[en] Anticipated transients without scram (ATWS) are anticipated operational occurrences followed by the failure of one reactor scram function. Current international practice requires that the capability of pressurized water reactors (PWRs) to cope with ATWS be demonstrated following a systematic evaluation of plants' defence in depth. Countries operating PWRs require design consideration of ATWS events on a deterministic basis. The regulatory requirements may concern either specific mitigating systems or acceptable plant performance during these events. The prevailing international practice for performing transient analysis of ATWS for licensing is the best estimate approach. Available transient analyses of ATWS events indicate that WWER reactors, like PWRs, have the tendency to shut themselves down if the inherent nuclear feedback is sufficiently negative. Various control and limitation functions of the WWER plants also provide a degree of defence against ATWS. However, for most WWER plants, complete and systematic ATWS analyses have yet to be submitted for rigorous review by the regulatory authorities and preventive or mitigative measures have not been established. In addition, it has also been recognized that plant behaviour in case of ATWS also relies on certain system functions (use of pressurizer safety valves for liquid discharge, availability of steam dump valve to both the condenser (BRU-K) and the atmosphere (BRU-A) for secondary side pressure control, and others) which have been identified as safety issues and need to be qualified for accident conditions. In all countries operating WWERs, the need for ATWS investigations is recognized and reflected in the safety improvement programmes. ATWS analysis for WWERs is not required for the licensing process in Bulgaria, the Czech Republic (with the exception of the Temelin nuclear power plant) and Russia. Design consideration of ATWS is required if expert assessments of probabilistic safety assessment (PSA) results show that the ATWS could contribute essentially to the probability of core damage or to off-site radioactive releases (Russia) or in cases where there is only one fast acting shutdown system for which the Regulatory Authority stipulates that failure must be assumed. The development of a national policy on the ATWS issue in line with international practices is recommended for national regulatory authorities. This policy should consider the systems necessary to mitigate consequences and/or requirements for acceptable plant performance in an ATWS event. This publication provides guidance on the performance of ATWS analyses of transients for licensing purposes, on the initiating events identified for those WWER reactors, and on the best estimate approach to the transient analysis of ATWS events. Further guidance is directed at reliability assessment of instrumentation and control (I and C) related to ATWS, including its common mode failure potential, and the qualification of systems and components necessary for operation under accident conditions to mitigate ATWS. While focusing on WWER-1000 reactors, this publication also provides guidance for ATWS events in WWER-440 reactors, taking into consideration the differences of this reactor type and its instrumentation and control