Radiation effect on partial pressure of fission product iodine

The knowledge of the partial pressure of fission product iodine is important for evaluating the FCCI (fuel cladding chemical interaction) in FBR. Thermodynamical analysis shows that iodine is believed to react with cesium so strongly that the partial pressure of iodine is kept too low to cause FCCI. A molecule of cesium iodide is, however, excited by radiation in the reactor and dissociates into radical and ion. In this work, the radiation effect on the partial pressure of iodine is theoretically studied for the gaseous cesium-iodine system. Fission fragment flux is considered as the radiation and the iodine partial pressure is evaluated based on the theory of gaseous reaction. The results of the calculation show that the partial pressure of iodine can be increased enough to cause FCCI in the radiation field. In the case of the FBR fuel pin, the partial pressure of iodine under radiation conditions can be calculated to be ~ 10^?2 Pa (~10^?7 atm), which is higher than that of the non-radiation condition ~10^?9 Pa (~10^?14 atm). These calculations were carried out under the assumption that the cladding inner surface temperature was 873 K (600°C) and the oxygen potential was ?418 kJ/mol (?100 kcal/mol).     

Axial static pressure variations in inner and side subchannels of a 61-tube wire-wrap bundle

Measurements of axial distribution of the static pressure in an inner and side subchannel of a 61 wire-wrap tube bundle obtained with water at atmospheric conditions are presented. The wire wrap configuration is different from those used by previous workers and more representative of a bundle for the blanket of a Gas Cooled Fast Reactor. The data display axial static pressure variations which are attributed to the interchannel cross flow induced by the wire-wrap configuration. The static pressure drop over one wire pitch agrees well with the bundle pressure drop based on a bundle average Reynolds number and a friction factor f = 0.436 Re^−0.263 (Re > 2000). The experimental data obtained with water provide a useful benchmark to model and check the accuracy of thermal-hydraulic codes used for the analysis of subchannel flow distribution and pressure drop in wire wrap tube bundle cooled with one-phase fluid.The nodal subchannel code COBRA-IV was modeled by adjusting the forced cross-flow function to match the measured axial static pressure distribution in an inner and side subchannel. Some discrepancy remained in the static pressure profile in the side channel attributed to the flow distortion at the bundle exit.     

Variation of pressure load on flexible plate due to interaction between an expansion wave and the flexible plate

The interaction between an expansion wave and a flexible plate was investigated as a model of the fluid-structure interaction which occurs during the loss-of-coolant accident in pressurized water reactors. The test section, which was connected to an expansion tube, was divided into two regions by the flexible dividing plate, and pressure variations in both regions were measured. Two-dimensional flow equations were numerically solved coupled with an equation of the deflection of the plate. The reduction of pressure loads on the plate due to the interaction between the plate and the expansion wave only occurred within the early stage when the pressure on the back side of the plate was constant. The pressure in the back-side region of the plate was based on the volume change in that region, and the net effect of the pressure change in that region was the reduction of the load on the plate. The pressure in the outlet-side region of the plate was not affected by the pressure in the back-side region. Pressure disturbances generated by the deflection of the dividing plate were found to propagate around the plate when the test section was not perfectly divided into two regions. It was concluded that the conditions around the structure (back-side volume of the structure, for example) should be considered in the evaluation of the pressure load on the structure when the fluid-structure interaction phenomena may be important.     

The effect of internal pressure on in-plane collapse moment of elbows

Elastic–plastic finite element analysis has been carried out to evaluate collapse moments of six elbows with elbow factors varying from 0.24 to 0.6. Collapse moment is obtained by twice elastic slope method from the moment vs. end-rotation curve. The effect of internal pressure on in-plane closing/opening collapse moment is studied. For various elbow factors and level of internal pressure, total 60 and 54 cases are analysed for closing and opening mode of bending moment, respectively. Based on these results, two closed-form equations are proposed to evaluate the collapse moments of elbows under combined internal pressure and in-plane closing and opening bending moment.     

Studies on diversion cross-flow between two parallel channels communicating by a lateral slot. II: Axial pressure variations

The axial pressure variations of two parallel channels with single phase flows communicating by a long lateral slot have been studied experimentally. Using mass and momentum conservation principles, the axial pressure variations have been derived in terms of two parameters k_d and k_r, for donor and recipient channels, respectively. These parameters include the combined effect of fluid transferred from donor to recipient channel, and drag force brought on by the connection gap, and are functions of the velocities and slot geometry parameters. A pressure difference oscillation between channels along the slot has been detected which is sinusoidal with wave lengths which seem to be a function of the gap clearance.     

Generation and propagation of pressure waves due to unconfined chemical explosions and their impact on nuclear power plant structures

In the field of external reactor safety possible violent incidents outside the reactor building need to be assessed with regard to their potential for doing damage to nuclear power plants. This paper deals with the generation and propagation of pressure waves due to unconfined chemical explosions and their impact on nuclear power plant structures. A survey is made of the procedures by which the effects of gas explosions on nuclear power plant structures can be assessed.     

Investigation on constraint effect of reactor pressure vessel under pressurized thermal shock

In recent years, the integrity of reactor pressure vessels (RPVs) under pressurized thermal shock (PTS) accident has been treated as one of the most critical issues. Under PTS condition, the combination of thermal stress due to a steep temperature gradient and mechanical stress due to internal pressure causes considerable tensile stress inside the RPV wall. As a result, cracks on the inner surface of RPVs can experience elastic-plastic behavior that can be explained using the J-integral. In such a case, however, the J-integral may possibly lose its validity due to the constraint effect. The degree of constraint effect is influenced by the loading mode, the crack geometry and the material properties. In this paper, three-dimensional finite element analyses are performed for various surface cracks to investigate the effect of clad thickness and crack geometry on the constraint effect. A total of 36 crack geometries are analyzed and results are presented by the two-parameter characterization based on the J-integral and the Q-stress.     

Small-angle neutron scattering study on the effect of hydrogen in irradiated reactor pressure vessel steels

Hydrogen uptake can enhance the neutron embrittlement of reactor pressure vessel (RPV) steels. This suggests that irradiation defects act as hydrogen traps. The evidence of hydrogen trapping was investigated using the small-angle neutron scattering (SANS) method on four RPV steels. The samples were examined in the unirradiated and irradiated states and both in the as-received condition and after hydrogen charging. Despite the low bulk content of hydrogen achieved after charging with low current densities, an enrichment of hydrogen in small microstructural defects could be identified. Preferential traps were microstructural defects in the size range of ≈ > 10 nm in the unirradiated and irradiated samples. However, the results do not show any evidence for hydrogen trapping in irradiation defects.     

The effect of stress levels on the coefficient of thermal expansion of a fine-grained isotropic nuclear graphite

Due to the fluctuation and non-uniform distribution of temperature within the core structure of high-temperature gas-cooled reactors (HTGRs), the thermal expansion behavior of graphite materials plays an important role in the design of graphite components, especially of large-scale components. In the present paper, in order to investigate the influence of stress levels on the coefficient of thermal expansion (CTE) of IG-110 graphite, the strain gauge method was used to measure the CTE on the cylindrical specimens under a series of loads applied using a universal tensile testing machine. In addition, a more precise measurement using a thermal dilatometer was employed to validate the tests using the strain gauge method. A good agreement has been obtained between the experimental results using these two methods. The results show that when the specimens were under compressive loads, the CTE along the loading direction of the specimens increased and that along the perpendicular direction decreased, with more changes in the former. The absolute changes of the CTE in the two directions increased with increasing applied load. When graphite specimens were subjected to a compressive load of 40 MPa, the axial CTE of specimens sectioned along the radial direction of the graphite brick as it is installed in the core structure increased from 4.13 × 10⁻⁶ to 5.35 × 10⁻⁶ K⁻¹, while the axial CTE of specimens sectioned along the vertical direction increased from 3.97 × 10⁻⁶ to 5.58 × 10⁻⁶ K⁻¹. Moreover, the residual change of the CTE, which was caused by the permanent residual strain after unloading, was observed. The change of the CTE with stress levels should be considered in the stress analysis and life prediction of the nuclear graphite components.     

The effect of flow direction and magnitude on CHF for low pressure water in thin rectangular channels

Critical heat flux (CHF) at low flow condition can become important in an MTR-type research reactor under a number of accident conditions. Regardless of the initial stages of these accidents, a condition which is basically the decay heat removal by natural convention boiling can develop. Under such conditions, burnout may occur even at a very low heat flux. In view of this, the CHF at low-flow-rate and low-pressure conditions has been studied for water flowing in thin rectangular channels.Experiments were carried out with two types of rectangular test sections, namely, the one heated from one wide side and the other heated from two opposite sides. In order to observe the effects of gravity, CHF was measured both in upflow and downflow. The CHF at complete bottom blockage was also studied.The results indicate that burnout can occur at a much lower heat flux than pool-boiling CHF or than predicted by the conventional correlations. There was observed a minimum CHF at complete bottom blockage and at very low downflow. The low CHF at very low downflow appears to be due to the stagnation of the bubble in the heated section. This fact indicates that special care should be taken in analyzing the boiling phenomenon which occurs when the coolant flow is very low in a low pressure system.     

The effect of internal pressure on the creep relaxation of a thin, straight pipe

The effect of constant internal pressure on the relaxation of bending moment on a long, thin, circular cross section cylindrical shell is determined. The inadequacy of several simple approximate analyses, previously proven useful in other creep applications, is established.     

Experimental evaluation of the effect of local wall thinning on the failure pressure of elbows

This study performed a series of burst tests using real-scale elbow specimens containing simulated local wall thinning to evaluate the effects of wall-thinning defects on the failure pressure of pipe bends and elbows. The tests were conducted under simple internal pressure at ambient temperature. The experiments included various wall-thinning geometries with different thinning depths, lengths, and circumferential angles, as well as various thinning locations such as extrados, intrados, and full-circumference. The failure pressure decreased exponentially with increasing axial thinning length and decreased almost linearly with increasing thinning depth. These tendencies are similar to those observed for wall-thinned straight pipe. The failure pressure also decreased and gradually saturated with increasing circumferential thinning angle, unlike the results of wall-thinned straight pipe. All specimens failed by bulging, followed by cracking. The axial crack always occurred at the center of the wall-thinned area in the extrados and intrados wall-thinning cases. For the full-circumference wall-thinning case, however, the crack location and pattern were dependent on the axial thinning length. A comparison of the failure pressure with the results of existing models showed that the existing models were excessively conservative in all cases and could not properly predict the dependence of failure pressure on the wall-thinning geometry.     

One- and two-dimensional standing pressure waves and one-dimensional travelling pulses using the US-NRC nuclear systems analysis code TRACE

A variety of nuclear system transients can lead to rapid and large local pressure changes that propagate along the hydraulic system at the speed of sound, both in single phase and in two-phase fluids. Because of the relevance for safety issues, nuclear system codes like TRACE need to be assessed with respect to their capabilities to predict pressure wave behaviour. Therefore, we have analyzed the propagation of pressure waves in one-dimensional and two-dimensional configurations, i.e. a pipe and a slab, filled with liquid water. The pressure waves are driven by one-sided pressure boundary conditions, in the one-dimensional case of harmonic or Gaussian shape and in the two-dimensional case also of harmonic shape. The selected harmonic pressure boundary conditions lead to standing pressure waves, while using the Gaussian shape boundary conditions one-dimensional pressure pulses are injected and propagate through the pipe. The agreement of the TRACE results with the analytical solutions are, in general very good to good for the one-dimensional cases with respect to the pressure maxima and a small difference is only obtained in the wave speed.At the resonance frequencies of the one-dimensional standing waves, the code is tested to the extreme and shows that enforcing small time step sizes is crucial for the performance of the code. Non-linear effects are observed in the code results for the large amplitudes encountered at the closest neighborhood of the resonances, where the analytical linear standing wave solution diverges and the linear approximation is outside of its validity range. Also for these non-linear standing waves TRACE yields qualitatively physically correct behaviour as the pressure amplitudes are limited and a plateau is reached.For the one-dimensional pressure pulse of Gaussian shape the change of pulse amplitude and shape was analyzed in a longer system. The maximum amplitude of the pulse is slightly reduced as the pulse travels along the pipe. The effect of numerical diffusion on leading and trailing fronts is slightly asymmetric due to the donor-cell approach used in the numerical integration scheme of TRACE. The accuracy of the code is not negatively influenced by the reflections of the pulse at the boundaries of the pipe. As for the standing waves, the accuracy of the travelling pulse solution calculated by TRACE is negatively affected when the time steps are too large, while the effects of the spatial discretization are rather minor.For the case of two-dimensional standing waves in a slab, a lowest spatial harmonic generated with one wave node in the direction parallel to the pressure driving boundary is considered. TRACE results show an overall good agreement with the linear analytical solution. This good agreement includes for low to medium excitation frequencies the damping properties of the skin effect perpendicular to the pressure boundary, which does not exist in one-dimensional pressure wave propagation, the transition to a harmonic shape of the wave also perpendicular to the pressure boundary and the frequency dependence of the resonance spectrum for further increased frequencies with the rapid changes of the wavelengths encountered. Effects of the model set-up and code limitations with respect to the two-dimensional TRACE model set-up using a TRACE VESSEL component in connection with pressure boundary conditions are discussed, in particular with respect to the underestimation of the damping in the skin effect frequency range and the numerical damping for higher frequencies. All in all, the TRACE code is able to calculate one- and two-dimensional pressure wave propagation in liquid water, when an appropriate spatio-temporal numerical discretization is chosen.     

Legendre expansion in multilayer slab geometry

The monoenergetic integral transport equation for a multilayer slab geometry has been solved by the Legendre expansion method. The method utilizes an expansion of the flux density in each layer in Legendre polynomials of the position co-ordinate. The use of these polynomials makes it possible to calculate most of the resulting matrix by means of recurrence formulae. These formulae have been obtained by a procedure which is an extension of Carlvik's method for a homogeneous slab. A code (MULREG) has been written for this purpose. Using MULREG a series of calculations have been performed for a homogeneous slab with vacuum boundary conditions. The slab has been divided into NR number of regions and in each region flux is expanded into Legendre polynomials of order NSI. For a particular value of NR, the NSI varies from 0 to 4. The effective multiplication factor k_eff of the slab is calculated. By comparing the computational time for all the cases, it is studied as how severe it is to consider the flat flux approximation (conventional collision-probability approach) as compared to a case when more terms in the flux expansion are considered per layer.     

Response of NPP structures to simultaneously acting air pressure loads and ground waves caused by a gas cloud explosion

Gas cloud explosions cause air pressure waves which propagate over the ground surface. The ground motion induced by these loads and their effect on structures are studied. The soil is modelled as a linear viscoelastic medium. A semianalytical method is used to compute the ground motion produced by a deflagration and by a detonation in a stiff and a soft layered soil. For a PWR reactor building subjected to the direct impact of an air pressure wave the additional effects of the ground waves on the motion of the building are studied. Whereas the vertical structural response is increased, the horizontal response decreases, when the effect of the ground waves is included. For the case studied the additional effect of the ground waves is small.     

Combined effect of thermal and irradiation embrittlement in the reactor pressure vessel CrNiMoV steel

Combined effects of segregation and irradiation embrittlement in reactor pressure vessel CrNiMoV steel were studied. The study deals with an analysis of conditions affecting the 15Ch2NMFA type CrNiMoV steel susceptibility and the development of microsegregation processes in connection with temper brittleness formed on repeated annealing cycles. Microstructural analysis and results of tensile and impact testing for all the treatment conditions are presented.     

Paramagnetic effect under the influence of high-frequency pressure and electron paramagnetic resonance in plasma

The interaction of uhf fields ( = 2· 10¹⁰ sec^–1) in a space resonator containing dense plasma (n 10¹³ – 10¹⁴ cm^–3) in a steady magnetic field was studied experimentally. Under the influence ofhf pressure a paramagnetic current arises in the plasma; the associated effect of an increase in the static magnetic field inside the plasma agrees closely with the calculated relation.For _H/ = 0.5 paramagnetic resonance of the electrons takes place; this leads to a sharp rise in plasma pressure p₀, up to =8p₀/H₀ ²0.2.Translated from Atomnaya Énergiya, Vol. 20, No. 5, pp. 401–407, May, 1966.     

Numerical studies of the voltage amplitude effect on plasma characteristics in atmospheric pressure driven by dual LF-RF frequency discharge

Dielectric barrier discharges(DBDs) are primarily utilized as efficient sources of large-volume diffuse plasmas. However, the synergistic interaction of certain key plasma factors limits their broader application. In the present paper, we report numerical investigations of the effects of voltage amplitude in dual-frequency excitation on atmospheric DBDs using a 50 kHz/5 MHz frequency combination. Our results indicate that varying the voltages for low frequency(LF)and radio frequency(RF) significantly influences the electron dynamics during discharge,resulting in distinct spatio-temporal distributions of electron and metastable particle densities.These findings contribute to the regulation of discharges under atmospheric pressure conditions and facilitate the attainment of non-equilibrium and nonlinear plasma parameters.