BACKGROUND On March, 11th 2011, Japan has suffered from the strongest 9 magnitude earthquake and a destructive tsunami. The NPP Fukushima Dai-ichi (Plant 1) was in an impact zone of the tsunami. Originally, station reactors did not suffer and were automatically shutdown at the moment of the earthquake. However, owing to shutdown of the centralized power supply and damage of backup diesel generators, the supply of cooling water in the reactors was stopped and that led to

V.G. Khlopin Radium InstituteState Atomic Energy Corporation

ROSATOMSt. Petersburg, Russian Federation

http://www.khlopin.ru email: pakhomov@khlopin.ru

Estimation of Nuclear-Energy Excursion Possibility during Fukushima-1 NPP Accident

Sergei A. Pakhomov, Yuri V. Dubasov CTBTO: Science and Technology 2013, 17-21 June, Hofburg Palace, Vienna, Austria

Fig.. 1. NPP Fukushima-1 before the disaster. Source: AREVA.

Fig. 2(a,b). NPP Fukushima-1 before and after the disaster. Source: Digital Globe.

the active zone overheat, partial fuel rods meltdown. As a result, this was followed by a series of hydrogen explosions in 4 (out of 6) Units of NPP Fukushima-1 and they have been partially destroyed. About 1.5 1019 Bq of xenon-133 [1] (more than 2 time more than in Chernobyl) was ejected into the atmosphere, along with high amounts of cesium, iodine and other radionuclides. The detailed description of Fukushima nuclear accident scenario remains a necessary condition of similar accidents prevention.

Fig.. 3. Computer diagram (Wiki-media)

REFERENCES[1] Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety “The Accident at TEPCO’s Fukushima Nuclear Power Stations”, Nuclear Emergency Response Headquarters, Government of Japan, June 2011 [2] Justin McIntyre, Steve Biegalski, Ted Bowyer, Matt Copper, Paul Eslinger, Jim Hayes, Derek Haas1, Harry Miley, J.P. Rishel, Vincent Woods. US Particulate and Xenon Measurements Made Following the Fukushima Reactor Accident. INGE 2011 Yogyakarta Workshop.[3] Presentation of Nuclear Emergency Response Headquarters Government of Japan, presented during IAEA Ministerial Conference on Nuclear Safety (Vienna, June 20-24, 2011).[4] TEPCO release “The Great East Japan Earthquake and Current Status of Nuclear Power Stations” http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/f12np-gaiyou_e_1.pdf[5] Evidence of neutron leakage at the Fukushima nuclear plant from measurements of radioactive 35S in CaliforniaAntra Priyadarshi, Gerardo Dominguez, and Mark H. Thiemens. PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1109449108[6] T.W. Bowyer a, S.R. Biegalski, M. Cooper, P.W. Eslinger a, D. Haas, J.C. Hayes, H.S. Miley, D.J. Stroma, V. Woods. Elevated radioxenon detected remotely following the Fukushima nuclear accident. Journal of Environmental Radioactivity 102 (2011) 681-687.[7] Ringbom A., Larson T., Axelsson A., Elmgren K., Johansson C. SAUNA – a System for Automatic Sampling and Analysis of Radioactive Xenon // Nucl. Instrum. and Methods in Physics Research. 2003, v. A508, p. 542-553. [8] Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety “The Accident at TEPCO’s Fukushima Nuclear Power Stations”, Nuclear Emergency Response Headquarters, Government of Japan, June 2011 [9] Lars Erik De Geer. The Xenon NCC method…FOI-R-2350—SE, October 2007[10] T. R. England and B. F. Rider, LA UR 94 3106 (ENDF 349), October, 1994‑ ‑ ‑ ‑[11] S.A. Pakhomov, Yu.V. Dubasov. Estimation of Explosion Energy Yield at Chernobyl NPP Accident. Pure and Applied Geophysics , Volume 167, Numbers 4-5 , 575-580.

The accident in Fukushima-1 NPP was a large-scale nuclear catastrophe like Chernobyl, with a provisional International Nuclear Event Scale (INES) level 7 (highest). The result of nuclides analysis in the water extracted from the spent fuel pool of Unit 4 indicated no mass damage to the fuel rods. Thus, all release of radioactivity was from the reactors of Units 1 – 3. The releases of radionuclides were estimated in the report [3]. Table 2. Estimation of releases from the Units 1 – 3 of Fukushima Dai-ichi NPP [3]

Fig. 4. Schematic of Mark I BWR [2].

Table 1. Main parameters of the Spent Fuel Pools (SFP) of Fukushima-1 NPP[1]

Table 2. Main sequences of the accident of Units 1 – 4 of Fukushima-1 NPP [3]

Radionuclide laboratories around the world carried out the monitoring of air contamination with radionuclides during period of emergency products emission. As early as March 12 the fission products, such as iodine-131 and caesium-137 as well as the radioactive noble gas xenon-133, were registered by RN38 laboratory in Takasaki (Japan), located at closest distance – 250 km to the southwest from Fukushima, being in structure of IMS network of CTBTO. This laboratory successfully carried out the measurements of atmospheric aerosols starting March 12 (excluding March 15, when registered levels of radioactive aerosols exceeded the possible ranges of measurements) and provided unique information of air contamination with emergency products in the maximum proximity from the damaged reactors .

All the reactor units of Fukushima Daiichi NPP 1 are BWRs designed by General Electric. They were designed about 40 years ago. On the day when the earthquake occurred, Units 1 - 3 of the Fukushima Daiichi NPP were in operation at the constant rated electric power. Fukushima Dai-ichi NPP Unit 4 was in periodic inspection outage, nuclear fuel was unloaded. [1]

According the press release on April 12, 2011 of Nuclear and Industrial Safety Agency (NISA) the amount of released radioactive materials in the case of Fukushima accident order of magnitude smaller in comparison with Chernobyl. Table 3. Releases from Fukushima Dai-ichi and Chernobyl NPPs [4]

Table 4. Average airborne concentration 15-19 March 2011.

Fig. 5. Airborne concentration in Tokio.

Significantly reinforce or reject the hypothesis of secondary criticality at the Fukushima-1 accident could be possible with the data about the content of atmospheric xenon radionuclides at the time of the accident at the Fukushima-1. That data was obtained at Pacific Northwest National Laboratory PNNL (USA), located in Richland, outside Washington. [6] The distance from Fukushima to Richland is about 7000 km (Fig. 1). The laboratory is equipped with SAUNA installation [7] manufactured by Swedish company GAMMADATA, providing uninterraptable measurements of radionuclides content in the atmosphere: 131mXe, 133mXe, 133Xe and 135Xe.

The highest values of radioactivity were indicated on March 16. Thus, the specific activity of short-lived iodine-135 (half-life period = 6,6 hour) reached value of 74 Bq/m3, and the ratio of iodine-135/iodine-131 activities reached value of 24 that pointed indicated on "fresh" fission products from the damaged reactors, and also testified to possibility of emerged uncontrollable nuclear reaction. Unfortunately, the data of radioactive noble gases concentrations were not correctly obtained at Takasaka's station because of too high levels of their content and equipment pollution.Interesting data in favor of the criticality is given in [5] where the results of the determination of radioactive sulfur-35 in sulfate aerosols and gaseous SO2 in the oceanic air on the Pacific coast in the village of La Jolla, California. This data indicate exposure of sea water to neutrons with fluence 4 x 1011 neutrons per m2. As reported, the sea water was used instead of temporarily unavailable fresh water for emergency core cooling the reactors and spent fuel storage pools. Radionuclide sulfur-35 is formed from seawater containing chlorine-35 by neutron irradiation. The resulting sulfur-35 atom is oxidized to gaseous 35SO2 and acquires the ability to transoceanic transport under favorable weather conditions that occurred in the 2nd and 3rd decade of March, 2011.

Fig. 7 indicates that accidental release products reach the North-West coast of the U.S. on March 16. On March 20 the xenon radionuclides reached maximumc oncentration, then air contamination started to decline gradually. The results of measurements on March 30 remain significantly above the background, as by the time the gaseous products polluted the air basin of the Northern Hemisphere, mixing with air masses.To verify the hypothesis of emerged uncontrollable nuclear reaction at NPP Fukushima-1 accident, the data was compared with the calculated data characterizing decrease of activity of fission products and changes in isotopes ratio of 133Xe/133mXe and 131mXe/133Xe after automatic shutdown of reactors at the moment of the earthquake. Calculation model considered possibility of «supply» of reactor nuclides with «fresh» products of fission due to emerged criticallity.

Operation of SAUNA is based on sampling using a non-cryogenic technologies, chromatographic purification of the sample from interfering impurities and sample analysis using the method of the beta-gamma coincidence. Xenon sampling is carried on activated carbon at ambient temperature after drying air and CO2 absorption using termoelectrocooler and molecular meshes. The device provides a continuous cycle of measurement and sampling: while the sample is measured, next one is being sampled and processed.In total, [8] contains data about 30 measurements of 131mXe, 133mXe and 133Xe radionuclides in the air from March 1 to 30, 2011.

Concentration, Bq/m3

Fig. 6. The relative position of the Fukushima-1 laboratory in Takasaki RN38 and laboratory PNNL in Richland (USA).

Fig. 7. Concentration of 131mXe, 133mXe and 133Xe in the air of Richland on March 2011.

The dynamics of 131mXe, 133mXe and 133Xe activity change both during reactor operation at constant power and its boost, can be calculated by solving the set of equations describing the radioactive transformations in isobaric chains. Graphically, schemes of radioactive transformations in isobaric chains M = 131 and 133 are shown in Fig. 8, numerical values of requied constants are available in [9,10].

Fig. 8. Schemes of isobaric chains M = 131, 133

The greatest interest is the data of isomers ratio of 133mXe and 133Xe, because it depends only on reactor dynamics and it's shutdown time. Therefore, this ratio can be used to determine the radiochemical "age" of emergency products. Underestimation of calculated «age» relatively to measured will testify for supply of «fresh» fission products and consequently about criticality. The same approach was succesfully implemented earlier during Chernobyl data analysis [11].

0 5 10 15 20

100

1000

133X

e/13

3mX

e ra

tion

Day after shut-down

35.4

26.0

R

T(days) = 5,43 lnR -17,7

Fig. 9. The ration of 133Xe/133mXe activity depending on time after a reactor shut-down.

Fig. 10. Calculated date of emergency Fukushima-1 shutdown, obtained on the base of experimental data fig. 7 and approximation fig.9.

Fig. 9 indicates ratio of activities 133Xe/133mXe calculated using special software and approximating line calculated by least square method.Fig. 10 indicates results of statistical processing of calculated values of dates of shutdowns, calculated using data from [6] and graph on fig. 9.It can be seen, that peak if frequency distribution of these dates corresponds to March 14, which «gets behind» real date, which speaks in favour of criticality hypothesis.

Fig. 11. Isomeric ratios of 133Xe/133mXe recalculaded on March 11.

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 220

1

2

3

4

5

6

7

8

Friq

uenc

y

Calculated shutdown date (Day of March, 2011)

Data: Count3_CountModel: GaussEquation: y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2)Weighting: y No weighting Chi^2/DoF = 0.7975R^2 = 0.93015 y0 -1.3026 ±4.30102xc 14.36303 ±0.54499w 9.73967 ±5.52203A 85.82392 ±95.81574

Xe133/Xe133m

(R0=27,1)

16 18 20 22 24 26 28

10

20

30

40

50

60

March, 2011

133X

e/13

3mX

e ra

tion

day

Fig. 11 indicates isomer ratios 133Xe/133mXe, recalculated on March 11, using mesurement results from Richland [6]. It can be seen, that values of ratios are in trend to decline from «reactor» value — 36 to value typical for «fresh» products — 10.Therefore, data on Fig. 11 also indicate possibility of emergency emission products supply with "fresh" fission products, i.e. supports a hypothesis of postshutdown criticality.