Frequency dependence of electrical conductivity and dielectric constant of UO2

The dielectric constant and electrical conductivity of single crystal and polycrystalline UO2 are found to be frequency dependent. The dielectric constant measured at low frequencies is anomalously large at room temperature but decreases to a limiting value (~25) below about 130 K. A knee observed in the temperature dependence of the conductivity of polycrystalline UO2 corresponds to a process having an activation energy of 0.15 eV

Transient stability analysis using potential energy indices for determining critical generator sets

In this paper, we propose the enhancement of existing power system stability analysis techniques through the use of a proposed set of potential energy indices, applied for observing the separation of generators into critical sets during transient events. This proposed potential-energy-based description of system transient stability behavior permits the formation of a critical generator cutset, which is then used in a quantitative single machine equivalent (SIME) energy-function analysis of system stability. The derivation of the method will show that the proposed potential energy indices do not rely on a detailed representation of the network model, making the indices suitable for use in a variety of applications. This method enhances the current capabilities of SIME analysis for pre-fault offline stability studies, but may also be useful for near-real-time stability analysis, owing to the lack of dependence of the proposed potential energy indices on the network parameters. The ability to utilize the proposed indices without the need for network parameters or fault location information, typically obtained from updated SCADA data, potentially allows the proposed method to be applied for real-time stability analysis utilizing only PMU input data

The frequency dependent response of the electrical impedance of UO2

AC impedance techniques in the frequency range 5 Hz to 5 MHz have been employed to measure dielectric properties of single crystal UO2 in the form of plate specimens. The dielectric constant has been measured down to 4 K, giving results consistent with previous reports. Both barrier and volume effects have been shown to contribute to the measured impedances. The barrier effects account for the anomalously large capacitances observed in previous attempts to measure the dielectric constant by the conventional plate technique. Activation energies for carriers in both boundary and bulk regions are similar (0.18 to 0.25 eV). The behaviour is consistent with the presence of electronic holes present in the concentrations to be expected from small deviations from stoichiometry

The dielectric constant of UO2 below the Néel point

We report measurements of the frequency-dependent dielectric constant of UO2 from 4.2 K to above the phase transition at 30 K. The static dielectric constant of 23.6 at 4.2 K is comparable with accepted values at higher temperatures: it is essentially identical in both phases. The effects of undergoing the transition on the dielectric constant are marginal (about 1%) and take place in the temperature range 29 K to 37 K. The displacement of the oxygen sublattice, which occurs at the Ne´el point, should produce only a 0.05% change on the dielectric constant and of the opposite sense to that measured. Hence the structural changes at the transition are not the primary source of the observed small difference between the dielectric constant in the two phases which probably accrues from the influence of the displacements on a defect-related contribution

Electrical conductivity of polycrystalline uranium dioxide

The electrical impedance of a disc-shaped sample of polycrystalline UO2 has been measured over a frequency range of 10 Hz to 10 MHz at temperatures between 108 and 380 K. Three distinct regions in the impedance profiles were observed; these have been associated with the region near the metallic electrodes, with the bulk material and with the grain boundaries. Activation energies for conduction have been determined in each of the three regions [0.17, 0.13 and 0.29 eV for the electrode, bulk and grain boundary contributions, respectively]. The impedance response has been modelled using a two-phase microstructure and an effective medium treatment. At low temperatures the boundary region is less conducting than the grain interior. However, at ambient temperatures and above, the boundary region dominates and electrical conduction takes place primarily through the boundaries

The electrical impedance of single-crystal urania at elevated temperatures

The electrical admittance of single-crystal urania has been measured from 300 to 1500 K, over a frequency range of 10 Hz to 10 MHz, using complex impedance spectroscopy. The data have been analyzed using a simple equivalent circuit of a parallel element comprising a conductance and a capacitance connected in series with a separate capacitance. The simple equivalent circuit also reanalyzes successfully the frequency dependence of the electrical conductivity found by Bates and his co-workers, giving results consistent with the present work. The conductance data show a distinct “kink” at about 1300 K, which is in good agreement with previous work, as are the activation energies: 0.12 eV (T 1300 K). Results are used to estimate the ambipolar contribution to the thermal conductivity above 1500 K

The pressure dependence of the dielectric constant and electrical conductivity of single crystal uranium dioxide

Complex impedance techniques, within the frequency range 10 Hz to 1 MHz, have been used to make high pressure studies of monocrystalline uranium dioxide at ambient temperature. These techniques have shown that for frequencies below 40 MHz the electrical properties of high pressure samples are dominated by a boundary layer. The impedance methods have enabled us to make the first determination of the pressure dependence of the static dielectric constant of uranium dioxide within the boundary layer. The experimental pressure dependence (−0.03 kbar−1) is in reasonable agreement with that calculated (−0.02 kbar−1) using standard interatomic potentials. We have also measured the conductivity in the boundary layer as a function of pressure (2.5 μS kbar−1). The pressure dependences of the conductivity and the dielectric constant have been used to obtain an estimate of the carrier binding and hopping energies, which have then been compared with values predicted using the shell model

Inequalities in reported cancer patient experience by socio-demographic characteristic and cancer site: evidence from respondents to the English Cancer Patient Experience Survey.

Patient experience is a critical dimension of cancer care quality. Understanding variation in experience among patients with different cancers and characteristics is an important first step for designing targeted improvement interventions. We analysed data from the 2011/2012 English Cancer Patient Experience Survey (n = 69,086) using logistic regression to explore inequalities in care experience across 64 survey questions. We additionally calculated a summary measure of variation in patient experience by cancer, and explored inequalities between patients with cancers treated by the same specialist teams. We found that younger and very old, ethnic minority patients and women consistently reported worse experiences across questions. Patients with small intestine/rarer lower gastrointestinal, multiple myeloma and hepatobiliary cancers were most likely to report negative experiences whereas patients with breast, melanoma and testicular cancer were least likely (top-to-bottom odds ratio = 1.91, P < 0.0001). There were also inequalities in experience among patients with cancers treated by the same specialty for five of nine services (P < 0.0001). Specifically, patients with ovarian, multiple myeloma, anal, hepatobiliary and renal cancer reported notably worse experiences than patients with other gynaecological, haematological, gastrointestinal and urological malignancies respectively. Initiatives to improve cancer patient experience across oncology services may be suitably targeted on patients at higher risk of poorer experience.This is the final version, originally published by Wiley in the European Journal of Cancer Care (http://onlinelibrary.wiley.com/doi/10.1111/ecc.12267/abstract)

Post-sampling mortality and non-response patterns in the English Cancer Patient Experience Survey: Implications for epidemiological studies based on surveys of cancer patients

BACKGROUND: Surveys of the experience of cancer patients are increasingly being introduced in different countries and used in cancer epidemiology research. Sampling processes, post-sampling mortality and survey non-response can influence the representativeness of cancer patient surveys. METHODS: We examined predictors of post-sampling mortality and non-response among patients initially included in the sampling frame of the English Cancer Patient Experience Survey. We also compared the respondents' diagnostic case-mix to other relevant populations of cancer patients, including incident and prevalent cases. RESULTS: Of 109,477 initially sampled cancer patients, 6273 (5.7%) died between sampling and survey mail-out. Older age and diagnosis of brain, lung and pancreatic cancer were associated with higher risk of post-sampling mortality. The overall response rate was 67% (67,713 respondents), being >70% for the most affluent patients and those diagnosed with colon or breast cancer and <50% for Asian or Black patients, those under 35 and those diagnosed with brain cancer. The diagnostic case-mix of respondents varied substantially from incident or prevalent cancer cases. CONCLUSIONS: Respondents to the English Cancer Patient Experience Survey represent a population of recently treated cancer survivors. Although patient survey data can provide unique insights for improving cancer care quality, features of survey populations need to be acknowledged when analysing and interpreting findings from studies using such data

What explains worse patient experience in London? Evidence from secondary analysis of the Cancer Patient Experience Survey

This is the final published version. Available from BMJ Publishing Group via the DOI in this record.Objective: To explore why patients with cancer treated by London hospitals report worse experiences of care compared with those treated in other English regions. Design: Secondary analysis of the 2011/2012 National Cancer Patient Experience Survey (n=69 086). Setting and participants: Patients with cancer treated by the English National Health Service (NHS) hospitals. Main outcome measures: 64 patient experience measures covering all aspects of cancer care (prediagnosis to discharge). Methods: Using mixed effects logistic regression, we explored whether poorer scores in London hospitals could be explained by patient case-mix (age, gender, ethnicity and cancer type). Because patients referred to tertiary centres and/or with complex medical problems may report more critical experiences, we also explored whether the experiences reported in London may reflect higher concentration of teaching hospitals in the capital. Finally, using the data from the (general) Adult Inpatients Survey, we explored whether the extent of poorer experience reported by London patients was similar for respondents to either survey. Results: For 52/64 questions, there was evidence of poorer experience in London, with the percentage of patients reporting a positive experience being lower compared with the rest of England by a median of 3.7% (IQR 2.5-5.4%). After case-mix adjustment there was still evidence for worse experience in London for 44/64 questions. In addition, adjusting for teaching hospital status made trivial difference to the case-mix-adjusted findings. There was evidence that London versus rest-of-England differences were greater for patients with cancer compared with (general) hospital inpatients for 10 of 16 questions in both the Cancer Patient Experience and the Adult Inpatients Surveys. Conclusions: Patients with cancer treated by London hospitals report worse care experiences and by and large these differences are not explained by patient case-mix or teaching hospital status. Efforts to improve care in London should aim to meet patient expectations and improve care quality.National Institute for Health Research (NIHR)Macmillan Cancer Suppor