[en] The constraints and criteria for the core design of the 600 MWe natural circulation simplified boiling water reactor are reviewed. A design satisfying all of the constraints is presented. Incentives and tradeoffs for further optimization of the core design are quantified

[en] The n-bar annihilation cross section on several nuclei (C, Al, Cu, Ag, Sn, Pb) has been measured by the OBELIX experiment at the Low Energy Antiproton Ring at CERN; n-bar's were produced by charge exchange reaction in the 50-400 Mev/c momentum range. The dependence of the cross section on the target mass number and on the n-bar momentum has been determined. Preliminary results on the final state of the interaction have been obtained and indications of the production of the ρ vector meson in n-bar-nucleus annihilation have been observed

[en] The object of the current study is a Main Feed Water Pump (MFWP) miniflow control valve installed on Unit 1 of Cernavoda NPP. During feedwater system commissioning unexpected and severe pipe vibration and noise problems were experienced leading to valve stem break, thought as being due to hydrodynamic vibration generated by cavitation phenomena taking place inside the valve itself. The complex turbulent flow occurring in the valve has been simulated through the Computational Fluid Dynamic (CFD) methodology. Such valves are characterized by high pressure drops achieved by the particular design of the valve internals which originate an high speed swirling turbulent flow. The FLOW3D computer code has been employed to solve the velocity and pressure field in such complex 3D domain. The valve internals (plug and blades) are fully modeled as well as a significant portion of the discharge line. A grid size of 50,000 nodes and the K-Epsilon turbulence (isotropic turbulence) model has been used in the calculation. (author). 12 figs., 9 refs

[en] The results of a study are described focusing on a comparison of the values of fast neutron fluence for the AP600 reactor pressure vessel, calculated in 3-D with two different libraries: SAILOR (ENDF/B-IV) and BUGLE-93 (ENDF/B-VI). The presence of a radial reflector composed of stainless steel has been accounted for. Besides the 3-D core and internals representation allow the emergence of a more accurate relationship between the specimens and the inner vessel fluence. The availability of a CRAY computer in FINMECCANICA makes the three-dimensional transport code TORT an attractive choice. The analysis confirms that for the presence of the stainless steel radial reflector in the AP600 the use of the BUGLE-93 library implies a more conservative estimation (about + 10%) of the pressure vessel neutron fluence with respect to the use of the SAILOR library. (R.P.)

[en] A new set of measurements of neutron emission from gas (D₂ and H₂) loaded Ti and Pd systems has been carried out in the TOFUS experiment. The temperature and pressure controls of the gas loading apparatus were improved. The results concerning the Ti/D system show the presence of a small 2.5 MeV neutron emission, with a signal having a statistical significance of ∼5σ. The results on the Pd/D system doesn't show a statistically significant signal (less ∼2σ). (author)

[en] Strong efforts are devoted, in the design of new nuclear power plants, to reduce costs and increase reliability. Systems and components optimization is part of the overall effort which requires amongst other, sophisticated computer programs and detailed analysis models. As an example, the Westinghouse AP1000 core shroud, the structure having the function to maintain the core centered on its axis, is being designed as a highly heterogeneous structure (fig.1). Its thermal-mechanical sizing must take into due account the internal heat generation. Its accurate determination can be effectively performed with 3D deterministic transport methods. These methods have been followed since the 3D synthesis methods, combining 2D and 1D calculations, imply the separation of spatial variables and are mainly applicable for fluxes in the reactor beltline region and lead to the introduction of unduly conservative margins. Moreover Monte Carlo methods do not allow an easy quantification the uncertainties related to overall calculation. 3D deterministic models, based on the discrete ordinate transport theory, have both a well known difficulty to create and tune the geometrical model in a reasonable time and require high computer resources to perform the calculation; this, until few years ago, required massive parallel computers available only to large national laboratories and selected industries The recent increment of computer power and the contemporary development, by ENEA Bologna, of a pre-post processor based on the combinatorial geometry (BOT3P, [2, 3, 4, 5, 6]) makes the three dimensional deterministic program TORT (part of the DOORS 3.2 Package, [7]) an attractive choice to perform this type of calculation. This paper illustrates the approach followed by ANSALDO Energia-Divisione Nucleare for the AP1000 Core Shroud. The calculation has to be performed at 67 energy groups (47 neutrons plus 20 gammas) in P3/P5 Legendre approximation and 2.000.000 meshes are to be used. The computational power required is massive: 2 GB of RAM and up to 50 GB of peak storage for each run with an S8 full-symmetric angular quadrature and 120 GB of peak storage with an S16 full-symmetric angular quadrature. By using a latest generation workstation based on an INTEL P-IV 3.2 GHz processor under LINUX operating system (SUSE 9.1), each run requires less than a week of full computer time. The code BOT3P has been used both for the preparation of the geometrical model and as post processor of the TORT program outputs. BOT3P permitted to cut the model development time by a significant factor and to retrieve efficiently large amount of data from the binary files, allowing a graphical review of the input and output data and, finally, resulting in a unique tool for Quality Control for complex 3D models . This computational experience can be considered a good working example for the 3D deterministic transport method revival by considering that this method is widely accepted by licensing authorities. (authors)

[en] A diffusion coarse mesh method for static (criticality) and dynamic LWR core calculations is presented. It is based on a low order polynomial formula for the nodal flux (actually a sum of 3^d degree polynomials in each of the space variables x,y,z, as in CUBBOX, with some modifications) which seems to achieve a good balance between speed and accuracy. The resulting linear system is solved by means of a suitable alternating direction implicit algorithm. This coarse mesh technique has originated a family of computer codes which, while quite efficient when performing accurate calculations, become particularly fast if larger errors, of the typical order of magnitude of those admitted in most routine calculations, are allowed

[en] Antineutron annihilation cross sections on C, Al, Cu, Ag, Sn and Pb targets were measured in the momentum range 50-400 MeV/c. The full set of data is very well described by a parameterization in which the target mass number and momentum dependence are factorized. The A dependence is, within 2%, consistent with a A^2/3 scaling law, indicating a pure surface process. The momentum dependence is, within 20%, consistent with that expected from a properly averaged contribution of the elementary cross section for nbarp and nbarn annihilation, indicating that the process is due to the interaction with single nucleons on the surface

[en] The FINUDA experiment at DAΦNE, Frascati, has found evidence for the neutron-rich hypernucleus _Λ"6H studying (π"+; π"-) pairs in coincidence from the K_s_t_o_p"- + "6Li → _Λ"6H + π"+ production reaction followed by _Λ"6H → "6He + π"- weak decay. The production rate of _Λ"6H undergoing this two-body π"- decay has been found to be (2.9 ± 2.0)*10"-"6/K_s_t_o_p"-. Its binding energy has been evaluated to be B_Λ( _Λ"6H) = (4.0 ± 1.1) MeV with respect to ("5H + Λ), jointly from production and decay. A systematic difference of (0.98 ± 0.74) MeV between B_Λ values derived separately from decay and from production has been tentatively assigned to the _Λ"6H 0_g_._s_."+ → 1"+ excitation. A similar investigation has been carried out for the neutron-rich hypernucleus _Λ"9He studying the K_s_t_o_p + "9Be → _Λ"9He + π"+ reaction in coincidence with the _Λ"9He → "9Li + π"- weak decay; an upper limit for the production rate of _Λ"9He undergoing the two-body π"- decay has been found to be 4.2*10"-"6/K_s_t_o_p"- (90% C.L.). (author)

[en] From the first beginning the weak decay of Λ-hypernuclei has been considered a very interesting physics case, since its study could shed light on several topics in nuclear and particle physics, among which the ΛN → NN four-baryon weak interaction not otherwise accessible. However, only in the last decade a substantial series of reliable experimental data samples has been produced thanks to the synergic effort of dedicated facilities at some laboratories in the world. The existing pattern of data on lifetimes, partial decay widths for mesonic and non-mesonic decay (both one- and two-nucleon induced) and energy spectra and correlations of the emitted particles is reviewed and compared with existing theoretical predictions. Updated tables and plots summarizing the existing experimental information are presented. For each item a brief account of possible new experimental efforts, approved, planned or futuristic is given. From these analyses the full pattern of partial decay widths for ⁴ ΛHe, ⁵ ΛHe, ¹¹ Λ B and ¹² Λ C is discussed. Rare decay channels and polarization studies are also briefly analyzed.