Principal Research Results

Numerical Simulation for Overflow Behavior of Water from Spent Fuel
Pool with Sloshing in the 2007 Niigata-ken Chuetsu-oki Earthquake
Background
In order to evaluate overflow behavior of water from liquid-storage tank with nonlinear sloshing, CRIEPI has developed andvalidated two-dimensional computational fluid dynamics code, SLOSH-2D, and its three-dimensional version, SLOSH-3D, using thevolume-of-fluid (VOF) technique, following the 2003 Tokachi-oki Earthquake. The VOF technique is one of the simulation methods
for liquid surface behavior. Since it is very simple and specific to simulate the separation and combination of liquid mass as well as toconserve the mass volume, the VOF technique is adopted in our codes. On the other hand, it was reported that, due to the 2007
Niigata-ken Chuetsu-oki Earthquake, the water of spent fuel pool (referred to as “SFP”) overflowed onto the operation floor in theUnit-1 to Unit-7 reactor buildings at the Kashiwazaki-Kariwa Nuclear Power Plants (referred to as “NNPs”) of Tokyo Electric Power
Company, Inc. A video record of the incident at Unit-3, showing overflow behavior and complex water free surface phenomena, wasmade public. Therefore, in order to contribute to the evaluation of seismic safety of the NPPs, it became necessary to clarify theoverflow behavior of the SFP water with nonlinear sloshing.

Objectives
The purpose of this study is to validate the applicability of the newly developed three-dimensional computational fluid
dynamics code, SLOSH-3D, and clarify the overflow behavior of the SFP water with nonlinear sloshing.

Principal Results
1. Validation of the applicability of the computer program
The video record indicated that the triangular waves with conical shape were generated on the water free surface, and theSFP water overflowed from the curb of the pool, with the seismic-induced sloshing (Fig. 1). Time-history response analyses wereperformed using the three components of the observed seismic motions on the operation floor (tri-axial excitation in north-south, eastwest and vertical directions), in order to simulate the overflow behavior of the SFP water with nonlinear sloshing. Based on the analysis results, the time-dependent sloshing behavior was investigated by three-dimensional free surface profiles at various time steps (Fig.
2). Consequently, it was found that the newly developed computer program, SLOSH-3D, could simulate the observed complex water
free surface phenomena with acceptable accuracy.
2. Overflow behavior of the SFP water with nonlinear sloshing(1) Sloshing response
The analysis results indicate that the dominant frequency components in the free surface oscillations correspond to the natural
frequencies of the first and second sloshing mode, calculated by assuming the SFP to be a rectangular container. As a result, it isfound that the first sloshing mode is generally the dominant mode of the oscillations, and the complex free surface shape of SFPwater, such as the triangular waves, are superimposed on the first sloshing mode shape.
(2) Overflow onto the operation floor
The SFP water began to overflow onto the operation floor in the latter half of a principal shock of the input seismic motion, andcovered nearly the entire floor area of the analytical model in about 10 seconds. Subsequently, the cumulative amount of
overflowed water volume became almost constant (Fig. 4).
Main Researcher:
Yukihiro Toyoda,Senior Research Engineer, Structural Engineering Sector, Civil Engineering Laboratory
Nobukazu Tanaka, Ph. D.,
Executive Research Engineer, Civil Engineering Laboratory
Reference
M. Sakai, et al, 2007, “Nonlinear sloshing response evaluation method for rectangular tank with overflowing water,” CRIEPI Report
N06031 (in Japanese).

92

5. Nuclear

Ground plan of the operation floor level

Refueling machine

Cask pit

SFPDS pool
Reactor
VesselCamera frame (yellow area)

Schematic view of the Unit 3 reactor building
Unit 3 After the earthquake occurredFig.1 Snap Shot of the video record (by courtesy of The Tokyo Electric Power Company, Inc.)

time2 = 12.5 stime1 = 10.4 s

Cask pit

Cask pit

Waterheight5
Overflowed water

Fig.2 Computed free surface profiles of the SFP water at two time steps

Fig.3 Fourier spectrum of the water height
responses

Fig.4 Time history of relative overflowed water
volume93