Anticipated transient without scram ATWS

A long evolving use of PSA was for Anticipated Transients without Scram (ATWS) which extended over 15 years to culminate in NUREG-0460 which was upset by the Salem failure-to-scram incident and the subsequent SECY Letter 83-28. Other special studies have been (a) value-impact analysis (VIA.) studies of alternative containment concepts (e.g., vented containment, NUREG/CR-0165), (b) auxiliary feedwater studies, (c) analysis of DC power requirements, (d) station blackout (NUREG/CR-3220), and (e) precursors to potential core-damage accident.s (NUREG/CR-2497), to name a few of the NRC sponsored studies. 

The presence of a signiflcant neutron flux together with a fast shutdown actuation signal (Anticipated Transient Without Scram (ATWS) case, that is a transient with the failure of the scram to operate). 

The version of the program described here does not foresee the study of Anticipated Transients Without Scram (ATWS) or the calculation of the pressure in a water tank where the primary liquid from the depressurization system is discharged. For additional calculations of this type, the following notes and formulae may be useful. 

KALIMER has enhanced safety features with the use of metallic fuel, Self-Actuated Shutdown System (SASS), Gas Expansion Module (GEM) in the core, and Passive Safety Decay Heat Removal System (PSDRS). Utilization of these enhanced safety features eliminates the need for diverse and redundant engineered safety systems and KALIMER accommodates unprotected anticipated transients without scram (ATWS) events without operator action, and without the support of active shutdown, shutdown heat removal, or any automatic system without damage to the plant and without jeopardizing public safety. 

It should be noted that transients such as PTS in PWRs or anticipated transient without scram (ATWS) in BWRs are considered to be beyond the... 

Anticipated Transients Without Scram (ATWS) are not included in the design basis of CANDU plants, because of the extremely low frequency expected for such events. CANDU plants have three systems with independent and diverse capability for automatic reactor shutdown. 

Code Of Federal Regulations, Title 10, Part 50, Section 62, "Requirements for Reduction of Risk From Anticipated Transients Without Scram (ATWS) Events for Light-Water-Cooled Nuclear Power Plants , Office of the Federal Register National Archives and Records Administration, June 26, 1984. 

RETRAN (REactor TRansient ANalysis) is a best-estimate transient thermal-hydraulic analysis computer program (sponsored by EPRI) designed to provide analysis capabilities for BWR and PWR transients, small-break LOCAs, balance-of-plant modeling, and anticipated transients without scram (ATWS). 

In the case of fast neutron spectrum reactors, the efficacy of inherent and passive safety features to anticipated transients without scram (ATWS) has been well established by testing in the EBR-II, FFTF, and Phoenix reactors [5 and 6], So too has passive decay heat removal. The severe accident hazard of reactivity addition upon core rearrangement in small fast reactors is addressed by a range of approaches including ... 

For the safety analysis of the 4S, beyond design basis events (BDBEs) have been selected and identified in a similar manner. The criteria for anticipated transients without scram (ATWS) and accidents without scram (AWS) are as follows ... 

The design basis events (DBE) considered in the 4S-LMR are similar to those analyzed for the previous designs of fast reactors however, the beyond design basis events (BDBE) considered in the 4S-LMR are those in which the reflector does not drop down for any reason, categorized as anticipated transients without scram (ATWS). 

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. 

The STAR-H2 reactor has a central safety rod and an active scram circuit. It also will have a decay heat removal path through the flibe loop. However, in order to achieve the levels of safety required for deployments of tens of thousands of STAR-H2 plants worldwide - sited near cities, it will be essential to avoid core damage even in the absence of a scram. The technology to achieve passive safety response to anticipated transients without scram (ATWS) events was well developed for the IFR concept [XXIV-15] and was famously demonstrated in tests conducted at the EBR-II reactor [XXIV-16]. 

First off, the balance of plant (BOP) would have no nuclear safety function. Moreover, the STAR-H2 heat source reactor is being designed not only for passive safety response to Anticipated transients without scram (ATWS) initiators but also for passive load follow. The only information flow path from the BOP to the reactor would be the fused salt intermediate heat transport loop, which will convey the BOP heat request to the reactor by means of its flow rate and return temperature (see Fig. XXIV-3). In this way, the reactor could passively adjust its power to match heat demand while remaining in a safe operating regime. The safety implication of passive load follow is that the reactor would safety respond to all possible combinations and timing of ATWS initiators taken more than one at a time it would also safety respond to all conceivable human errors of the maintenance crew and the operator. In summary, all faults exterior to the reactor vessel might be safely accommodated on the basis of passive thermo-structural feedbacks. 

The completion of PSA analysis for a large number of plants has allowed a more comprehensive and systematic assessment of the safety of the plants. Systems and situations to vdiich not much attention was paid before, are now considered relevant to safety (i.e. analysis of accidents in modes other than full power, anticipated transients without scram (ATWS), station blackout, spent fuel systems, specific configuration situations, maintenance rule, external hazards). Issues with an analysis source include GL 4, SS 8, CS 3, AA 4, AA 5, AA 6, AA 7, MA 11, EP 3, TR 3, FS 2. 

Installation of overpressure protection by equipment qualification, e.g. valves designed for anticipated transient without scram (ATWS) situations, coolant pressure control logic with separated limitation and control, addition isolation signal for the pressurizer relief valves with improved position indication ... 

Anticipated Transients without Scram (ATWS) are defined as accidents initiated by anticipated transients, which are assumed to proceed without scram. If the automatic reactor trip fails during these transients, it could have adverse effects on the integrity of physical barriers in a reactor. 

At the Ulchin NPP Units 3 and 4, the Diverse Protection System (DPS) provides a diverse method to trip the reactor to safety concerns relative to Anticipated Transient Without Scram (ATWS) (10CFR50.62) issues. 

The application ofthe N18.2 checklist has been reviewed against the APIOOO design and PRA, and been appropriately updated to reflect the plant s specific design features. On this basis, while potential Anticipated Transient Without Scram (ATWS) faults are included in ANSI N 18.2, for the APIOOO no causes for these have been identified within the Design Basis (i.e. the initiating event frequency is less than 10-5 per reactor year). These faults are addressed via PRA and Severe Accident Analysis (as described in Section 5.4 and 5.5 of this chapter). This issue is discussed in sub-section 4.4.1.3 ofthe APIOOO Fault Schedule (Reference 5.1). 

Analyses of reactor operational transients, anticipated-transient-without scram (ATWS) events, and local faults to establish margins of safety for metal-fueled IFRs Out-of-reactor tests on both unirradiated and irradiated fuel to establish key fuel behaviour data under upset conditions ... 

For reactor shutdown, two independent RSSs (primary and backup) are installed. In addition to the two independent systems, an additional passive shut-down system using a Curie point-type SASS is adopted. The SASS, which is schematically illustrated in Fig. 11.4, is a device that provides passive shut-down capability in the case of anticipate transient without scram (ATWS) such as unprotected loss of flow, unprotected transient over power, and unprotected loss of heat sink (LOHS). When the coolant temperature increases in ATWS, the SASS passively detaches control rods... 

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