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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 45 The event [1.1.3] “CORE COMPACTION” assumes that all the Fuel Assemblies, in the most unlike event, can be packed together with the complete closure of the clearance between each Fuel Assembly wrapper. 3.4.2 Decrease of fuel assembly heat removal These subsection addresses events that affect heat removal from a single fuel assembly and, as such may have a significant impact on the affected assembly while the effect on the core and on the primary system as negligible. Obviously, challenges to the fuel cladding integrity may derive also from other events discussed in next sections 3.5. The decrease of heat removal from a single fuel assembly can be attributed to the anomalies of the eutectic flowrate caused by fuel element failure. The MLD branch detailing the event “DECREASE of FUEL ASSEMBLY HEAT REMOVAL” is shown in the following Fig. 3.4-2; it identifies scenarios that represent level 3 of the MLD. Figure 3.4-2 MLD development starting from " DECREASE of FUEL ASSEMBLY HEAT REMOVAL " [1.2] DECREASE of FUEL ASSEMBLY HEAT REMOVAL level 2 [1.2.1] FUEL ASSEMBLY PARTIAL BLOCKAGE [1.2.2] FUEL ASSEMBLY MECHANICAL LOCK FAILURE level 3 level 2 events description The "DECREASE of THE FUEL ASSEMBLY HEAT REMOVAL", event [1.2], can be originated by the following causes leading to an insufficient refrigeration of a single fuel assembly: • FUEL ASSEMBLY PARTIAL BLOCKAGE; • FUEL ASSEMBLY MECHANICAL LOCK FAILURE. 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 46 level 3 events description The events included in this level then represent some of the lowest level events of the tree having as Top Event: “FUEL CHALLENGES”. In particular event [1.2.1] “FUEL ASSEMBLY PARTIAL BLOCKAGE” assumes a partial obstruction of the fuel assembly cross sectional flow area, the most likely location being the inlet region. The obstruction causes a coolant flow reduction and then an increase in fuel and cladding temperature. The event [1.2.2] “FUEL ASSEMBLY MECHANICAL LOCK FAILURE” assumes the failure of the mechanical lock at the fuel assembly foot. Due to this occurrence the fuel assembly moves upward, due to the buoyancy, till it hits and stops against the above core structure plate. This relocation causes a modification of the hydraulic characteristics of the affected fuel assembly (possibly including some degree of obstruction of both the inlet and outlet holes located in the fuel assembly foot and head) which may reduce the fuel assembly coolant flowrate. It is noted that there may also be a reduction in the power generated in the affected fuel assembly, since a fraction of the active region moves upwards to a region with significantly lower neutron flux. 3.5 REACTOR COOLANT SYSTEM AND TARGET UNIT COOLANT SYSTEM CHALLENGES The second path depicted in Fig. 3.3-1 aims to identify those phenomenologies that have the potential to affect the integrity of the second physical barrier to the release of radioactive fission products to the environment, namely the Reactor Coolant System and the Target Unit Coolant System. As mentioned in previous section 3.4.2, it should be kept in mind that some of the events challenging the Reactor Coolant System or the Target Unit Coolant System integrity also challenge the fuel cladding integrity (normally of several or all the fuel assemblies) and hence the postulated events described in the following should be looked at from both points of view. In particular the "REACTOR COOLANT SYSTEM CHALLENGES", which are a level 1 of the MLD of Fig. 3.3-1, can be originated from the following events (level 2 out the MLD): • REACTOR COOLANT SYSTEM PRESSURE and TEMPERATURE VARIATION [2.1]. • TARGET UNIT COOLANT SYSTEM PRESSURE and TEMPERATURE VARIATION [2.2] Event [2.1] refers to anomalies in the Reactor Coolant System pressure and temperature variations originate from malfunctions or failures of systems and components normally devoted to (directly or indirectly) control the RCS pressure or temperature. DNU 020/1
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