Probabilistic methodology for turbine missile risk analysis

A methodology has been developed for estimation of the probabilities of turbine-generated missile damage to nuclear power plant structures and systems. Mathematical models of the missile generation, transport, and impact events have been developed and sequenced to form an integrated turbine missile simulation methodology. Probabilistic Monte Carlo techniques are used to estimate the plant impact and damage probabilities. The methodology has been coded in the TURMIS computer code to facilitate numerical analysis and plant-specific turbine missile probability assessments. Sensitivity analyses have been performed on both the individual models and the integrated methodology, and probabilities have been estimated for a hypothetical nuclear power plant case study.     

A review of procedures for the analysis and design of concrete structures to resist missile impact effects

Concrete containment walls and internal concrete barrier walls are often required to withstand the effects of missile impact. Potential missiles include external tornado generated missiles (steel rods, steel pipes, wooden poles, and automobiles), aircraft crash, and internal accident generated missiles (turbine blade, and steel pipe missiles resulting from pipe break). Impacting missiles can be classified as either ‘hard’ or ‘soft’ depending upon whether the missile deformability is small or large relative to the target deformability. This paper only deals with the effects of ‘hard’ missile impact. Missile velocities between 100 and 1500 ft/sec are emphasized. ‘Hard’ missile impact results in both local wall damage and in overall dynamic response of the target wall. Local damage consists of spalling of concrete from the front (impacted) face and scabbing of concrete from the rear face of the target together with missile penetration into the target. If damage is sufficient the missile may perforate or pass through the target. This paper reviews the various empirical procedures commonly used for determining penetration depth, perforation thickness, and scabbing thickness for concrete targets subjected to ‘hard’ missile impact. Results obtained from these procedures are compared with test data results for low velocity impacts (200–1500 ft/sec). Design recommendations to prevent detrimental local wall damage are presented. Overall dynamic response of the target wall consists of flexural deformations and a potential flexural or shear failure if the strain energy capacity of the wall does not exceed the kinetic energy input to the wall by the striking ‘hard’ missile. Simplified procedures are defined for determining the dynamic response of the target wall and for preventing overall failure of the wall. Included are procedures for defining the effective target mass to be used in determining the fraction of the total missile kinetic energy which is transferred or ‘input’ into the target wall. Also included are procedures for defining the total strain energy capacity of the target wall as determined from the moment and rotational capacities of flexural yield hinges and the yield line deformation pattern of the wall. Lastly, criteria for preventing a premature shear failure are presented.     

Loading time history for tornado-generated missiles

Nuclear power plant structures in the USA are designed for impact by tornado-generated missiles. The design load for flexure and shear can be obtained from the deceleration of the missile on impact. The paper gives a simple method to determine the deceleration of the most critical pipe missile. Results, obtained by the simple method, are compared with full-scale test results. The comparisons between the predicted and actual deceleration time histories show excellent correlation.     

Tornado missile transport analysis

A methodology has been developed to simulate the initial release conditions and subsequent motion of objects transported by tornadoes. A probabilistic three-degree-of-freedom trajectory model which includes drag, lift, and side forces has been developed to simulate rigid body dynamics in turbulent tornado flow fields. Comparisons of this random orientation model to results from ballistic three-degree-of-freedom trajectory analysis are presented and the results suggest that the simpler models are potentially unconservative in predicting missile range and impact velocity. A missile injection methodology has also been developed which treats injection as the composite of all missile interactions in the near-ground domain and relies on a restraint force exceedance criterion to initialize missile release relative to the translating tornado. The aerodynamic forces acting on a potential missile during injection suggest a multi-peaked time history which is significantly influenced by missile offset position from the vortex center. A simulation study of missile injection has been performed to determine a conservative range for the assumed horizontal restraining force.     

Damage evaluation of 500 MWe Indian Pressurized Heavy Water Reactor nuclear containment for aircraft impact

Safety assessment of Indian nuclear containments has been carried out for aircraft impact. The loading time history for Boeing and Airbus categories of aircrafts is generated based on the principle of momentum transfer of crushable aircrafts. The case studies include the analysis of BWR Mark III containment as a benchmark problem and analyses of Pressurised Heavy Water Reactor containment (inner and outer containment) for impulsive loading due to aircraft impact. Initially, the load is applied on outer containment wall model and subsequently the load is transferred to inner containment after the local perforation of the outer containment wall is noticed in the transient simulation. The analysis methodology evolved in the present work would be useful for studying the behavior of double containment walls and multi barrier structural configurations for aircraft impact with higher energies. The present analysis illustrates that with the provision of double containments for Indian nuclear power plants, adequate reserve strength is available for the case of an extremely low probability event of missile impact generated due to the commercial aircrafts operated in India.     

Experimental and numerical studies of liquid dispersal from a soft projectile impacting a wall

A medium-scale IMPACT test programme is currently being implemented at the Technical Research Centre of Finland (VTT). In these tests, deformable cylindrical steel or aluminium projectiles impact a solid concrete wall or a steel force plate. One part of the test is conducted with a missile filled with liquid water to study liquid dispersal phenomena (i.e., wet missile tests).The fluid-filled missile ranged in length from 0.5 to 1.5 m, the water mass inside the missile from 15 to 68 kg, and the impact velocity of missile from 70 to 177 m/s.This paper describes the methods used to measure the liquid dispersal processes, and presents the main results for preliminary simulations of liquid spread. Because the IMPACT tests have focused on structural aspects, it was necessary to develop cost-effective methods for measuring liquid phenomena. The tests measured some important parameters associated with liquid: the discharge speed and direction of the liquid core released from the ruptured missile, propagation speed of the spray front, liquid pooling on the floor, extent of liquid dispersal away from the target, and the drop size of the liquid spray.The experimental findings indicate that the liquid release starts along the surface almost perpendicularly to the incoming direction of the missile and forms a fairly “flat” and uniform splash pattern around the missile. Although the discharge speed of the liquid core may be initially much higher than the impact velocity of the missile, the propagation speed of the spray front decreases rapidly with increasing distance from the source. Results of the preliminary simulations show that the Fire Dynamic Simulator (FDS) program is a usable tool for simulating two-phase flows involving high-speed droplets, provided that the initial conditions (angle and speed of liquid release, droplet size, and initial air speed) can be specified appropriately. Given these requirements, FDS can reasonably well predict the formation of the water spray cloud and final distribution of water.     

Impact load for tornado-generated missiles

Nuclear power plant structures are designed to resist impact loads from tornado-generated missiles. The paper gives a method to determine the contact pressure at the interface of the missile and the target, and the velocity and deceleration time-histories of the missile. It also gives a method to obtain the design load due to impact by tornado-generated missiles. The calculated design load compares very well with the total support reaction from a full-scale dynamic test. The comparison between the predicted deceleration and the deceleration recorded in a test is excellent. The method of determining the deceleration has also been used with very good results for earth-penetrating missiles.     

Tests of spinning turbine fragment impact on casing models

Ten 1/11-scale model turbine missile impact tests were conducted at a Naval spin chamber test facility to assess turbine missile effects in nuclear plant design. The objective of the tests was to determine the effects of missile spin, blade crush, and target edge conditions on the impact of turbine disk fragments on the steel casing. The results were intended for use in making realistic estimates for the initial conditions of fragments that might escape the casing in the event of a disk burst in a nuclear plant. The burst of a modified gas turbine rotor in a high-speed spin chamber provided three missiles with the proper rotational and translational velocities of actual steam turbine fragments. Tests of bladed, spinning missiles were compared with previous tests of unbladed, nonspinning missiles. The total residual energy of the spinning missiles, as observed from high-speed photographs of disk burst, was the same as that of the nonspinning missiles launched in a piercing orientation. Tests with bladed missiles showed that for equal burst speeds, the residual energy of bladed missiles is less than that of unbladed missiles. Impacts of missiles near the edge of targets resulted in residual missile velocities greater than for central impact.     

A synthesized windfield model for tornado missile transport

A tornado windfield model has been developed for use in a probabilistic assessment of the tornado missile hazard to nuclear power plants. Tornado flow characteristics have been identified which are significant in terms of missile transport phenomena. In order to account for both modeling uncertainty and the natural variability observed among tornadoes, serveral random variables are specified in the model, including: tornado intensity, path width, translational speed, radius to maximum tangential velocity, ratic of radial-to-tangential wind speeds, vertical variation of core size, and boundary layer thickness. Considering the lack of agreement regarding detailed tornado dynamics as well as the difficulty in establishing a priori conservative flow characteristics for missile transport, the windfield model was synthesized from theoretical, observational, and probabilistic considerations. A significant aspect of the model is that the parameters can be adjusted to make the intensity, size and velocity variables consistent with the tornado path width boundary specification. The modelling considerations are discussed, the windfield model and calculational procedure presented, sample windfield component velocity profiles illustrated, and missile velocity statistics given for a simulation case study involving several thousand missile histories.     

Local damage to reinforced concrete structures caused by impact of aircraft engine missiles Part 1. Test program, method and results

Structural damage induced by an aircraft crashing into a reinforced concrete structure includes local damage caused by the deformable engines, and global damage caused by the entire aircraft. Local damage to the target may consist of spalling of concrete from its front face together with missile penetration into it, scabbing of concrete from its rear face, and perforation of missile through it. Until now, local damage to concrete structures has been mainly evaluated by rigid missile impact tests. Past research work regarding local damage caused by impact of deformable missiles has been limited. This paper presents the results of a series of impact tests of small-, intermediate-, and full-scale engine models into reinforced concrete panels. The purpose of the tests was to determine the local damage to a reinforced concrete structure caused by the impact of a deformable aircraft engine.     

A simple model for predicting energy dissipation of thin plates being perforated by “hard” missiles

For the case of low velocity impact a simple model is derived for the determination of energy dissipation of thin plates being perforated by “hard” missiles. The predicted residual energy of the missile having passed through the target is compared with test results. The tests were carried out with plates made of wood-chips (a rather homogeneous and cheap material). For a projectile with large diameter relative to the thickness of the target it is shown that the energy absorption of the plate is essentially influenced by the fracture type.     

多传感器的数据融合落点定位方法研究

为了提高靶场末区落点定位的可靠性和准确性,提出了一种多传感器测量数据融合的定位方法。在分析了各传感器测量原理的基础上,以光电经纬仪和视听测量组成融合系统,给出这两类传感器数据融合定位的计算方法,并利用实测数据进行了计算,结果表明,该方法是切实可行的。     

Full-scale tornado-missile impact tests

Full-scale poles, pipes, and rods, representing postulated tornado-borne missiles, were rocket-propelled into reinforced concrete panels with thicknesses typical of walls and roofs in the auxiliary buildings of nuclear power plants. Data from the 18 tests can be used directly for structural design or for validating design and analysis techniques. The test panels, constructed with 3000-psi design strength concrete and minimum allowable reinforcement, were 12, 18, and 24 in. thick with 15 × 15-ft unsupported spans. The results show that a 1500-lb utility pole, 1-in rod, and 3-in. pipe are ineffective for producing significant local and structural damage even under the improbably severe tornado-missile impact conditions represented by the tests. The front several feet of the poles disintegrated upon impact without damaging the face of the panels. Although 12-in. pipes produced craters in the face of the panels, impact tests with these missiles showed that 18-in. thick walls are adequate for preventing backface scabbing (secondary missiles) in the highest tornado-intensity region of the US, while 12-in. thick walls are adequate in other regions. Data on penetration depths and scabbing thresholds for 12-in. pipe impacts could be fitted reasonably well only with a modified NDRC design formula. Examination of structural response was aided by electronic records of reaction forces, missile deceleration, panel velocity, and strains during impact. Contrary to the predictions of conventional structural design methods, which do not account for missile deformation, no overall permanent deflections of the panels were produced by any of the missiles.     

Missile impact on a reinforced concrete structure

In this analysis an attempt was made to study the behaviour of a reinforced concrete structure under missile impact loading. The local deformations in all directions including the wall thickness, the plasticity and the stress waves at and surrounding the impact point were taken into consideration. The data of the impacting steel missile and the shape of the target concrete wall were given. As the impacting time is short in comparison to the fundamental eigenfrequency of the structure, it is possible to study the local deformation by isolating the impacted zone and its surroundings from the total structure. The boundaries of this region were considered as fully clamped. The results justified this assumption as the stresses at and near the boundaries were negligible in comparison to their counterpart at the impacting point. The PISCES 2 DL code was considered suitable for this type of calculation. Special routines defining the material and yield models for reinforced concrete were integrated in the program system.     

Safety Margins of Spent Fuel Transport and Storage Casks Considering Aircraft Crash Impacts

Abstract

The safety of spent fuel transport casks in severe accident conditions is always a matter of concern. This paper surveys German missile impact tests that have been carried out in the past to demonstrate that German cask designs for transport and interim storage are safe even under conditions of an aircraft crash impact. A fire test with a cask beside an exploding propane vessel and temperature calculations concerning prolonged fires also show that the casks have reasonably good safety margins in thermal accidents beyond regulatory fire test conditions.     

Analytical studies on local damage to reinforced concrete structures under impact loading by discrete element method

This paper proposes a new analytical approach for assessing local damage to reinforced concrete structures subjected to impact load, by applying the discrete element method (DEM). It first outlines the basis concept and analytical formulation of the DEM. Next, it discusses the results of simulation analyses of concrete material tests, uni-axial compression tests and tensile splitting tests conducted to determine appropriate analytical parameters such as material constants, failure criteria and strength increase factors depending on strain rate. Finally, the adaptability of the DEM to local damage to reinforced concrete structures impacted by rigid and deformable missiles is verified through simulation analyses of various types of impact tests. Furthermore, the various impact response characteristics and failure mechanisms, such as impact forces, penetration behavior, reduction in missile velocity and energy transfer process, which are difficult to obtain experimentally, are analytically evaluated by the DEM.     

Numerical studies of impact on reinforced concrete beam of hard missile

The safety design of concrete containment structures in nuclear power stations has thus far covered only accidents due to internal pressure, temperature loading and earthquake loading. Recently, designers and researchers have become interested in the important effects of the impact load of a projectile on nuclear power stations. This paper develops an FEM model for analyzing the collision of a hard missile against reinforced concrete structures and compares the results with impact tests conducted at our institute.     

Comportement local des enceintes en beton sous l'impact d'un projectile rigide: Local behaviour of reinforced concrete walls under missile impact

Since 1974 CEA and EDF have developed in France a large programme with the aim to work out a means of computation reliable enough for the knowledge of the behaviour of reinforced concrete walls under missile impacts. The shots were performed on reinforced concrete slabs, the thickness of which were chosen to represent in a realistic way the thickness of the wall of a reactor containment. The scales used in this modeling were mainly one-half and one-third. The following parameters were kept constant: the properties of the concrete and the geometric shape of the missile (flat nose). Also studied were the effects of variation of parameters like missile velocity (25–450 m/s), its mass (20–300 kg), ratio of the missile diameter to the thickness of the slab (0.24–2.9) and characteristics of the steel reinforcement.The results of these tests may be summarized in a homogeneous perforation formula in the case of a velocity lower than 200 m/sec: where Vc is the minimum velocity for perforation, ø diameter of the missile, M its mass, σ the ultimate compressive strength of the concrete, its density and ρ its thickness.     

Model tests of turbine missile impact on reinforced concrete

The results of 25 impact tests on 1/11-scale models of reinforced concrete nuclear plant walls are presented. These tests determined experimentally the maximum velocity at which postulated turbine missiles are contained by typical reinforced concrete walls. The parameters varied were missile weight, velocity, orientation, and impact angle, as well as target design and thickness. The results showed that the NDRC perforation formula used extensively in current practice is overly conservative, whereas a newer empirical formula (CEA-EDF) gave reasonably conservative predictions of the test results. All but the most energetic postulated missiles are stopped by containment wall models, and steel liners on these walls are effective in suppressing backface concrete scabbing.