atw 2018-09v3


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



1. Tsuchiya A, Kondo T, Maruyama H.

Criticality calculation of fuel debris in

Fukushima Daiichi nuclear power station.

In: PHYSOR 2014. Kyoto, Japan; 2014.


| | 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.


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


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


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.


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

Reactors. In: Ken Nakajima, editor. Nuclear

Back-end and Transmutation Technology

for Waste Disposal. Springer Open;

2015. p. 251–60.

3. Nuclear Damage Compensation and

Decommissioning Facilitation Corporation.

Technical Strategic Plan 2016 for

Decommissioning of the Fukushima

Daiichi Nuclear Power Station of Tokyo

Electric Power Company Holdings, Inc.

2016 Jul.

4. Goorley, John T., James, Michael R.,

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.

5. GPU NUCLEAR. Three Mile Island

Nuclear Station Unit II Defueling

Completion Report. 1990.

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

Fukushima-Daiichi nuclear power plant.

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

debris bed generated in large-scale

fuel-coolant interaction experiments.

Nucl Eng Des. 2006;236(19):1998–



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

More magazines by this user
Similar magazines