atw Vol.

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

Bed

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,

2002.

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

cohesive sediment transportation. PhD

thesis, University of California, Davis,

California, 1974.

Authors

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

Young

Scientists'

Workshop

Awarded

473

AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP

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

calculations.

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

atw Vol. 63 (2018) | Issue 8/9 ı August/September 472 AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP | | Fig. 8. Time resolved washed off mass for different wash-off coefficients. Parameters according to Table 1 and ρ p = 5000 kg/m 2 . transport simulations last for 30 s. First the influence of the Weber number is investigated, see Figure 6. Increasing Weber numbers correspond to larger water velocities and an increasing percentage of wetted surface. Consequently for larger Weber numbers more particle mass is washed off. Thus two effects manifest in the results: first the larger velocities are able to wash-off even particles with larger density. And secondly the enlarged percentage of wetted surface enhances the particle wash-off, since much more particles can be eroded by the water. Figure 7 shows the variation of the particle density. Particles with larger density cannot be eroded that easily and hence the total washed off mass decreases with increasing particle density, as expected. In Figure 8 the influence of the wash-off coefficient is investigated. The total washed off mass, which is to a large extent determined by the area of wetted surface, does not change with different values of ~ r e but the temporal behavior does. For a large value of ~ r e a large fraction of particles erodes in a short timespan. Asymptotically for t → ∞ the total washed off mass converges always to the same amount. In order to compare the simulations with experimental data a parameter set based on Weber et. al [1] is chosen. Figure 9 displays the results of the simulation and the experimental data of test 4. In the experiments the particles are collected in intervals of 10 s for a total duration of 130 s. Due to this sampling strategy the time resolved washed off particle mass in the simulations is presented in the same manner and for the same duration. A good agreement for the temporal course of the wash-off as well as for the total washed off mass can be achieved. 7 Conclusions and discussion This paper presents a CFD particle wash-off model and particle transport by gravity driven flows. A parameter variation was conducted within the setting of a simplified geometry and with the geometry of the laboratory tests. The particle wash-off model, which is based on Shields criterion [12] and Weber et. al [1], shows the expected behavior for varying particle properties such as particle density and wash-off coefficient. One key influencing parameter for the resulting washed off mass is the percentage of area covered by water in each case, which differs with inclination and mass flow rate. First simulations with the laboratory geometry show satisfactory agreement when compared to the experiments. Nevertheless, the prediction of particle wash-off for a large variety of setups as in the laboratory experiments ( different inclinations, particle and surface properties and initial loadings) remains a great challenge and further comparisons for different parameter sets are current work in progress. This study contributes to the development of a semi-empirical model to quantify the aerosol washoff and the wetted surface area during an accident in a light water reactor. Acknowledgment The project underlying this report is funded by the German Federal Ministry of Economic Affairs and Energy under grant number 1501519 on the basis of a decision by the German Bundestag. The THAI project was carried out on behalf of the Federal Ministry for Economic Affairs and Energy under grant number 1501455 on the basis of a decision by the German Bundestag. We are also grateful for the support from Becker Technologies and the GRS. References | | Fig. 9. Comparison of test 4 of the laboratory experiments with the simulations of particle wash-off with inclination α = 20°, mass flow rate m = 11 g/s, initial loading c s = 12.5 g/m 2 , particle diameter d p = 2 μm, particle density ρ p = 5000 kg/m 3 and wash-off coefficient ~ r e = 0.025 s –1 . [1] G. Weber, F. Funke, W. Klein-Hessling, and S. Gupta. Iodine and silver washdown modelling in COCOSYS-AIM by use of THAI results. Proceedings of the International OECD-NEA/NUGENIA- SARNET Workshop on the Progress in Iodine Behaviour for NPP Accident Analysis and Management, 2015. [2] S. Gupta, F. Funke, G. Langrock, G. Weber, B. von Laufenberg, E. Schmidt, M. Freitag, and G. Poss. THAI Experiments on Volatility, Distribution and Transport Behaviour of Iodine and Fission Products in the Containment. Proceedings of the International OECD-NEA/NUGENIA-SARNET Workshop on the Progress in Iodine Behaviour for NPP Accident Analysis and Management, p. 1-4, 2015. [3] M. Freitag, B. von Laufenberg, M. Colombet, K. Amend, and M. Klein. Particulate fission product wash-down from containment walls and installation surfaces. Proceedings of the 47 th Annual Meeting on Nuclear Technology, Hamburg, 2016. AMNT 2018 | Young Scientists' Workshop Development and Validation of a CFD Wash-Off Model for Fission Products on Containment Walls ı Katharina Amend and Markus Klein

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 Bed 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, 2002. [13] R. Ariathurai. A finite element model of cohesive sediment transportation. PhD thesis, University of California, Davis, California, 1974. Authors 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 Young Scientists' Workshop Awarded 473 AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP 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 calculations. 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