Anticipated transients without scram

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. 

Control of low-pressure injection during an anticipated transient without scram (ATV i Dry well steel shell to prevent melt-through in a Mark 1 containment... 

The Canadian licensing philosophy requires that each accident, together with failure of each safety system in turn, be assessed (and specified dose limits met) as part of the design and licensing process. In response, designers have provided CANDUs with two independent dedicated shutdown systems, and the likelihood of anticipated transients without scram is negligible. 

Anticipated Transients without Scram for Light Water Reactors, Vol. 1-3, December 1978. Haasl, D. F, et al., Fault Tree Handbook, January 1981. 

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. 

Although very good reliability records exist for scram excitation, some failures of the gravity-driven control rod insertion have been recognized. The failures occurred for the different reasons in particular, the cases of insertion speed reduction and incomplete insertion due to fuel assembly deformation have been reported during last ten years (see for example [3]). Besides, some failure modes may be considered which could prevent all the control rods to insert, and it was the basis for designers to analyze Anticipated Transient Without Scram events. 

Anticipated transients without scram are the most significant group of BDBAs. These include failures of the reactor protection system in addition to the following anticipated operational occurrences ... 

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... 

The diversified means of control rod actuation and insertion (together with a generous reactor pressure relief capacity) in combination with a capability of rapid reduction in the recirculation flow rate (recirc. pump run-back) has led to regulatory acceptance of the system as being a sufficient ATWS (anticipated transient without scram) measure the CRD design is "ATWS proof. 

Reactor shutdown (ATWS Anticipated Transients Without Scram)... 

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. 

LOCA (Interfacing) e.g. Primary Heat Exchanger Tube Rupture ATWS Anticipated Transients Without Scram Primary Transients,... 

LOCA (Secondary ) Secondary Pipe Rupture (water or steam), LOCA (Interfacing) eg SGTR Steam Generator Tube Rupture, ATWS Anticipated Transients Without Scram,... 

LOCA (primary) Loss of Primary Coolant Accident LOCA (secondary) Secondary Pipe Rupture (Water) ATWS Anticipated Transients Without Scram Primary Transients (control rod assembly withdrawal) Secondary Transients (one or two loops failure)... 

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