSCEAR Report Annex A, (Exhibit JPN-210); see also 2015 IAEA DG Report Technical Volume 1, (Exhibit JPN-7), pp. 148-149; and Expert Meeting Transcript, pp. 6-8.

149 Buesseler et al. (2016), (Exhibit KOR-134), p. 6. The Cs-137 to Sr-90 ratio has gone from 12.5 at the FDNPP site in June 2011 to 3.8 in 2013. Korea's second written submission, para. 38.

150 2015 IAEA DG Report, (Exhibit JPN-2), p. 107.

2.5.1.3.1 Atmospheric dispersion

2.54. The transport of the atmospheric radioactive releases was directed mainly to the east and north of Japan, following the prevailing wind directions, and then around the globe.¹⁵¹ According to the models that were used to estimate the atmospheric transport of the various radionuclides and their deposition patterns, the activity concentration in the atmosphere decreased noticeably with increase in distance from the FDNPP.¹⁵² Highly sensitive radiation monitoring networks detected some radioactivity attributable to the accident as far away as Europe and North America.¹⁵³

2.55. Months after the FDNPP accident, Japan's Science Ministry reported that caesium had contaminated 11,580 square miles of the land surface of Japan, and about 4,500 square miles were found to have radiation levels that exceeded Japan's allowable exposure rate of 1 mSv/year.¹⁵⁴

2.5.1.3.2 Ocean dispersion

2.56. Most of the released and discharged radionuclides that entered into the Pacific Ocean from the FDNPP site moved eastward with the Kuroshio current¹⁵⁵ and were transported over large distances via the North Pacific Ocean gyre.¹⁵⁶ A number of oceanic transport models have been used to assess dispersion patterns of radionuclides in the ocean.¹⁵⁷ Studies have shown that dispersion within the ocean, for example whether the radionuclide stays on the surface or sinks to the sediment, varies according to the type of radionuclide. Testing in various areas of the ocean can be used to confirm whether radionuclides from the FDNPP accident have been dispersed there.

For example, the high caesium-activity ratios in samples from the North Western Pacific taken two years after the accident suggest that these samples were contaminated by caesium released from the FDNPP. On the other hand, plutonium fingerprints from the same area suggest that the plutonium contamination found is predominantly from other sources such as fallout from nuclear weapons use and testing.¹⁵⁸

2.57. The Fukushima prefecture and neighbouring prefectures have several river systems that flow from contaminated upland forests to coastal plains, and ultimately empty into the Pacific Ocean. Studies estimate that 17.1 TBq of total radionuclides were released into the Pacific Ocean from 1 June to 30 September 2012, which is only a fraction of the radiocaesium inventory of the upland forests of the Fukushima prefecture.¹⁵⁹ Some scientists hypothesize that river catchments will be a longer-term, ongoing source of radiocaesium to estuaries and coastal areas.¹⁶⁰

151 2015 IAEA DG Report, (Exhibit JPN-2), p. 11.

152 2015 IAEA DG Report, (Exhibit JPN-2), pp. 107-108.

153 2015 IAEA DG Report, (Exhibit JPN-2), p. 108.

154 Asahi Shimbun, "Contaminated regions having radiation doses of 1 mSv/year account for 3% of Japanese territory", 11 October 2011, (Exhibit KOR-28); S. Starr, "The Implications of The Massive Contamination of Japan with Radioactive Cesium" (2013),

http://www.ratical.org/radiation/Fukushima/StevenStarr.html, (Exhibit KOR-29).

155 The Kuroshio current is a northward flowing ocean current on the western side of the North Pacific Ocean that flows past the FDNPP.

156 The North Pacific Ocean gyre is one of the five major oceanic gyres, covering most of the North Pacific Ocean; it has a clockwise circular pattern and is formed by the North Pacific Ocean current to the north, the California current to the east, the north equatorial current to the south, and the Kuroshio current to the west.

157 2015 IAEA DG Report, (Exhibit JPN-2), p. 109.

158 W. Bu, M. Fukuda, J. Zheng, T. Aono, T. Ishimaru, J. Kanda, G. Yang, K. Tagami, S. Uchida, Q. Guo, M. Yamada, "Release of Pu isotopes from the Fukushima Daiichi Nuclear Power Plant accident to the marine environment was negligible", Environmental Science & Technology, (2014); Vol. 48, (Exhibit JPN-11.1(10)), pp. 9070-9078. See also para. 7.209.

159 Y. Yamashiki, Y. Onda, H. Smith, W. Blake, T. Wakahara, Y. Igarashi, Y. Matsuura, K. Yoshimura,

"Initial Flux of Sediment-Associated Radiocesium to the Ocean from the Largest River Impacted by Fukushima Daiichi Nuclear Power Plant", Scientific Reports, 21 November 2013 (Exhibit KOR-185), p. 2.

160 O. Evrad, C. Chartin, Y. Onda, H. Lepage, O. Cerdan, I. Lefevre, S. Ayrault, "Renewed Soil Erosion and Remobilization of Radioactive Sediment in Fukushima Coastal Rivers After the 2013 Typhoons", Scientific Reports, 3 April 2014, (Exhibit KOR-184).