atw 2018-09v3


atw Vol. 63 (2018) | Issue 8/9 ı August/September

[4] K. Amend and M. Klein. Modeling and

Simulation of Water Flow on Containment

Walls with Inhomogeneous

Contact Angle Distribution. ATW

International Journal for Nuclear

Power, 62(7):477-481, 2017.

[5] B. von Laufenberg, M. Colombet, and

M. Freitag. Wash-down of insoluble

aerosols Results of the Laboratory Test

related to THAI AW3 Test. Technical

report, Becker Technologies, 2014.

[6] K. Amend and M. Klein. Simulation of

Water Flow down inclined Containment

Walls. 14 th Multiphase Flow

Conference, Dresden, 2016.

[7] K. Amend and M. Klein. Influence of the

contact angle model on gravity driven

water films. 13 th Multiphase Flow

Conference, Dresden, 2015.

[8] R. K. Singh, J. E. Galvin, and X. Sun.

Three-dimensional simulation of rivulet

and film flows over an inclined plate:

Effects of solvent properties and contact

angle. Chemical Engineering Science,

142:244–257, 2016.

[9] A. Hoffmann. Untersuchung mehrphasiger

Filmströmungen unter

Verwendung einer Volume-Of-Fluidähnlichen

Methode. PhD thesis,

Technische Universität Berlin, 2010.

[10] Y. Iso, X. Chen. Flow transition behavior

of the wetting flow between the film

flow and rivulet flow on an inclined

wall. Journal of Fluids Engineering

133.9:091101, 2011.

[11] I. Ausner. Experimentelle Untersuchungen

mehrphasiger Filmströmungen.

PhD thesis, Technische

Universität Berlin, 2006.

A Preliminary Conservative Criticality

Assessment of Fukushima Unit 1 Debris


María Freiría López, Michael Buck and Jörg Starflinger

[12] J. Guo. Hunter Rouse and Shields

diagram. Advances in Hydraulic and

Water Engineering, 2:1096–1098,


[13] R. Ariathurai. A finite element model of

cohesive sediment transportation. PhD

thesis, University of California, Davis,

California, 1974.


Katharina Amend

Prof. Dr.-Ing. habil. Markus Klein

Responsible Professor

Institute for Numerical Methods in

Aerospace Engineering Universität

der Bundeswehr München

Werner Heisenberg Weg 39

85577 Neubiberg, Germany







1 Introduction On March 11, 2011, a big severe accident occurred at Fukushima Daiichi nuclear power plant

(NPP) in Japan resulting in largely melted cores of Units 1, 2 and 3. After the corium solidification, debris beds

were formed and they are considered to be distributed not only in the reactor pressure vessel but also in the primary

containment. If such debris enter in contact with water, recriticality becomes possible. To prevent recriticality, severe

accident mitigation measures prescribe the injection of borated water into the reactor core. However, some leakage of

cooling water and the inflow of groundwater into the reactor building make it very difficult to maintain the necessary

boron concentration to secure the subcritical condition. Currently, the subcriticality of the debris bed is being monitored

by measurements of short lifetime fission products gas (e.g. Xe 133 or Xe 135 ) and water temperature [1]. As no sign of

criticality has been detected until now, the fuel debris is estimated to be subcritical and no preventive measure against

a possible recriticality event is being taken [2]. Nonetheless, this apparently critical-stable condition can change at any

moment due to changes in debris conditions. During the retrieval operations, changes in the water level and debris

shape are expected to occur that will endanger this stability. Thus, using borated water is then planned to ensure the

subcriticality [3].

María Freiría López

was awarded with the

3 rd price of the 49 th

Annual Meeting on

Nuclear Technology

(AMNT 2018) Young

Scientists' Workshop.

A recriticality scenario would lead to a

power increase, new fission products

release and may have severe consequences

even causing a secondary

criticality accident. Prevention and

controlling core sub-criticality is

there fore one of the main accident

management objectives. A risk evaluation

of recriticality is necessary for

the safe preservation and handling of

fuel debris.

This study is part of a larger project,

which pursues to assess the

recriticality potential of fuel debris

after a severe accident taking

­Fukushima as reference. The final

aim is to develop a criticality map that

will be used to evaluate the potential

risk of criticality of a fuel debris

taking the debris conditions as input

parameters. The criticality situation of

Fukushima damaged reactors will be

assessed by placing onto the map the

fuel debris conditions revealed by

observations or sample analyses.

In this study, a conservative

criticality evaluation of the Fukushima

Daiichi Unit 1 debris bed was carried

out. Parameters, such as debris size,

porosity, particle size, fuel burnup

and the coolant conditions including

the water density and the content of

boron were considered. The effect of

these parameters on the neutron

multiplication factor was analysed

and safety parameter ranges, i.e.

zones where the recriticality can be

totally excluded, have been identified.

The objective is to fix some boundaries

for the selected parameters

and define the ranges in which the recriticality

could be an issue. This will

provide the starting point for a future

more detailed criticality evaluation.

The Monte Carlo code MCNP6.1

was used to model the hypothetical

debris bed and to calculate the

neutron multiplication factor (k eff )

[4]. The ENDF/B-VII.1 cross section

libraries were used to perform the


2 Criticality of debris bed

after a severe accident

After a severe accident (SA), recriticality

occurs when the whole or part of the

reactor becomes unintentionally critical

after the reactor shutdown. This

study focuses on the analysis of recriticality

in debris beds that are formed

either at the bottom of the reactor

vessel (in-vessel debris bed) or in the

reactor containment (ex-vessel debris

bed) after the cool down of the reactor.

Debris beds are formed during a SA

after the solidification of the melted

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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