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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 21 The heat generated by the spallation reactions is removed by natural convection. The LBE recirculates from the heat source to the heat exchanger located at a higher level; an arrangement typical of natural-circulation cooling circuits. A stable, properly directed natural circulation of the target LBE is possible also when the accelerator is off, owing to the crosswise arranged flow path from inner to outer annulus and viceversa, provided at the bottom of the Target Unit and illustrated by the arrows in Fig. 3.2- 7. With the reactor in hot shutdown and the Target Unit cooling circuit in operation, the primary coolant at 300 °C, as a distributed heat source, heats up the LBE in the outer annulus. The cold flow in the downcomer is diverted by baffles from the inner annulus to the outer annulus The LBE of the Target, in turn, by mean of intermediate heat exchanger, placed in the upper part of the Target, transfer the heat to a cooling circuit filled with organic diathermic fluid which is back-cooled by water. The cold flow in the downcomer is diverted by baffles from the inner annulus to the outer annulus in the bottom part of the Target Unit. With the onset of natural circulation, effective cooling of the window takes place from the very beginning of the spallation reactions. The use of a diathermic fluid gives higher flexibility in the choice of the thermal cycle of the Target LBE and allows remaining below 500 °C at the hottest spot of the window. Several window materials are currently tested in Europe in order to assess their suitability to withstand the severe environment conditions and lifetime expectation. The corrosion protection LBE-side of the window by means of controlled oxygen activity in the LBE is also given attention. The Windowless Target Unit In the Windowless Target Unit (Fig. 3.2-7) the proton beam impinges directly on the free surface of the liquid LBE target. In this case a natural circulation pattern in the cooling circuit is no longer possible, because the heat source near the free surface of the LBE is at a higher level due to the vacuum in the proton beam pipe. Thus the hotter LBE must be driven downwards to the heat exchanger by some means, in this case by two mechanical pumps providing each a half of the total head required. A stream of primary LBE is diverted from the cold plenum to the heat exchanger to serve as a cooling medium. In the Windowless Target Unit no structural material is exposed to the direct proton irradiation. This option has the advantage of overcoming issues related to material structural resistance, but presents issues related to the proton beam impact area, flow stability and evaporation of LBE. The earlier axisymmetrical design of the free surface has been disregarded because tests in water have substantiated the risk of stagnation about the coalescence zone so that an alternative target design will be tested presently. In particular a full-scale test section is 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 22 planned to be constructed and used for a test campaign in the CIRCE facility with the following main goals: • Study of the main thermal-hydraulic phenomena • Geometrical optimization of the target region • Definition of the main parameters range to prevent zones of Pb-Bi stagnation in the target region • Definition of the requirements of the Pb-Bi flowrate control system • Definition of the requirements of the pipe vacuum control system 3.2.5 Primary Cover Gas System A simplified sketch of the Primary Cover Gas System is shown in Figure 3.2-8. The Primary Cover Gas System consists of a loop through which a gas stream circulates coming from the Reactor Vessel cover gas space and, after processing, is returned to the Reactor Vessel. The cover gas, mainly made of Argon, is taken through a collector who penetrates the Reactor Roof: the 4" piping transports the gas from the cover gas volume of the Reactor Coolant System. Downstream the Reactor Coolant Boundary isolation valve, the hot gas is processed, shell side, through a High Temperature Gas Cooler. This cooler (the coolant, tube side, is diathermic oil) lowers the temperature of the cover gas stream from 400 °C to about 125 °C, thus permitting to condense most of the impurities carried over by the gas flow, like Polonium aerosol, while preventing the formation of dusts which could remain suspended within the main gas stream. The impurities are then collected within a leaktight trap located just below the High Temperature Gas Cooler. This trap is cooled (using nuclear component cooling water) due to the decay heat generated by these impurities. The gas then enters a second quencher, the Low Temperature Gas Cooler, in which the cooling fluid is nuclear component cooling water. This heat exchanger lowers the gas temperature down to about 38 °C. Scope of this heat exchanger is to reduce the temperature at which the gas stream has to further be processed (for example, the compressors suction temperature). A particle filter is provided downstream of the two heat exchangers, to eventually retain condensate droplets. The gas is then sucked by two 100% capacity, redundant reciprocating compressors, which provide the gas with the driving force for being pumped back to the Reactor Vessel. The compressors should be two-stage type, with inter-cooler fed by component cooling water. The sucked gas is slightly under vacuum, because of the pressure drops through the process. A side effect of the compression is to increase the Argon temperature to about 200°C. DNU 020/1
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