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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 43 3.4 FUEL CLADDING CHALLENGES The first path depicted in Fig. 3.3-1 aims to identify those phenomenologies that have the potential to affect the integrity of the first physical barrier to the release of the radioactive fission products to the environment, namely the fuel cladding. In particular the "FUEL CLADDING CHALLENGES", which are a level 1 of MLD, can be originated by the following events (level 2 of the MLD): • POWER ANOMALIES [1.1]; • DECREASE OF FUEL ASSEMBLY HEAT REMOVAL [1.2]. Event [1.1] “POWER ANOMALIES“ refer to anomalies in the power generation leading, in particular, to an increase in the power generated in the fuel rods. The MLD for these events will be shown in next section 3.4.1. Events [1.2] “DECREASE of FUEL ASSEMBLY HEAT REMOVAL” refer to anomalies in the heat removal process from the fuel cladding that correspond to a decrease of the heat extracted from the fuel rod. The MLD for these events will be shown in next section 3.4.2. 3.4.1 Power Anomalies In the XADS the power generated in the fission core is strictly related to the proton beam current. In particular an almost linear dependence of the generated power from the proton current intensity exists; thus increasing the proton beam current increases fission power (while, similarly, decreasing the proton beam current decreases fission power). The proton current intensity is anticipated to vary along the fuel cycle to compensate the fuel burn up. Thus “POWER ANOMALIES” in the XADS can be mainly associated to anomalies of the proton beam current, rather than to reactivity insertion effects which can be considered “weak”. The MLD branches detail the event «POWER ANOMALIES», reported in Fig. 3.4-1, identify three scenarios that have the potential to challenge the fuel clad integrity. They represent level 3 of the MLD. level 2 events description As the MLD shows, the event [1.1] "POWER ANOMALIES” can result from: 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 44 Figure 3.4-1 MLD development starting from "POWER ANOMALIES" [1.1] POWER ANOMALIES level 2 [1.1.1] UNCONTROLLED PROTON BEAM CURRENT INCREASE [1.1.2] STARTUP of the PROTON BEAM with COLD REACTOR [1.1.3] CORE COMPACTION level 3 level 3 events description • UNCONTROLLED PROTON BEAM CURRENT INCREASE • STARTUP of the PROTON BEAM with COLD REACTOR • CORE COMPACTION These accidents then represent some of the lowest level events of the tree having as Top Event: “FUEL CHALLENGES”. The event [1.1.1] “UNCONTROLLED PROTON BEAM CURRENT INCREASE” assumes a malfunction that leads the proton beam current to increase to the maximum value pertaining to the EOL condition of the core (namely 6mA) or to the maximum value allowed by suitable interlocks (if any). Note that, with the plant at nominal power and at BOL, the nominal current intensity is about 3 mA. The proton beam current should increase from 3mA to 6mA, the generated power would almost double then causing a significant increase in fuel and clad temperature (also as a consequence of the increased coolant temperature). The list of the Accelerator Complex System malfunctions that lead to this phenomenology are presented in Ref. [2]. The event [1.1.2] “STARTUP of the PROTON BEAM with COLD REACTOR” assumes a malfunction that leads to an inadvertent start-up of the accelerator system when the XADS primary system is not ready to accept and remove fission power. DNU 020/1
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