Optimal Design of NPP Containment Protection Against Fuel

Container Drop

Juraj Králik1, a 1Faculty of Civil Engineering STU in Bratislava, Radlinského 11, 81368 Bratislava, Slovakia

ajuraj.kralik@stuba.sk

Keywords: Impact, damping device, energy, container drop, interaction, fluid, NPP, ANSYS

Abstract. This paper presents an optimal design of damping devices for the protection of the reinforced concrete structure of a nuclear power plant (NPP) against the impact loads from a container of nuclear fuel of the type TK C30 drop. The finite element idealization of the building structure is used in space. The interaction of the soil-structure, as well as the fluid-structure of the deactivated basin is considered. A steel pipe damper system is proposed for the dissipation of the kinetic energy of the container’s free fall. The Newmark’s integration method is used for the solution of the dynamic equations.

Introduction

The International Atomic Energy Agency (IAEA) initiated a program in 1990 to assist the countries of Eastern Europe and the Soviet Union in evaluating the safety of their first generation WWER-440/230 nuclear power plants [3, 4]. The main objectives of the program were: to identify major design and operational safety issues, to establish an international consensus on priorities for safety improvements, and to provide assistance in the review of the completeness and adequacy of safety improvement programs. The scope of the program was amended in 1992 to include RBMK, WWER-440 and WWER-1000 plants in operation and construction.

Fig. 1 Scheme of a nuclear power plant WWER-440/230

The program was complemented by national and regional technical cooperation projects. The results, recommendations and conclusions resulting from the IAEA program were intended only to assist national decision makers who have to resolve responsibilities for the regulation and safe operation of their nuclear power plants. The Russian Federation's regulations as well as the IAEA NUSS Series documents, and rules, guidelines, safety standards and general review procedures

Advanced Materials Research Vol 688 (2013) pp 213-221 Online: 2013-05-14© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.688.213

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practiced in Western countries as well as in the Russian Federation and Slovakia were compared by IAEA [3, 4]. The Nuclear regulatory authority (NRA) of Slovak Republic has defined the new tasks to evaluate the safety and reliability of the NPP in Slovakia in the frame of the „stress test“ due to Fukushima accident [15]. One significant accident is a possible drop of the container during the replacement of the nuclear fuel in the reactor shaft and the emergency basin. This accident can arise due to the failure of the crane hanger. It is necessary to remark that this operation is carried out during a maintenance break. The reinforced concrete structure of the NPP containment can be penetrated as a consequence of the container’s drop. All penetrations through the containment should meet the same design requirement as the containment structure itself [1, 9, 10 and 11]. The containment should be protected against reaction forces stemming from pipe movements or accidental loads such as jet forces, pipe whip and missiles [10].

Fig. 2 a) Scheme of the containment ceiling b) Scheme of the basin and reactor shaft

One load case presented, i.e., the impact loads according to the free fall of a container in the time of the nuclear fuels replacement was considered in [9, 10 and 11]. The simulation of a reinforced concrete structure’s behavior during an impact load is considered in [1, 9 - 11, 14]. The interaction between a structure and the soil and the interaction between the structure and fluid in a deactivated basin were investigated in [2, 7, 8, 10, 11- 14]. The uncertainties of the model and structure resistance can be considered using the probabilistic assessment [5, 6 and 10].

Analyzing reinforced concrete structure of the containment for different kind of loads was performed using the ANSYS software. The building of a power block was idealized with a discrete model, consisting of the solid elements (SOLID45), shell elements (SHELL43), beam elements (BEAM4), linear actuator elements (LINK11), mass elements (MASS21), and solid fluid elements (FLUID80). A finite element model was created from 20,840 elements and 15,600 nodes (see Fig. 2). A FE model is the best estimation of the behavior of the real NPP structure (walls, plates, etc.) from the point of view of the geometric dimensions and material properties.

Free Fall Loading of TK-C30 Container

A hall crane transports the nuclear fuel in the TK - C30 steel container (Fig.3) under a ceiling plate at +18.90 m (Fig.4). The cylinder container has a diameter of 2 285 mm, a height of 4 367 mm and a weight of 89.5 t. In the case of an accident the container can fall to the containment plate. The accident scenario was defined by a technological engineer in the works [9, 10]. Two possible tracks were defined (Fig. 4):

1. Track - at lines 31, 32, 33, 37, 38, 36, 39, 41, 2. Track - at lines 31, 32, 33, 34, 35, 36, 39, 41.

214 Proceedings of the Conference on the Rehabilitation and Reconstruction ofBuildings CRRB 2012

We proposed three levels of the containers fall: 1. Fall on the plate at level +18.90 m from a height of 200 mm (points 31, 32, 33, 34, 35, 37, and

38) 2. Fall on the plate at level +18.90 m from a height 3 670 mm (point 36) 3. Fall on the plate at level +7.30 m from a height 15 270 mm (point 41)

Fig. 3 Scheme of TK-C30 container

Fig. 4 The proposed tracks of the container’s transport

The impact loads can be defined from the equality of the kinetic energy Ek of the container’s

weight mo before impact and the potential energy Ep of the plate’s deformation at the moment of maximal impact effect

Ek= Ep, 21

2k o oE m w= �

max

1

2pE kw= (1)

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where ow� is the velocity of the container’s fall at the moment of the plate-container contact; wmax is

the maximal amplitude of the displacement of the plate, k is the stiffness of the reinforced concrete plate (defined from the FEM model of structures). The velocity of the fall is:

2o ow gh=� (2)

where ho is the height of the free fall. The long time of the impact tr and the amplitude of force Fmax can be considered as the impact loads in the form of a half wave as follows:

or

mt

kπ= , max max.F k w= (3)

The impact forces on the plate of the steam generator (SG) box at level +18.9 m are presented in Table 1. In the case of the fall of the container into the basin, we must consider two phases of the fall – firstly, the fall above a water surface (ho), and secondly, underwater (hv). Moreover, the kinematic energy of the fall on the basin bottom is

2 21

1 1

2 2k o o oE m w m w= −� � (4)

where ow� is the velocity of the container’s fall on the water surface (from the height ho), and 1w� is

the velocity of the container fall on the basin’s bottom. The potential energy of the deformation is

( )2 2max

1 1

2 2p o v k k v kE kw m gh Agh Agh h hρ ρ= + − − − (5)

where wmax is the maximum displacement due to the deformation of the structure on the basin’s bottom due to the impact load; hv (alias hk) is the height of the basin water (alias the container), A is the section area of the container, ρ is the density of the water, g is the gravitation acceleration.

The total time of the impact can be considered as a half wave from the equation of the vibration in the form

( ) ( ) ( ) 0om w t kw t gAw tρ+ + =�� (6)

The fall of the container into a basin was tested by an experiment in the laboratory of CVUT Prague [2] in the case of 1:10 experimental model. Based on the experimental results, the factor of the water resistance was equal to Cz=1.5.

Fig. 5 Position and velocity of container fall in the basin from the experiment [2]

216 Proceedings of the Conference on the Rehabilitation and Reconstruction ofBuildings CRRB 2012

The reaction force of the water resistance during the fall of the container is

0

21

2z zF C A wρ= � , (7)

where Cz is the factor of the water resistance defined by the experiment. The velocity of the container’s fall on the basin’s bottom can be calculated from the equation

( ) 20 0

0 02z

m Az g C A wdw

dt m m

ρ ρ−= −

�� , (8)

The solution of (8) can be defined based on the experimental results in the form of an exponential function. The velocity iw� in the time step 1i it t t−= + ∆ and position 1i iz z w t−= + ∆� is

expressed as follows:

( ) 1

1

1ki

i

ki

Cw

C n

+=

−� , 1 221 0

1 0

1

1in n t

ki

n wC e

n w

− −= +

�

�, 1

0

zC An

m g

ρ= ,

( )02

0

im Az gn

m

ρ−= (9)

The solution of the differential equation (8) using the difference approximation gives us the behavior of the position and velocity of the container fall (Fig. 5). The impact forces on the plate of the SG box and the bottom of the basin, considering the water’s resistance in the basin on the basis of the experimental results, are presented in Table 1. All the information about the impact loads – height, velocity, impact force and period are shown there.

Table 1 The impact load on basin bottom Point (CSYS) Height [mm] Velocity [m/s] Impact force [MN] Period [s] 15498(CS41) 14010 15.31 583.07 (404.60)* 0.0007

15498(CS41)** 14010 10.70** 431.35 ( 58.23)* 0.0007 ** Calculation with water resistance, * Impact force considering the damping effect of damping devices

If we calculate with the water’s resistance in the basin during the container’s fall, the velocity of the movements of the container is diminished from a value of 15.31 m/s to a value of 10.70 m/s (see Table 1).

Design of Damping Device

The kinetic energy of a free fall container can be dissipated with the plastic energy of damping devices from the pipes in either one or two layers. This type of damping device was used in Germany.

We propose the kinetic energy of the container’s fall to the bottom of the basin in the form:

( )20 0

1

2 2v v s

k o v k k v k

v

F h DE m gh m gh Agh Agh h h

h

−= + − − − −ρ ρ , (10)

where Fv is the force of the water resistance during the container’s fall in the basin. The kinetic energy Ek must be absorbed by the elastic and plastic deformation of the damping pipe device. The dissipation energy of the plastic deformation of the pipe is expressed as follows:

( )0,03p m m sD F f D= − , (11)

where Fm is the resistance force of the pipe, and fm is the maximum cross pipe plastic deformation. On the basis of the experimental results we have:

2 12m F

s

F btD a

γσ=−

, 0,85m L sf n D= , (12)

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where Ds is the pipe diameter, t is the thickness of the pipe wall, a is the length of the U profile, b is the length of the pipe segment, γ is the safety factor, nL is the number of the pipe layers.

The potential energy Ep of the elastic and plastic deformation of damping devices is:

10,03 0,82

2p m L s m L sE F n D F n D= + (13)

The reliability condition for the design of damping devices is:

p kE E≥ (14)

Four types of damping devices (Table 3) were considered. Two of them (type – T1 and T3) are designed in one pipe layer, the rest (type – T2 and T4) are designed in two pipe layers.

The proposed damping devices are from short pipe elements mutually connected with the steel beams from the U-profile in a grid form (Fig. 6). The detail of the type T2 damping device is presented in Fig. 7.

This pipe element was tested by Bundesanstalt für Materialprüfung (see Table 2) for the plastic capacity of the device due to the impact load. The safe crosswise deformation of the pipe element was defined on the basis of the experimental results as 0.85Ds (Fig. 7).

Fig. 6 Configuration of pipe damping device

218 Proceedings of the Conference on the Rehabilitation and Reconstruction ofBuildings CRRB 2012

The plastic capacity of the pipe device (Fig. 7) is defined as follows:

( )22m F sF b d D aασ= − , (15)

where α is the reliability parameter (α = 1.1), σF - stress yield (σF = 350 MPa), b - length of pipe, d - thickness of pipe, Ds - diameter of pipe, a - height. The maximal plastic deformation of a pipe that can be used is 0,85Ds and the maximal diameter dilatation is a = 1.5Ds. Three layer damping devices from 2×24×219/20 – 150 mm pipes were proposed to dissipate the kinetic energy Ek = 2.702.5 kNm (free fall from height of 3 670 mm) with an efficiency of η = 107.9%. In the case of a basin bottom an effective damping device is designed as 2×52×219/18 - 150 mm, which dissipates the kinetic energy Ek = 5 120.0 kNm (free fall from height 14010 mm ) with an efficiency of η = 110.2%.

Fig. 7 Experimental test of the pipe’s resistance

Table 2 Experimental test of a pipe segment (Bundesanstalt für Materialprüfung) Specimen

No. Drop

[mm]

Deformation

[mm]

Potential Energy1)

[J]

Velocity Fall2) [m/s]

Deformation Energy

[J]

Load Impulse over 30ms

[kN] 1 1400 68 10369 5.2 9880 108 2 1600 82 11880 5.6 11300 107 3 1700 72 12516 5.8 11900 110 4 1800 90 13350 5.9 12700 106

1).This energy corresponds to the drop plus the pipe’s deformation calculated from the permi-ssible pipe crack

2) This value corresponds to the velocity of the test frame at the moment of contact with pipe

Table 3 Comparison of affectivity of damping devices Type Diameter of pipe Ek [kNm] Ep [kNm] η [%]

Free fall from height 3.670 mm T1 1×18×219/18 – 455 mm 2962.4 2688.0 90.7 T2 2×24×219/20 – 150 mm 2702.5 2917.4 107.9

Free fall from height 14.010 mm T3 1×24×219/22 - 380 mm 4753.2 4471.4 94.1 T4 2×52×219/18 - 150 mm 4644.4 5120.0 110.2

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Summary

This paper deals with the problem of the analysis of buildings of nuclear power plants for the case of their resistance to a possible accident during the transport of a TK C30 container [9 - 11] with nuclear fuel. Depending on the importance of these buildings, not only numerical analysis, but also experimental measurements [2] were provided. The dynamic transient analyses from the container’s drop during the accident simulation were realized using the ANSYS system. During a container’s drop simulation the influence of the load-bearing structure vibration is very significant for the bearing capacity of the steam generator suspensions, which are anchored in the ceiling. During the reconstruction of the containment structure damping devices from steel pipes were designed. The flooring plate of the containment was protected from the container drop by reducing the effect of the impact using the damping devices from the pipe elements. The damping devices were designed in accordance with the results of attests of Bundesanstalt für Materialprüfung. The kinetic energy was dissipated in the plastic deformation of the steel pipes.

Acknowledgement

The project was realized with the financial support of the Grant Agency of the Slovak Republic (VEGA). The project registration number is VEGA 1/1039/12.

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