atw Vol. 63 (2018) | Issue 8/9 ı August/September
References
476
1. Tsuchiya A, Kondo T, Maruyama H.
Criticality calculation of fuel debris in
Fukushima Daiichi nuclear power station.
In: PHYSOR 2014. Kyoto, Japan; 2014.
AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP
| | Fig. 4.
Debris size – Burnup Unit 1 Fukushima Daiichi criticality map.
critica lity cannot be reached. A boration
of 1,600 ppm B will ensure the
subcriticality independently of the
debris bed conditions.
Figure 4 provides criticality data
as function of the debris size and
burnup. It can be noticed how the k eff
decreases progressively with the
burnup of the core. If the SA happens
at the very end of a fuel cycle, when
the average burnup of the fuel is larger
than 53 GWd/t HM , recriticality will
not be reached under any conditions.
Additionally, the graph provides
the information about the criticality
condition of a debris bed depending of
its size. With these data, the critical
masses for the different burnups
can be calculated. The burnup of
Fukushima Unit 1 at the moment of
the accident was estimated to be
25.8 GWd/t HM . The minimum critical
size of a debris bed for this case is
about 55 cm. For these conditions, the
optimum porosity was calculated to
be 0.75. This results in critical mass of
226.5 kg, which represents only the
2.4 % of the core.
Conclusions
In this study, a conservative criticality
evaluation of the current debris bed
of Fukushima Daiichi Unit 1 was
performed. The lack of knowledge
regarding the debris bed properties
has compelled the use of very conservative
assumptions in the debris
bed models. Six of the most influencing
parameters on the k eff were considered:
debris size/mass, particle size,
porosity, water density and content of
boron in water. The effect of these parameters
on the criticality condition of
Fukushima Daiichi Unit 1 debris bed
was calculated and discussed. Finally,
it was concluded that recriticality can
be totally excluded if:
1. Porosity of the debris bed is lower
than 0.4 or
2. Void fraction of water is higher
than 78 % or
3. Debris mass is lower than 226.5 kg
or
4. Boration in water is equal or
greater than 1,600 ppm B
Additionally, for a reactor core with
UO 2 fuel and initial enrichment of
3.7 % wt 235 U it was found that if a
SA occurred at the very end of a fuel
cycle when the average burnup is
53 GWd/t HM or higher, recriticality is
not achievable under any conditions.
Taking severe accident scenarios
into account, the void fraction threshold
(2) and the debris mass threshold
(3) will be violated under almost all
circumstances. The molten mass
easily reaches values higher than
226 kg, which represents only 2 % of
the core mass, and the void fraction
does not stay at values higher than
78 % for the range of cool temperatures
considered. However, experiments
like DEFOR [11] or FARO [12]
indicate average porosities of about
38 %, which is slightly underneath
the “criticality safe” threshold (1) for
porosity.
As a next step, it is planned to
include new parameters, for example,
the presence of zirconium, control
rods or other reactor structural materials
in order to evaluate their
influence on the criticality of debris
beds. Additionally, new debris bed
configurations will be also investigated.
The first samples and explorations
of debris beds in Fukushima are
planned for this year 2018. This
will provide more information
about the debris characteristics and
will allow a less conservative
and more accurate criticality evaluation.
Acknowledgments
The presented work was funded by
the German Ministry for Economic
Affairs and Energy (BMWi. Project no.
1501533) on basis of a decision by the
German Bundestag.
2. Kotaro Tonoike, Hiroki Sono, Miki Umeda,
Yuichi Yamane, Teruhiko Kugo, Kenya
Suyama. Options of Principles of Fuel Debris
Criticality Control in Fukushima Daiichi
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Back-end and Transmutation Technology
for Waste Disposal. Springer Open;
2015. p. 251–60.
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Decommissioning Facilitation Corporation.
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Decommissioning of the Fukushima
Daiichi Nuclear Power Station of Tokyo
Electric Power Company Holdings, Inc.
2016 Jul.
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Booth, Thomas E., Brown, Forrest B., Bull,
Jeffrey S., Cox, Lawrence J., et al. Initial
MCNP6 Release Overview – MCNP6 version
1.0. Los Alamos National Laboratory
(LA-UR-13-22934); 2013.
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Nuclear Station Unit II Defueling
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6. Freiría López M, Buck M, Starflinger J.
Neutronic Modelling of Fuel Debris for a
Criticality Evaluation. In: PHYSOR 2018.
Cancun, Mexico; 2018.
7. International Atomic Energy Agency
(IAEA). The Fukushima Daiichi Accident
Technical Volume 1/5 Description and
Context of the Accident Annexes.
Vienna (Austria): International Atomic
Energy Agency (IAEA); 2015.
8. Nishihara K, Iwamoto H, Suyama K.
Estimation of fuel compositions in
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Japan Atomic Energy Agency; 2012.
9. Croff AG. ORIGEN 2.1. Oak Ridge
National Laboratory; 1991.
10. Nuclear Safety Standards Commission
(Kerntechnischer Ausschuss, KTA).
Storage and Handling of Fuel Assemblies
and Associated Items in Nuclear
Power Plants with Light Water Reactors.
2003 Nov. Report No.: KTA 3602.
11. Kudinov P, Karbojian A, Tran C-T,
Villanueva W. Agglomeration and size
distribution of debris in DEFOR-A
experiments with Bi2O3–WO3 corium
simulant melt. Nucl Eng Des.
2013;263(Supplement C):284–95.
12. Magallon D. Characteristics of corium
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2009.
Authors
María Freiría López
Dr.-Ing. Michael Buck
Prof. Dr.-Ing. Jörg Starflinger
Responsible Professor
Institute of Nuclear Technology
and Energy Systems (IKE)
University of Stuttgart
Pfaffenwaldring 31
70569 Stuttgart, Germany
AMNT 2018 | Young Scientists' Workshop
A Preliminary Conservative Criticality Assessment of Fukushima Unit 1 Debris Bed ı María Freiría López, Michael Buck and Jörg Starflinger
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