Structural Integrity Research of the Electric Power Research Institute. Palo Alto, California, USA

207 Pages·1984·11.65 MB·English

by  Stanley H. Fistedis (Eds.)

Structural Integrity R e s e a r ch
of the
Electric P o w er R e s e a r ch Institute
Palo Alto, California, U SA
EPRI
ELECTRIC P O W ER R E S E A R CH INSTITUTE
Edited by:
Stanley H. FISTEDIS
1984
NORTH-HOLLAND PHYSICS PUBLISHING - AMSTERDAM
© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 1984
All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means,
electronic, mechanical, photocopying recording or otherwise, without the prior written permission of the Copyright owner.
Reprinted from Nuclear Engineering and Design Vol. 77, No. 3 (1984)
Printed in The Netherlands
PREFACE
The thirteen papers of this issue give an overview of the Structural Integrity Research effort supported
by the Electric Power Research Institute, (EPRI).
The work described is a result of the mission of EPRI to conduct research and development which
promotes the safe, clean, and economical generation of power by the utility industry. Thus, the research
areas covered are the ones currently receiving the most attention. They address (1) nuclear plant design,
licensing and regulation questions, (2) design and construction costs of components and structures and the
overconservatism that has crept into the process, and (3) safety questions that are subject to probabilistic
risk assessment.
The material is arranged in five sectors: (1) methods of analysis and design, (2) earthquake response, (3)
fluid-structure interactions, (4) response of components and structures to impact, and (5) the performance
of components and structures.
In view of the current state of nuclear energy the material is very timely. Thanks are extended to all
authors of the EPRI contractors for the prompt preparation of their papers, and to EPRI for accepting my
invitation to initiate this special issue.
Stanley H. Fistedis
Principal Editor
Nuclear Engineering and Design 77 (1984) 207-227 207
North-Holland, Amsterdam
OVERVIEW OF EPRI RESEARCH IN STRUCTURAL INTEGRITY
H.T. TANG, G.E. SLITER, Y.K. TANG and LB. WALL
Electric Power Research Institute, 34 J 2 Hillview Avenue, Palo Alto, California 94303, USA
Received August 1983
This paper is an overview of the structural integrity research within the Nuclear Safety and Analysis Department of the
Electric Power Research Institute. This research addresses structurally related safety issues in light water reactors. Five major
technical areas are covered: Analysis/Design Methods, Seismic/Vibratory Response, Fluid/Structure Response, Impact/Im­
pulse Response, and Structure/Component Performance. Each technical area is briefly described and research results are
highlighted. This paper puts in perspective the research and development work described in this special issue of the journal in
addressing such safety and licensing issues as soil-structure interaction, seismic response of piping systems, hydrodynamic
loads in pipes and vessels, pipe rupture and whip, jet impingement, missile impact, and concrete containment integrity.
1. Introduction abreast of rapidly advancing technology. Another has
been the occurrence of unforeseen events, not antic­
The mission of the Electric Power Research Institute ipated in the original design, as operating experience
(EPRI) is to conduct research and development which grew in a maturing industry.
promotes the safe, clean, and economical generation of Example issues raised by advancing technology in­
power by the U.S. electric utility industry. In this con­ clude improved methods for addressing plant seismic
text, the focus of the Structural Integrity program within response, analysis of piping seismic stresses, PWR hy­
the Nuclear Power Division is upon improving the drodynamic loads, and BWR pressure suppression loads.
design, safety, licensing, and thereby the cost and avail­ Under NRC's Systematic Evaluation Program (SEP) [1],
ability of light water reactor power plants with respect operating plants designed with minimal seismic require­
to their structural performance under normal operating ments are being reviewed to assess their earthquake
conditions and postulated accident conditions. The mis­ resistance capacity. The use of advanced technology
sion-oriented nature of EPRFs research places stringent during the licensing of a U.S. plant [2] led to the
criteria on the selection of projects with near-term pay­ recognition of asymmetric hydrodynamic loads propa­
offs and a high success rate. gating across a PWR reactor core barrel under a pos­
The primary motivations for structural integrity re­ tulated loss of coolant accident. For BWRs, pressure
search include (a) evolving nuclear plant design and suppression system loads such as those from pool swell,
licensing issues, (b) the large and increasing costs of steam condensation, and chugging were identified. Fur­
plant structural components coupled with the prevailing ther examples in this category include missile impact,
observation that many design/licensing practices are pipe whip, and combinations of loads from operating
grossly conservative, and (c) insights into reactor safety and accident conditions.
from probabilistic risk assessments. These motivations Example issues raised by unanticipated plant operat­
are closely interrelated. ing events include water hammer (about 142 BWR events
In the past decade or so, the nuclear utilities have between 1969 and 1980 [3]), pressurized thermal shock
been confronted with increasing licensing requirements (3 events reported in 1982 alone [4]) and potential
imposed by the U.S. Nuclear Regulatory Commission overpressurization of containment under degraded core
(NRC) for assuring the safety operation of nuclear conditions (TMI-2 led to an interim rulemaking [5]
plants. Many of these requirements are related to struct­ which addresses this issue).
ural issues. One root cause for the changing regulatory These issues and resulting licensing requirements
requirements has been the desire to keep regulations have placed a significant demand on electric utility
0029-5493/84/$03.00 © Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
208 Η. Τ. Tang et al. / Overview of EPRI research in structural integrity
resources (and ultimately ratepayer costs) due to reanal- Evaluatio n Too ls PSlatnrutctures
ysis and retrofit, sometimes coupled with plant shut­
down and the expense of replacement power. A prevail­ A. naMl,v estihs/oDdess ian Applications StrucPteurrfeo/rCmoamnpcoen ent
ing question is whether requirements have gone beyond
their ability to pay off in terms of improved safety. For Applications Benchmark data Load definition
example, the cumulative conservatisms that have been for validation
applied in tracing seismic motion through the ground,
coupling with building response, and being amplified by
assuming low damping in piping systems have led to f
stiff, complex piping and cable support systems. Such
Seismic/Vibratory Fluid/Structure Impact/Impulse
systems are difficult to maintain and may be suppress­ Response Response Response
ing the flexibility commonly inherent in earthquake-re­
Loadin g Type sand Testing
sistant structures.
A typical LWR contains 45 miles of piping and 550 Fig. 1. Organization of the Structural Integrity program accord­
miles of cables which are supported by 6000 hangers ing to technical areas.
and snubbers each of which is seismically engineered. It
has been estimated [6,7] that the total costs for seismic
design and construction ranges from 6% ($160 million) leading to the consideration of using a less demanding
of capital cost for a 0.2g SSE (typical of eastern U.S. Design Basis Accident [11,12]. A further insight from
sites) to 15% ($400 million) for a 0.5g SSE (typical of a the study was that public risk is dominated by accidents
site in California). These estimates assume that the in which the fuel is severely degraded'. For such accident
engineering is only done once; in fact, changing require­ sequences, a critical parameter is the failure mode of the
ments probably result in a doubling or more of these containment when it is internally pressurized. The usual
capital costs. As described in section 3, EPRI is spon­ assumption [9,13] is a gross rupture of containment
soring several projects in the area of seismic design and which maximizes the public health consequences. How­
analysis which are identifying conservatisms and aiming ever, if the containment experienced a slow leakage, the
toward simplifications in piping support systems. public health consequences would be much smaller. As
Another design basis event that has had a substantial described in section 6.1, EPRI has initiated experiments
impact on piping, supports, and other structural compo­ and analyses in order to substantiate the containment
nents is the assumed double-ended guillotine break of failure mode.
the largest pipe in the primary coolant system. Typi­ With this perspective, EPRI initiated in 1974 a
cally, a current generation PWR can have 250 to 400 Structural Integrity program within the Nuclear Power
pipe whip restraints. Estimated total costs for design, Division to sponsor and manage a wide range of re­
procurement, and construction are in the range of $20 search projects to address structurally related safety
to 40 million per unit [8]. This amount does not include issues associated with light water reactors *. These re­
additional operating costs associated with in-service in­ search projects are organized into the following five
spection and maintenance due to difficult access and major technical areas:
other design problems. Possibly even more important, - Analysis/Design Methods,
the difficult access leads to increased occupational radi­ - Seismic/Vibratory Response,
ation doses. EPRI has under way several experiments to - Fluid/Structure Response,
measure pipe opening times, pipe whip, and jet im­ - Impact/Impulse Response,
pingement forces, as described in sections 4.2 and 5.2. - Structure/Component Performance.
Measurements to date have shown substantial margins Embedded in this organization is a balanced techni­
which, when incorporated in design practice and licens­ cal approach, with equal emphasis on analysis and
ing requirements, should significantly reduce the cost of testing. Fig. 1 is a graphic overview showing the rela­
pipe restraints. tionship among the five technical areas. The three areas
In 1975, NRC published the Reactor Safety Study
(WASH-1400) [9] which systematically analyzed the
probabilities and consequences of postulated reactor * A separate Structural Mechanics program within the Nuclear
accidents. The study found that public risk is dominated Power Division has as its goal improved reliability and
not by large pipe breaks but by small pipe breaks and availability, particularly with regard to material behavior and
transients (e.g., loss of offsite power [10]). This insight is analyses of pressure boundaries.
Η. Τ. Tang et al. / Overview of EPRI research in structural integrity 209
Table 1
Research topics covered under technical areas in the Structural Integrity program
Analysis/Design Methods Transient Continuum Mechanics Code (STEALTH)
Nonlinear Structural Analysis Code (ABAQUS)
Simplified Nonlinear Methods
Seismic/Vibratory Response Seismology
Soil/Structure Interaction
In-situ Earthquake Response Data
Seismic Base Isolation
Piping System Damping and Support Interaction
Simplified Piping Support System
Fluid/Structure Response BWR Pressure Suppression Venting Phenomena
PWR Reactor Vessel Hydrodynamic Loads
Pipe Rupture and Depressurization
Waterhammer
Impact/Impulse Response Tornado Missiles
Turbine Missiles
Pipe Whip
Jet Impingement
Structure/Component Performance Concrete Containment Integrity
Dynamic Capacity of Piping and Supports
that focus on structural response induced by ground design methods selected. These conservatisms provide
motion, fluid motion, or short-duration dynamic excita­ not only the desired margin of safety, but also a cushion
tion, consist mainly of testing to provide benchmark against calculational uncertainties and the unan­
data on load definitions which feed into the other areas. ticipated extreme loads that are invariably identified as
To provide reliable data, the program strongly em­ technology and experience grow. It is there that ad­
phasizes the conduct of experiments at the largest scale vanced analysis methods can play a key role by using
possible. The area on performance concerns load carry­ more sophisticated theories to account for realistic, but
ing capacity of plant structures. The area on methods complex modes of structural response that demonstrate
provides evaluation tools that can be applied in all of increased performance capability.
the other areas. Table 1 summarizes the topics covered Two types of general purpose nonlinear computer
in each area. codes have been developed within the Structural Integr­
In this paper, research needs and technical ap­ ity program. One, formulated in finite difference form,
proaches in each technical area are briefly described. is suited for addressing severe dynamic transients in­
Results achieved to date are highlighted. Papers in the volving wave propagation in continuum bodies of
rest of this special issue give more detailed information structural materials, soils, and fluids (e.g., PWR hydro-
on progress in selected research topics. dynamic load, BWR pool swell, seismic soil-structure
interaction, impact penetration, and water hammer).
The other, formulated in finite element form, is more
2. Analysis/Design Methods
suited for addressing static, quasi-static, and dynamic
A suitable set of analytical tools has always been a nonlinear structural response (e.g., containment over-
fundamental requirement for reactor design and safety pressurization, creep, pipe whip, structural response to
evaluation. Currently available design tools are often impact, and seismic response of buildings and piping).
supplemented with the application of more sophisti­ (Requests for information on the use and availability of
cated computer codes at the later stages of design. But, either of these codes described below should be addre­
because of the impracticality of using ultra-sophisti­ ssed to the Electric Power Software Center (EPSC),
cated methods for design, simplifying assumptions that 1930 Hi Line Drive, Dallas, TX 75207, USA, tel. (214)
err on the safe side are built into the analytical and 655-8883.)
210 Η. Τ. Tang et al. / Overview of EPRI research in structural integrity
Table 2 1
STEALTH family of codes (Version 4-1 A) 5
(artificial earthquake)
STEALTH-GEN General purpose version for 1-D, 2-D, Nonlinear calculation
and 3-D thermal-mechanical transient (STEALTH)
- Linear calculation (SHAKE)
analysis
4 —
STEALTH-PIPING Special 1-D version for piping flow
analysis
STEALTH-SEISMIC Special 1-D and 2-D version for soil-
structure interaction analysis Í A - ~\
Damping = 2%
STEALTH-FSI Special STEALTH and WHAMSE
coupled version for 1-D, 2-D, and
Í /
3-D fluid-structure interaction
analysis
STEALTH-IMPLICIT Special 1-D and 2-D version for
hydrodynamic response analysis
I
-- >\
2.1. Transient continuum mechanics code
01 10 10.0 100.0
Frequency (Hz)
The STEALTH * family of codes as shown in table 2
has been developed to analyze reactor transient events. Fig. 2. Surface response spectra displaying nonlinear soil effects
STEALTH-GEN (GENeral purpose) is the basic ver­ under strong ground motion [17].
sion that has both thermal and mechanical capabilities.
The rest are spin-offs for efficient analysis of special
classes of problems. Refs. [14] and [15] provide an
Y Check valve mmmmm Intra-branch pipe
overview of the STEALTH family of codes and their
capabilities. Al Tee junction • Feedpipe
STEALTH has had many applications. One paper in
^ Reducer
this issue [16] reports a particular application in the
soil-structure interaction area. It is shown that proper
account of nonlinearity in a strong motion environment
is very important in reproducing experimentally ob­
served characteristics, such as the downward shift of a
containment building's rocking frequency. This and the
result of an earlier parametric study [17] demonstrate
the inappropriateness of using linear methods to calcu­
late foundation input if indeed nonlinearity becomes
influential because of strong ground motion. As shown
in fig. 2, an important finding is that nonlinear effects
produce a substantial reduction of low frequency ampli­
tude. This difference could significantly impact predic­
ted dynamic structural response, particularly the low-
frequency response of reactor building structures and
components.
* Solids and Thermal Hydraulics Code for EPRI Adapted
U
from Lagrange Toody and Hemp, developed for EPRI by
Science Applications, Inc. Fig. 3a. Sketch of feedwater for steam generator.
Η. Τ. Tang et al. / Overview of EPRI research in structural integrity 211
η Ο Ο ν ν Ο ι
• ψ • Ο Ο ν ν Οι
ΟΟΎ • τ Ο ι
Boundaries Piping Component Losses
U — upstream pressure boundary • - 90 short radii elbows
D - downstream pressure boundary o - 90° long radii elbows
Control Volumes • - 45 long radii elbows
30° long radii elbows
V — check valve ν -
gate valve
Τ — tee junction ¥ -
R — reducer
Fig. 3b. Simulation schematic of feedwater branch used for STEALTH calculation of shutdown transient flow.
Other STEALTH applications have included simula­
tion of transient piping flow (fig. 3) [18] and missile
impact (fig. 4) [19]. Another paper in this issue [20]
FEEDURTER SK/TDOM, SLU&S-FT-SEC
describes the STEALTH-WHAMSE analysis of a PWR
asymmetric load situation to be discussed further in
section 4.1.
3.00E+05 1
Static or quasi-static problems can also be analyzed
with STEALTH by using a dynamic relaxation tech­
nique. The waste isolation study sponsored by the Office
of Nuclear Waste Isolation [21] and a small fuel-pin
thermal mechanical behavior study sponsored by EPRI
[22] are two examples.
In light of the many validation efforts performed, the
STEALTH family of codes is qualified to perform
sophisticated transient analysis to meet many reactor
design, safety and licensing needs.
2.2. Nonlinear structural analysis code
Subsequent to the initiation of the STEALTH code
work, EPRI recognized the need for developing a gen­
eral purpose code for nonlinear structural response
analysis, particularly in the dynamic situation, where
inertia rather than wave propagation dominates the
physics. The initial effort was to sponsor the develop­
0.00E+O0 S.OCE-01 i.OOE+00
ment of a general purpose nonlinear finite element code
Time (s) architecture with the special capability of handling large
Fig. 3c. Typical pressure history calculated by STEALTH (near rotation, elastic-plastic impact problems such as pipe
tee junction - 24.6 ft from check valve) [18]. whip [23]. Attention to the architecture aspect was
212 Η. Τ. Tang et al. / Overview of EPRI research in structural integrity
C/L (Axial symmetry) behavior. Numerically, this problem also offered a chal­
20.0
lenge in generating reliable and convergent solutions.
- Target support
The challenge of generating numerical code calcu­
lations that converge to an accurate and correct solution
10.0 has always plagued numerical analysts. This is espe­
cially true for nonlinear problems, for which it is known
that implicit operators are not unconditionally stable.
0.00 The user has had the burden of selecting appropriate
time or load steps, as well as solution accuracy. From
the very beginning of EPRI's finite element code devel­
-10.0 — opment, taking this judgment off the hands of users, if
at all possible, was given high priority. An automatic
time stepping algorithm was formulated in terms of a
-20.0 nodal equilibrium-balance parameter (R\/2) which is a
measure of desired solution accuracy. As shown in fig.
0.00 10.0 20.0 30.0 5, this algorithm leads to convergent solutions [24]. It
X — cm has also proven to be efficient because time steps are
(a) t = 162 μ$ continuously optimized during the solution execution.
20.0 The first version of the code that resulted from these
- Target support considerations is ABAQUS-ND (Nonlinear Dynamic)
[25]. The finite element formulation enables the code to
10.0 perform pipe whip analysis and general nonlinear dy­
namic piping and shell analysis. It should be noted that
the code is not suited for linear dynamic problems.
Following this initial effort, the need for a more
0.00
general code to include dynamic, static, temperature,
-10.0 Time Step Selection Criteria
R| /= i444.82 Ν (100 lb )
0.014 o R,/t= 22R24.1 Ν (5001b)
* i/4= 8896.4 Ν (20001b)
-20.0
-
0.00 10.0 20.0 30.0 0.012
X — cm
(b) t = 396 μ%
Fig. 4. Deformed STEALTH grid showing penetration of
turbine missile into reinforced concrete target at time (t) [19].
_ 0.008 " / /
/ / if 0.3 §
/ ^
motivated by the fact that most finite element codes had
/ </ °
been developed originally without the benefit of a good
data management structure. Consequently, subsequent /
development and qualification suffered from the inher­
ent inadequacies of the architecture. EPRI's aim, there­
fore, was not only to develop a user-oriented reliable
Beam length = 0 254 m (10 in)
code but also one versatile enough for efficient code
1 1 . I I I
development and maintenance. The pipe whip capa­ 0.6 0.8
bility offered a good starting point since it involves both Time, ms
geometric nonlinearity such as large pipe rotations, Fig. 5. Automatic time stepping results from
pipe-target impact, pipe crushing, etc. and material ABAQUS-EPGEN for a fixed-ended elastic-plastic beam with
nonlinearity such as plasticity and strain-rate dependent an initial velocity field [24].
Η. Τ. Tang et ai / Overview of EPRI research in structural integrity 213
fracture, and additional nonlinear features arose. In a ogy. The initial short-term effort will be the industry
cooperative effort with the code developers, Hibbitt, program to develop a basis for reassessing the seismic
Karlsson, and Sorensen, EPRI cosponsored the general hazard for all reactor sites in Eastern U.S. in response
purpose version of the ABAQUS code, which EPRI to the change of position [34] by the U.S. Geological
calls ABAQUS-EPGEN (EPRI GENeral purpose) [24] Survey on the 1886 Charleston earthquake. The existing
for licensing to utility users. This code has extensive licensing criteria embodied in 10 CFR 100 Appendix A
nonlinear capabilities for a wide class of applications. A are predicated upon the assumption that large earth­
paper in this issue describes it in more detail [26]. quakes in Eastern U.S. are temporally and geographi­
Application studies sponsored by EPRI using the cally stationary. Recent data has eroded this assumption
ABAQUS code include pipe whip, seismic, piping vibra­ and the new approach by NRC [35] and EPRI [36] will
tion [27], pressure vessel thermal shock [28], steam gen­ treat the different tectonic hypotheses within a prob­
erator tube denting [29], and concrete containment abilistic framework as a basis for reassessing the Safe
overpressurization [30]. Although the code development Shutdown Earthquake for each site.
is in an advanced, well-documented state at present, the
constant fine tuning so vital to improving solution ef­ 3.1. Soil-structure interaction
ficiency and accuracy of a nonlinear code continues to
be implemented. The primary effort in this area has been the develop­
ment of experimental data bases to quantify nonlinear
SSI under strong ground motion. These data bases were
3. Seismic/Vibratory response needed to validate analysis codes such as STEALTH-
SEISMIC, developed in conjunction with the
Because of increasing safety concerns, regulatory re­ Analysis/Design Methods effort.
quirements on seismic design of nuclear power plants The importance of SSI has long been recognized in
have become more stringent in the past several years. nuclear plant seismic design practice. During a strong
Under NRCs Systematic Evaluation Program, older motion earthquake, the dynamic coupling between mas­
plants which were designed with minimum seismic con­ sive plant buildings and their underlying soil media can
siderations are being evaluated for their seismic struct­ significantly influence the responses of structures, com­
ural and system integrity. Piping systems, which are ponents, and equipment. However, due to the complex­
important to safety, have been undergoing extensive ity of the seismic environment, and the lack of recorded
reviews with regard to seismic adequacy. Recent exam­ real earthquake data at plant sites, the interpretation of
ples include the algebraic summation procedure for SSI phenomena is subject to a high degree of uncer­
determining total piping response which led to the tainty. Recognizing the lack of actual earthquake and
temporary shutdown of five plants [31], the as-built controlled experimental data as a major barrier to im­
inspection and confirmation of all power plants [32], proving SSI technology, EPRI sponsored a series of
and the Diablo Canyon piping support design incident. strong-motion SSI experiments [37,38] by using explo­
In the overall risk assessment of Zion nuclear plant [13], sives to simulate earthquake motion. Containment mod­
seismic risk was identified as a major risk contributor. els ranging in scale from 1 /48 to 1 /8 were constructed
According to the categorization adopted in NRCs in the field with various embedment and foundation
Seismic Safety Margins Research Programs (SSMRP) conditions.
[33], four aspects of nuclear plant design are: In these SIMQUAKE (SIMulated earthQUAKE) ex­
- seismic input, periments, the detonation of vertical arrays of explo­
- soil-structure interaction (SSI), sives propagated wave motions through the ground to
- major structure response, the model structures (fig. 6). Underneath the founda­
- subsystem response. tion, accelerometers were installed around a "soil is­
This categorization follows the normal path of the land" boundary to pick up the input motion. This
seismic excitation starting with source motion and end­ controlled input motion together with the responses
ing with component motion. The Structural Integrity measured on the structures provide a base for analytical
program's research has emphasized nonlinear SSI and modeling and validation. Although the wave character­
subsystem response, particularly piping system re­ istics generated by explosives are not the same as those
sponse. These are described in the following sections. under actual earthquakes (the former is rich in P-waves
To address the aspect of seismic input, EPRI has and the latter is normally rich in S-waves), the
recently started a parallel research program on seismol­ soil-structure dynamic characteristics occurring during