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Kim, Rhee, Song, Kim, and Ha: Estimation of In-plant Source Term Release Behaviors from Fukushima Daiichi Reactor Cores by Forward Method and Comparison with Reverse Method

### Background

The purpose of this paper is to confirm the event timings and the magnitude of fission product aerosol release from the Fukushima accident. Over a few hundreds of technical papers have been published on the environmental impact of Fukushima Daiichi accident since the accident occurred on March 11, 2011. However, most of the research used reverse or inverse method based on the monitoring of activities in the remote places and only few papers attempted to estimate the release of fission products from individual reactor core or from individual spent fuel pool. Severe accident analysis code can be used to estimate the radioactive release from which reactor core and from which radionuclide the peaks in monitoring points can be generated.

### Materials and Methods

The basic material used for this study are the initial core inventory obtained from the report JAEA-Data/Code 2012–018 and the given accident scenarios provided by Japanese Government or Tokyo Electric Power Company (TEPCO) in official reports. In this research a forward method using severe accident progression code is used as it might be useful for justifying the results of reverse or inverse method or vice versa.

### Results and Discussion

The release timing and amounts to the environment are estimated for volatile radioactive fission products such as noble gases, cesium, iodine, and tellurium up to 184 hours (about 7.7 days) after earthquake occurs. The in-plant fission product behaviors and release characteristics to environment are estimated using the severe accident progression analysis code, MELCOR, for Fukushima Daiichi accident. These results are compared with other research results which are summarized in UNSCEAR 2013 Report and other technical papers. Also it may provide the physically based arguments for justifying or suspecting the rationale for the scenarios provided in open literature.

### Conclusion

The estimated results by MELCOR code simulation of this study indicate that the release amount of volatile fission products to environment from Units 1, 2, and 3 cores is well within the range estimated by the reverse or inverse method, which are summarized in UNSCEAR 2013 report. But this does not necessarily mean that these two approaches are consistent.

### Introduction

After the Fukushima Daiichi accident, which happened on March 11, 20111)[1], there have been many attempts to estimate fission product source term release to environment2,3)[2]. Figure 1A shows the temporal variation of ambient dose rate measured by a monitoring car at the frontal gate of Fukushima Daiichi site. Figure 1B shows the ambient dose rate measured at Fukushima Prefecture by NaI scintillation detector from March 12 to 17, 2011. Figure 2A shows water level measurement data at Fukushima Daiichi Unit 1. Figure 2B shows pressure measurement data at Fukushima Daiichi Unit 1.
We want to know which reactor unit and which radionuclide generate the peaks in Figure 1. We know already when explosions occurred from the mass media or reports1) [1]. But we don’t know when the fission products start to release to environment from each reactor. We also want to know which radionuclide contributes dominantly to each peak in Figure 1. For this end, fission product release characteristics are evaluated for Fukushima Daiichi Unit 1 as an example case in this study. The Unit 1 event chronology is summarized in Table 1. Chino et al. [3] first estimated atmospheric release of radioactivity to environment by reverse method based on the monitoring results. Tsumune et al. [4] first estimated direct release amount to Pacific Ocean by regional ocean model. Major release to ocean occurred during March 26 to April 6, 2011.
In pages 114–115 of the UNSCEAR 2013 Report [2], the purpose of source term estimation is described and were indicated two kinds of methods to estimate source term from NPP accident to environment impact.
Estimates of the source term (that is the time-dependent release of radioactive material to the environment) were made for two main purposes:
1. To indicate the amounts of radioactive material released to the environment;

2. To be used, in combination with models (for example, for atmospheric and marine dispersion), to support for inferring the dispersion and deposition of radionuclides at locations in the environment where measurements were not available or could no longer be made.

Estimates of the release of radioactive material to the atmosphere can be made using two complementary approaches: (a) one based on analyses of how an accident progressed by using severe accident progression analysis code; and (b) the other based on measurements of radioactive material in the environment and using reverse or inverse methods to reconstruct their transport through the atmosphere back to the source of the release. Both approaches are subjects to own limitations and uncertainties.
Source term release to environment of 131I and 137Cs is summarized in Table 2, which is originally summarized in pages 116–117 of UNSCEAR 2013 report.
The first approach, based on analyses of the progression of an accident, uses severe accident simulation codes, such as MELCOR4), ASTEC [5], MAAP5), etc. There are few publications based on this approach. IRSN6), NISA7,8), Hosh and Hirano [6] used accident progression method. Therefore, in this paper a trial estimation based on the severe accident progression analysis code, MELCOR code9), is presented. In-plant thermal hydraulics, core degradation analysis, and fission product transport in plant compartments are calculated with actual time sequences provided in publically available reports. As an illustration purpose, source term to environment is calculated for Fukushima Daiichi Unit 1 during first 184 hours (about 7.7 days) in detail. And then source term release to environment from Unit 2 and Unit 3 cores are also explained briefly at the end. The issues arising from the using of accident progression method are discussed.
The second approach is based on measurements of radioactive material in the environment and the use of reverse or inverse modelling: there are hundreds of publications based on this approach. Typical results are listed here. Chino et al. [3], Terada et al. [7], Katata et al. [8] used this reverse approach to estimate released amount of radionuclide from Fukushima Daiichi nuclear power plants to atmosphere. ZAMG10), TEPCO11), Mathieu et al. [9] Achim et al. [10] and Kobayashi et al. [11] used reverse method also. Stohl et al. [12], Winiarek et al. [13] and Saunier et al. [14] used inverse method to estimate 137Cs and 131I source term from Fukushima accident.
Source term release to environment of other radionuclides is also well summarized in papers of Lin et al. [15] and Steinhauser et al. [16]. The highest estimate of 131I is about 500 PBq, which is estimated by TEPCO12). The highest estimate of 137Cs is about 37 PBq, which is estimated by Stohl et al. [12].

### Materials and Methods

A severe accident analysis code MELCOR version 1.8.6 is used to estimate fission product generation in core, transport in reactor pressure vessel (RPV), release to primary containment vessel (PCV), and finally to the environment. Figure 3 shows the MELCOR nodalization for (a) RPV and (b) PCV, reactor building (RB), and environment (ENV). Reactor core and lower plenum are modelled with 4 radial rings and 16 axial levels to describe the core degradation and meltdown phenomena in RPV (Figure 4). Axial level 5 represents the core support plate. The release rate of fission products are heavily related on the in-plant accident progression and boundary conditions such as timing and area of RPV and PCV leakage (or rupture). After the thermal hydraulic and core relocation analyses based on the in-plant geometry and operating conditions of the core cooling systems such as isolation condenser, external water injection, and wetwell venting operation are performed, the fission product release to environment is predicted. The initial inventory of the radionuclides in the core estimated by ORIGEN code13) and summarized in JAEA 2012–018 report [17] is used for this study. The environmental release activity up to 100 hours is estimated by multiplying this initial inventory at core by the leakage fractions to environment up to 100 h estimated by MELCOR code. The thermal hydraulic and core degradation analyses for Units 1 and 2 used for this study are described in Kim et al. [18], Kim et al. [19], and OECD/NEA BSAF Phase-I Report (2015)14,15).
When the RPV pressure reaches the safety relief valve (SRV) opening set pressure, SRV is opened and the steam (and hydrogen and fission product aerosols, if generated) in RPV is released to the suppression chamber (SC) (it also called torus or wetwell). If RPV pressure decreases below the SRV closing set pressure, the SRV is reclosed. The SRV continues to cycle to open and close up to the time of steam exhaustion in PRV. The vacuum breaker is located in vent leg (VL) which is located between the SC and the drywell (DW). If the differential pressure between SC and DW exceeds the set pressure, the vacuum breaker (VB) opens. If the steam released to SC is not completely condensed in the SC; and VB opens, the non-condensable gas such as hydrogen and fission product aerosol can move to the pedestal and drywell regions via VB. PCV is inert by nitrogen (N2) gas filling so that hydrogen burning would not occur during at-power operation.
The control volumes in RPV, PCV, and RB are summarized in Table 3. All the possible transient leakage/rupture flow paths are summarized in Table 4. Normally opened flow paths in RPV and PCV are not shown here except in RB. Beside the normal opening flow paths from RB to TB or from RB to ENV, various transient leakages are assumed due to the increasing temperature and pressure of control volumes and heat structures. One thing to remind here is the modeling of DW head flange leakage when DW pressure increases above 0.75 MPa.

### 1. Thermal hydraulic and core meltdown behaviors

Initial and boundary conditions of Fukushima Daiichi Unit 1 used in MELCOR analysis are summarized in Table 5.
In the plant log as shown in Figure 2B and 6C, the RPV pressure was 7 MPa before 5 hours, and it decreased to 0.6 MPa right after 5 hours. Therefore, it is assumed that the SRV stuck open occurs at 5 hours, when the steam is exhausted in RPV. In the plant log it is recorded that the PCV pressure increased up to 0.9 MPa in 12 hours and SC venting was tried at about 23 hours after the reactor scram. Hydrogen explosion occurs at about 25 hours in the top operating floor (refueling bay) of the reactor building. It is stated that the fresh water injection started at 15 hours and the sea water injection started at 28 hours in the plant log. However, it seems to be difficult for the water to reach to the core because of high pressure in the reactor vessel and also there might be various bypass routes and valves may not work appropriately in the plant due to the strong earthquake and tsunami and loss of electrical power, etc. It was discussed in the biannual BSAF meetings16,17). Therefore, such events as hydrogen explosion at 25 hours, fresh water injection at 15 hours and sea water injection at 28 hours are not modeled explicitly in the MELCOR simulation because of the above reasons. Event timings on the reactor and containment thermal hydraulics, the core degradation progression, and the fission product release to environment analyzed by MELCOR code for Fukushima Daiichi Unit 1 are summarized in Table 6.
Figure 5 shows cumulative leakage flow mass for various leakage flow paths. Figure 5A shows integrated leakage flow mass from RPV to PCV via various leakage flow paths. It is assumed that there was total 140 tons of liquid inside the RPV initially (Table 5). Total 80 tons of liquid (water plus steam) leaks from RPV to PCV by cycling opening and closure of the SRV from 1 to 5 hours. The other leakage mechanisms are SRV stuck open, instrument pipe leak, MSL failure, and SRV gasket failure. Total of these leakages are about 60 tons from 5 to 10 hours. Total leakage from RPV to PCV is 140 tons from 1 to 10 hours. Therefore, the most dominant leakage failure mode from RPV to PCV is identified as SRV cycling open and closure.
Figure 5B shows integrated leakage flow mass from PCV to RB via various leakage flow paths. The most dominant failure mode is identified as DW head flange seal failure due to the pressure increase above 0.75 MPa at 19 hours. The leakage mass via this leakage path is about 23 tons. DW pressure increased to 0.84 MPa at 12 hours in the plant log (Table 5, Figure 2B, Figure 6C, Figure 6D). Such a high pressure in DW head flange region could lift shield plugs up. This lift up of shield plugs, hydrogen and non-condensable gases could be released to the top operating floor (refueling bay) of RB. The second and third leakage mechanism are PCV leakage starting at 30 and 77 hours. These assumptions are made for matching pressure trend with plant monitoring curve in the long term (from 30 to 140 hours) as shown in Figure 6D. SC venting at 23 hours is also assumed but it is not shown here because the leakage flow mass is too small due to the too small failure area (FL921 in Table 3). Total 70 tons of liquid leaked out from PCV to RB during 184 hours (Figure 5D).
Figure 5C shows integrated leakage flow mass from RB to ENV via various leakage flow paths. The most dominant failure mode is identified as leakage from the top operating floor (refueling bay) of the reactor building to environment due to the DW head flange seal failure starting at 19 hours. About 60 tons of liquid leaked out from RB to environment during 184 hours (Figure 5D).
Figure 6 shows the thermal hydraulic and core degradation parameters. Figure 6C shows RPV pressure behavior during first 25 hours. The pressure in RPV is decreased during the first one hour due to isolation condenser (IC) operation. The SRV opens and closes cyclically from 1 to 5 hours. It is assumed that the SRV stuck open at 5 hours. Only two RPV pressure points are recorded in the plant monitoring data set; one is 7 MPa at 5.33 hours and the other is 0.4 MPa at 5.72 hours (Figure 2B, Figure 6C, Figure 6D). Due to the repeated opening of SRV, about 140 tons of water in RPV moves to the SC from 1 to 12 hours (Figure 6B). Reactor vessel failure occurs at about 16 hours.
RPV water level decreased to core bottom at about 5 hours (Figure 6A). From this time core heat up and subsequent core relocation occurs due to the loss of coolant inventory. Hydrogen is generated from zirconium water reaction and from steel water reaction (Figure 6G). Metal water reaction is exothermic in nature. When the cladding temperature increases above 900 K, the metal water reaction starts.
##### (1)
$Zr+2H2O=ZrO2+2H2+Q$
The total heats generated by metal water reaction are 3 to 5 times higher than the core decay heat from 5 to 7 hours (Figure 6E). Fuel temperature increases above 2250 K (Figure 6H). Intact fuels collapse down to lower plenum at 7 hours due to the core support plate failure by over-temperature mode.
After reactor vessel fails at 16 hours, molten core concrete interaction (MCCI) occurs in the reactor cavity due to interaction of high temperature corium and concrete in pedestal and drywell floor. Non-condensable gases (CO, H2, CO2, H2O) are generated in cavities due to MCCI. By following BSAF recommendation, it is assumed that drywell head flange leakage occurs when the PCV pressure exceeds 0.75 MPa. It is occurred at 18 hours in the MELCOR simulation (Figure 6C and 6D). Fission product release to atmosphere occurs at the same time. Note that in the actual plant record, DW pressure reached to 0.84 MPa at 12 hours and hydrogen explosion occurred at about 25 hours (Table 5). The potential leakage of steam and non-condensable gases through drywell head flange to the reactor building roof (operating floor, refueling bay) might be the main reason of the hydrogen explosion of Unit 1.
Core components (UO2 fuel, Zircaloy cladding, and supporting material such as stainless steel) can be molten so that it can be engaged in relocation process when the core is heated. Figure 6E shows temporal change of decay power in reactor vessel, cavity 1 and cavity 2. Two cavity model is used in this analysis. Corium ejection from reactor vessel to cavity occurs between 17.5 to 19 hours (Figure 6F).

### 2. Fission product generation and movement behaviors

Release of radionuclides can occur from the fuel-cladding gap by exceeding a failure temperature criterion or losing intact geometry, from material in the core using the various CORSOR empirical release correlations1820) based on fuel temperatures. Release of radionuclides from fuel debris during core-concrete interactions in the reactor cavity is estimated by using the VANESA21) release model. After release to a control volume, masses may exist as aerosols and/or vapors, depending on the vapor pressure of the radionuclide class and the volume temperature. There are three models in CORSOR release correlations: 1) Original CORSOR model, 2) Modified COSOR model (CORSOR-M), and 3) CORSOR-Booth model22).
The original CORSOR model correlates the fractional release rate in exponential form,
##### (2)
$f˙=A exp (B T)for T≥Ti$
where is the release rate (fraction per minute), A and B are empirical coefficients based on experimental data, and T is the core cell component temperature in degrees Kelvin. Different values for A and B are specified for three separate temperature ranges. The lower temperature limit Ti for each temperature range and the A and B values for that range are defined for each class in sensitivity coefficient array to reflect the release rate from fuel. If the cell temperature is below the lowest temperature limit specified (900 K), no release is calculated. Ti is defined as 900, 1,400 and 2,200K for classes other than class 5 (Te). Ti is defined as 900, 1,600 and 2,000 K for classes other than class 5 (Te). Coefficients A and B are shown in page 5 of CORSOR user’s manual23).
The CORSOR-M model correlates the same release data used for the CORSOR model using an Arrhenius form, ko,
##### (3)
$k=ko exp (-Q/R T)$
Where, k is the release rate (min−1) at a given temperature T for a particular species, Q is the activation energy for the release process, R is the gas constant, and ko is the so-called preexponential factor. The values of ko, Q, and T are in units of min−1, kcal/mole, and K, respectively. The value of R is 1.987×10−3 in (kcal/mole)K−1. The values of ko and Q for each class are defined in page 7 of CORSOR user’s manual23).
Twelve radionuclide classes are defined in MELCOR code as shown in Table 7. Aerosol transport and deposition in reactor coolant system (RCS) and containment is handled by MAEROS model24). Thermal analysis during the molten core and concrete interaction (MCCI) process in the cavity are handled by CORCON-Mod325).
Release from fuel and release to environment are studied for some selected radioisotopes of volatile radionuclides (Kr, Xe, Cs, I, Te), non-volatile radionuclides (Sr, Mo, Tc, Sb, Ag) and actinides (Am, Cm, Pu) in this paper. Refer to Koo et al. [20] paper for the classification of fission products according to their volatilities.
The fission products generated from the damaged core are released to the suppression chamber (SC) and drywell (DW) at first by SRV cycling open and closures. After the failure of the lower head penetrations at 20 hours, fission products retained in the molten corium or debris beds are released to the pedestal. Finally they are released to the other locations such as reactor building (RB), turbine building and environment (ENV) through various leak paths already existed before the SC venting operation or after the venting. The explosion at the refueling bay might occur due to leakages of hydrogen from drywell head flange seals degradation due to high pressure and temperature in the drywell top region. Initial inventories of Fukushima Daiichi reactor cores at shutdown is summarized in page 118 in the book of Povinec et al.26) Initial inventories for Units 1, 2, and 3 cores at shutdown only for the volatile radionuclides such as 85Kr, 133Xe, 137Cs, 89Sr, 131I, and 132Te are summarized in Table 8. MELCOR code estimates released fraction to environment in according to the class. The elements in each class has the same release mechanism from fuel, transportation, and deposition mechanism in in-plant compartments. Cumulative released radioactivity for each radionuclide is obtained by the multiplication of initial inventory of each radionuclide by release fraction to environment of corresponding class number.
The release characteristics from fuel and the in-plant transport and deposition behaviors in 4 compartments, RPV, SC, DW, and ENV+, are shown in Figure 7 for 12 radionuclide classes defined in Table 7. Class 1 represents noble gases all which are released to the environment finally. Class 2 (Cs), class 4 (I), and class 5 (Te) show similar behavior. Class 2 deposits more than class 4 and class 5 in RPV. Class 3 (Sr) represents semi-volatile about 23% of inventory is released from fuel. Class 7 (Tc), class 11 (Sb), and class 12 (Ag) show similar Trends and less than 60% of the inventory are released from fuel before 100 h. About 0.5% of inventory of class 9 (La) and class 10 (U) are released from fuel. Less than 0.01% of inventory of class 6 (Ru) are released from fuel. About 0.15% of inventory of class 8 (Ce) are released from fuel.
Temporal variation of cumulative release fraction to environment of 5 volatile classes (class no. 1 to 5) are shown in Figure 8. Temporal variation of cumulative released radioactivity to environment for 6 radionuclides is shown in Figure 9. Temporal release rate of radionuclides to environment which is shown in Figure 10, are obtained by the derivative operation of cumulative release curve of Figure 9. For the short-lived radionuclides, such as 133Xe, 131I, and 132Te, decay in the plant is corrected during the course of release rate calculation.
Estimation for the radioactivity release to atmosphere from Unit 2 and Unit 3 is also conducted with similar way to Unit 1. Cumulative radioactivity released to environment up to 184 hours (7.7 days) after reactor trip for Fukushima Daiichi Units 1, 2, and 3 are summarized in Table 9. Core cooling for Unit 3 is performed by RCIC/HPCI up to 36 hours after reactor trip. Core cooling for Unit 2 is performed by RCIC up to 72 hours after reactor trip. Hydrogen explosion occurred at 68 hours in Unit 3. But, hydrogen explosion is not reported in Unit 2.
Source term release timing and radioactivity analysis results for 131I and 137Cs for Units 1, 2, and 3 are summarized in Table 10. The results are also compared with the results in UNSCEAR 2013 report. First radioactivity release timings estimated by MELCOR are 19, 43, and 98 hours for Unit 1, 3, and 2, respectively. For 131I, the results estimated in this study (754 PBq) is greater than the results reported UNSCEAR 2013 report (95-500 PBq). For 137Cs, the results in this study (29 PBq) is well within the results reported in the UNSCEAR 2013 report (6-37 PBq).
Radioactivity release peak timings to atmosphere are recorded at many monitoring posts at Fukushima Daiichi site and Fukushima Prefecture monitoring posts as shown in Figures 11 and 12, respectively. We know that the release rate peaks before 40 hours originated from Unit 1. Remarkable peak timings occurred roughly at 15, 20, 25, and 30 hours after reactor trip in the real monitoring posts. Monitoring result at Oono has about three hours delayed peaks compared to other monitoring results. The release start time to atmosphere is roughly at 20 hours after reactor trip in this study. Even though hydrogen explosion occurs at 25 hours at the plant, actual DW pressure peak occurs already at 12 hours at the plant (Figure 6C, Table 5).
We can also identify from these figures that the peaks during 40 to 95 hours are originated from Unit 3 release. It is also reported that multiple SC vent valve openings were tried at 42, 45, 54, 63, 97, 107, 150 hours in page 357 of reference TEPCO report [1]. The same assumptions are used in our MELCOR analysis. As shown by the multiple peaks in Figure 10B. Vent valve openings in Unit 2 were also tried at 78 and 81 hours. However, it is not assumed in our analysis that the peaks cannot be seen in Figure 10C.

### Conclusion

Using the initial and boundary conditions which are provided in open publications and OECD/NEA BSAF project, the fission product release timing and amount are estimated by using the MELCOR code. The timing to release to atmosphere estimated by MELCOR code seems to be reasonable compared to real monitoring post records at accident site or remote locations.
Estimated source terms for volatile radionuclides by forward accident progression analysis based on ORIGEN-MELCOR code frame are compatible with or well within the ranges estimated by international research report such as UNSCEAR-2013 and those estimated or summarized in the technical papers which are usually estimated by reverse or inverse method.
The biggest contributing radionuclides from Fukushima Daiichi accident to environment in early term (within a few weeks) are 133Xe, 131I and 132Te. Short half-lives (a few days) of those radionuclides, however, they diminish their strength very shortly. Thereafter, volatile radionuclides which has half-lives of a few years such as 86Kr, 134Cs, 137Cs, and 133I will dominate in the environment.
This estimate is, however, based on the initial 184 hours duration since reactor trip that release estimate would be much varied according to the conditions of the plant thereafter. One of the aspect which can impact to the estimated result is whether MCCI happen in cavity or not. Ground water intrusion into the plant is also one of the important scenario to the estimation. Reasonable estimation would be very helpful for the future work planning, for instance, decommissioning at the site.

### ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT, and Future Planning) (No. NRF-2017M2A8A4015280).

### Notes

1) Nuclear Emergency Response Headquarters (NERHQ). Government of Japan. Report of the Japanese Government to the IAEA Ministerial Conference on Nuclear Safety-The Accident at TEPCO’s Fukushima Nuclear Power Stations. 7 June, 2012. Available at http://japan.kantei.go.jp/kan/topics/201106/iaea_houkokusho_e.html.

2) Ministry of Education, Culture, Sports, Science and Technology, MEXT. Monitoring information of environmental radioactivity levels. http://radioactivity.mext.go.jp/en/.2012.

3) Ministry of Education, Culture, Sports, Science and Technology, MEXT. Environmental radioactivity database. http://search.kankyo-hoshano.go.jp/servlet/search.top.2012.

4) Sandia National Laboratories. R. Gauntt et al. MELCOR Computer Code Manuals Vol. 1: Primer and Users’ Guide. Version 1.8.6. NUREG/CR-6119. Rev 3. SAND2005-5713. September 2005.

5) Electrical Power Research Institute, EPRI. MAAP4 Applications Guidance. Technical Report. Palo Alto, California, 2006. Available at https://www.epri.com/#/pages/product/000000000001020236/.

6) IRSN. L’ IRSN publie une évaluation de la radioactivité rejetée par la centrale de Fukushima Daiichi (Fukushima I) jusqu’au 22 mars 2011. Institut de Radioprotection et de Sûreté Nucléaire. Available at http://www.irsn.fr/fr/actualites_presse/actualites/pages/20110322_evaluation-radioactivite-rejets-fukushima-terme-source.aspx.

7) NISA. INES (the International Nuclear and Radiological Event Scale) rating on the events in Fukushima Dai-ichi Nuclear Power Station by the Tohoku district - off the Pacific Ocean earthquake, April 12, 2011. Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry. Available at http://www.nsr.go.jp/archive/nisa/english/files/en20110412-4.pdf.

8) NISA. Regarding the evaluation of the conditions on reactor cores of Unit 1, 2 and 3 related to the accident at Fukushima Dai-ichi Nuclear Power Station, Tokyo Electric Power Co. Inc. Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry. Available at http://www.nsr.go.jp/archive/nisa/english/press/2011/06/en20110615-5.pdf.

9) Sandia National Laboratories. R. Gauntt et al. MELCOR Computer Code Manuals Vol. 1: Primer and Users’ Guide. Version 1.8.6. NUREG/CR-6119. Rev 3. SAND2005-5713. September 2005.

10) Zentralanstalt für Meteorologie und Geodynamik. Accident in the Japanese NPP Fukushima: Spread of radioactivity/first source estimates from CTBTO data show large source terms at the beginning of the accident/weather currently not favourable/low level radioactivity meanwhile observed over U.S. East Coast and Hawaii. Available at http://www.zamg.ac.at/docs/aktuell/Japan2011-03-22_1500_E.pdf.

11) Tokyo Electric Power Company. Press release (May 24, 2012): Estimation of the released amount of radioactive materials into the atmosphere as a result of the accident in the Fukushima Daiichi Nuclear Power Station. Available at http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120524e0201.pdf.

12) Tokyo Electric Power Company. Press release (May 24, 2012): Estimation of the released amount of radioactive materials into the atmosphere as a result of the accident in the Fukushima Daiichi Nuclear Power Station. Available at http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120524e0201.pdf.

13) Oak Ridge National Laboratory (ORNL). ORIGEN-ARP-2. 2004.

14) Nuclear Energy Agency. OECD/NEA. BSAF Project Website. Available at http://fdada.info.

15) Nuclear Energy Agency. OECD/NEA. Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant. Phase I Final Report. March 2015.

16) Nuclear Energy Agency. OECD/NEA. BSAF Project Website. Available at http://fdada.info.

17) Nuclear Energy Agency. OECD/NEA. Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant. Phase I Final Report. March 2015.

18) Battelle Memorial Institute. Kuhlman MR, Lehmicke DJ, Meyer RO. CORSOR User’s Manual. BMI-2122. NUREG/CR-4173. March 1985.

19) Battelle Memorial Institute. Ramamurthi M, Kuhlman MR. Final Report on Refinement of CORSOR—An Empirical In-Vessel Fission Product Release Model. October 1990.

20) Sandia National Laboratories. Gauntt RO. MELCOR 1.8.5 Modeling Aspects of Fission Products Release, Transport and Deposition. SAND2010-1635. Albuquerque, NM. April 2010.

21) Sandia National Laboratories. Powers DA, Brockmann JE, Shiver AW. VANESA: A Mechanistic Model of Radionuclide Release and Aerosol Generation During Core Debris Interactions with Concrete. NUREG/CR-4308. SAND85-1370. Albuquerque, NM. July 1986.

22) Battelle Memorial Institute. Ramamurthi M, Kuhlman MR. Final report on refinement of CORSOR: An empirical in-vessel fission product release model. October 1990.

23) Battelle Memorial Institute. Kuhlman MR, Lehmicke DJ, Meyer RO. CORSOR User’s Manual. BMI-2122. NUREG/CR-4173. March 1985.

24) Sandia National Laboratories. Gelbard F. MAEROS User Manual. SAND80-0822. NUREG/CR-1391. Albuquerque, NM. December 1982.

25) Sandia National Laboratories. Bradley DR, Gardner DR. CORCON-Mod3: An Integrated Computer Model for Analysis of Molten Core-Concrete Interactions. User’s Manual. SAND92-0167. NUREG/CR-5843. Albuquerque, NM. October 1993.

26) Povinec P, Hirose K, Aoyama M. Fukushima Accident: Radioactivity Impact on the Environment. Elsevier. 2016.

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##### Fig. 1
Gamma dose rate measurement data at Fukushima Daiichi site and Fukushima Prefecture. (A) Measurements at Fukushima Daiichi site from March 11 to 21, 2011, (B) Measurements at Fukushima Prefecture from March 11 to 19, 2011.
##### Fig. 2
Water level and pressure measurement data at Fukushima Daiichi Unit 1. (A) Water level measurement data, (B) Pressure measurement date.
##### Fig. 3
MELCOR nodalization for (A) RPV and (B) PCV, RB, TB, and ENV.
##### Fig. 4
Core part nodalization.
##### Fig. 5
Cumulative leakage flow mass for various leakage flow paths. (A) RPV to PCV, (B) PCV to RB, (C) RB to ENV, and (D) Summary of RPV to PCV, PCV to RB, RB to ENV.
##### Fig. 6
Temporal variation of thermal hydraulic and core degradation parameters estimated by MELCOR code. (A) RPV water level, (B) RPV total liquid mass, (C) RPV pressure (up to 25 h), (D) PCV pressure (up to 140 h), (E) Decay heat varioan, (F) Mass change of core materials, (G) Hydrogen generation in RPV, (H) Maximum fuel and core temperatures.
##### Fig. 7
Temporal transport and deposition behaviors of 12 classes in in-plant compartments (Unit 1) up to 100 h (4.2 days) after reactor trip. (A) Class 1, (B) Class 2, (C) Class 3, (D) Class 4, (E) Class 5, (F) Class 6, (G) Class 7, (H) Class 8, (I) Class 9, (J) Class 10, (K) Class 11, (L) Class 12.
##### Fig. 8
Temporal variation of cumulative release fraction to environment of five volatile fission product classes up to 184 hours (7.7 days) after reactor trip. (A) Unit 1, (B) Unit 3, and (C) Unit 2.
##### Fig. 9
Cumulative radioactivity release to environment up to 184 hours (7.7 days) after reactor trip. (A) Unit 1, (B) Unit 3, (C) Unit 2, and (D) Sum of three units.
##### Fig. 10
Radioactivity release rate to environment up to 184 hours (7.7 days) after reactor trip. (A) Unit 1, (B) Unit 3, (C) Unit 2, and (D) Sum of three units.
##### Fig. 11
Gamma ray dose rate monitored at Fukushima Daiichi site from 0 to 130 hours after reactor trip.
##### Fig. 12
Dose rate monitored by ion chamber (IC) at Fukushima Prefecture monitoring posts from 0 to 130 hours after reactor trip.
##### Table 1
Date/time Time since reactor trip (hr) Event Explicit use in MELCOR analysis?
2011-03-11 14:46 0.00 Earthquake occurs No
2011-03-11 14:47 0.00 Reactor trips occur in Units 1, 2, and 3 Yes
2011-03-11 14:52 0.08 Isolation condenser (IC) starts (A&B) Yes
2011-03-11 15:03 0.27 Isolation condenser (IC) stops (A&B) Yes
2011-03-11 15:27 0.67 Arrival of first tsunami No
2011-03-11 15:35 0.80 Arrival of second tsunami No
2011-03-11 15:37 0.83 Complete loss of AC power (SBO) occurs No
2011-03-12 02:30 11.72 DW pressure reached 840 kPa[abs] No
2011-03-12 05:46 14.98 Fresh water injection starts No
2011-03-12 14:30 23.40 Suppression chamber (SC) vent starts No
2011-03-12 14:50 24.05 Suppression chamber (SC) vent stops No
2011-03-12 15:36 24.82 Hydrogen explosion occurs at top operating floor (refueling bay) of reactor building No
2011-03-12 19:04 28.28 Sea water injection starts No
2011-03-12 20:45 29.97 Sea water injection mixed with boric acid No

* Nuclear Emergency Response Headquarters (NERHQ). Government of Japan. Report of the Japanese Government to the IAEA Ministerial Conference on Nuclear Safety-The Accident at TEPCO’s Fukushima Nuclear Power Stations. 7 June, 2012. Available at http://japan.kantei.go.jp/kan/topics/201106/iaea_houkokusho_e.html.

##### Table 2
Source Terms Estimated for Fukushima Daiichi Accident by Various Sources
Reference Date of publication 131I 137Cs Approach used Time period of data
IRSN* 22 March 2011 90 10 Accident progression 12 to 22 March 2011
ZAMG 22 March 2011 400 33 Reverse modelling 12 to 15 March 2011
NISA 12 April 2011 130 6 Accident progression 12 to 18 March 2011
NISA§ 6 June 2011 160 15 Accident progression 12 to 31 March 2011
Chino et al. [3] July 2011 150 13 Reverse modelling
NISA 16 February 2012 150 8 Accident progression
Stohl et al. [12] 1 March 2012 - 37 Inverse modelling
Winiarek et al. [13] 9 March 2012 190–380 12 Inverse modelling
TEPCOII 24 May 2012 500 10 Reverse modelling 12 to 31 March 2011
Terada et al. [7] 19 June 2012 120 9 Reverse modelling
Mathieu et al. [9] June 2012 197 20.6 Forward and Reverse modelling
Katata et al. [8] July 2012 130 11 Reverse modelling
Hosh and Hirano [6] 17 September 2012 250–340 7.3–13 Reverse modelling 11 to 17 March 2011
Achim et al. [10] September 2012 400 10 Reverse modelling
Kobayashi et al. [11] 15 March 2013 200 13 Reverse modelling
Saunier et al. [14] 25 November 2013 106 15.5 Inverse modelling

* IRSN. L’ IRSN publie une évaluation de la radioactivité rejetée par la centrale de Fukushima Daiichi (Fukushima I) jusqu’au 22 mars 2011. Institut de Radioprotection et de Sûreté Nucléaire. Available at http://www.irsn.fr/fr/actualites_presse/actualites/pages/20110322_evaluation-radioactivite-rejets-fukushima-terme-source.aspx.

Zentralanstalt für Meteorologie und Geodynamik. Accident in the Japanese NPP Fukushima: Spread of radioactivity/first source estimates from CTBTO data show large source terms at the beginning of the accident/weather currently not favourable/low level radioactivity meanwhile observed over U.S. East Coast and Hawaii. Available at http://www.zamg.ac.at/docs/aktuell/Japan2011-03-22_1500_E.pdf.

NISA. INES (the International Nuclear and Radiological Event Scale) rating on the events in Fukushima Dai-ichi Nuclear Power Station by the Tohoku district - off the Pacific Ocean earthquake, April 12, 2011. Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry. Available at http://www.nsr.go.jp/archive/nisa/english/files/en20110412-4.pdf.

§ NISA. Regarding the evaluation of the conditions on reactor cores of Unit 1, 2 and 3 related to the accident at Fukushima Dai-ichi Nuclear Power Station, Tokyo Electric Power Co. Inc. Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry. Available at http://www.nsr.go.jp/archive/nisa/english/press/2011/06/en20110615-5.pdf.

II Tokyo Electric Power Company. Press release (May 24, 2012): Estimation of the released amount of radioactive materials into the atmosphere as a result of the accident in the Fukushima Daiichi Nuclear Power Station. Available at http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120524e0201.pdf.

##### Table 3
Control Volume (CV) Assignment
CV number CV Name
RPV CV310 Downcomer
CV320 Lower plenum
CV330 Core bypass
CV340 Core channel
CV350 Shroud dome
CV360 Steam dome

PCV CV100 Pedestal
CV101 Drywell (DW)
CV105 Vent leg (VL)
CV200 Wetwell (SC)

RB/TB/ENV CV401 Torus room
CV402 to CV407 Reactor building (RB)
CV408 Refueling bay/Operating floor (OF)
CV409 Turbine building (TB)
CV410 Environment (ENV)
##### Table 4
Leakage and Rupture Flowpath (FL) Assignment
FL number From CV To CV FL name Opening condition Maximum Flow area (m2)
From RPV to PCV FL362 360 200 SRV cyclic open and closure Open at 7.3 MPa, reclose at 7.0 MPa 7.54×10−3
FL999 360 200 SRV stuck open At 5 h 7.54×10−3
FL598 360 101 SRVs gasket leakage Steam dome temperature >773 K 2.26×10−4
FL901 320 101 Instrument pipe leakage Lower plenum temperature >1,000 K 1.4×10−4
FL903 360 101 MSL flange leakage Pipe creep rupture condition
PRPV–PPCV>200 kPa under
PPCV >7 MPa
1.36×10−3
FL399 320 100 Vessel breach RPV lower head penetration failure 1.0×10−2

From PCV to RB FL398 101 408 DW head flange seals failure PPCV >7.5 MPa 1.2×10−2
FL907 101 402 PCV leakage at 30 hr after reactor trip Time >30 hr 2.0×10−4
FL909 101 402 PCV leakage at 77 hr after reactor trip Time >77 hr 2.5×10−4
FL921 200 402 SC venting 23.76 hr<Time<24.73 hr 3.63×10−3

From RB to ENV FL404 402 410 RB to ENV Normal leakage 2.5×10−2
FL406 403 410 RB to ENV Normal leakage 2.5×10−2
FL409 404 410 RB to ENV Normal leakage 2.5×10−2
FL413 406 410 RB to ENV Normal leakage 2.5×10−2
FL414 407 410 RB to ENV Normal leakage 2.5×10−2
FL415 408 410 RB to ENV For RB explosion simulation 22.3
FL416 408 410 RB to ENV Normal leakage 1.09×10−1
FL417 409 410 RB to ENV Normal leakage 2.9×10−1
##### Table 5
Initial and Boundary Conditions of Fukushima Daiichi Unit 1 Used in MELCOR Analysis
Parameters (unit) Value
Initial conditions Thermal power (MWth) 1,380
Initial RPV water level (m) 13.5
Initial RPV pressure (MPa) 7.0

Free volume RPV (m3) 258
Drywell (m3) 3,543
Suppression chamber (m3) 2,620
Reactor building (m3) 63,522

Initial liquid mass RPV (ton) 141
Suppression chamber (ton) 1,025

Core material mass UO2 (ton) 82.1
Zircaloy (ton) 30.5
Stainless Steel (ton) 54.4
B4C (Control Rod Poison) (ton) 1.28
##### Table 6
Thermal Hydraulic and Core Degradation Event Timings Analyzed by MELCOR Code
No. Events Time (sec) Time (hr) Date
1 Reactor trip occurs 0 0.00 2011-03-11 14:47
2 SRV starts to open and close cyclically 3,960 1.10 2011-03-11 15:53
3 Uncovery of top of active fuel (TAF) occurs 10,182 2.83 2011-03-11 17:36
4 SRV gasket seals failure 15,961 4.43 2011-03-11 19:13
5 Gap release occurs in Ring 1 16,481 4.58 2011-03-11 19:21
6 Gap release occurs in Ring 2 16,484 4.58 2011-03-11 19:21
7 Gap release occurs in Ring 3 16,505 4.58 2011-03-11 19:22
8 Gap release occurs in Ring 4 16,772 4.66 2011-03-11 19:26
9 Uncovery of bottom of active fuel (BAF) occurs 17,953 4.99 2011-03-11 19:46
10 Core instrument pipe (CIP) leakage occurs 18,273 5.08 2011-03-11 19:51
11 SRV stuck open (assumed) 18,720 5.20 2011-03-11 19:59
12 Main steam line (MSL) leakage occurs 18,877 5.24 2011-03-11 20:01
13 Core support plate (CSP) failure by over-temperature occurs (Ring 1) 21,368 5.94 2011-03-11 20:43
14 Debris quenching in lower plenum (Ring 1) 21,369 5.94 2011-03-11 20:43
15 CSP failure by over-temperature occurs (Ring 3) 25,309 7.03 2011-03-11 21:48
16 Debris quenching in lower plenum (Ring 3) 25,312 7.03 2011-03-11 21:48
17 CSP failure by over-temperature occurs (Ring 2) 25,330 7.04 2011-03-11 21:49
18 Debris quenching in lower plenum (Ring 2) 25,331 7.04 2011-03-11 21:49
19 CSP failure by over-temperature occurs (Ring 4) 25,785 7.16 2011-03-11 21:56
20 Debris quenching in lower plenum (Ring 4) 25,788 7.16 2011-03-11 21:56
21 Lower plenum dryout (Liquid level <10 CM) 35,629 9.90 2011-03-12 0:40
22 CSP failure by loss of mass occurs (Ring 1) 37,495 10.42 2011-03-12 1:11
23 RPV pressure equals PCV pressure 37,980 10.55 2011-03-12 1:20
24 CSP failure by loss of mass occurs (Ring 2) 39,382 10.94 2011-03-12 1:43
25 CSP failure by loss of mass occurs (Ring 3) 39,808 11.06 2011-03-12 1:50
26 CSP failure by loss of mass occurs (Ring 4) 46,003 12.78 2011-03-12 3:33
27 Lower head penetration failure occurs (Ring 4) 56,485 15.69 2011-03-12 6:28
28 Lower head penetration failure occurs (Ring 3) 57,163 15.88 2011-03-12 6:39
29 Lower head penetration failure occurs (Ring 1) 59,911 16.64 2011-03-12 7:25
30 Lower head penetration failure occurs (Ring 2) 61,594 17.11 2011-03-12 7:53
31 Debris ejection to cavity occurs 63,259 17.57 2011-03-12 8:21
32 Cavity 1 wake up 63,259 17.57 2011-03-12 8:21
33 Drywell head flange seals failure occurs (0.75 MPa) 64,459 17.91 2011-03-12 8:41
34 Cavity 2 wake up 65,178 18.11 2011-03-12 8:53
35 End of debris quenching in Ring 4 66,465 18.46 2011-03-12 9:14
36 End of debris quenching in Ring 3 67,248 18.68 2011-03-12 9:27
37 End of debris quenching in Ring 2 67,341 18.71 2011-03-12 9:29
38 End of debris quenching in Ring 1 68,049 18.90 2011-03-12 9:41
39 Leakage area from PCV to RB increased 107,998 30.00 2011-03-12 20:46
40 Drywell head flange seals failure occurs 109,297 30.36 2011-03-12 21:08
41 Leakage area from PCV to RB increased 277,196 77.00 2011-03-14 19:46
42 End of MELCOR simulation 662,400 184.00 2011-03-19 6:47
##### Table 7
Radionuclide classes used in MELCOR fission product release analysis
Class Number and Name Member Elements
1. Noble Gases Xe, Kr, (RN), (He), (Ne), (Ar), (H), (N)
2. Alkali Metals Cs, Rb, (Li), (Na), (K), (Fr), (Cu)
3. Alkaline Earths Ba, Sr, (Be), (Mg), (Ca), (Ra), (Es), (Fm)
4. Halogens I, Br, (F), (Cl), (At)
5. Chalcogens Te, Se, (S), (O), (Po)
6. Platinoids Ru, Pd, Rh, (Ni), (Re), (Os), (Ir), (Pt), (Au)
7. Transition Metals Mo, Tc, Nb, (Fe), (Cr), (Mn), (V), (Co), (Ta), (W)
8. Tetravalents Ce, Zr, (Th), Np, (Ti), (Hf), (Pa), (Pu), ©
9. Trivalents La, Pm, (Sm), Y, Pr, Nd, (Al), (Sc), (Ac), (Eu), (Gd), (Tb), (Dy), (Ho), (Er), (Tm), (Yb), (Lu), (Am), (Cm), (Bk), (Cf)
10. Uranium U
11. More Volatile Main Group Metals (Cd), (Hg), (Pb), (Zn), As, Sb, (Tl), (Bi)
12. Less Volatile Main Group Metals Sn, Ag, (In), (Ga), (Ge)

* The elements in ( ) does contribute to less than one percent of total decay heat.

##### Table 8
Initial Core Inventory at Shutdown for Fukushima Daiichi Unit 1, 2, 3 Core
RN Class No. RN Half Life (Unit) Unit 1 (PBq) Unit 2 (PBq) Unit 3 (PBq) Sum (PBq)
1 85Kr 10.756 years 23 31 29 83
1 133Xe 5.24 days 2,710 4,670 4,670 12,050
2 134Cs 2.065 years 190 277 252 719
2 137Cs 30.04 years 203 256 241 700
3 89Sr 50.53 days 1,360 2,210 2,350 5,920
4 131I 8.02 days 1,350 2,340 2,330 6,020
5 132Te 3.2 days 1,950 3,360 3,370 8,680
Total 7,786 13,144 13,242 34,172
##### Table 9
Radioactivity Released to Environment up to 184 hours (7.7 days) after Reactor Trip for Fukushima Daiichi Unit 1, 2, 3
RN Unit 1 (PBq) Unit 2 (PBq) Unit 3 (PBq) SUM (PBq) Unit 1 (%) Unit 2 (%) Unit 3 (%) Sum (%)
85Kr 23 28 26 77 100.0 90.3 89.7 92.8
133Xe 2,014 2,432 3,171 7,617 74.3 52.1 67.9 63.2
134Cs 6 12 12 30 3.2 4.5 4.6 4.2
137Cs 6 12 11 29 3.2 4.5 4.6 4.1
89Sr 43 23 36 102 3.2 1.0 1.5 1.7
131I 118 377 259 754 8.7 16.1 11.1 12.5
132Te 68 157 281 506 3.5 4.7 8.3 5.8
Total 2,278 3,041 3,795 9,115 29.3 23.1 28.7 26.7
##### Table 10
Summary of Atmospheric Source Term Estimates for Fukushima Daiichi Units 1, 2, and 3
Unit Core cooling method Duration of core cooling Hydrogen explosion timing recorded at the plant log First radioactivity release timing estimated by MELCOR Cumulative radioactivity release to environment up to 184 hr (7.7 days)

131I 137Cs
Unit 1 IC 1 hr 25 hr 19 hr 118 6

Unit 2 RCIC 72 hr - 98 hr 377 12

Unit 3 RCIC/HPCI 36 hr 68 hr 43 hr 259 11

Total (this study) 754 29
UNSCEAR 2013 report 95–500* 6–37*

* These values are thought to be estimated from the various date sources in Table 2 which may be not necessarily consistent one another as the measurement locations, times, period and methods are different, thus can be regarded to be subject to not negligible uncertainties.

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