The influence of cavitation damage upon high temperature creep under stationary and non-stationary loading conditions: Part II: Tensile creep at constant load and constant true stress resp

The object of this investigation is to examine the influence of different tensile loading procedures and that of high temperature cavitation upon plastic flow. In the preceding Part I the phenomenological damage model (A-model) was introduced. In the present paper the creep behaviour of materials (in the absence of work hardening) is examined for specimens which are subjected to tensile loading at constant load F and constant true stress s respectively. It is shown that, independent of the material, in accordance with experiments, the lifetimes t_f,F and t_f,s (in creep at F = 0 and s = 0 respectively) obey the relationship t_f,F= 0.63 t_f,s. For comparable conditions in the same time t the material is more damaged in creep at constant load than in creep at constant true stress. The volume change $\text{δV}\text{V}{}_0$ calculated for creep at F = 0 is in good agreement with experiments.The A-model is compared to Kachanov's damage model (ω-model) and it is shown that although from both the models the same life fraction rule can be deduced, the HTC behaviour described by the ω-model is in disagreement with experiments.     

The influence of cavitation damage upon high temperature creep under stationary and non-stationary loading conditions: Part IV: Cyclic creep at constant load amplitude and constant true stress amplitude

The aim of the present paper is to investigate the influence of high temperature cavitation upon the creep behavior of ideally plastic materials subjected to tensile load cycling. In the present treatment the oscillating tensile stress is replaced by a static equivalent stress which is a function of the loading conditions. Creep equations are derived for cavitating and non-cavitating specimens respectively. The volume change for constant load amplitude cycling is calculated. The main results derived for different loading procedures are also compared.     

Internal stress buildup in SIPA creep

A discrete simulation of internal stress effects, arising during stress-induced preferred-absorption (SIPA) irradiation creep, has been conducted for randomly oriented linear-elastic fcc polycrystals. Of particular interest are the intercrystalline incompatibility stresses arising during the development of faulted interstitial loops (Frank loops). These simulations suggest that a unique state of internal stress can develop at a sufficiently large loop volume-fraction. As this state is approached the deviatoric creep-rate reduces to zero. Existing microstructural data suggest, however, that unfaulting processes limit loop volume-fractions to levels consistent with a maximum reduction in creep-rate of ～18%. A substantial coupling effect is predicted between faulted loop SIPA and other SIPA mechanisms, such as preferred absorption at perfect dislocations (SIPAD), and climb controlled glide (CCG). Faulted loop SIPA coupled with CCG creep leads to an enhanced creep-rate. The opposite effect is predicted for the coupling with SIPAD creep.     

Preliminary measurements of the prompt neutron decay constant in MASURCA

Pulse counting techniques have been used to measure the prompt decay constant = (β - ) / Λ in the MASURCA reactor of CEA at critical state. The data has been analyzed in time domain using Rossi- and Feynman- techniques, and in frequency domain using the cross power spectral density.

The Rossi- technique has been studied using one and two detectors. Due to the strong inherent spontaneous fission source, the one-detector variant gives a very strong white-noise signal, which is absent in the two-detector method. Because each neutron detected recorded not only a pulse, but also an echo after 120 ns, corrections had to be made to the theory applied.

The Feynman- technique is even more sensitive to the echo in the signals, and quite large corrections had to be made. Nevertheless the results obtained are in reasonable agreement with those of the correlation methods. For both measurement techniques, experiments of long duration are needed to get accurate results. The results obtained agree within 10% with calculations.

The prompt decay constant has also been measured with a continuous current technique. From the cross power spectral density thus obtained, the -value is in agreement with that of the pulse counting techniques.     

Axial strains in fuel cladding associated with creep and fast neutron irradiation

Commercially produced CANDU-PHWR (CANada Deuterium Uranium-Pressurized Heavy Water Reactor) Zircaloy-4 and Zr—2.5% Nb fuel cladding was biaxially creep-tested in the laboratory and in the WR-1 reactor. The axial strains measured have been interpreted, through a knowledge of the texture, to provide evidence supporting prismatic slip as the major process for axial contraction at high stresses and temperatures and that this is the same process which gives rise to axial lengthening of pressure tubes. At low stresses and temperatures, axial lengthening of fuel cladding appears to be associated with Coble creep in elongated and flattened grains.     

Transient induced creep for irradiated fuels

Creep of nuclear fuels under irradiation has generally been considered as an athermal process. A transient state of irradiation induced creep, however, was treated with kinetic equations based on thermodynamics. For this purpose, the kinetic equations for enhanced diffusion, annihilation of excess vacancies and migration probability under irradiation were derived. Effective works were introduced to the above processes as a result of the fission damage, and reduced activation energies for each process were defined. Based on the knowledge obtained in the transient state, the effective activation energies in the steady state were discussed. The above concepts were examined using the experiments of Clough.     

Simplified creep analysis of perforated plates under steady creep condition

A study is performed on the creep analysis of perforated plates based on the equivalent solid plate concept.A series of creep analyses by FEM for various materials, ligament efficiencies and loading conditions is performed on a perforated plate with a triangular penetration pattern of circular holes. Based on the results of the analysis, prediction methods for both the equivalent solid plate property and the stress-strain concentration around the hole are proposed.The prediction method for the equivalent solid plate property is compatible with the elastic property shown in the ASME B&PV Code Sec. III A-8000. The prediction method for the stress—strain concentration is based on Neuber's rule together with Hoff's analogy, and is compatible with the elastic peak stress multiplier in the ASME B&PV Code Sec. III A-8000.     

能谱变化对239Pu裂变室测量快中子注量的影响

热中子和共振区的中子在快中子临界装置中所占的份额很小,但是由于其相对大的截面,在慢化物存在的情况下,热中子和共振中子份额的微小变化,对^239Pu裂变室测量中子注量的结果影响很大。通过测量^239Pu裂变电离室在包镉和包硼、周围有无慢化物等情况下的反应率,Au、In活化片的镉比,S活化片在能谱变化下与^239。Pu的反应率比等,分析了快中子临界装置中热中子和共振区中子的分布,讨论了中子能谱变化对^239Pu裂变室测量快中子注量的影响及解决办法。     

Measurements of neutron induced nucleation

The paper describes new experimental evidence on the effects of fast neutron irradiation on the nucleation of bubbles in superheated demineralized water during sudden depressurization. The experimental range covered strating pressures of (0.33–6.0) MPa. It was shown that fast neutron irradiation causes significant increases in the bubble formation for specified conditions of pressure, temperature and rate of blowdown. A mechanistic interpretation of the measurements based on a thermodynamic model was included, and comparison was made, in terms of void fraction, with the experimental results.     

Mechanisms of radiation induced creep and growth

Irradiation creep occurs primarily because the applied stress causes the evolving microstructure to respond in an anisotropic fashion to the interstitial and vacancy fluxes. On the other hand, irradiation growth requires the response to be naturally anisotropic in the absence of applied stress. Four fundamental mechanisms of irradiation creep have been conjectured: stress induced preferred absorption (SIPA) of the point defects on the dislocations, stress induced preferred nucleation (SIPN) of point defects in planar aggregates (edge dislocation loops), stress induced climb and glide (SICG) of the dislocation network and stress induced gas driven interstitial deposition (SIGD). These mechanisms will be briefly outlined and commented upon. The contributions made by these mechanisms to the total strain are not, in general, mutually separable and also depend on the prevailing (and changing) microstructure during irradiation. The fundamental mechanism of irradiation growth will be discussed: it is believed to arise by the preferred condensation of point defects and climb of dislocation loops and network on certain crystallographic planes. The preferred absorption and nucleation is thus a consequence of natural crystallographic anisotropy and not due to any external stresses. Again the effectiveness of this mechanism depends on the prevailing microstructure in the material. In this connection attention will be particularly drawn to the significance of solute trapping, segregation at grain boundaries, dislocation bias for interstitials and transport parameters for an understanding of irradiation growth in materials like zirconium and its alloys; the relevance of recent simulation studies of growth in such materials using electrons to the growth under neutron irradiation will be discussed in detail and a consistent model of growth in these materials will be presented.     

Irradiation induced creep of ceramic nuclear fuels

This contribution gives a review of the experimental results and accompanying theoretical considerations. The mechanisms considered for irradiation creep are: relaxation of elastic stresses by fission spikes, promotion of dislocation slide by thermal spikes, preferential, stress-orientated nucleation of dislocation loops and preferential growth of dislocation loops. A survey over the irradiation creep rates attributed to steady-state creep shows $\text{ε}{}_{\mathrm{irr}}\text{~ σ · F}$ for oxide fuel in the stress and fission rate ranges of $\text{σ = 10–50}\text{MN/m}{}^2$ and $\text{F = 3 × 10}{}^{12}\text{–1 × 10}{}^{14}\text{f/cm}{}^3\text{· s}$ at burnups < 3%. There seems to be a continuous increase of the irradiation creep rate of oxide fuels with increasing temperature. However, that increase cannot be directly interpreted through a thermally activated process. It seems that the irradiation creep rate will also depend on fuel porosity, on plutonium distribution in mechanically blended UO₂-PuO₂, but not substantially on the plutonium content per se. Some results were already given for carbide and nitride fuels, which show the irradiation creep rate to be lower by about a factor of 10 than for oxide fuel under comparable conditions. Primary irradiation creep has been observed up to $\text{(3–5) × 10}{}^{19}\text{f/cm}{}^3$ and could prevail up to $\text{1 × 10}{}^{20}\text{f/cm}{}^3$.