Research and Development Relating to the Accident at the TEPCO’s Fukushima Daiichi NPS

1-17 Calculation of Decay Heat Causing Core Meltdown－ Three-Dimensional Decay Heat Analysis for the TEPCO’s Fukushima Daiichi NPS －(a) Calculation method
Use of the MOSRA code systemdeveloped by JAEA and the Japaneselatest nuclear data library JENDL-4.0(b) Decay heat distribution
at the time of core shutdown（水平断面図）12341312977869 10 11 12 13 145 4 32 169877869 10 11 12 13 14543210.55 0.66 0.60 0.72 0.60 0.60 0.72 0.80 0.60 0.60 0.80 0.72 0.60
0.68 0.82 0.75 0.90 0.75 0.75 0.90 1.00 0.75 0.75 1.00 0.90 0.75

220.77 0.92 0.83 1.01 0.83 0.83 1.01 1.12 0.83 0.83 1.12 1.01 0.83

1.2
1.00.80.81 0.89 1.19 0.98 1.07 1.19 0.98 1.07 1.19 0.98 1.07 0.98 1.19

A 15230.81 0.89 1.07 1.19 0.98 1.07 1.19 0.98 1.07 1.19 0.98 1.19 1.07

2411 10240.81 0.98 0.98 1.07 1.19 0.89 1.07 1.19 0.89 0.89 1.19 0.98 0.89

33

0.81 0.81 0.98 0.89 0.89 0.98

0.81 0.89 0.98 1.07 1.19 0.98 1.07 0.89 0.89 1.07 1.19 0.89

42

0.36 0.43 0.39 0.47 0.39 0.39 0.47 0.52 0.39 0.39 0.52 0.47 0.39

0.81 0.81 0.81 0.89 0.89 0.98 1.07 1.19 0.89

0.81 0.89 1.19 0.98 1.07 1.19 0.89 1.07 1.19 0.98 1.07 0.98

5114 13 1225

0.81 1.07 0.98 1.07 1.19 0.98 1.07 1.19 0.98 1.07

0.81 1.07 1.19 0.98 0.98 1.19 0.98 1.07 1.19 0.98 1.19

6

at the time of core shutdown
Z＼X0.81 0.98 0.89 0.89 0.98 1.07 1.19 0.89 1.07 1.19 0.89

725 axial regions

80.81 0.89 0.89 1.19 0.98 1.07 1.19 0.98 1.07 0.89

8

Fuel assembly

Burnup distribution

6

0.81 0.81 0.81 0.81

10Void distribution

91411

The Following are calculated by region,(1) Neutron spectrum (200-group)(2) Reaction rate (e.g. number of fissions)(3) Nuclide-wise concentration (>1000 nuclides)(4) Decay heat (Sum of the nuclide-wise decay heat)5Y＼X 14 13 12 11 100.81 0.98 0.89 1.07 0.89 0.89 1.07 1.19 0.89 0.89 1.19 1.07 0.89

after about 80 h123456789 10 11 12 13 14
Y＼X 14 13 12 11 10 9 8 7 6 5 4 3 2 114210.81 0.97 0.88 1.07 0.88 0.88 1.07 1.19 0.88 0.88 1.19 1.07 0.88

200.83 1.00 0.91 1.10 0.91 0.91 1.10 1.22 0.91 0.91 1.22 1.10 0.91

190.86 1.03 0.93 1.13 0.93 0.93 1.13 1.25 0.93 0.93 1.25 1.13 0.93

180.88 1.05 0.95 1.15 0.95 0.95 1.15 1.28 0.95 0.95 1.28 1.15 0.95

170.89 1.07 0.97 1.17 0.97 0.97 1.17 1.30 0.97 0.97 1.30 1.17 0.97

16

0.90 1.08 0.98 1.18 0.98 0.98 1.18 1.31 0.98 0.98 1.31 1.18 0.98

15

0.91 1.09 0.99 1.20 0.99 0.99 1.20 1.33 0.99 0.99 1.33 1.20 0.99

14

0.92 1.10 1.00 1.21 1.00 1.00 1.21 1.34 1.00 1.00 1.34 1.21 1.00

13

0.92 1.10 1.00 1.21 1.00 1.00 1.21 1.34 1.00 1.00 1.34 1.21 1.00

12

0.92 1.11 1.00 1.21 1.00 1.00 1.21 1.35 1.00 1.00 1.35 1.21 1.00

110.92 1.11 1.00 1.22 1.00 1.00 1.22 1.35 1.00 1.00 1.35 1.22 1.00

100.93 1.12 1.01 1.22 1.01 1.01 1.22 1.36 1.01 1.01 1.36 1.22 1.01

0.94 0.94 0.94 0.94

90.94 1.12 1.02 1.23 1.02 1.02 1.23 1.37 1.02 1.02 1.37 1.23 1.02

120.94 0.94 1.03 0.99 0.99 1.03

80.93 1.12 1.01 1.23 1.01 1.01 1.23 1.36 1.01 1.01 1.36 1.23 1.01

110.94 0.94 0.94 0.99 0.99 1.03 1.05 0.98 0.99

1070.93 1.11 1.01 1.22 1.01 1.01 1.22 1.35 1.01 1.01 1.35 1.22 1.01

0.94 1.05 1.03 1.05 0.98 1.03 1.05 0.98 1.03 1.05

60.91 1.10 0.99 1.20 0.99 0.99 1.20 1.33 0.99 0.99 1.33 1.20 0.99

50.89 1.07 0.97 1.18 0.97 0.97 1.18 1.30 0.97 0.97 1.30 1.18 0.97

40.84 1.01 0.92 1.11 0.92 0.92 1.11 1.23 0.92 0.92 1.23 1.11 0.92

30.76 0.91 0.83 1.00 0.83 0.83 1.00 1.11 0.83 0.83 1.11 1.00 0.83

20.63 0.76 0.69 0.84 0.69 0.69 0.84 0.92 0.69 0.69 0.92 0.84 0.69

10.48 0.57 0.52 0.63 0.52 0.52 0.63 0.69 0.52 0.52 0.69 0.63 0.52

13

90.94 0.99 0.99 0.98 1.03 1.05 0.98 1.03 1.05 0.99

80.94 1.03 0.99 0.99 1.03 1.05 0.98 0.99 1.05 0.98 0.99

7

0.94 1.05 0.98 1.03 1.03 0.98 1.03 1.05 0.98 1.03 0.98

60.94 0.99 0.98 1.03 1.05 0.98 0.99 1.05 0.98 1.03 1.05 1.03

51.21.00.94 0.99 1.03 1.05 0.98 1.03 1.05 0.99 0.99 1.05 0.98 0.99

40.94 1.03 1.03 1.05 0.98 0.99 1.05 0.98 0.99 0.99 0.98 1.03 0.99

30.94 0.99 1.05 0.98 1.03 1.05 0.98 1.03 1.05 0.98 1.03 0.98 1.05

20.94 0.99 0.98 1.03 1.05 0.98 1.03 1.05 0.98 1.03 1.05 1.03 0.98

10.94 1.03 0.99 1.05 0.99 0.99 1.05 0.98 0.99 0.99 0.98 1.05 0.99

Horizontal distribution

0.8

1.37
0.86
0.36

1.00

Axial distribution(A-A’ cross-sectional surface)(normalized to core average)

Fig.1-38 Calculation method and sample result of decay heat distribution in 1F2
The 3D decay heat distribution was calculated considering the distributions of the neutron spectra, burnup, and void fraction.
The fuel loading pattern was based on that of a typical boiling water reactor.

Even after a nuclear reactor is shut down, nuclides such as
fission products (FPs) decay, and decay heat is generated. The
accident at the TEPCO’s Fukushima Daiichi NPS (1F) wascaused by a failure of the ultimate decay heat removal becauseof the loss of electric power due to the tsunami.
Understanding the core melt progression in accidents
enables the location of melted fuel to be estimated andsupports further improvements in the safety of nuclear
reactors. The detailed analysis of the core melt progression
begins with a prediction of the decay heat distribution in thecore at the time of shutdown. Although there are some simple
formulae for estimating the decay heat, they do not yield an
accurate time-dependent decay heat distribution in the core.
Thus, we newly developed data and a code to predict thetime-dependent three-dimensional decay heat distribution.
They were applied to the decay heat analysis of each core of
1F1, 1F2, and 1F3. In this method, each core is divided into
four or five horizontal regions corresponding to different
fuel assembly types or refueling batches, and each region isfurthermore divided into 25 axial regions that have different

burnup values and void fractions, as shown in Fig.1-38 (a).
For each region, the nuclide-wise concentration is calculated
for more than 1000 nuclides considering the operating
conditions of each plant until core shutdown. Then, we canobtain the nuclide-wise decay heat in each region using thedecay and heat generation constants. The region-wise decay
heat is calculated by summing the nuclide-wise decay heat.
Fig.1-38 (b) shows the obtained decay heat distribution of
1F2, the core melting time of which is estimated to be about
80 h after core shutdown. The decay heat at the time of core
shutdown is approximately proportional to the power density
under normal operating conditions because the decay heat justafter shutdown is dominated by the concentrations of shortlived FPs, which are in equilibrium according to the power
density of each region. After that, the decay heat distribution
gradually becomes flatter owing to the disappearance of theshort-lived FPs. The obtained three-dimensional decay heat
information is useful for future detailed analyses of the coremelting progression because the decay heat is an importantparameter for the movement of melted materials.

Reference
Okumura, K. et al., Nuclear Data for Severe Accident Analysis and Decommissioning of Nuclear Power Plant, Proceeding of the 2012 Symposium on
Nuclear Data, November 15-16, 2012, Research Reactor Institute, Kyoto University, Kumatori, Japan, JAEA-Conf 2013-002, 2013, p.15-20.

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JAEA R&D Review 2013