Int. J. Pres. Ves. & Piping 44 (1990) 117-135

A Thermal-Transient Test of an FBR Piping-Bellows Model

T. Saito,* H. Umeda, S. Kanazawa, K. Watashi~ & A. Imazu

Oarai Engineering Center, Power Reactor & Nuclear Fuel Development Corp., 4002 Narita-cho, Oarai-machi, Ibaraki-ken 311-13, Japan

(Received 3 May 1990; accepted 27 February 1990)

A BS TRA C T

This paper describes the results of the thermal-transient test of the piping- bellows models consisting o fan internally pressurized type of bellows and an externally pressurized type of bellows. The piping bellows were subjected to cyclic cold and hot thermal transients by sodium in a temperature ranging .from 250 to 600°C. Heat-transfer and thermal-elastic-stress analyses were carried out for evaluating the creep-fatigue strength. The method of evaluating creep-fatigue damage in the Design Guide of Japan was demonstrated to be applicable to ensuring a high degree of integrity of the spec(fic structure of the bellows type of expansion joints.

1 I N T R O D U C T I O N

In the design of the liquid-metal fast-breeder-reactor (FBR) components, a high degree of integrity at elevated temperatures and a further reduction in the construction cost are required. The study of FBR main-coolant piping containing bellows expansion joints has been performed to cope with such requirements.

The bellows expansion joints for FBR piping usually consist of bellows and hardware structures that connect bellows and piping. The bellows are required to absorb thermal deformation of the piping system and to

* Present address: Toshiba Corp., Yokohama, Japan. To whom correspondence should be addressed.

117

hit. J. Pres. Ves. & Piping 0308-0161/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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A thermal-transient test of an FBR piping-beUows model 119

withstand internal and external pressures, seismic loadings, and so on. Creep-fatigue tests by mechanical loadings, buckling tests by internal and external pressures, and vibration tests of the bellows have been conducted for the purpose of grasping the characteristics of the bellows itself.

For thermal-transient loadings, the bellows are a thin-walled uniform structure, so the thermal stress is generally small. Hardware structures, however, incorporate various structural discontinuities, such as Y-shaped and E-shaped junctions. The thermal stress originating from the temperature difference between adjoining members is significant in usual cases. The important issue in the design of the bellows expansion joints for thermal-transient loadings is the creep-fatigue failure of hardware structures due to repeated thermal stresses. Moreover, thermo-hydraulic behavior in the annular part between the bellows and shell structures through which sodium stagnates or flows very slowly are expected to be extremely complicated, especially under transient conditions. Some uncertainties are expected in the prediction of the thermal-boundary condition of this part, which provides the basis of the calculation of the metal temperatures.

This paper describes the results of thermal-transient tests of piping- bellows models with Y-shaped and E-shaped junctions for evaluating the design method for creep-fatigue failure, as one of the R & D items of the feasibility studies of the piping bellows for FBRs in the Power Reactor & Nuclear Fuel Development Corporation (PNC).

2 EXPERIMENTS

The configuration of the piping-bellows models is shown in Fig. 1. The typical structural discontinuities of Y-shaped and E-shaped junctions in the bellows expansion joints were incorporated. The internally pressurized type of bellows model (vertical type) consists of Y-shaped junctions, thin-walled bellows, a thick-walled section, and thick-walled bellows, etc., and the externally pressurized type of bellows model (horizontal type) consists of thin-walled bellows and Y-shaped and E-shaped junctions. The diameter of the piping is 24in (610mm), and the wall thickness is 8 mm.

Argon gas is discharged into the space between the boundary bellows whose inner surface was exposed to sodium and the back-up bellows. Inner vessels are installed in these models, which provide a faster sodium-flow rate by decreasing the flow area of sodium.

The Y-shaped and E-shaped junctions were fabricated from type 304 forged austenitic stainless steel, and the bellows are made of type 316 austenitic stainless steel. The fatigue and creep tests were performed at

120 T. Saito et al.

elevated temperatures with the same lot materials as were used with the type 304 forged austenitic stainless steel of the piping-bellows models. Comparing these results with the material properties of standard type 304 austenitic stainless steel, the authors confirmed that the fatigue life is 20-30% longer than that at 600°C and the creep-rupture time is three times its value at 600°C.

2.1 Test procedure

The thermal-transient test of the piping-bellows models was conducted by using TTS (thermal-transient test facility for structures) at Oarai Engineering Center of PNC.~ TTS has two independent sodium loops, and successive operation of valves produces the thermal cold and hot transients in the model. A number of thermal-transient tests of structural models have been conducted by using TTS, and some of the experimental results and evaluation of creep-fatigue damage were reported earlier. 3A

The piping-bellows models were subjected to cyclic cold and hot thermal transients by sodium at the constant flow rate of 1 m3/min, which causes a sodium temperature-change rate of approximately 20°C/s at the inlet nozzle of the vertical type. In a cold transient of 250°C, sodium flows into the model during a period of 30 min, and, in a hot transient of 600°C, sodium flows into the model during a period of 150 min. This thermal-transient condition is more severe than that of FBR plants. The tests were continued to 427 cycles, by which time sodium leak had occurred in the horizontal type.

Thermocouples of the alumelmhromel type are installed on the inner and outer surfaces of the models to measure the temperature of the sodium and metal in the models. The number of thermocouples is 125 for the vertical

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A thermal-transient test of an FBR piping-bellows model 121

type and 147 for the horizontal type. The outputs of these thermocouples were recorded by a data-acquisition system.

2.2 Results

The sodium-temperature changes of the vertical-type model are shown in Fig. 2. The sodium temperature at the inlet nozzle changes rapidly at the beginning of the thermal cold and hot transients. In the hot transient, the sodium temperature reaches about 600°C within a minute, which causes the maximum temperature-change rate of approximately 20°C/sec. The sodium temperatures at different locations change more slowly towards the down

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122 T. Saito et al.

stream and show axisymmetric change (axisymmetric flow) in the vertical- type model.

Figure 3 shows the temperature changes at the Y-shaped junction in the upper stream (the first Y-shaped junction) of the vertical-type model. The sodium temperature is measured by the thermocouple TV-4a, and the temperatures on the outer surface are measured by TVO-3, TVO-4, and TVO-5. The sodium temperatures of TV-4a and the metal temperature of TVO-3 change rapidly, and the temperature gradient through the wall thickness of the piping is not significant. The thermocouples of TVO-4 and TVO-5 change moderately, and the maximum temperature difference between TVO-3 and TVO-4 reaches about 190°C after 90 seconds from the beginning of the cold transient. For the hot transient, the maximum temperature difference is about 140°C after 90 seconds of the hot transient.

These results revealed that the temperature difference between adjoining members at junctions is significant, and the temperature gradient through the wall thickness is very small. The temperature changes on the inner and outer surfaces of the bellows, the wall thickness of which is 2 mm, are much the same.

Figure 4 shows the sodium-temperature changes of the E-shaped junction (the first E-shaped junction) of the horizontal-type model. These temperatures are measured by the thermocouples TH-11a-e, which were arranged at intervals of 45 ° around the circumference in such a way that the thermocouple TH-1 la was mounted at the top (0 ~') and TH-11e was at the bot tom (180°). A non-uniform temperature distribution around the circumference is observed in the cold transient. The temperature of TH-1 le drops below 300°C within 6min, while T h - l l a changes very slowly and shows about 500°C after 6min of the cold transient. The temperature difference between the top and the bot tom is more than 250°C. The temperature difference along the circumference is most significant around the top in the cold transient, whereas the most significant temperature difference is around the bot tom in the hot transient as shown in Fig. 4.

The sodium-temperature changes of the third Y-shaped junction shows that a considerable thermal stratification is observed at the top. The temperature changes, with the exception of that at the top, are the same in the third Y-shaped junction, while the temperature of the top changes very slowly. There is a temperature difference of more than 250°C between the top and the other points, and the stratification behavior occurs locally at the top. The temperature distribution on the circumference of the third Y- shaped junction is more severe than that of the E-shaped junction.

Test results showed that structural-design analysis of the vertical-type bellows model subjected to thermal-transient loadings can be successfully performed by thermal-transfer analysis and thermal-stress analysis by using

A thermal-transient test o f an F B R piping-bellows model 123

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Sodium leak had occurred at the weldment that connects the piping with the third Y-shaped junction, which is indicated by W in Fig. 5. A remark- able thermal stratification is observed at the top of this portion as described

124 T. Saito et al.

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Fig. 5. A through-wall cracking in the horizontal-type bellows model.

previously. After the destructive examinations, a through-wall crack existed, which is shown in the figure. The crack was initiated at the welding toe on the outer surface and was propagated towards the inner surface. The dye- penetrant examination after the test indicated some fine cracks, the depths of which were less than 1 mm, on the inner surface of the thick-walled section for the vertical-type model, while no crackings were found in the particular structure of the bellows expansion joints, such as the Y-shaped and the E- shaped junctions.

3 ANALYSES

The heat-transfer and thermal-elastic-stress analyses of the bellows- expansion-joints models were carried out by using the measured- temperature data. Axisymmetric analyses were carried out for the vertical- type model because of the axisymmetric sodium flow. As for the horizontal- type model, three-dimensional calculations are needed on account of the temperature distribution around the circumference, and two local-structure models were adopted for analyses: one Y-shaped junction (the third Y- shaped junction) and one E-shaped junction (the first E-shaped junction). At the same time, axisymmetric analyses were carried out by assuming that the measured temperatures in a specific meridional section were distributed uniformly in the circumferential direction, in order to investigate the effect of three-dimensional temperature distribution on thermal stresses. Such axisymmetric analyses were conducted for three meridional sections, namely 0 ° (top), 90 ° (side), and 180 ° (bottom).

Three-dimensional analysis was also carried out for the piping model containing the welding at which the through-wall crack occurred in order to

A thermal-transient test o f an FBR piping-bellows model 125

investigate the crack initiation and growth. This structure is not a particular structure such as the Y-shaped and the E-shaped junctions of the bellows expansion joints.

These calculations were performed by using a finite-element analysis computer code FINAS, developed by PNC. 5 As for the axisymmetric analyses, axisymmetric solid 8- and 6-nodes isoparametric elements were used, while 20- and 15-nodes isoparametric elements were used for three dimensional-analyses.

3.1 Heat-transfer analyses

Heat-transfer analyses were carried out for the cold and hot transients by using the measured sodium-temperature data. The coefficients of heat transfer at various locations on the inner surfaces exposed to sodium were determined in such a way that temperatures of metal calculated on the outer surfaces coincided with measured values. A constant value was used for the coefficient of heat transfer at each location during the cold and hot transients. Thermal radiation was considered in the space in which argon gas was charged. The remaining surfaces were assumed to be adiabatic. As for the axisymmetric analyses of the vertical-type model, these coefficients of heat transfer were determined for 27 surfaces, and representative sodium temperatures obtained from the experiment for these surfaces were used for the heat-transfer analyses. For the three-dimensional heat-transfer analyses of the horizontal-type model, the sodium temperature measured at intervals of 45 ° around the circumference for each location on the meridional line were interpolated in the circumferential direction as well as in the meridional direction, so as to provide the temperature of sodium at any location.

The analytical results of the temperatures on the outer surfaces are in good agreement with observed ones, as shown in Fig. 6(a). These distributions of calculated temperature along the meridional line on the outer surface of the first Y-shaped junction of the vertical-type model are shown for various times during the cold transient in Fig. 6(b). A significant temperature gradient takes place in the Y-shaped junction, and the temperature difference between the adjoining members is most remarkable at 90 s. The results of axisymmetric heat-transfer analyses for the third Y- shaped and first E-shaped junctions show good agreement with those derived from three-dimensional analyses.

3.2 Thermal-stress analyses

Thermal-elastic-stress analyses were performed for each model after unsteady-heat-transfer analyses. Internal pressures and piping-reaction forces were ignored in the calculation, since they had little effect on the results.

126 T. Saito et al.

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90 s, this caused a large bending stress at the section Y, as was mentioned before in relation to Fig. 6. Figure 7(b) shows the distributions of stress components along the meridional line of the first Y-shaped junction at that time. A significant compressive axial stress occurs locally near the structural discontinuity, while a tensile hoop stress is relatively small. Figure 7(b) shows that low stresses are produced at the piping portion away from the Y-shaped junction, which is thought to be caused only by the temperature distribution along the wall thickness. In the hot transient, the largest tensile axial stress is produced on the inner surface. In general, the cold transient produces tensile axial and hoop stresses on the inner surface of the thick-walled vessel by the

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A thermal-transient test o f an FBR piping-bellows model

TABLE 1 Comparison of Equivalent Stress Ranges in the E-shaped Junction

129

Analysis Equivalent stress range ( M P a )

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temperature distribution through the wall thickness, while the temperature difference in the cold transient of the Y-shaped junction produces compressive axial and tensile hoop stress. The difference in the behavior in the stress state depends on whether a temperature gradient is mainly in the wall or in the adjoining members.

Figure 8 shows an example of the elastic-analysis results for the E-shaped junction. Figure 8(a) is the stress distribution along the meridional line facing the air in the 90 ° section of the first E-shaped junction model by three- dimensional analysis. The maximum stress intensity in this junction takes place at the position denoted by E in the figure, which is produced by the temperature gradient between adjoining members. It is very useful if the thermal stresses of axisymmetric shells with a non-uniform temperature distribution in the circumferential direction are estimated with sufficient accuracy by axisymmetrical analysis, assuming that the temperatures of the specific meridional section are uniform in the circumferential direction. Figure 8(b) shows the axisymmetrical-analysis results assuming the temperature distribution in the 90 ° section. Figure 8(b) is very close to Fig. 8(a). A comparison of the equivalent stress ranges between the three- dimensional and axisymmetrical analyses is made for the three meridional sections of the first E-shaped junction in Table 1. Both results support the adequacy of the axisymmetrical-analysis methods described above. It is assumed that the thermal stress arising from the temperature distribution in the circumferential direction damps down remarkably along the circum- ference in such a thin shell structure. It is thought to be reasonable to evaluate the creep-fatigue damage by using these axisymmetrical-analysis results.

3.3 Thermal analyses of the piping model

The three-dimensional thermal-stress analyses were also used to evaluate the thermal response of the piping model. An analytical model simulates the detailed geometry of the welding, where a through-wall crack occurs in the horizontal-type model, which is shown in Fig. 5. For the thermal-boundary conditions, two extremes at which the temperature gradient is noticeable in

130 7". Saito et al.

CONTOUR VALUES (MPa)

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each transient were selected for evaluating the stress range. The observed temperature distribution around the top (0-20 ° ) is used for the analyses. For the remaining portion, the temperature distribution is the same with a temperature of 20 °. In the cold transient, a thermal-elastic-stress analysis is performed for a temperature of 550°C at the top and 300°C at 20 °. In the hot transient, a temperature of 390°C at the top and 550°C at 20 ° is used. Figure 9 shows the result of the axial stress at the 0 ° section (top) for the piping model at the hot transient. The largest stress intensity occurs at the welding toe, which is caused by the geometrical discontinuity. Figure 10 shows the distributions of stress components along the circumference of the welding toe at the hot transient. A significant tensile axial stress, which is caused by the thermal-stratification behavior, occurs locally near the top. Thermal- elastic-stress analyses reveal that the maximum stress range is beyond 1600 MPa. This stress is produced by the geometrical discontinuity and the severe temperature distribution around the top on the circumference. The results of these analyses reveal that the crack is supposed to initiate and grow by the cyclic thermal stress at the welding toe of the top, where the through-

A thermal-transient test of an FBR piping-bellows model 131

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wall crack occurs. However, this behavior is a special case and not essentially concerned with the characteristics of the particular structures of the bellows expansion joints.

4 EVALUATION OF CREEP-FATIGUE D A M A G E

The structural design of FBR components is required to have a high degree of integrity at elevated temperature. PNC has developed 'Elevated Temperature Structural Design Guide for Class 1 Component of Prototype Fast Breeder Reactor' (ETSDG) in Japan. z

The object of this study is to confirm the safety margin against the allowable limit described in 'Limits to creep-fatigue damage' in the Guide. Evaluation of the creep-fatigue damage of the models of the bellows expansion joints based on the Guide was carried out with the results of thermal-elastic-stress analyses.

4 . 1 E v a l u a t i o n m e t h o d

The Guide includes a rule for evaluating the total strain range between two extremes of events by use of Neuber's equation and elastic follow-up of secondary stresses. The total strain range, et, for the case of low-level long- term primary stresses is provided by the following equations.

et = K,.~, + KT.E v (1)

K~ = max [(S*/,~). Kz, K . Ke'] (2)

Ke' = 1 + (q -- 1)[1 -- (3Sm/Sn)] q = 3 (3)

132 T. Saito et al.

where e. is the primary and secondary strain range, ev is the thermal-peak- strain range, K~ and KT are the strain-concentration factors of e, and ev, respectively, and K is a stress-concentration factor. The factor Ke' is the factor considering the total strain-range increase due to elastic follow-up, with use of the elastic-follow-up parameter, q = 3.

The Guide provides a limit on the qeft-hand side of fatigue damage, D r, plus creep damage, De, which is less than or equal to D:

D r + D c < D (4)

Fatigue damage, D r , is given by the following equation:

De = n/Nd(et) (5)

The Guide prepares three design-fatigue curves for different strain rates. In this study, the design-fatigue curve (B) for thermal-transient events is used for calculating an allowable number of cycles to fatigue failure N~(~,), and n is the number of cycles of thermal transients applied. Fatigue damage for a total strain range and at the maximum metal temperature is given by the following equation:

Df = 427/N~(~t) (6)

The fundamental limit to creep damage De in the ETSDG is provided by the following equation:

D~ = f2 dt/tR(a) (7)

The factor of 2 in the ETSDG is a safety factor for the creep-damage evaluation, corresponding to the factor K', which is equal to 0"9 in ASME CC N47 and is provided to evaluate creep damage under constant stresses. The time margin of 10 is the same.

Creep-damage evaluation by using elastic analyses for the case of low- level long-term primary stresses in the ETSDG is provided by the following equations:

D~ = Dot + De2 + Dcp

De, = f 2 dt/tR(Sg)

(8)

(9)

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Dc2 = D* + D* Dcp = n. D**

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A thermal-transient test o f an F B R piping-bellows model 133

which is associated with stress relaxation down to the stress level Sg. Creep- damage evaluation described in the Guide is explained in detail in ref. 2.

4.2 Calculated creep-fatigue damage

Creep-fatigue damage was calculated by evaluating the total strain range in accordance with the procedure in the Guide. These safety margins of material strength were applied to the Guide on the basis of standard material properties. Table 2 shows the results of creep-fatigue evaluation at four sections, which are selected as the most significant section in each particular structure with structural discontinuity in the piping-bellows models. The creep-fatigue-damage results in the third Y- and E-shaped junctions of the horizontal-type model are evaluated by the axisymmetric analyses of the 90 ° section. Creep-fatigue damage in the first and second Y- shaped junctions is most significant, which shows that the creep-fatigue damage is beyond 90 and the primary- and secondary-stress ranges, S n, are 1300-1400 MPa. Other portions of interest are also evaluated, but the creep- fatigue damage of them is relatively small, except for the thick-walled section at which the safety-margin value of damage reaches approximately 50. Some fine cracks were observed only at the thick-walled section, as described before. The results of creep-fatigue damage are shown in Fig. 11. The experimental data of the previous study reveal that crackings can be observed over the safety margin of 50 against the allowable limit on the damage diagram. This coincides with the results o f the thick-walled section, whereas the previous study indicates that no crackings exist even for the safety margin of 100 in the case of the hardware structures with structural discontinuities of the piping-bellows models.

For the particular structure in the piping-bellows models, the safety margin against the allowable limit in the E T S D G Guide is more than 100 for the Y-shaped junct ion and 70 for the E-shaped junction. However, these values of creep-fatigue were calculated from the standard material properties that applied to the Guide. In fact, the creep-rupture time and the

TABLE 2 Results of Evaluation of Creep-Fatigue Damage

Section S. (MPa) e, F (mm/mm) K K~ e, t (mm/mm) Df D c D

First Y 1 364 1.3 × 10 -4 1-00 2-67 2-4548 × 10 -2 70.2 26"1 96"3 Second Y 1 319 1.2× 10 -4 1.00 2-66 2-363 1 × 10 -z 65"8 24'5 90"4 ThirdY 1086 0'9×10 -4 1"00 2"58 1"8865×10 -1 44.3 17.0 61.3 First E 908 0.4 × 10 -4 1.13 2-83 1-7199 × 10 -2 37-4 14.7 52'2 Thick-walled 557 2.9 × 10 - 3 1"28 2-81 1"3316 x 10 -2 24"6 10'2 34'8

134 T. Saito et al.

100.11 -~ ~'~ o ~,t v -o- t,L E~o'l Q ~d v(o ' ) / R E A C T O R o ~ v -c.,, E "-^" - ~d v-oo" / VESSEL ~bTI-ItCK.WALI_I[D'~r lst 1[<180") ~ a ,d v(~so'l.,(~ MODEL

10.0

1"0 - r ~ A [ . . " \ I [ t sAFETY / I I I MARGIN

LLOWABLE 110 20 150 100 I \"'"

0.1l ' ~ , 0 .1 1 .0 10 .0 100 .0

FATIGUE DAMAGE, Df Fig. 11. Creep- fa t igue d a m a g e o f the be l lows-expans ion - jo in t mode l s a cco rd ing to

E T S D G .

u/ (9 <

< O eL ul I11 ae 19

fatigue life of this material were a little longer than that of the standard one. From these points, the essential safety margins against the allowable limit of damage are thought to be slightly lower than these values. But no crackings were found in the specific structure of the piping-bellows models. These results reveal that there is a sufficient margin for the allowable limit in the particular structure.

The evaluation method of creep-fatigue damage in the ETSDG can assess a high degree of integrity of the specific structure of the piping bellows.

5 CONCLUSIONS

The important issue in the design of the bellows expansion joints for thermal loadings is the creep-fatigue failure of the hardware structures. A thermal- transient test of the piping-bellows models was conducted. These models were subjected to more severe cyclic thermal transients than those of the plant conditions. After the destructive examination, no crackings were found in the particular structures of the piping-bellows models, such as the Y-shaped and E-shaped junctions.

The sodium temperatures show the axisymmetrical change in the vertical- type bellows model, with three-dimensional change in the horizontal-type model. The results revealed that the temperature difference between adjoining members at a junction is significant, and the temperature gradient through the wall thickness is very small. The largest axial-bending stress occurs near the structural discontinuity in the Y-shaped junction. Moreover, thermo-hydraulic behavior in the annular part in which sodium stagnates or

A thermal-transient test of an FBR piping-bellows model 135

flows very slowly is extremely complicated under the thermal transient. Though three-dimensional analyses are needed for the horizontal-type model on account of the temperature distribution around the circumference, axisymmetric analysis, assuming the temperature on the specific meridional section, is adequate for the thin-shell structure in this study.

An evaluation of creep-fatigue damage based on the Guide was carried out with these results, and it was confirmed that there is a sufficient margin of the particular structure with structural discontinuity of the piping bellows against the allowable limit of creep-fatigue damage included in the Guide. The evaluation method of creep-fatigue strength adopted in the Guide was demonstrated to be applicable to ensuring a high degree of integrity of the specific structures of the piping bellows.

A C K N O W L E D G E M E N T

The authors would like to acknowledge the contribution of Toshiba Corporation to the design and fabrication of these testing models and to thank Century Research Center Ltd for programming assistance.

REFERENCES

1. Nakanishi, S., et al., Thermal transient test facility for structures. In 8th S M i R T , E2/1, 1985, pp. 37-42.

2. Iida, K., et al., Simplified analysis and design for elevated temperature components of monju. Nuclear Engng & Design, 98 (1987) 305-17.

3. Watashi, K., et al., Creep-fatigue test of thick-walled vessel under thermal transient Ioadings. In 9th S M i R T , L4, 1987, pp. 207-12.

4. Watashi, K., et al., Creep-fatigue strength evaluation of thick-walled vessel under thermal transient loadings. In 9th S M i R T , L2, 1987, pp. 93-8.

5. Iwata, K., et al., General purpose nonlinear analysis program FINAS for elevated temperature design of FBR components. A S M E Pressure Vessel Technol., 66 (1982) 119-37.