Le calcul des coefficients de diffusion dans un réseau suppose la connaissance d'un mode de pondération correct des libres parcours des différents constituents. On définit ici sans ambiguité ce mode de pondération á partir du calcul des fuites hors d'une zone de réacteur. La formulation obtenue, simple et générale, ramène le calcul des coefficients de diffusion à celui des probabilités de collision dans les différents milieux; elle fait apparaître dans l'expression du coefficient radial la série des termes de corrélation angulaire (termes rectangles), mis en évidence récemment par plusieurs auteurs. Cette formulation est ensuite appliquée au calcul pratique d'un réseau classique, composé d'un modérateur et d'un élément combustible entouré d'une cavité; la comparaison analytique et numérique des expressions obtenues avec celles déduites de la théorie de Behrens fait apparaître l'importance de plusieurs termes nouveaux, dont certains sont liés à la transparence de l'élément combustible; les termes rectangles sont calculés jusqu' à l'ordre 2. Un formulaire pratique est donnéà la fin de cette ètude.
Two distinct ways in which the variational method may be used to obtain approximate solutions of the equations which are involved in neutron transport theory are discussed. In the first, a method is given whereby an estimate of an eigenvalue or some weighted average of the solution of the equation is obtained. Whilst the second way is a method whereby a relatively complicated equation involving multi-dimensional variables is reduced to a number of simpler equations each in a fewer number of independent variables. Examples to illustrate both methods are given.
Side-channel collision attacks have shown good efficiency on breaking unprotected or reused-mask implementations of block ciphers. Depending on the relationship between “Hamming distance of values” and “average distance of traces”, this work presents a novel bit-policy based chosen-plaintext collision attack to break a software implementation of reused-mask AES. Some factors affecting the success rates of the proposed scheme and their reasons are analyzed, such as how to measure the distance of traces, signal-to-noise ratio (SNR), how to select points of interest (POI) and the number of traces (n). In this paper, two measurements, the least square method (LSM) and the central moment product method (CMP), are adopted to calculate the distance of traces. Then their probability distribution and efficiency are theoretically analyzed and simulated. As a result, CMP outperforms LSM, and all the other three factors, i.e. SNR, POI, n, are relevant to the distribution of LSM and CMP. When LSM and CMP are combined with the proposed scheme and the compared ones, both theoretical and practical analysis are given to illustrate how the number of traces impacts on the success rates at different levels of SNR; as a result the developed attack has the best performance among all the compared schemes at both low and high level of SNR, especially at high level of SNR. To gain a success rate of 95%, at SNR of 0 dB and −8 dB, our optimal scheme and the compared optimal scheme have 800 and 4500 less energy traces, respectively. Furthermore, our attack requires no detailed knowledge about the design of SBoxes, nor does it require templates for profiling.
Galanin's proof of the homogenization method for an infinite reactor lattice is extended to the case of the finite bare reactor lattice. It is shown that homogenization is rigorously valid only for a rectangular reactor in which the reactor boundary has a certain specified relationship to the lattice, but that homogenization is a good approximation in any reactor containing a large number of lattice cells. The applicability of Wigner's second fundamental theorem to a heterogeneous reactor is discussed.
Side-channel collision attacks have shown good efficiency on breaking unprotected or reused-mask implementations of block ciphers. Depending on the relationship between “Hamming distance of values” and “average distance of traces”, this work presents a novel bit-policy based chosen-plaintext collision attack to break a software implementation of reused-mask AES. Some factors affecting the success rates of the proposed scheme and their reasons are analyzed, such as how to measure the distance of traces, signal-to-noise ratio (SNR), how to select points of interest (POI) and the number of traces (n). In this paper, two measurements, the least square method (LSM) and the central moment product method (CMP), are adopted to calculate the distance of traces. Then their probability distribution and efficiency are theoretically analyzed and simulated. As a result, CMP outperforms LSM, and all the other three factors, i.e. SNR, POI, n, are relevant to the distribution of LSM and CMP. When LSM and CMP are combined with the proposed scheme and the compared ones, both theoretical and practical analysis are given to illustrate how the number of traces impacts on the success rates at different levels of SNR; as a result the developed attack has the best performance among all the compared schemes at both low and high level of SNR, especially at high level of SNR. To gain a success rate of 95%, at SNR of 0 dB and −8 dB, our optimal scheme and the compared optimal scheme have 800 and 4500 less energy traces, respectively. Furthermore, our attack requires no detailed knowledge about the design of SBoxes, nor does it require templates for profiling.
Irradiation of rubberlike fluorine copolymers gives a rubber with poor physicochemical properties, but with fillers it is possible to obtain rubbers with valuable technical properties. The surface activity of the filler has a strong effect on the properties of the rubbers obtained. The most valuable properties are those of radiation rubbers that contain active channel black. They have a higher resistance to heat aging, are more stable to solvents and static deformation than the usual peroxide rubbers. In contrast to polytetrafluoroethylene and polytrifluorochloroethylene, rubberlike fluorine copolymers of the Kel-F type are cross-linked under the action of ionizing radiation. The degree of cross-linking of the fluorine copolymers is determined by their composition and the irradiation dose and medium. The presence of oxygen promotes a greater degree of cross-linking.
Experimental and theoretical work on the Doppler effect in thermal reactors is reviewed for uranium metal, UO2, thorium metal, and ThO2. The experimental values of a, the fractional increase in resonance capture per °C, have a spread many times the quoted errors. The use of different slowing-down spectra has contributed to the discrepancies. For uranium metal, approximate corrections are made to obtain the coefficient α0 appropriate to a 1E spectrum. The spread in the corrected values α0 is smaller than that for α, but remains unsatisfactory. Other experimental difficulties arise in reactivity normalizations, in obtaining the statistical weight of samples and from spurious temperature effects. Theory and experiment agree on an increase of α0 with increasing surface-to-mass ratio and that this is caused by an increase in the contribution of lower-energy resonances to the Doppler effect. It is also in agreement with the theoretical interpretation of the radial dependence of the Doppler effect in a lump. However in the region of practical interest where the surface-to-mass ratio is small, α0 is almost constant. Experimental evidence on the temperature behaviour of α0 is unsatisfactory but indicates that α0, decreases with increasing temperature. Theory predicts that α0 will vary approximately as T−12 where T is the Kelvin temperature. In the case of non-uniform temperature distribution in a fuel element, both experimental and theoretical effort is needed.
The effective cross-sections, \̂gs and cadmium ratios, RCd for the reaction 133Cs(n,γ)134Cs (2·17 yr) have been measured in three different positions of the NRX reactor and the results interpreted by means of the Westcott convention to give values for the 2200 m/sec cross-section, σ0, and the resonance integral, Σ. Measurements were made relative to the cross-sections for the reaction 59Co(n,γ)60Co (5·28 yr). \̂gs varies from 34 barns in a graphite shielded position outside the reactor calandria to 41 barns in a vacant fuel rod position. The values of σ0 and Σ derived are 33·4 ± 2·6 barns and 370 ± 50 barns respectively, where Σ is evaluated from 0·5 eV upwards and includes the 1v part. The errors are standard deviations and include the errors in the measurements and the errors in the input data; they do not include errors which could arise in the interpretation of the data.
On décrit un modèle stochastique susceptible d'exprimer le destin d'individus évoluant dans un milieu multiplicateur tel qu'un réacteur nucláire. Grâce à ce modèle il est possible d'obtenir les lois de probabilité de plusieurs variables aléatoires liées au processus, et notamment l'équation différentielle stochastique exprimant l'évolution de la densité de la population ainsi que l'équation de diffusion de la probabilité qui lui est associée. On peut alors, dans le cas stationnaire, en déduire le spectre énérgétique des fluctuations. Toute une serie de resultats obtenus par divers auteurs est ainsi retrouvée, unifiée et complétée.
A description is given of a hydrogen condensation pump of output 3·7 × 104 sec−1 which has its own liquifier. The limiting vacuum is 10−8–10−9 mm Hg. The total power consumed (allowing for the liquid nitrogen) is 17 which is less than for oil-diffusion pumps of the same performance.
The scattering of neutrons from a rotating rigid molecule has been analysed for incident neutron energy of the same order of magnitude as the rotational level separations, without making any approximations.
The yields of 188Ba, 138Sr, 90Sr, 89Y and of five zirconium isotopes formed in the fission of 233U have been measured. There is a fine-structure in the yield as a function of mass number. The independent yield of 136Cs has been measured, while that of 86Rb has been estimated, thus completing important parts of the yield curve. The boundary between the region where a light fragment emits one neutron and the region where it emits two is established from the curve. It is concluded that the structure is determined by events at the instant of fission.
Side-channel collision attacks have shown good efficiency on breaking unprotected or reused-mask implementations of block ciphers. Depending on the relationship between “Hamming distance of values” and “average distance of traces”, this work presents a novel bit-policy based chosen-plaintext collision attack to break a software implementation of reused-mask AES. Some factors affecting the success rates of the proposed scheme and their reasons are analyzed, such as how to measure the distance of traces, signal-to-noise ratio (SNR), how to select points of interest (POI) and the number of traces (n). In this paper, two measurements, the least square method (LSM) and the central moment product method (CMP), are adopted to calculate the distance of traces. Then their probability distribution and efficiency are theoretically analyzed and simulated. As a result, CMP outperforms LSM, and all the other three factors, i.e. SNR, POI, n, are relevant to the distribution of LSM and CMP. When LSM and CMP are combined with the proposed scheme and the compared ones, both theoretical and practical analysis are given to illustrate how the number of traces impacts on the success rates at different levels of SNR; as a result the developed attack has the best performance among all the compared schemes at both low and high level of SNR, especially at high level of SNR. To gain a success rate of 95%, at SNR of 0 dB and −8 dB, our optimal scheme and the compared optimal scheme have 800 and 4500 less energy traces, respectively. Furthermore, our attack requires no detailed knowledge about the design of SBoxes, nor does it require templates for profiling.
La recherche d'un optimum économique d'un réacteur nécessite la connaissance précise des variations du bilan de réactivité suivant les caractéristiques géométriques ou physiques du réseau. Il est donc nécessaire de disposer d'un nombre d'expériences critiques avec une bonne précision relative.
An important quantity in nuclear fission and in process taking place in a nuclear reactor is the mass spectrum of fission fragments. The droplet model of the nucleus predicts asymmetric distribution of masses as the most probable, but this contradicts experiment. In the present paper the mass distribution of fission fragments is calculated for several nuclei from the energy of the fissile nucleus prior to cleavage, with due allowance for shell effects. It is shown that the energy minimum corresponds to asymmetric fission.
One of the essential characteristics of nuclear fission and of processes occuring in nuclear reactors is the distribution of fission fragments by mass. According to the drop model of the nucleus, a symmetrical mass distribution of the fragments is most probable, but this, however, is contrary to experiment.
The thermal neutron absorption cross-section of 238Th and the resonance absorption integrals of 232Th, 233Th and 69Co have been measured as 1450, 85, 400 and 75 barns, respectively, by neutron activation measurements using cadmium filters with reactor spectrum neutrons.
