ATWS Analysis

As described in Sec. 6.7.2, eight transients are accompanied by a reactor scram. These transients make up the ATWS events of the Super LWR. The same analysis sequences as used in the abnormal transient analyses are applied with the exception of the reactor scram occurrence. 

Alternative actions for the ATWS mitigation are credited in the analyses of LWRs [27-39]. PWRs need the ATWS Mitigating System Actuation Circuitry (AMSAC) to initiate a turbine trip and the AFS function as alternative actions so as to satisfy the pressure limit [29]. Opening the ADS induces strong coolant flow 

Until the ADS is initiated, the reactor behavior is analyzed with SPRAT-DOWN. After that, the blowdown is analyzed with SPRAT-DOWN-DP. Since SPRAT-DOWN-DP does not distinguish between the hot and average channels, only the hot channel parameters are transferred. After initiating the ADS, the reactor behavior for all the ATWS events is similar to the behavior described in Fig. 6.7 [1] because the depressurization is an intense phenomenon that is not influenced by the condition before it. The ATWS events having relatively fast responses before initiating the ADS are important here. Representative results are shown below. 

Only the partial loss of reactor coolant flow is accompanied by a decrease in the main coolant flow rate before initiating the ADS. The ADS is actuated at 5 s by the ATWS signal which is reactor coolant pump trip and reactor power ATWS permissive for 5 s . The calculation results are shown in Fig. 6.48. A decrease in the flow rate leads to an increase in the coolant temperature due to the power and flow mismatch. The cladding temperature increases due to the coolant heat-up and a decrease in the heat transfer coefficient. The net reactivity and the reactor power decrease due to coolant density feedback. The increase in the cladding temperature is about 120°C, which is the highest value of all the ATWS events with the alternative action. 

The loss of turbine load is a t5 ical pressurization event The turbine bypass is not credited. The ADS is initiated at 5 s by the ATWS signal of the turbine cmitrol valve quickly closed and reactor power ATWS permissive for 5 s. The calculation results are shown in Fig. 6.49. The pressure increases due to the closure of the turbine control valves. As described in Sect. 6.7.1.3, the inherent characteristics of the Super LWR design make the reactivity insertion and the power increase very small. The peak power is only 104% of the initial value. When the SRVs open, the pressure begins to decrease. After initiating the ADS as the alternative action, the pressure, power, and cladding temperature decrease. The increase in the cladding temperature is about 50°C and the peak pressure is about 26.8 MPa. They are exactly the same as those obtained in the abnormal transient analysis with a reactor scram (see Sect. 6.7.1.3). 

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Incomplete or lacking ATWS analysis or their unavailability at plants would it make impossible to understand which primary safety fiinction(s) would be affected and which corrective measures need to be implemented to cope with ATWS. This issue affects control of accidents within the design basis (level 3 of protection of plant s defence in depth). 

ATWS analysis were carried out in a broad range for all credible anticipated operational occurrences in order to tune the functional design of a Diverse Protection System (DPS) which is being implemented at Temelin NPP (WWER-1000 NPP). The ATWS analyses and evaluations were performed in accordance with NUREG-0460 guidelines, using a best-estimate approach. 

Safety Transient and accident analysis at supercritical-and subcritrical pressure (SPRAT-F, SPRAT-DOWN), ATWS analysis (SPRAT-DOWN), LOCA analysis (SCRELA,SPRAT-DOWN-DP), Time-dependent subchannel analysis Start-up (sliding pressure and constant pressure)... 

Safety analysis of the Super LWR is described in ref. [121]. The SPRAT-DOWN code for the analysis of downward flowing water rods and the SPRAT-DOWN-DP code for depressurization in an LOCA were prepared. The LOCA analysis of the Super LWR was performed in combination with SPRAT-DOWN-DP and the reflooding module of SCRELA. ATWS analysis is also described in ref. [121]. The momentum equation is included in the SPRAT-DOWN code from the ATWS analysis. The design of the two-pass core of the Super LWR and the safety analysis at subcritical pressure during startup are described in ref. [122]. 

Y. Ishiwatari, Y. Oka and S. Koshizuka, ATWS Analysis of Supercritical Presstne Light Water Cooled Reactor, Proc. Global 2003, New Orleans, LA, November 16-20, 2003, 2335-2341 (2003)... 

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. 

Erdmann, R. C. et al., 1976, ATWS A Reappraisal Part If Evaluation of Societal Risks Due to Reactor Protection System Failure Vol IIBWR Risk Analysis EPRINP-265,. ugust. 

Kdly, J. E. et al 1976, ATWS, A Reappraisal Part II, Evaluation of Societal Risks due to Reactor Protection System Failure Vol.3 PWR Risk Analysis, EPRINP 265, August. 

Luckas, W. J. et al., A Human Reliability Analysis for the ATWS Accident Sequence at the Peach Bottom Atomic Power Station, BNL Technical Report A3272, May 1986. 

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). 

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 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. 

International practice considers the analysis of ATWS for a variety of initiating events such as loss of feedwater, loss of load, turbine trip, loss of condenser vacuum, loss of off-site power, closure of main steamline isolation valves, uncontrolled boron dilution, inadvertent control rod withdrawal, etc. ATWS analyses are performed in general by using best-estimate tools to determine the preventive (e.g. a diverse scram system) or mitigative measures (e.g. initiation of turbine trip and emergency feedwater supply) which need to be implemented for strengthening plants defence in depth. 

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). 

The economics of a closed fuel cycle within the already mentioned system with 20 LWRs and 3 PEACER parks with 12 PEACER reactors (PEACER system) was evaluated in comparison with the present-day fuel cycle of an advanced LWR (ALWR). A preliminary economic analysis has been conducted using cost figures suggested in the accelerator-driven transmutation of waste (ATW) roadmap the cost of pyro-processing within the PEACER system was conservatively assumed to be twice the value suggested in this roadmap. The overall fuel cycle cost of the PEACER system was preliminarily evaluated to be about 24% lower than that of the ALWR fuel cycle, as presented in Table XXIV-7. 

The safety analysis of the TMSR-SF has also drawn much attention. Three types of transient conditions including ULOF, UOC, and UTOP were examined on the TMSR-SF by an FHR safety analysis code named the FHR Safety Analysis Code (FSAC Xiao et al., 2014). The station blackout anticipated transient without scram (SBO-ATWS) accident was analyzed by the modified RELAP5/MOD 4.0 code with the responses of the passive residual heat removal (PRHR) system (Jiao et al., 2015). 

R. M. Harrington and S. A. Hodge, Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratory, ATWS at Browns Ferry Unit One - Accident Sequence Analysis, USNRC Repon, NUREG/CR-3470 (ORNL/TM-8902), July 1984. 

The SSC-K code was used for assessment of the inherent safety features in the KALIMER conceptual design. The SSC-K aims not only at extensive analysis capability and flexibility, but also at sufficiently fast running to simulate long transients in a reasonable amount of computer time. The code is capable of handling a wide range of transients, including normal operational transients, shutdown heat removal transients, and hypothetical ATWS events. The SSC-K code is currently used as the main tool for system transient analysis in the KALIMER development. 

Abnormal transients are defined as events that will lead to the situation in which the nuclear plant cannot maintain the normal operation due to an external disturbing factor that may occur during the life span of the nuclear plant under the operational conditions including single failure or malfunction of the devices or single operational errors by operators, and to the abnormal situation in which the nuclear plant is not planned to operate and that may occur with the same probability as the former. A set of abnormal transients and accidents as standard safety analysis of the current LWRs is studied for the Super LWR, including the loss of coolant accident (LOCA) and the anticipated transients without scram (ATWS) (see Chap. 6 for details). The requirements for the Super LWR are same as those of LWRs ... 

The most important (severest) ATWS events for the Super FR are the loss of flow type ones, the same as in the Super LWR. As an example, the loss of offsite power is analyzed here without a reactor scram. The analysis results without the alternative action of the ADS are shown in Fig. 7.116. Due to the small heat... 

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