摘要:

AbstractIn the mathematical models of ceramic fuel pin performance that have been formulated to date, the fuel is divided into a number of radial zones, usually three, having different properties and operating conditions. It is assumed that fuel behavior can be adequately characterized in this way for purposes of pin performance modeling.In this set of papers, emphasis is on the fuel material itself and the phenomena that occur during reactor operation. These include recrystallization and the effects of fission product production, especially the motion of fission gas. The first paper discusses the known mechanisms, and outlines the status of the entire subject. The next four discuss, in different degrees of mathematical detail, processes by which fission gas affects fuel behavior. In the next paper (by Notley) is a detailed evaluation of the interaction between fuel and cladding that illustrates the complexity of the relationships involved. The model described in the final paper covers mechanical and thermal behavior of complete pins, taking into account three dimensional volume variations of fuel. The fuel material under consideration in all papers is oxide, except for inclusion of some data on UC (in the paper by Ritzman et al.).

ISSN:0550-3043    

DOI:10.13182/NT70-A28802

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe development of models of fuel-element performance requires, among other things, a detailed, quantitative understanding of fuel swelling and fission gas release. The current state of knowledge on these phenomena is reviewed. It is shown that gas bubbles play an important role in both swelling and gas release, that a knowledge of fuel operating history is essential to quantitative predictions, and that the migration of cesium will greatly influence local swelling rates. Finally, an indication is given of the areas toward which future studies should be directed.

ISSN:0550-3043    

DOI:10.13182/NT70-A28803

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractCurrent fuel swelling models based upon the growth of fission gas bubbles do not consider effects due to radiation induced re-solution phenomena. This paper describes a fission gas swelling model which assumes that fission fragments will destroy existing gas bubbles and maintain the resulting gas atoms in supersaturated solid solution. Such a model should be particularly applicable to fuels operating at low temperatures and high fission rates. Bubble nucleation and growth then take place until another fission fragment again passes through the same region. Bubble growth is calculated using reaction rate theory over the period of time in which no radiation damage occurs. The model predicts bubble growth significantly smaller than that experimentally determined in UO2. This discrepancy is attributed to assumptions made in defining the re-solution mechanism. The model implies that fission gas bubble growth is a state junction independent of path.

ISSN:0550-3043    

DOI:10.13182/NT70-A28804

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractA computer simulation of a fuel region has been developed based on movement of fission gas bubbles in solids under a thermal gradient. Within this region, fission gas events are followed via a Monte Carlo technique. Individual bubbles are followed through their time history from nucleation to release from the fuel, with interactions at dislocations and grain boundaries. Saturation in gaseous swelling at elevated temperature is predicted. A maximum in swelling is predicted at intermediate temperatures for a given burnup. These swelling and gas release predictions at high temperatures are in good agreement with experimental results. A low temperature modification of dislocation density is required to allow for effects dominant at low temperature which are not included in the current version of the program. With this low temperature modification, swelling predictions are in good agreement with experimental observations over the entire operating temperature range.

ISSN:0550-3043    

DOI:10.13182/NT70-A28805

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe swelling of the refractory nuclear fuels, UO2, UN, and UC, at temperatures of the order of 1700°C, is related to the behavior of the fission gases as these gases make their way out of the fuels. In all three fuels, the fission gases first precipitate to form a two-phase system consisting of solid fuel and gaseous precipitates. These precipitates or bubbles grow in-reactor mainly by the accretion of new fission gas atoms. New gas atoms diffuse to the older bubbles (which are formed in the first few minutes of irradiation) causing these bubbles to grow and to swell the fuel. In UC, this process continues with very little change to produce rather large amounts of swelling. However, at about 10 vol% swelling of UC, bubbles begin to overlap to form channels and gas release begins to accelerate. In UN and UO2at about 1700°C, UN and UO2vapors begin to migrate across the growing bubbles as a result of the temperature gradients usually found in fuels in-reactor. This process, which is a form of zone refining, results in rapid and efficient gas collection and release. This gas release may reduce the stresses applied by UN and UO2to their claddings; but the zone refining process does not completely eliminate swelling stresses because swelling precedes gas release. Also, the zone refining or“restructuring”process tends to eliminate built-in porosity; so that while built-in porosity is very effective in limiting the swelling of UC, such porosity is relatively ineffective in reducing the swelling of UN and UO2at temperatures where restructuring is rapid. These processes are described in terms of classical chemical and physical metallurgical models; and the validity of the models is illustrated, although not necessarily proved, by the results of high-temperature irradiation experiments.

ISSN:0550-3043    

DOI:10.13182/NT70-A28806

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe critical size for a bubble pinned to a dislocation to continue motion has been evaluated. The result differs from earlier line-tension models. A statistical method for treating bubble coalescence has been developed in which bubble-size distribution is defined in terms of the number of gas atoms in a bubble on a logarithmic scale. Based on this method, a swelling and gas-release model is developed and discussed. The resulting model can be readily adapted to numerical computation.

ISSN:0550-3043    

DOI:10.13182/NT70-A28807

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractA computer program is described which calculates the behavior of UO2pellet fueled elements during irradiation. Whenever possible, prediction and experimental observation are compared to verify that the models developed for calculation are reasonable. The interaction of variables when operating with high internal fission gas pressures (and, hence, reduced fuel-sheath heat transfer coefficient) is emphasized.

ISSN:0550-3043    

DOI:10.13182/NT70-A28808

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe COMETHE II computer program has been formulated to predict the thermal and mechanical behavior of fuel pins during their irradiation life. It calculates, in particular, the temperature distribution, the radial and axial fuel swelling and expansion, the fission gas release, and the stresses and strains in the cladding.The program involves many models for this purpose. Some of these have been separately tested and calibrated with respect to available experimental results in the literature. Calibration of the whole program is also being currently performed. COMETHE II results are compared with experimental data for different burnups and thermal ratings. The agreement obtained with the experiments is rather good, the theoretical data lying generally in the margin of error of the experimental data.The capabilities of the COMETHE II program are illustrated by a parametric study of the gap width influence on the maximum center temperature and on the strain of the sheath. This example shows that the COMETHE II program is a useful tool for the design of fuel pins for fast or thermal power reactors.

ISSN:0550-3043    

DOI:10.13182/NT70-A28809

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractCalculations show that a 10% isotopic abundance of238Pu in a bare spherical239Pu-H2O system increases the minimum critical mass by ~48%, thereafter the increase continues exponentially. These results, in addition to several subcritical neutron multiplication experiments with238Pu in both solution and dioxide form, confirm that238Pu behaves as a neutron poison in a well-moderated medium. Neutron multiplication experiments were conducted with four SNAP-19B (IRHS) heat sources with ~1 kg238Pu per source. The results indicated that a neutron multiplication<1.2 existed when the heat sources were assembled in different array configurations. The criticality unit value for238Pu in either solution or dioxide form has been conservatively established at 2.0 kg. Thermocouples installed on process vessels have proven to be a new criticality control technique for determining gradual238Pu sludge buildup. Heterogeneous water-filled storage vaults provide the safest method for storing large quantities of PuO2containing at least 76%238Pu.

ISSN:0550-3043    

DOI:10.13182/NT70-A28810

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractIt is shown that an accurate determination of the fission rate in fuel elements can be obtained by the technique of solid-state track detectors. The method gives: (a) the radial variation and (b) the absolute value of the fission rate, by application of the measured value of the mean-path-length of the fission products in the fuel. The quality of the results is independent of the neutron spectrum during the irradiation.In a particular case (thermal flux), results deduced from measurements performed with solid-state track detectors are compared with those obtained by the fissile detector (activation) technique. The agreement is excellent.

ISSN:0550-3043    

DOI:10.13182/NT70-A28811

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor