Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.     

Primary creep of UO2 and the effect of amorphous grain boundary phases

Creep experiments were conducted on nearly stoichiometric UO₂ helical springs from 1000 to 1600°C and 2.1 to 80 MPa. Entirely transient behaviour was measured in all experiments with the plastic strain,ε = (Aσ/d^1.5) exp(−Q/RT)t^m, where A is a constant that depends on purity, d is the grain size, σ is the applied stress, Q is the apparent activation energy, t is the time, m is a constant, and the other terms have their usual meaning. At T > 1200°C, Q 100 kcal mol⁻¹, but at T < 1200°C, Q increased dramatically and became strain dependent. The value of m for most experiments was 0.8, but at σ > 48 MPa, m decreased, and for d < 10 μm, it increased. Amorphous or glassy grain boundary phases were observed by transmission electron microscopy in all specimens: specimens containing the largest concentrations of Fe and Si sometimes had anomalously high creep rates. The phases existed as discontinuous, lenticular bodies on grain faces and a continuous network along triple grain junctions. Some instances of precipitation of UO₂ from the phase were observed. At T > 1200°C, glassy phases may accelerate Coble creep by providing short circuit diffusion paths along the grain boundaries or may accelerate superplastic deformation by diffusion along the continuous glassy phase triple line junctions. At low temperatures the glassy phase appears to control grain-boundary sliding.     

Contribution of uranium diffusion on creep behavior of uranium dicarbide

Compressive creep tests of uranium dicarbide (UC₂) have been conducted. The general equation best describing the creep rate over the temperature range 1200–1400°C and over the stress range 2000–15000 psi is represented by the sum of two exponential terms ge =A(σ/E)^0.9 exp(−39.6 ± 1.0/RT) + B(σ/E)^4.5 exp(−120.6 ± 1.7/RT), where pre-exponential factors are A(σ/E)^0.9 = 12.3/h at low stress region (3000 psi) and B(σ/E)^4.5 = 3.17 × 10¹³/h at high stress region (9000 psi), and the activation energy is given in kcal/mol. Each term of this experimental equation indicates that important processes occurring during the steady state creep are grain-boundary diffusion of the Coble model at low stress region and the Weertman dislocation climb model at high stress region. Both mechanisms are related to migration of uranium vacancies.     

Enthalpy and heat capacity of LiAlO2 between 298 and 1700 K by drop calorimetry

The enthalpy of γ-LiAlO₂ was measured between 403 and 1673 K by isothermal drop calorimetry. The smoothed enthalpy curve between 298 and 1700 K results in H⁰(T) − H⁰(298 K)=−37 396 + 93.143 · T + 0.00557 · T² + 2 725 221 · T⁻¹ J/mol. The standard deviation is 2.2%. The heat capacity was derived by differentiation of the enthalpy curve. The value extrapolated to 298 K is C_p,298=(65.8 ± 2.0) J/K mol.     

Design study of the secondary-beam line at RCNP

A beam line for use with the new RCNP ring cyclotron (K = 400) has been designed to separate unstable nuclei produced in high energy heavy-ion reactions. The separation principle takes advantage of A- and Z-dependence of the energy loss of ions in matter (energy degrader), and enables us to obtain the reaction products in the form of a secondary beam. Ion-optical consideration has been made for the separator by treating the degrader as one of the ion-optical elements. By this new treatment it has been made possible to optimize the optics of the system according to experimental requirements. The methods (i) to achieve an achromatic focusing of the secondary beams and (ii) to obtain beams with small energy spread are discussed. The effects of energy straggling in the degrader are shown to be the principal factor to limit the isotopic resolution of the separator. Ion-optical calculations have been performed for the secondary-beam line taking into account the transfer matrix elements up to third order. The design specifications are described and the expected profiles of the secondary beams are presented.     

Development of an SCRFQ heavy-ion linac for RI beams

A split coaxial RFQ (SCRFQ) is being developed to accelerate RI beams from 1 to 170 keV/u as part of the Japanese Hadron Project (JHP). Our SCRFQ is equipped with modulated vanes. On the basis of the studies on a cold model and a proton accelerating one, a 25.5 MHz prototype for the JHP SCRFQ has been constructed. The prototype, consisting of three module cavities, is 2.1 m in length and 0.9 m in inner diameter, and accelerates ions with a charge-to-mass ratio (q/A) greater than 1/30 from 1 to 45.4 keV/u. The unloaded Q-value of the cavity is 6400, corresponding to about 84% of the calculated value, and the field imbalance between vanes is within ±0.6%. The designed intervane voltage of 109 kV for ions with q/A = 1/30 is achieved with a 70 kW peak power. By using ions of three species, N₂⁺, N⁺ and Ne⁺, acceleration tests are conducted. The transmission efficiency attained with a N⁺ beam is better than 80% at normalized intervane voltages higher than 1.2.     

Thermal recovery of defects in neutron irradiated UO2

Thermal recovery of both lattice and volume expansions of UO₂ irradiated to a dose of less than 9.97 × 10¹⁷ fissions/cm³ was studied in a temperature range from 200 to 1000°C. Two or three steps were observed in the recovery of lattice expansion. Volume expansion was also recovered in two or three steps. Recovery behavior of volume expansion of the specimen irradiated to 9.97 × 10¹⁷ fissions/cm³ was different from those irradiated to lower doses, and an abrupt volume increase was observed in the temperature range from 500 to 700 °C. This behavior might be related to the bubble swelling due to fission product gases. From the results on the recovery of lattice and volume expansions, the behavior of lattice point defects and defect clusters was discussed.     

Systematics for (n,p) excitation functions in the neutron energy between 13.3 and 15.0 MeV

Systematics of (n,p) excitation functions in the neutron energy between 13.3 and 15.0 MeV were studied on the basis of experimental data measured by the Nagoya and Fusion Neutronics Source groups. The empirical formulae of a cross section (σ₁₄) at 14.0 MeV and a relative slope (S) of excitation functions were deduced. These formulae covered the mass range between 19 and 188. The empirical formula of S was expressed as a function of (N—Z)/A and threshold energy, where N, Z and A are the mass, proton and neutron numbers numbers for the target nuclei, respectively. The empirical formula of σ₁₄ was expressed by a simple formula with two fitting parameters. By using the proposed empirical formulae, the partial excitation functions between 13 and 15 MeV were reproduced. Comparing the experimental data with the calculated excitation functions, we concluded that the accuracy of the proposed empirical formulae was ± 20%.     

Thermodynamic studies on Cs4U5O17(s) and Cs2U2O7(s) by emf and calorimetric measurements

The oxygen potentials over the phase field: Cs₄U₅O₁₇(s)+Cs₂U₂O₇(s)+Cs₂U₄O₁₂(s) was determined by measuring the emf values between 1048 and 1206 K using a solid oxide electrolyte galvanic cell. The oxygen potential existing over the phase field for a given temperature can be represented by: Δμ(O₂) (kJ/mol) (±0.5)=−272.0+0.207T (K). The differential thermal analysis showed that Cs₄U₅O₁₇(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs₄U₅O₁₇(s) was calculated from the above oxygen potentials and can be given by, Δ_fG⁰ (kJ/mol)±6=−7729+1.681T (K). The enthalpy measurements on Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were carried out from 368.3 to 905 K and 430 to 852 K respectively, using a high temperature Calvet calorimeter. The enthalpy increments, (H⁰_T−H⁰₂₉₈), in J/mol for Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) can be represented by, H⁰_T−H⁰_298.15 (Cs₄U₅O₁₇) kJ/mol±0.9=−188.221+0.518T (K)+0.433×10⁻³T² (K)−2.052×10⁻⁵T³ (K) (368 to 905 K) and H⁰_T−H⁰_298.15 (Cs₂U₂O₇) kJ/mol±0.5=−164.210+0.390T (K)+0.104×10⁻⁴T² (K)+0.140×10⁵(1/T (K)) (411 to 860 K). The thermal properties of Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were derived from the experimental values. The enthalpy of formation of (Cs₄U₅O₁₇, s) at 298.15 K was calculated by the second law method and is: Δ_fH⁰_298.15=−7645.0±4.2 kJ/mol.     

Measurements of the rate of the uranium-water vapour reaction

The rate of the uranium-water vapour reaction has been measured between 30 and 80°C. The measured reaction rate obeys the rate equation: k = 3.0 × 10⁹r^1/2 exp(−15.5 kcal/RT) mg U/cm ² H = 4.0 × 10⁸r exp(−15.5 kcal/RT) mg weight gain/cm² h, where r is the fractional relative humidity.

This rate equation agrees remarkably well with the literature equation which was derived from much more limited experimental evidence and so the present equation is preferred.     

Release of tritium from boron carbide irradiated with reactor neutrons

The release of tritium from irradiated boron carbide in a pure Ar atmosphere was investigated between 500 and 900°C. The sintered B₄C samples with densities between 75 and 95% of the theoretical density were irradiated with reactor neutrons with total neutron doses up to 5 × 10²⁰/cm². Effective diffusion coefficients, D_eff, were derived from the release data using the model “diffusion out of a sphere”. D_eff decreases by about 3 orders of magnitude with increasing total neutron dose, levels off at about 10¹⁸n/cm² and increases at very high doses ( > 10²⁰ n/cm²). The decrease in the tritium mobility is attributed to the radiation defects formed in the B₄C. The activation energy of 210 ± 30 kJ/mol for the tritium diffusion in the irradiated B₄C is much higher than the value found for unirradiated material. D_eff depends also very strongly on the density of the sintered material.     

Design of the new superconducting microprobe system in Munich

At the Munich 15 MV tandem accelerator a new two-stage microprobe system is under construction. It will demagnify a beam spot by a factor 200 in both transversal directions for ions up to an energy of 200 MeV · q²/A. The second stage of the system is a superconducting quadrupole doublet lens, with overlayed 2n-poles, n = 4, 6 and 8. The multipole components of this lens have been determined by a ray tracing program with a special treatment of the fringing fields. As a result of these calculations, the overall geometrical aberrations at the focal point are 180 nm horizontally and 280 nm vertically at full angular acceptance (image coordinates ±9 and ±20 mrad, respectively). A main part of the superconducting lens is a central ceramic tube which defines the exact positions of the poles. Superconducting NbTi foils are used to shape the field to the desired one. Superconducting current loops automatically serve for a suppression of all odd harmonic field components.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

The diffusivities of fission-created or thermally-doped tritium in UO2

The diffusion behavior of tritium in UO₂ was studied. Two methods were adopted for the introduction of tntium into UO₂: one via ternary fission of ²³⁵U and the other via thermal doping. In the former, the diffusion constants decreased with increase in sample weight. The diffusion constants obtained from the pellet with the same specification (9 mm in diameter, 5 mm high) were D″_bulk = 3.03 × 10⁻³(^+0.369_−0.003) exp[−163±43(kJ/mol)/RT](cm²/s) for fission-created tritium and D′_bulk = 0.15(^{+ 0.94}_−0.13) exp[−76±13 (kJ/mol)/RT](cm²/s) for thermally-doped tritium. The difference of the diffusion constants between two systems was discussed in terms of the effects associated with the recoil processes of energetic tritium.     

Accelerator mass spectrometry with time-of-flight measurement

A time-of-flight detection system for the measurement of microscopic concentrations of long-lived radioisotopes has been set up at the Munich MP tandem accelerator. The technique allows an unambiguous mass determination for heavy ions up to the actinides. As a first test of the system, the concentration of ¹²⁹I in iodine has been measured for several samples of mineral and biological origin. The smallest ¹²⁹I/¹²⁷I ratios measured were a few times 10⁻¹³. The high sensitivity of the method was achieved by using a 90° injector with a good mass resolution, a special technique of stabilizing the terminal voltage with an auxiliary beam, and a time-of-flight system with a mass resolution of 800.     

Effects on channeling of radiation damage due to 28 GeV protons

For an irradiation of 2.9 × 10¹⁹ protons at a beam energy of 28 GeV, the channeling minimum yield in a silicon single crystal increased from 2.3% to 4.1%. The radiation damage occurred with a proton fluence of (4.1 ± 1.4) × 10²⁰/cm². The degradation was measured with MeV-range He ions using Rutherford backscattering. The relevance to bent crystal extraction of TeV beams is discussed.     

Status report of the Leuven isotope separator on-line (LISOL)

The mass separator LISOL, on-line to the CYCLONE cyclotron, is described as it is presently configured. The main development has been in the ion-guide operation in conjunction with light-ion induced fusion and fission. The plans to obtain intense (nA) beams of light (A ≤ 30), short-lived nuclei in the framework of the ARENAS³ project are also described.     

Standard molar Gibbs free energy of formation of PbO(s) over a wide temperature range from EMF measurements

The EMF of the following galvanic cells,

(render)

Kanthal,Re,Pb,PbOCSZO₂ (1 atm.),Pt

(render)

Kanthal,Re,Pb,PbOCSZO₂(1 atm.),RuO₂,Pt

were measured as a function of temperature. With O₂ (1 atm.), RuO₂ as the reference electrode, measurements were possible at low temperatures close to the melting point of Pb. Standard Gibbs energy of formation, Δ_fG⁰_mβ-PbO was calculated from the emf measurements made over a wide range of temperature (612–1111 K) and is given by the expression: Δ_fG⁰_mβ-PbO±0.10 kJ=−218.98+0.09963T. A third law treatment of the data yielded a value of −218.08 ± 0.07 kJ mol⁻¹ for the enthalpy of formation of PbO(s) at 298.15 K, Δ_fH⁰_mβ-PbO which is in excellent agreement with second law estimate of −218.07 ± 0.07 kJ mol⁻¹.     

High energy resolution PIXE with high efficiency using the heavy ion microbeam

An X-ray crystal spectrometer using a position sensitive proportional counter combined with tandem microbeam line at Osaka National Research Institute have been developed. This system realizes high energy resolution PIXE analysis using a heavy ion microbeam (E < 6 MeV) with reasonable detection efficiency. The design of the spectrometer, such as detection geometry, detectable energy range and energy resolution, are described. This system was applied to high energy resolution PIXE analysis of Ti, SUS and Si with 2 MeV proton and 5 MeV Si³⁺ focused or collimated beams. The best energy resolution was 2 eV for the Si K line.