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Divisione Nucleare Progetto Project PDS-XADS <strong>AnsaldoEnergia</strong> Identificativo Document no. XADS 20 TRIX 009 Rev. Rev. 1 Cl. ris. class Pagina Page 27 radiation heat absorption. In conclusion, the RVCS is substantially configured as a large tubular air heat exchanger. Heat transfer from the Reactor Vessel to the RVACS takes place through different heat transfer modes: 1. conduction through the Reactor Vessel walls; 2. conduction, natural convection and radiation in the gap between the Reactor Vessel and the Guard Vessel; 3. conduction through the Guard Vessel wall; 4. conduction, convection and radiation heat transfer in the stagnant air in the Reactor Cavity; 5. conduction through the ascending hot air pipe wall facing the Guard Vessel; 6. convection inside the ascending hot pipes. So the decay heat can be rejected to the ultimate heat sink via fuel assemblies → primary lead-bismuth coolant → main reactor vessel → Guard Vessel → Reactor Vessel Auxiliary Cooling System (RVACS) → external atmosphere. Also this flow path relies on natural circulation only, i.e. on heat transfer by natural mechanisms, consistent with the passivesafety philosophy, which is a principle of the XADS design. It relies on natural circulation of the primary lead-bismuth coolant inside the Primary Vessel, heat conduction through the Primary Vessel wall, heat convection and radiation between reactor vessel and guard vessel, heat conduction through the guard vessel, heat convection and radiation from guard vessel to the air pipes of the RVACS, heat conduction through the air pipes, natural convection pipe-side to the atmospheric air. The hot, exhaust air from the inlet header runs through four stacks, internal and concentric to the annular ducts, and arrives over the roof of the Reactor Building at about 35 m elevation, where it is discharged to the atmosphere. The internal and external annular ducts are thermally insulated with each other in order to optimize the overall heat transfer efficiency. The system performance (i.e. the heat removed by the system) as well as the comparison of the global efficiency among the various loops, is continuously monitored as the enthalpy change in the air flowing through the system. This is calculated from the air mass flow rate, humidity, and the temperature differential between inlet and outlet flows. One temperature element, installed on each air intake opening, is provided with a temperature alarm, thus revealing the abnormal ingress of hot air from the environment, such as in case of smoke due to a fire outside of the Reactor Building. In addition the concrete temperature of the bottom of the Reactor Cavity is continuously monitored by means of temperature sensors. DNU 020/1
Divisione Nucleare Progetto Project PDS-XADS <strong>AnsaldoEnergia</strong> Identificativo Document no. XADS 20 TRIX 009 Rev. Rev. 1 Cl. ris. class Pagina Page 28 Each stack is provided with radioactivity measures installed at the exit of each of the four RVACS stacks. These instruments provide information for the potential for being breached or the actual breach of the containment and for assessing the release of the radioactive materials (type C and E measurements according to the R.G. 1. 97). 3.2.8 Accelerator The accelerator is basically a high intensity proton machine, delivering beam on a spallation target to provide an externa neutron flux source for a sub-critical core. Two accelerator options available: the cyclotron and the linear accelerator Linac). Although the cyclotron could probably fulfill the demonstrator needs, the linac option has been selected for the XADS. The main reason is that a power cyclotron will be closely approaching its technological limits. For future industrial power ADS systems, it will therefore be unavoidable to use linacs. Consequently, it makes more sense to favor the demonstration of the linac accelerator that offers much more capability for future upgrade and extension to higher energies and to higher current beams. Any high intensity linear proton accelerator can be divided in three main parts. The one is the injector including the source (usually delivering a continuous current) followed bunching structure like a RadioFrequency Quadrupole (RFQ). The beam comes out from injector at energies between 2 to 10 MeV. The second part, labeled here "Intermediate Part", brings up the beam to energies in the vicinity of 100 MeV. At that energy, although the proton is not yet fully relativistic (β = v/c = 0.428), its speed is high enough to use elliptical cavities. The final (and most efficient) part of the linac using these elliptical cavities is the high-energy section. It starts from around 100 MeV up to any desired final energy. Both the injector section and the high-energy section are quite straightforward. The source is usually an Electron Cyclotron Resonance (ECR) ion source based on the experience accumulated in many laboratories around the world. ECR sources have demonstrated delivering reliable proton beams exceeding 100 mA, an order of magnitude higher than the requirements needed for the XADS. The source is followed by an RFQ that has two main functions: It prepares the particles in bunches separated by the RF period (the beam is now microscopically pulsed1 in burst of particles flowing at the rate of the RFQ frequency) and it accelerates the beam to an energy of a few MeV while maintaining a strong confinement. In a similar manner, there is a general agreement that the high-energy section should be made of superconducting elliptical cavities. These have been demonstrated to be extremely effective (accelerating fields exceeding 25 MV/m have been experimentally obtained in many laboratories) and a lot of experience and knowledge has been accumulated. Moreover, large size machines using Superconducting RF Cavities (SCRF) have been constructed or are currently under construction (like the Spallation Neutron Source, SNS at Oak Ridge, USA) that may bring some confidence in the achievable performance for a XADS type accelerator. DNU 020/1
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