It is well known that adding a moderator in the blanket or strongly absorbing reflector in a fast reactor increases the decay time of the neutron density from the neutron pulse. Measurements during the international experiment BERENICE [1] on determining the effective delayed-neutron fraction Beff of the temporal distribution of prompt neutrons by the ~-Rossi method gave an unusual result. The computed lifetime of the prompt neutrons in the core and blanket of the MASURCA-R2 critical assembly (Cadarache, France) was equal to -0.35 and 1/~see, respectively, and it was expected that the decay time ~-of the main component of the neutron density under suberiticality conditions p = 0.15/3eft will be close to 40-45 tzsec.In reality, the measured temporal distribution was described by two exponentials with decay times r I = 22.5 + 1.5 and r 2 = 127 + 6 psee. In [2] it was hypothesized that the appearance of these two exponentials is due to two small polyethylene blocks in the blanket. Numerical calculations using the model of two groups of prompt neutrons in a reactor with a reflector [3] confirmed the possibility of such an explanation. A weak point was the lack of measurements in an assembly with no polyethylene blocks.To check the hypothesis experimentally, measurements were performed on the KBR-22 critical assemblies [4]. Its main characteristics are: 235U thorium (approximately 20% enrichment) core, depleted uranium dioxide blanket, 61-cm high and -80 cm in diameter core, 40-cm thick radial blanket, 20 cm thick axial blanket. A section of the assembly along the central plane is displayed in Fig. 1. The same neutron detectors (3He counters) as in the experiment on the MASURCA-R2 critical assembly were placed inside the empty channels.The a-Rossi temporal distribution was measured for four compositions. The first is the assembly described above. The second composition was obtained by introducing two polyethylene blocks into the blanket. Each block was obtained by replacing the uranium dioxide by polyethylene in three cylindrical tubes. The centers of the 25-cm high polyethylene cylinders in these tubes were located in the central plane. The third composition contains four blocks (two more blocks in the right-hand side of the blanke0. The fourth composition was obtained by separate placement of 12 polyethylene cylinders in the second row of the tubes in the blanket. The measurements were performed with 0.15/3el f subcriticality.The measured c~-Rossi temporal distribution is displayed in Fig. 2. The figure also shows the short-lived correlated part, obtained by subtracting out the long-lived part. Table 1 contains r 1 and r2 and the ratio of the "areas" under them $2/S r The characteristic neutron lifetime r r in the polyethylene blocks and cylinders was measured in an additional experiment. It turned out to be 65 and 50 ~,sec, respectively, in the uranium dioxide environment.Therefore it has been established experimentally that introducing into the blanket of a fast reactor polyethylene blocks with neutron lifetime l...
The thermal stability of amorphous Ti4,Zr4, Si,, alloys is studied by small-angle neutron scattering. In as-quenched samples inhomogeneities of 8.1 nm in radius are observed which disappeared during a low temperature (250 "C) annealing treatment. The annealing carried out a 500 "C for different times (10, 30, 60 min, and 24 h) leads to a considerable increase of the scattered intensity. Because of the interparticle correlation the dimensions of the particles must be determined by the least-squares fitting of the total scattering cross-section formula to the measured intensity pattern. The dimensions of the particles obtained in this way do not explain the non-monotonous time course of the scattered intensity. Therefore it is concluded that the scattering length density of the particles, that is their composition, has changed during the high temperature annealing treatment.La stabilite thermique des alliages amorphes Ti41Zr41Si18 a ete etudiee par diffusion aux petits angles. Dans I'echantillon trempe, des heterogtneites de 8.1 nm ont kte observees et disparaissent lors d'un traitement de revenu a basse temperature (250 "C). Le traitement thermique a 500 "C a etk effectue duree du temps differents (10, 20, 60 min et 24 h) en resultant une augmentation considerable de I'intensite diffusee. A cause de la correlation interparticulaire les dimensions des particules dans l'echantillon ont ete determintes par l'ajustement type moins quarrt de I'intensitk mesuree i la formule de la section eficace total de la diffusion. Ces dimensions obtenues n'expliques pas la tenue non-monotonique en temps de l'intensite diffusee. En consequence le densite de la longueur de diffusion, cet-a-dire la composition des particules changeait au cour du traitement a haut temperature.
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