In-Pile and Out-of-Pile Grain Growth Behavior of Sintered UO2 and (U,Gd)O2 Pellets

Grain growth behavior of UO₂ and (U, Gd)O₂ fuel pellets was investigated with the data from the out-of-pile isothermal heating experiments and the irradiation test at the Halden Boiling Water Reactor. The laboratory data gave best-fitted equations by employing the following fourth power rate equations :

UO₂ : D²-D⁴ ₀=3.79×10¹⁸ exp(-142,000/RT)t,

(U, Gd) : D²-D⁴ ₀=4.98×10¹⁷ exp(-140,000/RT)t,

where, D ₀ and D are initial and final three-dimensional diameters (μm), respectively, R the gas constant (=1.987 cal/mole/K), T the absolute temperature (K) and t the time (h) (gadolinia content: 3~10%, temperature range: 1,700~2,000°C).

The calculated grain diameter with the above equations revealed an overestimation on specimens which involved noticeable fission gas bubbles on their grain boundaries. It was demonstrated that the in-pile grain growth model, as was given in the following equation, which took account of the retarding effects of growth by precipitated intergranular bubbles could describe the grain growth of the irradiated samples :

where f: Grain boundary fractional coverage (-).     

Sodium Vapor Deposition,on Simulated Model of LMFBR Rotating Shield Plug Annulus, (I)

Experimental study was made to investigate the controlling factors on the vapor deposition rate on reactor operational shield plug annulus, which is exposed to the vapor entrained cover gas during reactor operation. Two simulated test assemblies having annuli were made for this purpose and were installed into a small test vessel. In the experiment, the average deposition rates on the annular walls of the test assemblies were measured for various pool temperatures, and their dependents upon such parameters as pool temperature, T_s (or the saturated vapor pressure P_s at pool surface), cover gas pressure P_g , and temperature drop ΔT_a across cover gas, were studied.

The results revealed that the dominant controlling factor was the vapor pressure P_s at pool surface. Dependent of the average deposition rate φbar_d. upon the above parameters was simply expressed by: φbar_d=Bσp_sD_sΔT_G , where, σ_s is the saturated vapor concentration at pool surface, D_s , the vapor diffusion coefficient, and B the proportional constant.

To these experimental results, the previously published evaporation rate data and the theoretical evaporation rate equation based on Epstein & Rosner's theory were reviewed. Then correlation between the deposition and the evaporation rates was discussed.     

An experimental investigation on thermal striping: Mixing phenomena of a vertical non-buoyant jet with two adjacent buoyant jets as measured by ultrasound Doppler velocimetry

An experimental investigation on the thermal mixing phenomena of three quasi-planar vertical jets, with the central jet at a lower relative temperature than the two adjacent jets, was conducted. The central jet was unheated (‘cold’), while the two adjacent jets were heated (‘hot’). The temperature difference and velocity ratio between the heated (h) and unheated (c) jets were, ΔT_hc=5°C, 10°C and r=V_cold,exit/V_hot,exit=1.0 (isovelocity), 0.7, 0.5 (non-isovelocity) respectively. The typical Reynolds number was Re_D=1.8×10⁴, where D is the hydraulic diameter of the exit nozzle. Velocity measurement of a reference single-jet and triple-jet arrangement were taken by ultrasound Doppler velocimetry (UDV) while temperature data were taken by a vertically traversed thermocouple array. Our UDV data revealed that, beyond the exit region, our single-jet data behaved in the classic manner. In contrast, the triple-jet exhibited, for example, up to 20 times the root-mean-square velocity values of the single-jet, especially in the regions in-between the cold and hot jets. In particular, for the isovelocity case (V_exit=0.5 m/s) with ΔT_hc=5°C, we found that the convective mixing predominantly takes place at axial distances, z/D=2.0–4.5, over a spanwise width, x/D|2.25|, centered about the cold jet. An estimate of the turbulent heat flux distribution semi-quantitatively substantiated our results. As for the non-isovelocity case, temperature data showed a localized asymmetry that subsequently delayed the onset of mixing. Convective mixing however, did occur and yielded higher post-mixing temperatures in comparison to the isovelocity case.     

Gaussian Approximation for Phonon Frequency Distribution Function for PuO2 and UO2

The phonon frequency distribution function for PuO₂ and UO₂ has been approximated by a one-parameter Gaussian distribution. By comparing the limiting forms of the specific heat and the Debye-Waller factor calculated with this approximate distribution function with the corresponding limiting values of these quantities in the Debye model, the value of the parameter is found to vary inversely as the temperature T. The constant of proportionality is found to be of the order of Debye's temperature.

Values of the parameter for UO₂ are calculated from its Debye's temperature and its experimental data on Debye-Waller factor, Debye's temperature and specific heat. The value of the parameter is also calculated for PuO₂ from its specific heat data.     

Determination of Oxygen Solubility in NaK-78 by Vacuum Distillation Method

Oxygen solubility in Na-K (22 w/0-78 w/0) was determined by a vacuum distillation method. A NaK sampling apparatus was installed in a forced convection NaK loop with a cold trap. Subtracting the oxygen blank from oxygen amount calculated from chemically determined quantities of Na and K in the distillation residue and using the least squares method, a following equation for oxygen solubility in NaK over a temperature range of 90–210°C was derived:

Log₁₀ S=7.09–2,795/T

where S is the oxygen solubility (wppm) and T the absolute temperature (K). This equation was close to those derived for oxygen solubility in Na.     

Fragmentation of continuous molten copper droplets penetrating into sodium pool

The progression of hypothetical core disruptive accidents (CDAs) in metal fuel cores is strongly affected by exclusion of molten metal fuel from the core region due to molten fuel–coolant interaction (FCI). As a basic study of FCI, the present paper focuses on the fragmentation characteristics of continuous molten copper droplets with a total mass from 20 to 50 g penetrating into a sodium pool. The results show that the fragmentation of the continuous molten copper droplets is sensitive to the change of the hydrodynamic and thermal conditions when the instantaneous contact interface temperature (T_i) is lower than the turning point (T_tp) and insensitive at T_i > T_tp. Compared with the fragmentation of a single droplet, the fragmentation of continuous droplets is accelerated and enhanced due to the collision between the droplets and the upward microjets. The present mass median diameter (D_m) or dimensionless mass median diameter (D_m/D₀) of continuous copper droplets shows a distribution with smaller values than those of single copper droplet, and larger values than those of copper jets under similar thermal and hydrodynamic conditions. These results are promising to assure the termination of accidents in CDAs and useful to the core design with enhanced safety in FBRs.     

Fragmentation of a Single Molten Metal Droplet Penetrating Sodium Pool I Copper Droplet and the Relationship with Copper Jet

The progression of hypothetical core disruptive accidents in metallic fuel fast breeder reactors is strongly affected by the fragmentation of molten metallic fuels due to the molten fuel-coolant interaction (FCI). As a basic study of FCI, the present paper focuses on the fragmentation of a single molten copper droplet with mass from 1 to 5 g, whichpenetrated a sodium pool at instantaneous contact interface temperatures (T_i) from 995 to 1,342°C. Intensive fragmentation of a single molten copper droplet was clearly observed even if T_i values are below the melTsing point (1,083°C) of copper besides the higher T_i range. The intensive fragmentation shows that the mass median diameters (D_m) of copper droplets with a fivefold difference in mass or the same mass have little difference, i.e., they are nearly the same. Under the lower T_i condition, the D_m data of droplet fragments of both the same and different masses scatter widely. It is found that the present D_m/D₀ data of mass median diameter normalized by the diameter before touching sodium (D0) give a distribution with larger values than those of molten copper jets with large mass from 20 to 300 g under the lower T_i condition, which were previously reported by the authors, because of the limited amount of heat of droplets. The present Dm=D0 data under the higher T_i condition are found to show an effecTive fragmentation compared with those of molten copper jets with a large mass of 4 kg.     

Hydrogen Concentration Dependence of Tritium Tracer Diffusion Coefficient in Alpha Phase of Niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients D_t of tritium in alpha phase of hydrogenated and deuterized niobium (α-NbH_xT_y and α-NbD_xT_y (x<0,8,y<<x)) have been measured at 473 K, 493 K and 553 K. The data on D_t show typical hydrogen concentration dependence: D_t of tritium for both α-NbH_xT_y, and α-NbD_xT_y, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of D_t of tritium differs from that of D_t of protium in α-NbH_x or of deuterium in α-NbH_xT_y. On the other hand, no appreciable differences in the concentration dependence of D_t of tritium between α-NbH_xT_y, and α-NbD_xT_y, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that D_t of tritium for a tritiated niobium (α-NbT_x) is not very different from that for α-NbH_xT_y and α-NbD_xT_y. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained D_t of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.     

Transient Cooling Process of Fuel Rod in Reactivity Initiated Accident

Out-of-pile experiments were performed with Zircaloy-4 rods in subcooled water environment to study the basic phenomena occurring in the transient cooling process undergone by a fuel rod during a reactivity-initiated accident (RIA) affecting a light water reactor (LWR). The experimental results show that the cooling process of the fuel rod during an RIA can be divided into three phases separated by the quenching temperature T_q and the rewetting temperature T_q .

It is also noted from the experimental results that with increasing degree of subcooling, T_q tends to rise to levels far exceeding the maximum liquid superheat temperature of water; T_q , on the other hand, is little affected by the cooling water temperature, and remains close to that of the maximum superheat temperature.

Numerical calculations indicate conclusively that radial heat transfer to coolant water is the dominant factor that governs the transient cooling process in an RIA affecting the cold start-up of a BWR, rather than the axial heat conduction through rod which is considered to be the basic mechanism of cooling that governs the reflooding process during a LOCA.     

Thermophysical properties of Th1−xUxO2 pellets prepared by spark plasma sintering technique

Th_1?x U _x O₂ solid solutions were synthesized by solid-state reaction and pelletized using the spark plasma sintering (SPS) technique. Pellets with >90% theoretical density were easily obtained within 40 min of sintering without any additive. The thermal conductivity, Young's modulus, Debye temperature, Vickers hardness, and heat capacity were systematically investigated, and the values for ThO₂ agree with the literature data. The thermal conductivity of Th_1?x U _x O₂ decreased with increasing U content up to x =～0.5. This tendency corresponds to phonon-point defect scattering theory. The Young's modulus and Debye temperature linearly decreased with increasing U content. The Debye temperature and standard molar entropy derived from the low-temperature heat capacity agree with the reported values.     

Heat Capacity of Thorium Nitrides from 450 to 850K

The throium nitrides (ThN and Th₃N₄) were prepared by solid-gas reaction with thorium hydride and nitrogen.

The heat capacity of these samples were measured from 450 to 850K by a high-temperature double- adiabatic calorimeter and were determined as functions of temperature as follows:

Th₃N₄: C _p=41.30–8.47×10^?3 T-6.37×10⁵ T ^?2

ThN: C _p=12.50–2.66×10^?3 T-2.44×10⁵ T ^?2     

Transformation and fragmentation behavior of molten metal drop in sodium pool

In order to clarify the fragmentation mechanism of a metallic alloy (U–Pu–Zr) fuel on liquid phase formed by metallurgical reactions (liquefaction temperature = 650 °C), which is important in evaluating the sequence of core disruptive accidents for metallic fuel fast reactors, a series of experiments was carried out using molten aluminum (melting point = 660 °C) and sodium mainly under the condition that the boiling of sodium does not occur. When the instantaneous contact interface temperature (T_i) between molten aluminum drop and sodium is lower than the boiling point of sodium (T_c,bp), the molten aluminum drop can be fragmented and the mass median diameter (D_m) of aluminum fragments becomes small with increasing T_i. When T_i is roughly equivalent to or higher than T_c,bp, the fragmentation of aluminum drop is promoted by thermal interaction caused by the boiling of sodium on the surface of the drop. Furthermore, even under the condition that the boiling of sodium does not occur and the solid crust is formed on the surface of the drop, it is confirmed from an analytical evaluation that the thermal fragmentation of molten aluminum drop with solid crust has a potential to be caused by the transient pressurization within the melt confined by the crust. These results indicate the possibility that the metallic alloy fuel on liquid phase formed by the metallurgical reactions can be fragmented without occurring the boiling of sodium on the surface of the melt.     

Results on research of templates from Kozloduy-1 reactor pressure vessel

The studies on the specimens manufactured from the templates cut out from the weld 4 of Kozloduy NPP Unit 1 reactor vessel have been conducted. The data on chemical composition of the weld metal have been obtained. Neutron fluence, mechanical properties, ductile to brittle transition temperature (DBTT) using mini Charpy samples have been determined. The phosphorus and copper content averaged over all templates is 0.046 and 0.1 wt.%, respectively. The fluence amounted up to 5×10¹⁸ n cm⁻² within 15–18 fuel cycles, and about 5×10¹⁹ n cm⁻² for the whole period of operation. These values agree well with calculated data. DBTT was determined after irradiation (T_k) to evaluate the vessel metal state at the present moment, then after heat treatment at the temperature of 475°C to simulate the vessel metal state after thermal annealing (T_an), and after heat treatment at 560°C to simulate the metal state in the initial state (T_k0). As a result of the tests the following values were obtained: T_k, +91.5°C; T_an, +63°C; and T_k0, 54°C. The values of T_k and T_an obtained by measurements were found to be considerably lower than those predicted in accordance with the conservative method accepted in Russia (177°C for T_k and 100°C for T_an). Thus, the obtained results allowed to make a conclusion that it is not necessary to anneal Kozloduy NPP Unit 1 reactor vessel for the second time. The fractographic and electron-microscopic research allowed to draw some conclusions on the embrittlement mechanism.     

Solubility of Tributylphosphate in Plutonium Nitrate Solution and Highly Radioactive Liquid Waste Solution

Abstract

The solubility of tri-n-butylphosphate (TBP) in aqueous solutions of plutonium nitrate (PuN) and in highly radioactive liquid waste (HRLW) of PUREX nuclear fuel reprocessing was investigated. By an empirical formula the solubility of TBP in PuN solutions was described in the range of 0–0. 1 M Pu and 1–8M HNO₃ concentrations. The following items were elucidated:

(1) The logarithm of TBP solubility (S) in the solution of interest varies inversely in proportion to the concentration of Pu(IV) in the range of 0–0.1M PU(IV) at a constant concentration of HNO₃, indicating that Pu(IV) simply behaves as an electrolyte for the salting-out of TBP. Log S subsequently levels off with increasing Pu concentration, which would be due to a change in the principal dissolution form of TBP having an interaction with Pu (IV).

(2) The variation in S in PuN solutions (0–0.1M PU) with nitric acid concentration shows almost the same tendency as that in HNO₃ solution.

(3) A dependency of S on fission product metal ions in HNO₃ for HRLW similar to that for PuN was observed.

(4) The logarithm of the ratio of TBP solubility in water to that in solution of interest was nearly proportional to l/T for HRLW solution or for low concentration of PuN solution. That deviates from the linear relation at high temperature when the concentration of PuN is increased, which can be explained by the change in ionic form of Pu.     

Irradiation of Aqueous Solution of Thiocyanate by Radiation Emitted from 10B(n, α)7 Li Reaction

A dual temperature hydrogen isotopic exchange reaction system between water and hydrogen gas is numerically analyzed. The system has two features; high efficiency of isotope exchange reaction and operation under atmospheric pressure. To achieve them, the low temperature section of the system is composed of water mist and hydrogen gas co-current reactor units. For the high temperature section, a multistage-type reactor, in which a bubble plate, superheater and catalyst bed are alternatively arranged, is applied.

From a material balance between these reactors, enrichment and decontamination factors for the system are expressed as functions of seven parameters: unit number of the low temperature co-current reactor (X); stage number of the high temperature section (Y); flow ratio of tritium enriched water to decontaminated water (P/W), flow ratio of feed water to hydrogen gas (F/G); reaction temperatures of the low and high temperature sections (T _c , T _h ); and bubble plate temperature (T _b ). Numerical calculations show that enrichment factor depends remarkably on F/G and T _b as well as X and Y.

In order to understand the separation characteristics visually, the McCabe-Thiele diagrams for the present system are drawn and compared with the results calculated.     

Oxidation kinetics and oxygen diffusion in low-tin Zircaloy-4 up to 1523 K

This paper deals with the study of oxidation kinetics and the identification of oxygen diffusion coefficients of low-tin Zy-4 alloy at intermediate (973 K ? T ? 1123 K) and high temperatures (T ? 1373 K). Two different cases were considered: dissolution of a pre-existing oxide layer in the temperature range 973 K ? T ? 1123 K and oxidation at T ? 1373 K. The results are the following ones: in the temperature range 973-1123 K, the oxygen diffusion coefficient in α_Zr phase can be expressed as D_α = 6.798 exp(−217.99 kJ/RT) cm²/s. In the temperature range 1373-1523 K, the oxygen diffusion coefficients in α_Zr, β_Zr and ZrO₂, were determined using an ‘inverse identification method’ from experimental high temperature oxidation data (i.e., ZrO₂, and α_Zr(O) layer thickness measurements); they can be expressed as follows: D_α = 1.543 exp(−201.55 kJ/ RT) cm²/s, D_β = 0.0068 exp(−102.62 kJ/ RT) cm²/s and D_ZrO2=0.115exp(−143.64kJ/RT)cm²/s. Finally an oxygen diffusion coefficient in α_Zr in the temperature range 973 K ? T ? 1523 K was determined, by combining the whole set of results: D_α = 4.604exp(−214.44 kJ/RT) cm²/s. In order to check these calculated diffusion coefficients, oxygen concentration profiles were determined by Electron Probe MicroAnalysis (EPMA) in pre-oxidized low-tin Zy4 alloys annealed under vacuum at three different temperatures 973, 1073 and 1123 K for different times, and compared to the calculated profiles. At last, in the framework of this study, it appeared also necessary to reassess the Zr-O binary phase diagram in order to take into account the existence of a composition range in the two zirconia phases, α_ZrO2 and β_ZrO2.     

Consideration on Effective Diffusivity of Strontium in Granite

Abstract

Diffusive behavior of strontium and certain kinds of divalent cations in Inada granite were studied by a through-diffusion method. In order to examine the effect of sorption onto overall diffusive behavior, two kinds of solutions were used: 0.1M KCl solution and deionized water. The effective diffusion coefficient (D_e ) and rock capacity factor (a) were (2.0–3.6) xl0–¹³ m²/s and less than 0.022 in 0.1M KCl solution and (0.32–1.7) x 10 ^?13m²/s and 1.5–2.4 in deionized water respectively. The D_e , and α in deionized water were much larger than those in 0.1M KCl solution. These results are well explained by taking into account the diffusion of sorbed ion or the surface diffusion. In support of this mechanism, most D_e , values of Sr reported for various rocks are found proportional to the sorptivity ( _ρR_d )-In the case that the sorptivity is low, D_e of Sr depends on porosity like that of nonsorbed iodide. The effective diffusion coefficient of Sr in rocks was well explained by taking into account pore and surface diffusion and was expressed as D_e=2.1 xl0^?10 _? ^1.3+3.5xl0″^?12 _ρR_d . The effective diffusion coefficient of divalent cations in the granite was found proportional to their diffusion coefficients in bulk solution.     

Translational and rotational energy measurements of desorbed water molecules in their vibrational ground state following 157 nm irradiation of amorphous solid water

Water ice is the major solid component in a variety of astrophysical environments, e.g., cold and dense molecular clouds. Photodesorption plays a dominant role in consuming ice in such cold regions. In this study, photodesorption of vibrationally ground-state H₂O(v = 0) from amorphous solid water has been investigated at 157 nm. Using a resonance-enhanced multiphoton ionization technique, the translational and rotational energy distributions of photodesorbed H₂O(v = 0) were measured, i.e., Boltzmann distributions at 1800 and 300 K, respectively. These energies are in good accordance with those predicted by classical molecular calculations for water photodesorption due to a kick-out mechanism following absorption of a single photon; hot H atom released by photodissociation of H₂O in ice transfers enough momentum to another H₂O molecule to kick it off the surface. Desorption of D₂O(v = 0) following 193 nm photoirradiation of a D₂O/H₂S mixed ice was investigated to provide further direct evidence for the operation of a kick-out mechanism. The other desorption mechanisms were also discussed in the context of possible photodesorption of vibrationally excited H₂O.