Effect of Microstructure on Failure Behavior of Light Water Reactor Coolant Piping under Severe Accident Conditions

In a severe accident of light water reactors, the reactor coolant system (RCS) piping might be subjected to thermal loads caused by the decay heat of the deposited fission products and the heat transfer from the hot gases, with an internal pressure in some accident sequences. Tests on the RCS piping failure were performed along with high temperature tensile and creep rupture tests including metallography to investigate the failure behavior. The prediction of the 0.2% proof stress by Arrhenius equation is in good agreement with the measured stress above 800°C for served RCS piping materials. The modified Norton's Law for the short term creep rupture model agrees with the experimental values between 800 and 1,150°C for type 316 stainless steel. The microstructural change was discussed with the effect of the very rapid formation and resolution of the precipitation on the strength at high temperature. The result of the piping failure tests which simulated the severe accident conditions, i.e., in short-term at high-temperature, could support the plastic limit load prediction of the flow stress model using the 0.2% proof stress.     

Finite element analysis for creep failure of coolant pipe in light water reactor due to local heating under severe accident condition

During severe accident of a light water reactor (LWR), the piping of the reactor cooling system would be damaged when the piping is subjected to high internal pressure and very high temperature, resulted from high temperature gas generated in a reactor core and decay heat released from the deposit of fission products. It is considered that, under such a condition, short-term creep at high temperatures would cause the piping failure. For the evaluation of piping integrity under a severe accident, a method to predict such high temperature short-term creep deformation should be developed, using a creep constitutive equation considering tertiary creep. In this paper, the creep constitutive equation including tertiary creep was applied to nuclear-grade cold-drawn pipe of 316 stainless steel (SUS316), based on the isotropic damage mechanics proposed by Kachanov and Ravotnov. Tensile creep test data for the material of a SUS316 cold-drawn pipe were used to determine the coefficients of the creep constitutive equation. Using the constitutive equation taking account of creep damage, finite element analyses were performed for the local creep deformation of the coolant piping under two types of conditions; uniform temperature (isothermal condition) and temperature gradient of circumferential direction (non-isothermal condition). The analytical results show that the damage variable integrated into the creep constitutive equation can predict the pipe failure in the test performed by Japan Atomic Energy Research Institute, in which failure occurred from the outside of the pipe wall.     

Material Properties of a Natural UF6 Transport Cylinder Vessel at High Temperature

Abstract

The IAEA Regulations for the Safe Transport of Radioactive Material are to be revised in 1996 and the fire test (800°C for 30 min) could become a requirement for the natural UF₆ transport cylinder. ASME SA 516 carbon steel is used as the structural material for this type of cylinder. It is very important to obtain high temperature data for SA 516 steel to be able to evaluate the integrity of the UF₆ transport cylinder vessel in the fire test. CRIEPI has therefore conducted material tests on SA 516 at high temperatures. The AC₁ and AC₃ transformation points of actual SA 516 steels have been measured. Tensile tests up to 900°C were conducted using USA, French and Japanese manufactured materials and the influence of phase transformation assessed. Preliminary creep tests show that assessment by creep strength can give a more conservative estimation than using the tensile strength. Creep deformation equations have been obtained using uniaxial creep tests and internal pressure creep tests. In addition, by the use of internal pressure creep rupture tests, the relation between the circumferential stress, the test temperature and the rupture time has been obtained.     

Microstructural influence on high temperature creep flow of Zr-1%NbO alloy in near-α, (α + β), and β temperature ranges in a high vacuum environment

Uniaxial tensile creep tests were carried out at 650-1100 °C in a high vacuum environment on Zr-1%NbO tubes with various microstructures. The effect of microstructure on creep flow in the (α + β) temperature range is significant (the creep rate being modified by up to three orders of magnitude) under stresses lower than 10 MPa, that is, for stress values of one order of magnitude lower than those characteristic of prototypical Loss-of-coolant-accident (LOCA) conditions. Under stresses higher than about 20 MPa, this effect is much smaller. No transformation-induced plasticity was detected from anisothermal creep tests, once the creep strain was thoroughly taken into account to process experimental strain vs. time data.     

An improvement in creep resistance of Zr-2.5%Nb tubes

Specimens from a Zr-2.5%Nb pressure tube were heat-treated in the range 650 to 1050°C and then creep-tested at 450°C. Water quenching produced anisotropic behaviour for soaking temperatures from 650 to 850°C, and isotropic behaviour above this range. A lower ‘intermediate cooling rate’ produced anisotropic behaviour for the whole soaking range. Creep resistance improved with increasing soaking temperature, particularly for transverse intermediate-cooled specimens. At soaking temperatures of 880°C and higher, a 100-fold reduction in creep rate was achieved with these specimens compared with as-cold-worked. An attempt is made to relate the creep data to crystallographic texture. In the second part of the program, the 880°C intermediate-cool heat treatment was chosen for further testing. It was confirmed that the material exhibits superior creep resistance compared with cold-worked Zr-2.5%Nb, at all stresses from 34 MN/m² to the ultimate tensile strength, in the temperature range from 300 to 450°C. As expected, the ultimate tensile strength was reduced by this heat treatment. At the upper end of the stress range, at 300°C, a discontinuity occurs in creep data. The instability causing the discontinuity and leading to failure at relatively low stress is attributed to a twinning mechanism. Large twins encompassing hundreds of grains are observed.     

High temperature design curves for high nitrogen grades of 316LN stainless steel

Nitrogen alloyed low carbon grade 316L(N) stainless steel (SS) is a major structural material for high temperature structural components of sodium cooled fast reactors. With a view to significantly enhance the high temperature mechanical properties of 316L(N) SS and thereby increase the design life of structural components from 40 years to 60 years, the influence of nitrogen content on the tensile and creep properties of this steel has been investigated. Four heats of 316LN SS with 0.07, 0.11, 0.14, and 0.22 wt.% nitrogen were used in this investigation. Tensile tests were carried out at various temperatures between room temperature and 850 °C. Creep tests were carried out at 650 °C at various stress levels in the range of 140-225 MPa. The maximum rupture life in these tests was 16,000 h. The tensile and creep data were analysed according to RCC-MR nuclear code procedures and the design curves have been generated. The tensile and creep strength of 316L(N) SS have been found to improve significantly by increasing the nitrogen content.     

Fracture and general yield for carbon steel pipes with local wall thinning

Monotonic bending tests without internal pressure are conducted on 4 and 3.5 in. diameter full-scale carbon steel pipe specimens with local wall thinning in order to evaluate the structural integrity of power plant piping. The local wall thinning is simulated as erosion/corrosion metal loss. The eroded area of the wall thinning is subjected to tensile or compressive stress by applied bending moment. The maximum moments obtained from the tests are compared with the plastic collapse moments based on the net-section stress approach. The net-section stress approach based on flow stress σ_f gives a conservative estimation, sometimes overly conservative. Although the net-section approach based on ultimate tensile strength σ_u gives a slightly non-conservative estimation for some cases, the calculated values are close to the experimental data. Using the net-section stress approach based on σ_u, the eroded depth and the angle at which a pipe undergoes general yielding were obtained.     

Evaluation of anelasticity and plastic creep limit for alloy 800

The elevated temperature tensile, anelastic and creep properties of a precipitation strengthened Ni---Cr austenitic steel (alloy 800) have been evaluated with respect to LMFBR application. These properties have been estimated and formulated for small strains and temperatures on the order of 823 K. It has been shown that despite suppression of the in-service stresses to beneath the alloy's yield strength, plastic deformation will occur at such temperatures and the stress below which plastic strain is effectively zero (plastic creep limit) is negligible for a reactor life time. The plastic creep limit at a given time and temperature markedly depends on the degree of precipitation hardening or prior cold deformation for alloy 800. The contribution from anelastic deformation is less than 0.05% and only becomes important for strict dimensional control and constrained parts.     

Creep Properties with Short Period Excessive Loadings on a Nickel-Base Heat-Resistant Alloy Hastelloy XR

A series of constant load & temperature creep tests and constant temperature creep tests with short period excessive loadings was carried out on a nickel-base heat-resistant alloy Hastelloy XR, which was developed for applications in the High-Temperature Engineering Test Reactor (HTTR), at temperatures ranging from 900 to 1,000°C. The excessive loading levels were set at the design stress intensity values Sm for Hastelloy XR indicated in the HTTR high-temperature structural design code.

Five to six time excessive loadings did not cause significant changes of the minimum creep rate or the time to onset of tertiary creep. Excessive loadings repeated around ten times did not cause significant changes of the time to rupture or the rupture elongation. The results suggest that the design stress intensity values Sm for Hastelloy XR have been determined reasonably.     

Effects of 14 MeV neutron irradiation on creep of nickel and niobium

Flux, stress and temperature effects on the creep strength of nickel and niobium were observed in situ at the RTNS-II 14 MeV neutron source at Lawrence Livermore National Laboratory. Creep tests were done on Ni and Nb near 0.3 $\text{T}{}_{\mathrm{m}}$ with stresses to 280 MPa in a high vacuum test unit using a digital computer for control and data acquisition. Cyclic flux tests produced dramatic changes in creep rate. This creep behavior is attributed to the point defect fluctuations in the crystal structure. Analysis of creep and stress relaxation under steady state flux indicates that an intermediate temperature, thermally activated deformation mechanism is rate controlling.     

The prediction of failure strain for Zircaloy-4 subjected to non-stationary loading conditions

A modified Monkman-Grant relationship (MMG) is applied to predict — as far as the life time is known — the failure strain of Zry-4 subjected to tensile rupture test at load as well as temperature ramp conditions, respectively. As the analysis has shown in the first case, a simple relationship exists between the minimum creep rate and the stress rate. Thus, this quantity appearing in the MMG is phenomenologically connected with the test conditions. For failure strain predictions in temperature ramp tests the introduction of an effective temperature has shown to be advantageous. As compared to the peculiarities of the problem, the agreement between experiments and calculations is encouraging.     

Creep behaviour of modified 9Cr-1Mo ferritic steel

Creep deformation and fracture behaviour of indigenously developed modified 9Cr-1Mo steel for steam generator (SG) tube application has been examined at 823, 848 and 873 K. Creep tests were performed on flat creep specimens machined from normalised and tempered SG tubes at stresses ranging from 125 to 275 MPa. The stress dependence of minimum creep rate obeyed Norton’s power law. Similarly, the rupture life dependence on stress obeyed a power law. The fracture mode remained transgranular at all test conditions examined. The analysis of creep data indicated that the steel obey Monkman-Grant and modified Monkman-Grant relationships and display high creep damage tolerance factor. The tertiary creep was examined in terms of the variations of time to onset of tertiary creep with rupture life, and a recently proposed concept of time to reach Monkman-Grant ductility, and its relationship with rupture life that depends only on damage tolerance factor. SG tube steel exhibited creep-rupture strength comparable to those reported in literature and specified in the nuclear design code RCC-MR.     

Long-term creep rupture behavior of smoothed and notched bar specimens of low-carbon nitrogen-controlled 316 stainless steel (316FR) and their evaluation

Low-carbon, nitrogen-controlled 316 stainless steel is regarded as a principal candidate for a main structural material of future fast breeder reactor plants in Japan. To grasp creep deformation and rupture behavior of this steel whose modeling is indispensable in the design of high-temperature components, a number of uniaxial tensile creep tests have been conducted for four products of this steel at 550 °C and higher temperatures. Long-term creep rupture data up to about 94,000 h were obtained and used to examine the applicability of rupture and deformation estimation methods developed earlier. In addition, two tests were conducted using round-bar specimens with circumferential notches to make investigation of the effect of stress multiaxiality on creep damage.     

High-temperature tensile and creep data for Inconel 600, 304 stainless steel and SA106B carbon steel

We report results from high temperature tensile and creep tests performed on Inconel 600, 304 stainless steel and SA106B carbon steel, materials used to construct components which penetrate the lower heads of reactor vessels. Testing temperatures for the Inconel and stainless and carbon steels exceeded 1360 K, 1350 K and 1040 K, respectively. These data are applicable, but not limited, to severe accident analysis.     

The effects of large concentrations of helium on the mechanical properties of neutron-irradiated stainless steel

Tensile and creep properties have been determined on specimens of type 316 stainless steel irradiated in the High Flux Isotope Reactor in the range 380 to 785°C. Irradiation of type 316 in this reactor partially simulates fusion reactor irradiation, with displacement damage levels up to 120 dpa and helium contents up to 6000 appm achieved in two years. Samples irradiated in the annealed condition to about 100 dpa and 4000 appm helium showed an increased yield strength between 350 and 600°C and, except at 350°C, a reduced ultimate tensile strength compared with values for the unirradiated material. Samples irradiated in the 20%-cold-worked condition showed decreases in both yield and ultimate tensile strengths at all test temperatures. The irradiated samples of both annealed and cold-worked material exhibited little strain hardening, and total elongations were small and became zero,for tests at 650° C. Tensile tests at 575°C and creep-rupture tests at 550°C showed strong effects of fluence on strength and ductility for helium contents above about 30 appm. Optical metallography showed extensive carbide precipitation at all temperatures and precipitation of a second phase, believed to be sigma, at the higher temperatures.     

Application of fracture mechanics methods in safety analysis of piping components in subcreep and creep behavior

A survey and review program for the application of fracture mechanics methods in elevated temperature design analysis and safety evaluation was initiated in December 1976. The first report [1] surveyed and assembled the material for a critical review of the theories of fracture and the application of fracture mechanics methods to life prediction and safety analysis of piping components. The second report [2] provided the basic concepts and a review of the problem areas associated with the development of analytical and experimental programs for a systematic evaluation and comparison of the currently available fracture mechanics theories. The basis for such an evaluation was described in terms of a series of benchmark problems which accurately specify conditions of geometry, loading and environment characteristic of large diameter piping systems in nuclear service.The objective of this third report is to establish a data base and detail the additional analytical techniques needed to confirm the validity of existing analytical methods and improve the state of the art in current problematic areas effecting the interpretation and extension of safety evaluation methods. The need for such a program in the elevated temperature field has been demonstrated by a number of independent surveys on various safety aspects of LMFBR related structural analysis methods and matetials problems. The results of this program, however, will be applicable not only to reactor plants operating at elevated temperatures, but will also lead to improvements of light water reactor evaluation methods for operating and accident conditions.The current state of elevated temperature reactor design technology is embodied in the standards and codes which provide guidance and minimum requirements for systematic design and evaluation procedures. These, however, do not necessarily provide specific absolute values which, if satisfied in the course of design, will guarantee thirty to forty years of uninterrupted life. There are numerous assumptions and approximations embodied in these standards concerning materials behavior, damage mechanisms, and failure modes at elevated temperature. There are also numerous areas of uncertainty and conflicting opinion in the interpretation of the existing test data and in the analysis and evaluation methods. Furthermore, the standards and codes leave some areas to the judgement of the designer, some of which require explicit justifications, but no standards or rules are provided.The overall safety therefore lies, at the present time, in the combination of rigorous enforcement of current standards, judicious application of experience with high temperature equipment even if not in nuclear service, and the surveillance of actual operating conditions. In the past, one criterion proposed for elevated temperature design has been that the time for crack initiation should exceed the design life. However, due to the complexities of the piping structures and the nature of the stress history during service, the evaluation of initiation times is difficult and often leads to uneconomical designs. In addition defects may exist in the component before it enters service. Hence, the knowledge of the growth rates of cracks and the residual strength of the components containing cracks is important in a realistic design evaluation. For more brittle materials and lower temperature applications where plasticity is restricted, linear elastic fracture mechanics methods have been developed. For more ductile materials where the plastic zones near the cracks are larger, linear fracture mechanics methods are not directly applicable, but in these nonelastic cases the opening displacement and J integral methods of assessment have been proposed. In the complex situation encountered in nuclear power plant design, the analysis must also account for cyclic thermal strains, time dependent creep, and the effect of harmful environments which are not explicitly treated in the above-mentioned methods. In this report an in-depth review is presented in sufficient detail to illustrate the degree of agreement between the theoretical and empirical methods available in the literature and indicate the scope of the additional analyses and experimental work needed for the development of reliable safety evaluation methodology.For pure cylindrical bending, cracks perpendicular to the load start to grow when reaches a critical value which is generally larger than the corresponding critical uniaxial tension value. There appears to be a thickness effect in the bending case which is probably due to interference from the compressive sides of the crack.For a circular plate with lateral pressure and small lateral displacements, results agree with the bending data when using the nominal bending stress . For larger displacements when bulging occurs, the results agree with the tensile data when the nominal tensile stress is used.For curves surfaces, such as a cylinder under internal pressure, the data agree with the expression developed by Folias both for axial cracks under hoop stress σ and for circumferential cracks under axial stress σ Generally, the expressions were accurate up to , showing a tendency to be lower than the experimental data at higher values of the parameter. The parameter is a promising one.To study the influence of cracks at different angles to the applied load, analysis and data are available including the stress component parallel to the crack in the stress field around a crack tip. This, together with the concept of a critical circumferential stress at a critical distance (α = 0.1) ahead of the crack provides improved correlation with fracture predictions for both the angle of fracture and the critical stress intensity factor for the angled cracks in flat plates.For a hollow cylinder under torsion with angled cracks, the best correlation was given by the same analysis although the results were not as conclusive as for the flat plate. From elastic theory useful curves for the variation of K₁, K₂, and K₃ around the border of an elliptically shaped crack are available.In a plane stress fracture the addition of a biaxial stress produces an increase in the apparent fracture toughness compared with the uniaxial case. However, there is as yet no evidence to show that there would be the same increase in a plane strain situation. Hence, in the absence of biaxial information the uniaxial fracture data may be the most conservative for flat plates. However, for shells there will also be a curvature effect.In an analogous manner, fatigue crack propagation rates appear to be less rapid under biaxial stresses than under uniaxial stress. However, this shift is not great and generallly will be masked by other effects such as environment and temperature service situations.The analysis of cracks in weldments with residual stress effects are also available. In the case of a crack in a weld the estimated residual stress distribution agreed reasonably well with some experimental data for elastic conditions. Results indicate that there can be a tensile stress intensity factor even when the original residual stress distribution has changed to compressive. A point to remember is that residual stresses near welds can be beyond yield.An analysis based on Lagrangean mechanics is useful for indicating the different effects of liquids and gases as pressurizing media in hollow pipes. The results show that whereas gases maintain their pressure as a crack begins to propagate, the pressure in the liquid can quickly decrease so that subsequent catastrophic failure is less likely even in large diameter piping.     

Effect of dynamic strain aging on the leak-before-break analysis in SA106 Gr.C piping steel

The characteristics of dynamic strain aging (DSA) on material properties used in leak-before-break (LBB) analysis were discussed. Using these material data, the effect of DSA on the LBB analysis was estimated through the evaluation of leakage-size crack and flaw stability in SA106 Gr.C piping steel. Also, the results were represented as a form of ‘LBB allowable load window'. In the DSA temperature region, the leakage-size crack length was smaller than that at other temperatures and it increased with increasing tensile strain rate. In the results of flaw stability analysis, the lowest instability load appeared at the temperature corresponding to the minimum J–R curve which was caused by DSA. The instability load depended on the loading rate of J–R data, and decreased with increasing tensile strain rate at the plant operating temperature. These are due to the strain hardening characteristic and strain rate sensitivity of DSA. In the ‘LBB allowable load window', the LBB allowable region at the temperature and loading conditions where DSA occurs was decreased by about 30% compared with that in other conditions.     

Fracture criteria of reactor graphite under multiaxial stesses

New fracture criteria for graphite under multiaxial stresses are presented for designing core and support materials of a high temperature gas cooled reactor. Different kinds of fracture strength tests are carried out for a near isotropic graphite IG-11. Results show that, under the stress state in which tensile stresses are predominant, the maximum principal stress theory is seen as applicable for brittle fracture. Under the stress state in which compressive stresses are predominant, there may be two fracture modes for brittle fracture, namely, slipping fracture and mode II fracture. For the former fracture mode the maximum shear stress criterion is suitable, but for the latter fracture mode the following mode II fracture criterion including a restraint effect for cracks is verified to be applicable, where δ₁ and δ₃ are the maximum and minimum principal stresses, δ_t and δ_c are the tensile and compressive strengths and K_Ic and K_IIc are the mode I and II fracture toughness values, respectively. The above equation is similar in form to the Coulomb-Mohr criterion.Also a statistical correction for brittle fracture criteria under multiaxial stresses is discussed. By considering the allowable stress values for safe design, the specified minimum ultimate strengths corresponding to a survival probability of 99% at the 950 confidence level are presented.     

Crack growth determination on laboratory components

The present work outlines the reasoning behind the selection of laboratory component tests for the validation of design and remanent life models governing crack growth behaviour. For the case of creep crack growth a ferritic and an austenitic alloy have been studied and a reference stress based solution used to successfully relate the stress rupture behaviour of internally and externally, axially and circumferentially notched, tubular components to base line creep data. Using the same reference stress based approach, it has been demonstrated that the notched component creep crack growth rates exhibit the same C^* dependence as conventional compact tension specimens. For 316L stainless steel components subjected to thermal fatigue conditions simulative of the fusion reactor first wall, a modified version of the superposition method of Buchalet and Bamford has been applied to estimate the stress intensity range as a function of crack length during the test. By this approach the crack growth rate dependency on stress intensity range for a variety of notch geometries is seen to be broadly in line with the conventional specimen mechanical fatigue data. Recent studies of crack growth under combined creep and thermal fatigue conditions are described and some early results are reported.