Reduction of heat flux on divertor plates by remote radiative cooling in doublet III

Using a single null divertor configuration, heat flux intensity and its profile on the divertor plates as a function of plasma current and density were measured with an infrared camera and thermocouples. The vertical width of the heat flux on the divertor plates 2λ is ≈ 10 cm at the lower separatrix and is ≈ 5.5 cm at the upper separatrix. A diffusion coefficient D which is obtained from the measurement of the diffusion length across the scrape-off field lines is roughly proportional to and its magnitude is on the order of Bohm diffusion. The heat flux on the plates decreases by more than a factor of 5 with increasing electron density in the main plasma and is much smaller than that on the limiters in non-diverted plasmas. Only 3% of ohmic input power goes into the divertor plates at high density of the main plasma, while ≈ 20% goes in at low density. The decrease of heat flux is in good agreement with the increase of radiation loss in the divertor region. The heat flux on the divertor plates can be reduced by remote radiative cooling in high density discharges.     

Effect of thermal radiation on rewetting of top spray emergency coolant

A general physical model for top spray rewetting during an emrgency core cooling (ECC) transient is proposed which takes into account thermal radiation in the dry region. The model is employed to study the effect of thermal radiation on rewetting a single rod and a 3 × 3 rod bundle up to 2100°F. The results show that rewetting in a bundle is slower than for an isolated rod, due to reduced thermal radiation heat transfer in the dry region. Also, there is a definite correlation between the decreased radiation heat flux Δq_R and the corresponding decrease in rewetting velocity Δu. Values of Δu are not significant unless Δq_R is larger than 6000 Btu/hr ft², where Δq_R cannot exceed a value of 6000 Btu/hr ft² below a temperature of 1100°F, even in the most adverse conditions. Hence, it is concluded that radiant heat transfer does not significantly affect rewetting velocities up to an initial rod temperature of 1100°F. Beyond this temperature, the rewetting velocities change by more than 1.5% and hence radiation must be included in the model for top spray rewetting.     

Photo-Neutron Production in Heliotron-E

Photo-neutron emission in H₂ and He discharges was observed in the initial ohmic heating experiments of the Heliotron-E. Typical total neutron yield was 10⁹ neutrons per pulse under high level of runaway electrons (≧10 MeV). Neutron flux was localized near the limiters. Energy spectrum of neutrons was continuous up to about 2 MeV. The radioactive nuclides in the limiters and the vacuum chamber irradiated by runaway electrons showed that ⁵⁸Ni(γ, n) ⁵⁷Ni and ⁵³Cr(γ, n)⁵¹Cr reactions had occurred, proving that the photo-disintegration process was the source of neutron flux.     

Primary research of heat flux deposition pattern on the surface of HT-7 movable limiter

This paper presents a method of characterizing the heat flux deposition pattern on HT-7 movable limiter, a new component in the spring experimental campaign of year 2006. A new modeling establishment combines with heat transfer simulation with ANSYS code, and a shot of long pulse discharge is chosen with small radius 265 mm of movable limiter, which is 5 mm smaller than that of the main toroidal limiters. Both parallel and perpendicular heat flux are taken into account and different ratios of them are also tested in simulations. The simulation temperature values match well with the results of the IR-camera. Temperature distribution shows that ?_||/?_⊥ = 5 is quite suitable in HT-7 device, and the maximum heat flux is about 10 MW/m² and average heat flux is about 5 MW/m² on the movable limiter. This method can be also used in other tokamak devices with limiter configuration and circular cross-section shape.     

Film boiling heat transfer from a vertical cylinder in forced flow of liquids under saturated and subcooled conditions at pressures

Forced convection film boiling heat transfer on a vertical 3-mm diameter and 180-mm length platinum test cylinder located in the center of the 40-mm inner diameter test channel was measured. Saturated water, and saturated and subcooled R113 were used as the test liquids that flowed upward along the cylinder in the test channel. Flow velocities ranged from 0 to 3 m s⁻¹, pressures from 102 to 490 kPa, and liquid subcoolings for R113 from 0 to 60 K. The heat transfer coefficients for a certain pressure and liquid subcooling are almost independent of flow velocity and of a vertical position on the cylinder for the flow velocities lower than ≈1 m s⁻¹ (the first range), and they become higher for the velocities higher than ≈1 m s⁻¹ (the second range). Slight dependence on a vertical position being nearly proportional to z^−1/4, where z is the height from the leading edge of the test cylinder, exists for the flow velocities in the second range. The heat transfer coefficients at each velocity in the first and second ranges are higher for higher pressure and liquid subcooling. Correlation for the forced convection film boiling heat transfer with radiation contribution on a vertical cylinder was derived by modifying an approximate analytical solution for a two-phase laminar boundary layer model to agree better with the experimental data. It was confirmed that the experimental data of film boiling heat transfer coefficients in water and R113 were described by the correlation within ±20% difference.     

Modeling of heat deposition on the W/Cu movable limiter in HT-7

The thermal behavior of a directly water-cooled W/Cu movable poloidal limiter was investigated in HT-7, a medium-sized superconducting tokamak with limiter configuration, major radius R = 1.22 m, and minor radius a = 0.27 m. The W/Cu movable limiter (ML) was exposed to the plasma at various radial positions at r < a. The surface and bulk temperatures were monitored by an IR-camera and the thermocouples, respectively. The heat flux deposited on the limiter was evaluated by an ANSYS code using the measured surface temperatures as boundary conditions. It was found that the maximum heat flux incident on the ML was less than 1 MW/m² in the Ohmic discharges, but reached up to 5-7 MW/m² in the discharges with lower hybrid current drive (LHCD). A simple model was developed to understand heat transport to the W/Cu ML, taking into account the “funnel effect”.     

CHF and dryout point in vertical narrow annuli

Experimental study associated with CHF and dryout point in narrow annuli is conducted with 1.5 mm and 1.0 mm gap, respectively. Distilled water is used as work fluid. The parameters examined were: pressure from 2.0 MPa to 4.0 MPa; mass flux from 26.0 kg/(m² s) to 69.0 kg/(m² s); heat flux from 10 kW/m² to 70 kW/m²; exit equilibrium mass quality from 0.52 to 1.08.It is found that CHF monotonously increases with mass flux in internally heated annuli and bilaterally heated annuli. However, the observed trends are not similar to that in externally heated annuli. The CHF is not affected significantly by mass flux.Critical qualities of dryout point (X_DO) decreases with mass flux and increases with inlet qualities. Under the same conditions X_DO in outer tube are always larger than that in inner tube. According to experimental data, a criterion for the appearance of dryout point for bilaterally heated has been presented.The comparison with the correlations [КУТАТЕЛАДЗЕ, C.C., 1979. Тедплоэнергетика, No. 6] and experimental data indicates that the existing correlations applied to tube cannot predict X_DO in narrow annuli well. Based on experimental data, a new correlation is developed.     

Creep and creep-fatigue cracking behaviour of two structural steels

The present study deals with crack initiation and crack growth, not only under creep and creep-fatigue conditions but also under more complex thermomechanical cyclic loadings, in both 316L and 1Cr-1Mo-0.25V steel.In these creep ductile materials, most studies have focused on the creep crack growth rates, da/dt and load-geometry parameter C* correlations. In this paper, the creep crack initiation time is defined as the time T_i necessary for a defect to grow by a small critical distance X_c (X_c ≈ 50 μm for example). This initiation stage may represent a large part of the rupture life of a cracked component. The importance of such studies is discussed in the first part.In the second part, an attempt is made to present a simplified method based on the fracture mechanics of creeping solids to define the relevant load-geometry parameters for crack initiation and crack growth under creep-fatigue loadings. In particular, it is shown that da/dN-ΔK correlations apply only when the hold time t_h is smaller than the transition time t_tr between small-scale and large-scale viscoplasticity. Conversely, for long hold times, it is suggested that the T_i-C* correlation be used to predict the fatigue.     

Study on the characteristic points of boiling curve by using wavelet analysis and genetic neural network

Local singularity of a signal includes a lot of important information. Wavelet transform can overcome the shortages of Fourier analysis, i.e., the weak localization in the local time- and frequency-domains. It has the capacity to detect the characteristic points of boiling curves. Based on the wavelet analysis theory of signal singularity detection, Critical Heat Flux (CHF) and Minimum Film Boiling Starting Point (q_min) of boiling curves can be detected by using the wavelet modulus maxima detection. Moreover, a genetic neural network (GNN) model for predicting CHF is set up in this paper. The database used in the analysis is from the 1960s, including 2365 data points which cover a range of pressure (P), from 100 to 1000 kPa, mass flow rate (G) from 40 to 500 kg m⁻² s⁻¹, inlet sub-cooling (ΔT_sub) from 0 to 35 K, wall superheat (ΔT_sat) from 10 to 500 K and heat flux (Q) from 20 to 8000 kW m⁻². GNN mode has some advantages of its global optimal searching, quick convergence speed and solving non-linear problem. The methods of establishing the model and training of GNN are discussed particularly. The characteristic point predictions of boiling curve are investigated in detail by GNN. The results predicted by GNN have a good agreement with experimental data. At last, the main parametric trends of the CHF are analyzed by applying GNN. Simulation and analysis results show that the network model can effectively predict CHF.     

MHD Properties of Low-aspect Ratio RFP in RELAX

The low-aspect-ratio (A) reversed field pinch (RFP) offers attractive properties such as enhanced bootstrap current and simpler MHD mode dynamics. The RELAX (REversed field pinch of Low-Aspect ratio eXperiment) machine with the world’s lowest A of 2 (R/a = 0.5 m/0.25 m) has been constructed to explore the RFP properties in low-A regime. In flat-topped low-A RFP discharges in RELAX, plasma current of ~50 kA has been attained with discharge duration of ~2 ms. In round-topped discharges with plasma current of ~70 kA, quasi-periodic growth of a single helical mode has been observed. When the dominant m = 1/n = 4 mode grows, the toroidal mode spectrum looks like that of the quasi-single helicity (QSH) RFP state with higher amplitude. MHD equilibrium analyses using a reconstruction code have shown that the bootstrap current fraction is lower than ~5% in the present RELAX plasmas, and it will be ~25% if we could achieve the plasma density of 4 × 10¹⁹ m⁻³ and electron temperature of 300 eV at plasma current of ~100 kA.     

Limits of validity of trajectory simulation: Correlation of the error with density of scatterers and particle wavelength

To a first approximation, the elastic scattering of long wavelength particles in amorphous matter may be modelled as scattering in a volume filled with a density n of N point scatterers in random positions. For not too large N (up to about 2×10³), the error in trajectory simulation (classical transport theory) due to the neglect of interference effects can then be determined in detail by means of a comparison with an exact quantum calculation of the plural or multiple scattering process. A relative error RE is defined and calculated for the scattering in different directions as well as for the distribution of scattering events inside the volume. A very strong correlation is found between the relative error and the ratio λ/d_nn, where λ is the wavelength of the incident particle and d_nn=n^-1/3 is an average distance between nearest neighbour scatterers. For scattering in a volume of dimensions large compared to the particle wavelength, present calculations suggest that the correlation can be described as RE≈a·(λ/d_nn)^b, where the parameters a<0.05 and b2 depend on the s-wave phaseshift δ₀ in the scattering process. The condition for validity of trajectory simulation, defined in terms of a limit of validity L (maximum acceptable relative error), may thus be written λ/d_nn<ξ, where ξ=(L/a)^1/b1. For λ/d_nn<1, the relative error is generally less than 5%, and trajectory simulation may be regarded as valid with at least 95% accuracy. In the exact quantum calculation, two features of pronounced quantum character are observed in the distribution of scattering events: oscillations due to quantum interference in finite volumes, and, for small negative δ₀, randomly localized peaks due to proximity resonance.     

Experimental study of onset of subcooled annular flow boiling

An experimental study on the onset of nucleate boiling (ONB) is performed for water annular flow to provide a systematic database for low pressure and velocity conditions. A parametric study has been conducted to investigate the effect of pressure, inlet subcooling, heat and mass flux on flow boiling. The test section includes a Pyrex tube with 21 mm inner diameter and a stainless steel (SS-304) rod with outer diameter of 6 mm. Pressure, heat and mass flux are in the range of 1.73 < P < 3.82 bar, 40 < q < 450 kW/m² and 70 < G < 620 kg/m² s, respectively. The results illustrate that inception heat flux is extremely dependent on pressure, inlet subcooling temperature and mass flux; for example in pressure, velocity and inlet subcooling as 3.27 bar, 230 kg/m² s and 41.3 °C; consequently q_w,ONB is 177.3 kW/m². In other case with higher inlet temperature of 71.5 °C and with P, 3.13 bar and G, 232 kg/m² s the inception heat flux reached to 101.6 kW/m². The data of ONB heat flux are over estimated from the existing correlation, and maximum deviation of wall superheat (ΔT_w,ONB) from correlations is 30%. Experimental data of inception heat flux are within 22% of that predicted from the correlation.     

Critical heat flux in a vertical tube at low and medium pressures. Part II — new data representation

An analysis of the physical processes taking place in a dispersed-annular flow which govern dry-out type CHFs has been carried out. The analysis has shown that the number of variables required to describe the critical phenomena can be reduced by the introduction of a new parameter: the length over which dispersed-annular flow takes place, L_dan. In this case only, for a given tube diameter, pressure and mass flux, the critical heat flux may be expressed in terms of a single variable: L_dan. A correlation which may be used to determine this length has also been developed. The representation of the CHF data obtained at low pressures in terms of the coordinate system (L_dan, q″_cr) has shown that the dispersion of the data about the regression curves is considerably reduced as compared with the traditional presentation of the critical heat flux as a function of the thermodynamic quality at the end of the heated length.     

Effects of an external circuit on the dispersion of soluble matter in a magnetohydrodynamic channel flow with homogeneous and heterogeneous reactions

An analysis of the effects of the first-order homogeneous irreversible chemical reaction and the heterogeneous chemical reaction on the effective Taylor diffusion coefficient is carried out in a magnetohydrodynamic channel flow characteritic of a MHD generator. On taking into account the external circuit, the effects of M (the Hartmann number), γ (homogeneous reaction rate parameter), β (heterogeneous reaction parameter), R^* (resistance), and V_g^* (generator voltage) are discussed quantitatively in all cases of the magnetohydrodynamic channel flows. Results are compared with those when γ = 0, β = 0.It is observed that the effective Taylor diffusion coefficient is affected more significantly by the first-order homogeneous chemical reaction than that by the heterogeneous chemical reaction.     

Recent Results in JT-60 Advanced Experiment JT-60 Team,JAERI (Presented by Naoyuki MIYA)

Major efforts in the recent JT-60 experiments have been concentrated on the improved confinement of plasmas with profile control and on the steady state operation study. Peaked density profiles were produced with the successive pellet injection. The energy confinement time was improved by 40% as large as that with the gas fuelled discharges. The fusion products n _e(0)τET _i(0)reached 1.2 × 10²⁰m^?3·s·keV, which was twice that of gas fuelled discharges. High-β_p, plasmas were obtained in low-I _p discharges with improved confinement and a high ion temperature T _i, (0) of 12 keV. The bootstrap current reached 80% of the total plasma current at β_p=3.2. The new concept of a steady-state tokamak power reactor has been proposed on the basis of this result. The maximum current drive efficiency ηCD of 3.4 × 10₁₉m^?2·MA/MW was obtained in the LH current drive experiments. Helium ash exhaust experiments using He-beam injection into H⁺ plasmas showed promising results for α-particle exhaust in a fusion power reactor.     

Some problems for bundle CHF prediction based on CHF measurements in simple flow geometries

A comparison of critical heat flux (CHF) fuel bundles data with CHF data obtained in simple flow geometries was made. The base for the comparison was primary experimental data obtained in annular, circular, rectangular, triangular, and dumb-bell shaped channels cooled with water and R-134a. The investigated range of flow parameters (pressure, mass flux, and critical quality) in R-134a was chosen to be equivalent to modern nuclear reactor water flow conditions (p=7 and 10 MPa, G=350–5000 kg (m² s)⁻¹, x_cr=−0.1–1). The proper scaling laws were applied to convert the data from water to R-134a equivalent conditions and vise versa. The effects of flow parameters (p, G, x_cr) and the effects of geometric parameters (D, L) were evaluated during comparison. The comparison showed that no one simple flow geometry can be used for accurate and reliable bundle CHF prediction in wide range of flow parameters based on local (critical) conditions approach. The comparison also showed that the limiting critical quality phenomenon is unique characteristic for each flow geometry which depends on many factors: flow conditions (pressure and mass flux), geometrical parameters (diameter or surface curvature, gap size, etc.), flow obstructions (spacers, appendages, turbulizers, etc.) and others.     

Requirements for a radiation monitoring system in connection with risk and dose

Conclusions In summary, we have arrived at the seemingly paradoxical conclusion that the requirements for the radiation-monitoring system in region 2, where by definition the average individual dose is less than in region 1, are higher. Conversely, it would seem that if the dose load is smaller, then there is no need to complicate the monitoring system; for example, it is sufficient to establish a monitoring level that is the same as in region 1. The paradox is solved if one takes account of the fact that the lower dose loads in region 2 are associated with the lower content of radionuclides in objects in the environment, i.e., the relatively small useful signals, detected by the given monitoring system. This requires better monitoring systems, since weak signals must be detected and discriminated under conditions of random noise, due to background sources of inoizing radiation and other types of noise. In conclusion, we note that the materials presented in this paper make it possible to distinguish the radioecological environment in two regions quantitatively. Specifically, it must be assumed that regions 1 and 2 have the same radioecological state ifK ₁ ^* <K ₂ ^* ;D ₂<D ₁;N ₁≪N ₂ and ln(K ₁ ^* /K ₂ ^* )<K _thr(K ₂ ^* −K ₁ ^* )/(K ₁ ^* K ₂ ^* ). The latter approximate relation follows from Eq. (3) and the conditiong ₁(K _thr)<g ₂(K _thr), whereK _thr is the threshold value or the control level of the yearly effective collective dose andg(K) is the distribution density of the effective collective dose. Under these conditions, the requirements imposed on the radiation monitoring system in region 2 are more stringent than for an analogous system in region 1. Moscow Scientific and Industrial Association “Radon.” Translated from Atomnaya énergiya, Vol. 88, No. 6, pp. 476–480, June, 2000.     

Transient thermal response in nuclear waste repositories

In this paper, it is shown that a previously reported non-linear, one-dimensional, theoretical approximation simplifies — from a computational point of view — the calculation of the time-decay temperature field in nuclear waste repositories (NWR). This conclusion has been reached after solving, by using the control volume numerical method, the full three dimensional, transient, non-linear heat diffusion equation. The transient thermal field in a rock salt repository, is analytically solved and numerically predicted, along 100 years, after the disposal of a high-level waste (HLW). The nuclear waste, with a half-life of 32.9 years, releases an exponentially time dependent heat flux with 12 W m⁻² as the initial thermal load. Two cases are studied, in the first one it is assumed that the conductivity (k) and the volumetric heat capacity ρc_p of the host rock (diffusion domain) remain constant (linear case), whereas in the second one, a more realistic situation is analysed. In this last case, the conductivity of the rock salt varies as a function of the temperature field and the product ρ×c_p remains constant (non-linear case). In order to observe the effect of the salt conductivity (constant or variable) on the repository temperature distribution, a comparison of both cases is performed. It is concluded, that the theoretical model, which provides an analytical solution of the thermal fields may be a powerful low cost method for design purposes.     

2-D temperature distribution and heat flux of PFC in 2011 KSTAR campaign

KSTAR has reached a plasma current up to 630 kA, plasma duration up to 12 s, and has achieved high confinement mode (H-mode) in 2011 campaign. The heat flux of PFC tile was estimated from the temperature increase of PFC since 2010. The heat flux of PFC tiles increases significantly with higher plasma current and longer pulse duration. The time-averaged heat flux of shots in 2010 campaign (with 3 s pulse durations and I_p of 611 kA) is 0.01 MW/m² while that in 2011 campaign (with 12 s pulse duration and I_p of 630 kA) is about 0.02 MW/m². The heat flux at divertor is 1.4–2 times higher than that at inboard limiter or passive stabilizer. With the cryopump operation, the heat flux at the central divertor is higher than that without cryopump. The heat flux at divertor is proportional to, of course, the duration of H-mode. Furthermore, a software tool, which visualizes the 2D temperature distribution of PFC tile and estimates the heat flux in real time, is developed.     

Enrichment of 6Li by Ion Exchange

Elementary solutions to the energy-dependent Boltzmann equation with a one-term degenerate scattering kernel are derived in plane geometry, and the weight function W (z) is obtained which makes these solutions mutually orthogonal over the half-range interval of the continuum. The weight function greatly facilitates determination of the expansion coefficients in general solutions and is applied to the problems in infinite half space.

The diffusion length (discrete space eigenvalue) υ₀ is exactly expressed by using the halfrange characteristic function X(z). In a 1/υ-absorbing medium, as the absorption concentration q increases from zero to a critical value g^*, the diffusion length decreases from infinity to the end of the continuum 1/σ_min. For q≥q^*, v₀ vanishes and the neutron density can be represented by the transient term alone, whose exact expression is obtained.