Nonbuilding and Special Structures
Structures for Nuclear Power Plants (NPP) are designed to protect from the effects of tornado including impact of wind generated missiles. This is achieved by locating the components and system inside a robust structures consisting of reinforced concrete. It is necessary, that the concrete structure will have sufficient strength and thickness to survive wind forces and the effects of missile impact. This paper deals with determining the concrete thickness necessary to stop tornado generated missiles considering deformation of missiles due to impact.
Different types of tornado generated missile for NPP design are specified in the NRC US Regulatory guide 1.76 (Ref. 1). This guide includes a number of missiles, their shapes, sizes, weight and their velocity of impact. The most damaging missile for the design of the structure is found to be the 6” diameter 15 feet long Schedule 40 steel pipe. For Zone III the missile can impact the barrier with a velocity of 41 m/sec. The required thickness of concrete barrier to prevent perforation or spalling is determined using empirical formulae developed on the basis of impact tests performed in research laboratories in the last two decades. NDRC, Bechtel or pother formulae have been used for many years for such calculation. The empirical formulae that are used in the industry are developed on the basis impact of rigid missiles on to concrete targets. A schedule 40 pipe with wall thickness of 0.28 inches does not behave as a rigid missile and actually behaves like a flexible missile. Use of formulae that are based on impact of rigid missiles can provide very conservative results. A simulation of non-linear impact effect using a finite element formulation using the LSDYNA computer program has been performed to determine the degree of deformation of the pipe missile. It is found that the impacting end of the pipe undergoes gross deformation and rupture end becomes blunt there by losing its ability to penetrate into concrete.
Therefore the concrete thickness determined using NDRC or other similar formulae can be excessive and may not represent the actual behaviour. One way will be to use a reduction factor to account for the flexibility effect of the missile. Such an approach is technically justifiable and provides realistic results. This will reduce the barrier thickness that in turn will reduce the construction materials and will a better engineering solution.
This paper presents the analytical model and results of the non-linear impact analysis using computer analyses and performs limited study by varying parameters to determine the effects. Based on the study, reduction factors are proposed to be used along with the available formulae for rigid missiles. The author points out the need for more research work in this area to establishing realistic barrier thickness calculation procedures for actual missiles that are not rigid.
1) US NRC Regulatory Guide 1.76, Design Basis Tornado and Tornado Generated Missiles for Nuclear Power Plants, Revision 1, March 2007.