1. magnetic confinement - ENEA - Fusione

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78 3. FUSION TECHNOLOGY 3.5 Neutronics percent level, are not considered in D1S. It was concluded that the shutdown dose rate for the outer vessel region is well predicted within 25% by the R2S and D1S methods with the FENDL-2 library. 3.5.3 Design of the neutron cameras for ITER ITER will have two neutron cameras for measuring the neutron emission distribution. This diagnostic system has to provide absolute neutron yield, fusion power, alphaparticle birth profile and ion temperature, besides the neutron source profile. The radial camera, located in a horizontal port, consists of a fan-shaped array of flight tubes (totalling 12×3 ) viewing the plasma through a slot at the blanket/shield level, intersecting at a common aperture (focal point) defined by a specialised shielding plug, and penetrating the vacuum vessel, cryostat and biological shield through stainless-steel windows. Each flight tube culminates in a set of neutron detectors (both flux detectors and compact spectrometers) housed in a massive shielded structure outside the biological shield. The geometry of the radial camera is fixed by the port size; as a result, the plasma fraction covered is rather limited. The vertical camera has a different configuration: the arrays of 15 chords viewing the plasma downward are located at four different toroidal locations. Each array of chords views the plasma through the first collimators in the upper radial port plug and through the second collimators above the upper cryostat lid. Flight tubes are placed in the vacuum vessel, above the plug of the upper radial port. The upper collimators, the detectors and beam dumps are located between the cryostat lid and the top bioshield and are housed in a massive shielded structure to prevent neutron scattering and to limit the cryostat activation to allowable levels. The measurement capability of the system was evaluated for relevant neutron source profiles [3.36]. In particular, the chord integrals of the neutron emissivity and the resulting fluxes at the detectors were calculated for both the radial and the vertical camera, for the reference operation scenario (ELMy H-mode) and for the more peaked neutron emissivity profiles. The results showed that the accuracy of the absolute value of total neutron yield measured by the radial camera alone would not be better than 20% due to the very limited plasma coverage. The combination of the radial and vertical cameras will increase the accuracy of the absolute neutron yield to better than 10%, as required. The minimum number of sightlines in the vertical camera and the effectiveness of the most external sightlines were analysed, taking into account the neutron backscattering from the first wall. As a result, it was found that the most external channels of the vertical camera are still effective (although considerable corrections have to be applied) in the case of the reference ELMy H- mode operation scenario, which is characterised by a very flat neutron emissivity profile. In the case of more peaked emissivity profiles, the most external channels and the ones adjacent to them lose their effectiveness and can cause a significant level of noise due to backscattering neutrons. [3.36] P. Batistoni, Design of the radial and vertical neutron camera for ITER, in preparation The size of the collimator diameters was optimised in the range of variation in the neutron production rate to improve the measurement capability. Flux monitors suitable for the ITER camera requirements were identified. As for compact spectrometers, a number of possible candidates exist; however, they require further investigation and development before they can meet the ITER requirements for energy and time resolution in neutron energy spectra measurements. ENEA is investigating the capability of organic liquid scintillators (NE213) to provide an effective energy resolution of about 2-3% at 2.45-MeV neutron energy and 1% at 14 MeV in tokamak conditions, i.e., proving neutron/gamma-ray and pulse-height discrimination at high counting rates. In collaboration with PTB Braunschweig, Germany the feasibility of the method is being investigated, and the capability of the
3. FUSION TECHNOLOGY 79 3.5 Neutronics system to reach useful energy resolution will be tested during D-D and D-T operations at JET. 3.5.4 Evaluation of neutron cross sections for fusion materials (EFF project) The correct design of a fusion reactor requires the availability of a complete nuclear database extending up to 20 MeV in neutron energy. The ENEA Fusion and Applied Physics Divisions participated in the European Fusion File (EFF) Project by updating neutron cross-section data. In 2001, the carbon and oxygen cross sections were newly evaluated on the basis of the latest experimental and theoretical findings. The neutron capture cross sections were re-evaluated in the entire range of 10 -5 eV up to 20 MeV of incident neutron energy. Model calculations based on state-of-the-art nuclear structure and nuclear reaction models were employed together with a global analysis of the latest experimental information. Inverse photo-neutron reaction data were utilised to cover the energy range above a few MeV. These data and the model calculations were used for the transitions leading to excited states of residual nuclei. The resulting nuclear cross-section data were compiled and organised into ENDF 6 nuclear data format. The files were integrated with complete existing data libraries (including all the reaction channels other than capture). For 12 C, the JENDL 3.2 file was chosen as a basis; for 16O, the JEF-2 file was selected. The data files were made available to the community for testing. Preliminary tests were done with standard format checking codes (FIZCON, PSYCHE, and CHECKR). 3.5.5 Neutronics benchmark experiment on SiC (EFF project) [3.37] P. Batistoni et al., Measurements and analysis of reaction rates and of nuclear heating in SiC, Final report of task TTMN-002 (1)-001, Report FUS- TEC- MA- NE-R-2001 Fig. 3.14 - The SiC block in front of the FNG target. Silicon carbide (SiC) is one of the candidate structural materials for a fusion reactor because it has excellent low-activation, low-decay-heat properties. To validate the SiC neutron cross-section data in the EFF library, a benchmark experiment was started in 2000 at FNG. A block of sintered SiC (457 mm x 457 mm, 711-mm thick, 470 kg total weight, 127 pieces) lent to ENEA by JAERI was used (fig. 3.14). The experiment was completed in 2001 in collaboration with TUD, FZK and the Josef Stefan Institute of Ljubljana [3.37]. Several nuclear quantities, including neutron and gamma-ray spectra, nuclear heating and activation rates, were measured at different penetration depths inside the block irradiated with 14-MeV neutrons (up to about 58 cm, corresponding to about 10 mean free paths for 14-MeV neutrons). The measurements were compared with the same quantities calculated using MCNP-4C and the deterministic 2-D code DORT with EFF-2.4, the new evaluated cross sections for Si-28 included in EFF-3.0, and the international FENDL-2 and Japanese JENDL-FF nuclear data libraries. Comparison shows that the European files and JENDL- FF well reproduce the measured quantities, within the total uncertainty, while FENDL-2 tends to significantly under-estimate the fast neutron flux, as shown in figure 3.15 where the C/E values are given for the neutron flux in the energy range E > 10 MeV. The experiment was also used to validate, through deterministic and Monte
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