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Tests in support of LMFBR
Nuclear Engineering and Design 42 (1977) 1-186 ©North-Holland Publishing Company
89
TESTS IN SUPPORT OF LMFBR G. R. Abrahamson, D. J. Cagliostro, and A. L. Florence Stanford Research Institute Menlo Park, California 94025
Tests in support of LMFBR projects for potential incidents involving a hypothetical core disruptive accident or sodium-water interactions in steam generators have two possible goals: (i) to evaluate the integrity of the primary containment after the design is frozen or (2) to investigate design options in support of primary "containment design. The test planning approach differs depending on the goal. The Fast Flux Test Facility tests had the first goal, the current Clinch River Breeder Reactor tests have the second goal. This paper discusses test planning, sources for simulating loads, modeling, and instrumentation. The main source described involves controlled venting of explosive gases. The source avoids generation of undesirable shock waves and facilitates calibration because the reaction rate of the explosive is independent of the response. It is indicated that the test program requiring the least time and cost involves a mixture of three types of models: small simple models, small complex models, and large complex models. Instrumentation is discussed that is required for validation of loading, response measurements for comparison with calculations, and response measurements on critical members where predictions are not possible.
i.
Introduction
This paper concerns tests to investigate the integrity of LMFBR primary containment under certain potential incidents. Figure 1 shows the main elements of a fast breeder plant. The primary containment consists of the reactor vessel and cover, the piping between the vessel and the intermediate heat exchanger (IHX), and the IHX. The potential incidents considered in this paper arise from hypothetical core disruptive accidents (HCDA) or from sodiumwater interactions in the steam generator. These incidents result in the sudden generation of gases, producing high pressures that produce loads on the primary containment. Other potential incidents such as earthquakes, sabotage, and plant missiles are not considered here. Because of the size and complexity of LMFBRs, the only practical approach to testing the primary containment against such incidents is to use models at about i/i0 scale or smaller. Also, because the incidents originate in complex physical processes that are impractical to reproduce in scaled models, the sudden generation of gases must be simulated. Thus, the two main problems are how to model the primary containment and
how to simulate the incident. We first discuss the question of test planning, next we turn to load simulation, and finally we consider modeling and instrumentation. 2.
Test Planning
Tests in support of LFMBR projects have two possible goals: (i) Evaluation of the integrity of the primary containment after the design is frozen, or (2) Investigation of design options in support of primary containment design. The test planning approach dif,fers considerably depending on the goal. If the goal is t o e v a l u a t e the integrity of the primary containment after the design is frozen, the central objective is to perform a credible test on a realistic model without surprises and at a reasonable cost. If the goal is to investigate design options, the central objective is to get valid test results that evaluate the critical aspects of the competing designs within time constraints such that the information can be accommodated in the design. The tests of the Fast Test Reactor (FTR) of the Fast Flux Tesu Facility (FFTF) had the first goal; the current work on the C l i n c h R i v e r Breeder Reactor (CRBR) has
90
G.R. Abrahamson et al./Tests in support of LMFBR
SUPERHEATER ..-~_ ,_i PRIMARY PUMP ~,
REACTOR
vI - -
INTERMEDIATE
I
~
INTERMEDIATE PUMP
! Fig.
1.
~ STEAM TO TURBINE
I
EVAPORATORS
MA-317583-58
Schematic of Clinch River Breeder Reactor Vessel and Primary and Secondary Heat T r a n s p o r t Systems
the second goal. Schematics of these reactors are shown in Figures 2 and 3. Because incidents that may affect the primary c o n t a i n m e n t design are not of an o p e r a t i o n a l nature, they are generally considered after the essential operational features have been accommodated. This is both r e a s o n a b l e and proper. Moreover, a system design m u s t be well along before a m e a n i n g f u l test can be planned. Because of these constraints, the time w i n d o w during w h i c h test results can be useful in selecting design options is very restricted, and advance p r e p a r a £ i o n is needed to obtain valid results quickly. A f t e r the design is frozen, great care must be exercised in avoiding surprises in the final test, because the c r e d i b i l i t y of the entire test p r o g r a m may be lost if surprises occur. Surprises can be v i r t u a l l y eliminated by following a p r o g r a m that proceeds through stages. For the FFTF, m o d e l testing p r o c e e d e d through three stages: simple models, small complex models, and a large complex model. Tests w i t h simple models p r o v i d e the o p p o r t u n i t y to assess load simulation techniques, to become familiar w i t h overall response phenomena, and to test instrumentation. Because tests w i t h simple models are inexpensive and fast, many tests can be performed to d e v e l o p a b a c k g r o u n d for more complex and expensive tests. Tests w i t h small complex models provide a refined a s s e s s m e n t of load
simulation, response phenomena, and ins t r u m e n t a t i o n performance. Such models should incorporate all the essential features of the large model to be tested later. Materials, structural features, and joining m e t h o d s should be as close as possible to those to be used in the large model. Structural features such as the internals above the core and the detailed closures on the cover not u s u a l l y found in simple models w o u l d be included in small complex models. At the conclusion of the small complex model tests, the results expected from the large model should be well known. If all has been done p r o p e r l y to this point, the large model test will show no surprises. A l t h o u g h the three stages of model testing ar e sequential, m u c h of the preparation work goes on in parallel. For example, c o n s t r u c t i o n of the small and large complex models can proceed w h i l e the simple model tests are under way. The design of the complex models is u s u a l l y a d i f f i c u l t p r o b l e m that requires many compromises and approximations. The m a i n r e s p o n s i b i l i t y for the design should be w i t h the L M F B R designer, w i t h input from the testing organization. Problems g e n e r a l l y arise from attempts to include too much detail in the models, resulting in excessive cost and fabrication time. A reasonable degree of quality assurance should be included, however, to avoid p o s s i b l e spurious or ambiguous results.
91
G.R. Abrahamson et al./Tests in support of LMFBR
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2.
C u t a w a y V i e w of Liquid Metal Fast Breeder Reactor
I n s t r u m e n t a t i o n of the complex models, p a r t i c u l a r l y the large model, is a n o t h e r area of p o t e n t i a l difficulty. Care should be taken to avoid u s i n g m o r e i n s t r u m e n t a t i o n than is essential and yet p r o v i d e r e d u n d a n t m e a s u r e m e n t s for key quantities. For i n v e s t i g a t i n g design options, the testing p h i l o s o p h y differs from that p r e s e n t e d above, in that the c o m p l e x models m u s t be u n d e r t a k e n sooner to be useful. However, the cost of a surprise in the c o m p l e x m o d e l s is not as great, b e c a u s e the d e s i g n is p r e s u m a b l y at a stage at w h i c h changes can be accommodated. Tests of simple small m o d e l s are still e s s e n t i a l to successful e x e c u t i o n of the c o m p l e x m o d e l tests. Thus, for m a x i m u m efficiency, it is important that the schedule p r o v i d e for simple m o d e l tests and that they not be s a c r i f i c e d to the logistics of acquiring the c o m p l e x models.
3.
Load S i m u l a t i o n
The two p o t e n t i a l incidents of concern here are the H C D A and the sodiumw a t e r i n t e r a c t i o n in the steam generator. Both are too complex to attempt to reproduce in structural tests. Hence, w e m u s t rely on theory to define the level of the incident and the aspects to be simulated in testing the integrity of the p r i m a r y containment. Once the incident c h a r a c t e r i s t i c s to be simulated have b e e n defined, the simu l a t i o n task is to r e p r o d u c e the effects to be simulated w i t h as little disturbance as p o s s i b l e to secondary load generation m e c h a n i s m s (e.g., slug impact) and to the p r i m a r y containment. For the HCDA, this means p u t t i n g the Source in the corer for the s o d i u m - w a t e r interaction, it means producing pulses of the proper c h a r a c t e r i s t i c s to p r o p a g a t e into the piping and the IHX.
92
G.R. Abrahamson et al./Tests in support o f LMFBR
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Fig.
3.
Clinch River Breeder Reactor
Because of the explosive nature of the effects to be simulated, it is not surprising that m o s t simulation methods involve explosives or rapid release of gases. In early w o r k on source development, the total energy release was m a t c h e d using high explosives. This rapid energy release was thought to provide an upper bound on the loads that would be generated for a given energy release. While this may be true in part, the overloaded region near the explosive may respond d i f f e r e n t l y from more realistic loading rates. Hence, high explosive sources have two main problems: (i) the loading rate is too high, possibly giving
wrong response modes and hence incorrect strain energy distributions, and (2) the m e c h a n i c a l work potential of the explosive may not be correct. In later w o r k [13 attempts were made to control energy release rates, but the d e v e l o p m e n t of sources that match the m e c h a n i c a l work potential has occurred only w i t h i n the past five years. [2~ For m a x i m u m usefulness, sources should introduce m i n i m u m extraneous effects and should be amenable to calibration. Explosive sources that introduce shock waves are u n d e s i r a b l e because the effects of the shock waves are not readily explained. Unambiguous calibration
93
G.R. Abrahamson et al./Tests in support of LMFBR
Figure 4 shows the source located in the core of a C R B R m o d e l and Figure 5 shows the source attached to part of the c a l i b r a t i o n apparatus. The energy source is PETN powder m i x e d w i t h p l a s t i c m i c r o b a l l o o n s to reduce the d e t o n a t i o n pressure by an order of m a g n i t u d e to below about i0 kbar. The high pressure gas generated by d e t o n a t i o n flows through the gaps b e t w e e n the stacked steel rings into the air volume surrounding the canister to p e r f o r m m e c h a n i c a l w o r k on the c o o l a n t slug and core structure. Energy and pulse are c o n t r o l l e d by selection of explosive m i x t u r e weight, volumes inside and o u t s i d e the canister, and ring gaps. Two other types of energy sources w e r e d e v e l o p e d [3,43 as part of a program on the r a ¢ i o l o g i c a l c o n s e q u e n c e s for an HCDA. Of interest is the bubble physics following an H C D A as the bubble rises to the reactor head. Figure 6 shows the first of these sources developed to p r o v i d e a c o m p l e t e l y condensible high p r e s s u r e bubble. A stoichiometric m i x t u r e of h y d r o g e n and o x y g e n is m i x e d
is m o s t r e a d i l y a c h i e v e d for sources w h o s e o u t p u t does not depend on the response. For example, a chemical energy source in w h i c h the r e a c t i o n rate depends on the r e s p o n s e w o u l d be d i f f i c u l t to calibrate p r e c i s e l y except by testing in the p a r t i c u l a r s t r u c t u r a l c o n f i g u r a tion in w h i c h it is to be used. Thus, the c a l i b r a t i o n could change significantly in going from small simple models to small c o m p l e x m o d e l s and to a large complex model. Hence w i t h such a source m u c h of the u s e f u l n e s s of small model tests is lost. The problems ci£ed above are avoided by u s i n g c o n t r o l l e d v e n t i n g of explosive gases as the source. The r e a c t i o n rate of the e x p l o s i v e is i n d e p e n d e n t of the response, and shock w a v e s are e l i m i n a t e d by proper design of the canister containing the explosive. Such a source was used in the FFTF tests and is c u r r e n t l y being used in the C R B R tests. The calib r a t i o n of this source is d e s c r i b e d elsewhere. [23 It has p r o v e d to be reliable and easy to use in m a n y applications. MYLAR DIAPHRAGM STEEL CANISTER ASSEMBLY
CORE STUDS
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Fig.
4.
H C D A Load Simulator
in Core of a C R B R Model
94
G.R. Abrahamson et al./Tests in support Of LMFBR
!
Fig.
5. H C D A Load Simulator A t t a c h e d to C a l i b r a t i o n Apparatus
under low p r e s s u r e in a thin-walled cylinder. D e t o n a t i o n creates high pressure steam that expands the cylinder radially against sharp spikes to accelerate rupture. The steam rapidly fills the surrounding air, thereby p r o v i d i n g a pressurized core. Figure 7 shows the second of the sources d e v e l o p e d to investigate the HCDA bubble physics as a flashing source. W a t e r filling a steel c h a m b e r b e l o w the core is heated under p r e s s u r e to the desired t h e r m o d y n a m i c state. The w a t e r is then allowed to flash into the core through fast sliding doors. These sources have not been used to study structural response; they are less conv e n i e n t than the P E T N / m i c r o b a l l o o n mixture in e x p e r i m e n t a l design. Figure 8 shows s c h e m a t i c a l l y the c o n f i g u r a t i o n of the P E T N / m i c r o b a l l o o n source for g e n e r a t i n g pulses to be propagated along f l u i d - f i l l e d pipes. 4.
Models
Tests in support of LMFBR projects imply models r e a s o n a b l y r e p r e s e n t a t i v e of a p a r t i c u l a r design. In fact, the degree of p r o t o t y p i c a l i t y is the d i s t i n g u i s h i n g c h a r a c t e r i s t i c b e t w e e n investigations in general and those for direct support of LMFBR projects, Before d i s c u s s i n g the various types of models, we digress b r i e f l y to consider scaling laws. It is shown elsewhere [53 that structures of d i f f e r e n t sizes show the same response (stresses, strains, relative displacements) if the models are g e o m e t r i c a l l y similar, are made of the same materials, and if rate effects and g r a v i t a t i o n a l effects are negligible. LMFBR materials are u s u a l l y insensitive enough to allow strain rate to be
neglected. Also, g r a v i t a t i o n a l acceleration is u s u a l l y small compared with the a c c e l e r a t i o n s induced by the incidents of interest and hence can also be neglected. A p h e n o m e n o n that m a y be different in models of d i f f e r e n t sizes is fracture. M o d e r n fracture mechanics defines a minim u m crack size that will propagate under a given tensile stress. All materials are known to c o n t a i n cracks, and it is more likely that large pieces will contain larger cracks than small pieces. Since the stress d i s t r i b u t i o n s are the same in models of d i f f e r e n t sizesi it is concluded that larger pieces will fracture at lower stresses than small pieces, on the average. It is apparent that stress gridients become steeper as the model becomes smaller. As long as the stress gradients do not produce variations in the scale of the inherent cracks in the material, there should be no effect on scaling. The model materials for testing at room temperature cannot be the same as those of the L M F B R prototype operating at high temperatures. In the FTR and CRBR programs, the s t r e s s - s t r a i n curve of stainless steel at the o p e r a t i n g temperature was a p p r o x i m a t e d well by that of nickel 200 at room temperature. As m e n t i o n e d above, three stages of m o d e l i n g are common in test programs to examine the p r i m a r y c o n t a i n m e n t of an LMFBR: small simple models, small complex models, and large complex models. The small simple models may be further subdivided into rigid (no plastic deformation) models and flexible models. The rigid models are useful for rapid e v a l u a t i o n of simulation techniques, slug motion, and i n s t r u m e n t a t i o n for m e a s u r i n g loads. Flexible models are useful for studying d e f o r m a t i o n and related i n s t r u m e n t a t i o n and the distribution of m e c h a n i c a l energy. The flexible models can be made simple enough to provide data for code verification. This is important because the complex models are too detailed to be a c c u r a t e l y modeled in calculations. Figures 9 and i0 show the simple rigid and flexible FTR models used in the FFTF e x p e r i m e n t a l p r o g r a m at 1/30 and i/i0 scale. The energy source was calibrated in the rigid m o d e l and it was shown to be scalable by comparing the results at the two d i f f e r e n t scales. The flexible model results showed that the entire experiment scaled. [6~ Figures Ii and 12 show the small complex FTR models, and Figure 13 shows general views of the large complex F T R model. [73 Figure 12 also d e m o n s t r a t e s scaling by comparing the d e f o r m a t i o n experienced by the instrument trees from the small and large models. Figure 13 shows that the large model was used to add greater detail, p r i n c i p a l l y in the form of the three primary piping loops.
95
G.R. Abrahams on et al./Tests in support of LMFBR
2.80"
THIN WALLED CYLINDER (1.75" O.D. x 3.25 Lg) ,BRIDGE WIRE
~ RUPTURESPIKE 16 ARRAYS, 9 SPIKES EACH)
0.125 O.D. x 0,049" I.D. S. S. SEAMLESS TUBE
I
MA-3929-17
Fig. 5.
6.
Schematic of Condensible Bubble Source in Reactor Model Core
Instrumentation
Transient measurements are made on LMFBR model tests to validate the applied loading, to provide data for comparison with response predictions, and to provide data on critical members for which response predictions are not possible. Since validation of the applied loading is crucial to obtaining a credible test, backup measurements are essential. The measurements must include source pressure and upper surface motion
of the coolant slug. In simple models without a cover, the required instrumentation is shown in Figures 9 and i0. Response data consist of pressures, accelerations, velocities, displacements, and strains. Figures 14 and 15 show instrumentation layouts for future tests on the simplest and most complex models of the current CRBR program in which the sensor outputs will be monitored by tape recorders augmented by oscilloscopes. Examples of oscillographs for pressures and strains are shown in Figure 16, obtained from a simple flexible FTR model
96
3.R. Abrahamson et al./Tests in support of LMFBR
PASSAGE TO UPPER CORE BARREL ELECTRICAL FEEDTHROUGH
O-RING SEAL AT DOOR
¢,SLIDING DOORS
BASE PLATE
I NSU LATO RS
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INCONEL HEATING ELEMENT
LOWER CORE/ PRESSURE VESSEL
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MA-3g2g-70A Flashing Source
test. [6,7] Experiments are usually monitored by high speed cameras, especially for observation of coolant leakage. Tests on models such as SM-2, shown in Figure 14, provide data for comparison with code predictions, whereas tests on models with the complexity of that shown in Figure 15 provide data where predictions are not possible. One of the most important aspects of LMFBR model tests does not require transient response measurement but simply compares pretest and posttest observation and dimensional measurement. Figure 17 gives an example in which final FTR wall measurements are plotted [6,73; observations of instrument tree damage are seen in Figure 12. A rapid assessment of safety may be obtained with such posttest observations.
97
G.R. Abrahamson et al./Tests in support of LMFBR
References [I]
[23
[33
[4]
[53
[6]
R. W. Gates, Containment of fragments from a runaway reactor, SRI Final Report to USAEC (February 1964). D. J. Cagliostro, A. L. Florence, G. R. Abrahamson, and G. Nagumo, Characterization of an energy source for modeling hypothetical core disruptive accidents, Nucl. Eng. Design 27 (1974) 94-105. D. W. Ploeger and D. J. Cagliostro, Experimental study of heat transfer from a simulated hypothetical core disruptive accident bubble, SRI Technical Report 1 to ERDA (November 1975). D. W. Ploeger and D. J. Cagliostro, Development and characterization of a liquid-vapor bubble source for modeling HCDA bubbles, SRI Technical Report 2 to ERD~J (March 1977). J. N. Goodier, Dimensional analysis, in Handbook of Experimental Stress Analysis by M. Hetdnyi (John Wiley and Sons, New York, 1950). A. L. Florence, G. R. Abrahamson, and D. J. Cagliostro, Hypothetical core disruptive accident experiments
[7]
VENTED CHARGE CANISTER DET( INATOR
A3 ,
.~1
t CHARGE 'CHAMBER
PETN/MICROSPHE RES 90110 BY W E I G H T
on simple fast test reactor models, Nucl. Eng. Design 38 (1976) 95-108. A. L. Florence and G. R. Abrahamson, Simulation of a hypothetical core disruptive accident in a fast flux test facility, SRI Final Report HEDL-SRI-I (May 1973).
LOADING CHAMBER f FLEXIBLE i DIAPHRAGM RIGID PIPE
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Schematic of Pressure Pulse Source for Piping Tests
98
G.R. Abrahamson et al./Tests in support of LMFBR
DURCE
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9.
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Fig.
10.
Simple Flexible FTR Model
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Fig. ii.
Assembly of Small Complex FTR Model
99
G.R. Abrahamson et al./Tests in support of LMFBR
1/30-SCALE MODEL. PRIOR TO ASSEMBLY OF COVER PLATE
1/30-SCALE MODEL COVER PLATE AND INSTRUMENT TREES (3 EACH) AFTER TEST
1/30-SCALE INSTRUMENT 1/30-SCALE INSTRUMENT TREE PRIOR TO TEST TREE AFTER TEST
1/10- AND 1/30-SCALE INSTRUMENT TREES AFTER TESTING (SMALL TREE IS THE SAME AS AT LEFT) MP-317583-5
Fig.
12.
Small Complex FTR Model (Top), and Comparison of Permanent Deformation of Instrument Trees from Small and Large Models (Bottom)
i00
G.R. Abrahamson et al./Tests in support of LRFBR
(a)
OBLIQUE FRONT VIEW
(b) OBLIQUE OVERHEAD VIEW MP-J 109-I 03
Fig. 13.
Views of Large Complex FTR Model with Piping
i01
G.R. Abrahamson et al./Tests in support of LMFBR
HOLDDOWN STUDS (72 PLACES) / 9 ~E ;)
SIMPLE ,COVER GA~ WATER St
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~ 21.85in.
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/
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Fig.
14.
Schematic
of CRBR Model SM-2
102
G.R. Abrahamson et al./Tests i n support of LMFBR
(~) (~) (~ W
Acceler PresU~ Strain Water I Transdl (~)T Tee R Mp.-3U2~J-1l~J
Fig. 15.
Schematic of CRBR Model SM-5
103
G.R. Abrahamson et al./Tests in support of LMFBR
1 1 6 0 0 #sec ( F R O M
DETONATION)
H.D.
AIR ~LINING -VESSEL
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(b) COVER PRESSURE BASE
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VESSEL LINING: VESSEL:
COVER:
HOLD-DOWN BOLTS:
-800
#sec ( F R O M D E T O N A T I O N )
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(c) COVER STRAIN ) p.sec
304 Stainless Steel Thickness 44 mils 4.96 in. O.D. x 4 in. hgt. Lead 1/32-in. Thick 304 Stainless Steel Thickness 58 mils 8 in. O.D. x 17.2 in. inside hgt. A36 Steel 1/2-in. Thick x 10-in. dia. Weight (steel + lead) 50 lb. Nickel 205 24 Bolts @ 0.192-in. dia. x 2.3-in. Stretch Bolt Circle 9-1/4-in. dia.
STRAIN (CIRCUMFERENTIAL STRAIN 1-1/4.-in. BELOW
(d) VESSEL
VESSEL TOP) 800
STRAIN (CIRCUMFERENTIAL STRAINS 4-in. AND 6-9/16-in.
(e) VESSEL
(a) CORE PRESSURE
BELOW VESSEL TOP) MP-1109-78A
Fig. 16.
Typical Records from FTR Test 208
104
G,R, Abrahamson et al./Tests in support of LRFBR
• 17
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Fig. 17.
Vessel Deformation Profiles from Three FTR Tests
89
TESTS IN SUPPORT OF LMFBR G. R. Abrahamson, D. J. Cagliostro, and A. L. Florence Stanford Research Institute Menlo Park, California 94025
Tests in support of LMFBR projects for potential incidents involving a hypothetical core disruptive accident or sodium-water interactions in steam generators have two possible goals: (i) to evaluate the integrity of the primary containment after the design is frozen or (2) to investigate design options in support of primary "containment design. The test planning approach differs depending on the goal. The Fast Flux Test Facility tests had the first goal, the current Clinch River Breeder Reactor tests have the second goal. This paper discusses test planning, sources for simulating loads, modeling, and instrumentation. The main source described involves controlled venting of explosive gases. The source avoids generation of undesirable shock waves and facilitates calibration because the reaction rate of the explosive is independent of the response. It is indicated that the test program requiring the least time and cost involves a mixture of three types of models: small simple models, small complex models, and large complex models. Instrumentation is discussed that is required for validation of loading, response measurements for comparison with calculations, and response measurements on critical members where predictions are not possible.
i.
Introduction
This paper concerns tests to investigate the integrity of LMFBR primary containment under certain potential incidents. Figure 1 shows the main elements of a fast breeder plant. The primary containment consists of the reactor vessel and cover, the piping between the vessel and the intermediate heat exchanger (IHX), and the IHX. The potential incidents considered in this paper arise from hypothetical core disruptive accidents (HCDA) or from sodiumwater interactions in the steam generator. These incidents result in the sudden generation of gases, producing high pressures that produce loads on the primary containment. Other potential incidents such as earthquakes, sabotage, and plant missiles are not considered here. Because of the size and complexity of LMFBRs, the only practical approach to testing the primary containment against such incidents is to use models at about i/i0 scale or smaller. Also, because the incidents originate in complex physical processes that are impractical to reproduce in scaled models, the sudden generation of gases must be simulated. Thus, the two main problems are how to model the primary containment and
how to simulate the incident. We first discuss the question of test planning, next we turn to load simulation, and finally we consider modeling and instrumentation. 2.
Test Planning
Tests in support of LFMBR projects have two possible goals: (i) Evaluation of the integrity of the primary containment after the design is frozen, or (2) Investigation of design options in support of primary containment design. The test planning approach dif,fers considerably depending on the goal. If the goal is t o e v a l u a t e the integrity of the primary containment after the design is frozen, the central objective is to perform a credible test on a realistic model without surprises and at a reasonable cost. If the goal is to investigate design options, the central objective is to get valid test results that evaluate the critical aspects of the competing designs within time constraints such that the information can be accommodated in the design. The tests of the Fast Test Reactor (FTR) of the Fast Flux Tesu Facility (FFTF) had the first goal; the current work on the C l i n c h R i v e r Breeder Reactor (CRBR) has
90
G.R. Abrahamson et al./Tests in support of LMFBR
SUPERHEATER ..-~_ ,_i PRIMARY PUMP ~,
REACTOR
vI - -
INTERMEDIATE
I
~
INTERMEDIATE PUMP
! Fig.
1.
~ STEAM TO TURBINE
I
EVAPORATORS
MA-317583-58
Schematic of Clinch River Breeder Reactor Vessel and Primary and Secondary Heat T r a n s p o r t Systems
the second goal. Schematics of these reactors are shown in Figures 2 and 3. Because incidents that may affect the primary c o n t a i n m e n t design are not of an o p e r a t i o n a l nature, they are generally considered after the essential operational features have been accommodated. This is both r e a s o n a b l e and proper. Moreover, a system design m u s t be well along before a m e a n i n g f u l test can be planned. Because of these constraints, the time w i n d o w during w h i c h test results can be useful in selecting design options is very restricted, and advance p r e p a r a £ i o n is needed to obtain valid results quickly. A f t e r the design is frozen, great care must be exercised in avoiding surprises in the final test, because the c r e d i b i l i t y of the entire test p r o g r a m may be lost if surprises occur. Surprises can be v i r t u a l l y eliminated by following a p r o g r a m that proceeds through stages. For the FFTF, m o d e l testing p r o c e e d e d through three stages: simple models, small complex models, and a large complex model. Tests w i t h simple models p r o v i d e the o p p o r t u n i t y to assess load simulation techniques, to become familiar w i t h overall response phenomena, and to test instrumentation. Because tests w i t h simple models are inexpensive and fast, many tests can be performed to d e v e l o p a b a c k g r o u n d for more complex and expensive tests. Tests w i t h small complex models provide a refined a s s e s s m e n t of load
simulation, response phenomena, and ins t r u m e n t a t i o n performance. Such models should incorporate all the essential features of the large model to be tested later. Materials, structural features, and joining m e t h o d s should be as close as possible to those to be used in the large model. Structural features such as the internals above the core and the detailed closures on the cover not u s u a l l y found in simple models w o u l d be included in small complex models. At the conclusion of the small complex model tests, the results expected from the large model should be well known. If all has been done p r o p e r l y to this point, the large model test will show no surprises. A l t h o u g h the three stages of model testing ar e sequential, m u c h of the preparation work goes on in parallel. For example, c o n s t r u c t i o n of the small and large complex models can proceed w h i l e the simple model tests are under way. The design of the complex models is u s u a l l y a d i f f i c u l t p r o b l e m that requires many compromises and approximations. The m a i n r e s p o n s i b i l i t y for the design should be w i t h the L M F B R designer, w i t h input from the testing organization. Problems g e n e r a l l y arise from attempts to include too much detail in the models, resulting in excessive cost and fabrication time. A reasonable degree of quality assurance should be included, however, to avoid p o s s i b l e spurious or ambiguous results.
91
G.R. Abrahamson et al./Tests in support of LMFBR
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2.
C u t a w a y V i e w of Liquid Metal Fast Breeder Reactor
I n s t r u m e n t a t i o n of the complex models, p a r t i c u l a r l y the large model, is a n o t h e r area of p o t e n t i a l difficulty. Care should be taken to avoid u s i n g m o r e i n s t r u m e n t a t i o n than is essential and yet p r o v i d e r e d u n d a n t m e a s u r e m e n t s for key quantities. For i n v e s t i g a t i n g design options, the testing p h i l o s o p h y differs from that p r e s e n t e d above, in that the c o m p l e x models m u s t be u n d e r t a k e n sooner to be useful. However, the cost of a surprise in the c o m p l e x m o d e l s is not as great, b e c a u s e the d e s i g n is p r e s u m a b l y at a stage at w h i c h changes can be accommodated. Tests of simple small m o d e l s are still e s s e n t i a l to successful e x e c u t i o n of the c o m p l e x m o d e l tests. Thus, for m a x i m u m efficiency, it is important that the schedule p r o v i d e for simple m o d e l tests and that they not be s a c r i f i c e d to the logistics of acquiring the c o m p l e x models.
3.
Load S i m u l a t i o n
The two p o t e n t i a l incidents of concern here are the H C D A and the sodiumw a t e r i n t e r a c t i o n in the steam generator. Both are too complex to attempt to reproduce in structural tests. Hence, w e m u s t rely on theory to define the level of the incident and the aspects to be simulated in testing the integrity of the p r i m a r y containment. Once the incident c h a r a c t e r i s t i c s to be simulated have b e e n defined, the simu l a t i o n task is to r e p r o d u c e the effects to be simulated w i t h as little disturbance as p o s s i b l e to secondary load generation m e c h a n i s m s (e.g., slug impact) and to the p r i m a r y containment. For the HCDA, this means p u t t i n g the Source in the corer for the s o d i u m - w a t e r interaction, it means producing pulses of the proper c h a r a c t e r i s t i c s to p r o p a g a t e into the piping and the IHX.
92
G.R. Abrahamson et al./Tests in support o f LMFBR
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LARGE ROTATING PLUG
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SMALL ROTATING PLUG
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FUEL TRANSFER STORAGE FUEL ASSEMBLIES CORE
W INLET MODULES X Y
SODIUM INLET FROM INTERMEDIATE HEAT EXCHANGER GUARD VESSEL
MA-1960-228A
Fig.
3.
Clinch River Breeder Reactor
Because of the explosive nature of the effects to be simulated, it is not surprising that m o s t simulation methods involve explosives or rapid release of gases. In early w o r k on source development, the total energy release was m a t c h e d using high explosives. This rapid energy release was thought to provide an upper bound on the loads that would be generated for a given energy release. While this may be true in part, the overloaded region near the explosive may respond d i f f e r e n t l y from more realistic loading rates. Hence, high explosive sources have two main problems: (i) the loading rate is too high, possibly giving
wrong response modes and hence incorrect strain energy distributions, and (2) the m e c h a n i c a l work potential of the explosive may not be correct. In later w o r k [13 attempts were made to control energy release rates, but the d e v e l o p m e n t of sources that match the m e c h a n i c a l work potential has occurred only w i t h i n the past five years. [2~ For m a x i m u m usefulness, sources should introduce m i n i m u m extraneous effects and should be amenable to calibration. Explosive sources that introduce shock waves are u n d e s i r a b l e because the effects of the shock waves are not readily explained. Unambiguous calibration
93
G.R. Abrahamson et al./Tests in support of LMFBR
Figure 4 shows the source located in the core of a C R B R m o d e l and Figure 5 shows the source attached to part of the c a l i b r a t i o n apparatus. The energy source is PETN powder m i x e d w i t h p l a s t i c m i c r o b a l l o o n s to reduce the d e t o n a t i o n pressure by an order of m a g n i t u d e to below about i0 kbar. The high pressure gas generated by d e t o n a t i o n flows through the gaps b e t w e e n the stacked steel rings into the air volume surrounding the canister to p e r f o r m m e c h a n i c a l w o r k on the c o o l a n t slug and core structure. Energy and pulse are c o n t r o l l e d by selection of explosive m i x t u r e weight, volumes inside and o u t s i d e the canister, and ring gaps. Two other types of energy sources w e r e d e v e l o p e d [3,43 as part of a program on the r a ¢ i o l o g i c a l c o n s e q u e n c e s for an HCDA. Of interest is the bubble physics following an H C D A as the bubble rises to the reactor head. Figure 6 shows the first of these sources developed to p r o v i d e a c o m p l e t e l y condensible high p r e s s u r e bubble. A stoichiometric m i x t u r e of h y d r o g e n and o x y g e n is m i x e d
is m o s t r e a d i l y a c h i e v e d for sources w h o s e o u t p u t does not depend on the response. For example, a chemical energy source in w h i c h the r e a c t i o n rate depends on the r e s p o n s e w o u l d be d i f f i c u l t to calibrate p r e c i s e l y except by testing in the p a r t i c u l a r s t r u c t u r a l c o n f i g u r a tion in w h i c h it is to be used. Thus, the c a l i b r a t i o n could change significantly in going from small simple models to small c o m p l e x m o d e l s and to a large complex model. Hence w i t h such a source m u c h of the u s e f u l n e s s of small model tests is lost. The problems ci£ed above are avoided by u s i n g c o n t r o l l e d v e n t i n g of explosive gases as the source. The r e a c t i o n rate of the e x p l o s i v e is i n d e p e n d e n t of the response, and shock w a v e s are e l i m i n a t e d by proper design of the canister containing the explosive. Such a source was used in the FFTF tests and is c u r r e n t l y being used in the C R B R tests. The calib r a t i o n of this source is d e s c r i b e d elsewhere. [23 It has p r o v e d to be reliable and easy to use in m a n y applications. MYLAR DIAPHRAGM STEEL CANISTER ASSEMBLY
CORE STUDS
SEGMENTED
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Fig.
4.
H C D A Load Simulator
in Core of a C R B R Model
94
G.R. Abrahamson et al./Tests in support Of LMFBR
!
Fig.
5. H C D A Load Simulator A t t a c h e d to C a l i b r a t i o n Apparatus
under low p r e s s u r e in a thin-walled cylinder. D e t o n a t i o n creates high pressure steam that expands the cylinder radially against sharp spikes to accelerate rupture. The steam rapidly fills the surrounding air, thereby p r o v i d i n g a pressurized core. Figure 7 shows the second of the sources d e v e l o p e d to investigate the HCDA bubble physics as a flashing source. W a t e r filling a steel c h a m b e r b e l o w the core is heated under p r e s s u r e to the desired t h e r m o d y n a m i c state. The w a t e r is then allowed to flash into the core through fast sliding doors. These sources have not been used to study structural response; they are less conv e n i e n t than the P E T N / m i c r o b a l l o o n mixture in e x p e r i m e n t a l design. Figure 8 shows s c h e m a t i c a l l y the c o n f i g u r a t i o n of the P E T N / m i c r o b a l l o o n source for g e n e r a t i n g pulses to be propagated along f l u i d - f i l l e d pipes. 4.
Models
Tests in support of LMFBR projects imply models r e a s o n a b l y r e p r e s e n t a t i v e of a p a r t i c u l a r design. In fact, the degree of p r o t o t y p i c a l i t y is the d i s t i n g u i s h i n g c h a r a c t e r i s t i c b e t w e e n investigations in general and those for direct support of LMFBR projects, Before d i s c u s s i n g the various types of models, we digress b r i e f l y to consider scaling laws. It is shown elsewhere [53 that structures of d i f f e r e n t sizes show the same response (stresses, strains, relative displacements) if the models are g e o m e t r i c a l l y similar, are made of the same materials, and if rate effects and g r a v i t a t i o n a l effects are negligible. LMFBR materials are u s u a l l y insensitive enough to allow strain rate to be
neglected. Also, g r a v i t a t i o n a l acceleration is u s u a l l y small compared with the a c c e l e r a t i o n s induced by the incidents of interest and hence can also be neglected. A p h e n o m e n o n that m a y be different in models of d i f f e r e n t sizes is fracture. M o d e r n fracture mechanics defines a minim u m crack size that will propagate under a given tensile stress. All materials are known to c o n t a i n cracks, and it is more likely that large pieces will contain larger cracks than small pieces. Since the stress d i s t r i b u t i o n s are the same in models of d i f f e r e n t sizesi it is concluded that larger pieces will fracture at lower stresses than small pieces, on the average. It is apparent that stress gridients become steeper as the model becomes smaller. As long as the stress gradients do not produce variations in the scale of the inherent cracks in the material, there should be no effect on scaling. The model materials for testing at room temperature cannot be the same as those of the L M F B R prototype operating at high temperatures. In the FTR and CRBR programs, the s t r e s s - s t r a i n curve of stainless steel at the o p e r a t i n g temperature was a p p r o x i m a t e d well by that of nickel 200 at room temperature. As m e n t i o n e d above, three stages of m o d e l i n g are common in test programs to examine the p r i m a r y c o n t a i n m e n t of an LMFBR: small simple models, small complex models, and large complex models. The small simple models may be further subdivided into rigid (no plastic deformation) models and flexible models. The rigid models are useful for rapid e v a l u a t i o n of simulation techniques, slug motion, and i n s t r u m e n t a t i o n for m e a s u r i n g loads. Flexible models are useful for studying d e f o r m a t i o n and related i n s t r u m e n t a t i o n and the distribution of m e c h a n i c a l energy. The flexible models can be made simple enough to provide data for code verification. This is important because the complex models are too detailed to be a c c u r a t e l y modeled in calculations. Figures 9 and i0 show the simple rigid and flexible FTR models used in the FFTF e x p e r i m e n t a l p r o g r a m at 1/30 and i/i0 scale. The energy source was calibrated in the rigid m o d e l and it was shown to be scalable by comparing the results at the two d i f f e r e n t scales. The flexible model results showed that the entire experiment scaled. [6~ Figures Ii and 12 show the small complex FTR models, and Figure 13 shows general views of the large complex F T R model. [73 Figure 12 also d e m o n s t r a t e s scaling by comparing the d e f o r m a t i o n experienced by the instrument trees from the small and large models. Figure 13 shows that the large model was used to add greater detail, p r i n c i p a l l y in the form of the three primary piping loops.
95
G.R. Abrahams on et al./Tests in support of LMFBR
2.80"
THIN WALLED CYLINDER (1.75" O.D. x 3.25 Lg) ,BRIDGE WIRE
~ RUPTURESPIKE 16 ARRAYS, 9 SPIKES EACH)
0.125 O.D. x 0,049" I.D. S. S. SEAMLESS TUBE
I
MA-3929-17
Fig. 5.
6.
Schematic of Condensible Bubble Source in Reactor Model Core
Instrumentation
Transient measurements are made on LMFBR model tests to validate the applied loading, to provide data for comparison with response predictions, and to provide data on critical members for which response predictions are not possible. Since validation of the applied loading is crucial to obtaining a credible test, backup measurements are essential. The measurements must include source pressure and upper surface motion
of the coolant slug. In simple models without a cover, the required instrumentation is shown in Figures 9 and i0. Response data consist of pressures, accelerations, velocities, displacements, and strains. Figures 14 and 15 show instrumentation layouts for future tests on the simplest and most complex models of the current CRBR program in which the sensor outputs will be monitored by tape recorders augmented by oscilloscopes. Examples of oscillographs for pressures and strains are shown in Figure 16, obtained from a simple flexible FTR model
96
3.R. Abrahamson et al./Tests in support of LMFBR
PASSAGE TO UPPER CORE BARREL ELECTRICAL FEEDTHROUGH
O-RING SEAL AT DOOR
¢,SLIDING DOORS
BASE PLATE
I NSU LATO RS
WATER COOLED PRESSURE TRANSDUCER PORT
INCONEL HEATING ELEMENT
LOWER CORE/ PRESSURE VESSEL
WATER PASSAGE" THERMOCOUPLE PORT I INSULATED ELECTRICAL FEEDTHROUGH Fig. 7.
MA-3g2g-70A Flashing Source
test. [6,7] Experiments are usually monitored by high speed cameras, especially for observation of coolant leakage. Tests on models such as SM-2, shown in Figure 14, provide data for comparison with code predictions, whereas tests on models with the complexity of that shown in Figure 15 provide data where predictions are not possible. One of the most important aspects of LMFBR model tests does not require transient response measurement but simply compares pretest and posttest observation and dimensional measurement. Figure 17 gives an example in which final FTR wall measurements are plotted [6,73; observations of instrument tree damage are seen in Figure 12. A rapid assessment of safety may be obtained with such posttest observations.
97
G.R. Abrahamson et al./Tests in support of LMFBR
References [I]
[23
[33
[4]
[53
[6]
R. W. Gates, Containment of fragments from a runaway reactor, SRI Final Report to USAEC (February 1964). D. J. Cagliostro, A. L. Florence, G. R. Abrahamson, and G. Nagumo, Characterization of an energy source for modeling hypothetical core disruptive accidents, Nucl. Eng. Design 27 (1974) 94-105. D. W. Ploeger and D. J. Cagliostro, Experimental study of heat transfer from a simulated hypothetical core disruptive accident bubble, SRI Technical Report 1 to ERDA (November 1975). D. W. Ploeger and D. J. Cagliostro, Development and characterization of a liquid-vapor bubble source for modeling HCDA bubbles, SRI Technical Report 2 to ERD~J (March 1977). J. N. Goodier, Dimensional analysis, in Handbook of Experimental Stress Analysis by M. Hetdnyi (John Wiley and Sons, New York, 1950). A. L. Florence, G. R. Abrahamson, and D. J. Cagliostro, Hypothetical core disruptive accident experiments
[7]
VENTED CHARGE CANISTER DET( INATOR
A3 ,
.~1
t CHARGE 'CHAMBER
PETN/MICROSPHE RES 90110 BY W E I G H T
on simple fast test reactor models, Nucl. Eng. Design 38 (1976) 95-108. A. L. Florence and G. R. Abrahamson, Simulation of a hypothetical core disruptive accident in a fast flux test facility, SRI Final Report HEDL-SRI-I (May 1973).
LOADING CHAMBER f FLEXIBLE i DIAPHRAGM RIGID PIPE
1
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Fig. 8.
Schematic of Pressure Pulse Source for Piping Tests
98
G.R. Abrahamson et al./Tests in support of LMFBR
DURCE
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R r
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9.
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Fig.
10.
Simple Flexible FTR Model
)
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Fig. ii.
Assembly of Small Complex FTR Model
99
G.R. Abrahamson et al./Tests in support of LMFBR
1/30-SCALE MODEL. PRIOR TO ASSEMBLY OF COVER PLATE
1/30-SCALE MODEL COVER PLATE AND INSTRUMENT TREES (3 EACH) AFTER TEST
1/30-SCALE INSTRUMENT 1/30-SCALE INSTRUMENT TREE PRIOR TO TEST TREE AFTER TEST
1/10- AND 1/30-SCALE INSTRUMENT TREES AFTER TESTING (SMALL TREE IS THE SAME AS AT LEFT) MP-317583-5
Fig.
12.
Small Complex FTR Model (Top), and Comparison of Permanent Deformation of Instrument Trees from Small and Large Models (Bottom)
i00
G.R. Abrahamson et al./Tests in support of LRFBR
(a)
OBLIQUE FRONT VIEW
(b) OBLIQUE OVERHEAD VIEW MP-J 109-I 03
Fig. 13.
Views of Large Complex FTR Model with Piping
i01
G.R. Abrahamson et al./Tests in support of LMFBR
HOLDDOWN STUDS (72 PLACES) / 9 ~E ;)
SIMPLE ,COVER GA~ WATER St
f DIAPHRAGM 0 in.
~ 21.85in.
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I
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/
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J
CANISTER" SUPPORT STAND
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Fig.
14.
Schematic
of CRBR Model SM-2
102
G.R. Abrahamson et al./Tests i n support of LMFBR
(~) (~) (~ W
Acceler PresU~ Strain Water I Transdl (~)T Tee R Mp.-3U2~J-1l~J
Fig. 15.
Schematic of CRBR Model SM-5
103
G.R. Abrahamson et al./Tests in support of LMFBR
1 1 6 0 0 #sec ( F R O M
DETONATION)
H.D.
AIR ~LINING -VESSEL
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(b) COVER PRESSURE BASE
TEST NO: SCALE: CHARGE: CHARGE WEIGHT: CORE: CANISTER: CORE BARREL:
VESSEL LINING: VESSEL:
COVER:
HOLD-DOWN BOLTS:
-800
#sec ( F R O M D E T O N A T I O N )
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(c) COVER STRAIN ) p.sec
304 Stainless Steel Thickness 44 mils 4.96 in. O.D. x 4 in. hgt. Lead 1/32-in. Thick 304 Stainless Steel Thickness 58 mils 8 in. O.D. x 17.2 in. inside hgt. A36 Steel 1/2-in. Thick x 10-in. dia. Weight (steel + lead) 50 lb. Nickel 205 24 Bolts @ 0.192-in. dia. x 2.3-in. Stretch Bolt Circle 9-1/4-in. dia.
STRAIN (CIRCUMFERENTIAL STRAIN 1-1/4.-in. BELOW
(d) VESSEL
VESSEL TOP) 800
STRAIN (CIRCUMFERENTIAL STRAINS 4-in. AND 6-9/16-in.
(e) VESSEL
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BELOW VESSEL TOP) MP-1109-78A
Fig. 16.
Typical Records from FTR Test 208
104
G,R, Abrahamson et al./Tests in support of LRFBR
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Fig. 17.
Vessel Deformation Profiles from Three FTR Tests