Unprotected transient overpower

Inadvertent withdrawal of all control rods without automatic scram or manual actuation (unprotected transient overpower)... 

This scenario is typically called an unprotected transient overpower (UTOP). 

The unprotected transient overpower (UTOP) events were analyzed to estimate the allowable external reactivity insertion. In the 4S-LMR, slow upward movement of the reflector is used to compensate the bum-up reactivity loss. An UTOP is then the event initiated by an inadvertent reflector lifting without scram. The analytical assumptions used for the reference UTOP case are summarized below ... 

The analytical results for unprotected loss of flow and unprotected transient overpower as design basis accidents are presented below beyond design basis accidents were not considered at this stage. However, risk of a core disruptive accident due to local blockage of the core is expected to be low since there are no wrapper tubes in the RAPID core. 

Unprotected transient overpower (UTOP) accidents - a transient overpower accident with failure to scram. 

HTGRs with pebble bed fuel and continuous refuelling (PBMR, HTR-PM) provide a relatively small reactivity margin for fuel bum-up (Level 1 in Table 4), resulting in the reactor capacity to survive, without core damage, an unprotected transient overpower caused by the ejection of a control rod. 

In all designs of liquid metal cooled SMRs, the fundamental safety functions related to control of accidents within the design basis (Level 3 in Table 4) are achieved by the redundant and diverse combinations of active and passive systems, with a strong role of the inherent safety features in reactor self-control. All design descriptions provide the results of safety analyses indicating that several unprotected transients, such as the unprotected transient overpower, the unprotected loss of flow, the unprotected loss of heat sink, or the unprotected total NPP blackout could be safely controlled by the appropriate combinations of reactivity effects only. In many cases, such accidents are included in the design basis of liquid metal cooled SMRs. 

Establish the limits securing the core integrity and ensure that these limits will not be exceeded during the unprotected loss-of-flow (ULOF), unprotected loss-of-heat-sink (ULOHS) and unprotected transient overpower (UTOP) scenarios. 

In a protected loss of flow, moving the reflector to a lower position helps accomplish the reactor shutdown and the decay heat is then removed passively. As it was already mentioned, an anticipated transient without scram (ATWS) for the 4S-LMR was defined as an accident in which the active reactor shutdown system does not work. Therefore, the unprotected loss of flow (ULOF) and transient overpower (UTOP) are categorized as ATWS. 

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