Cooling failure mode

All areas of the cooling water system where a specific form of damage is likely to be found are described. The corrosion or failure causes and mechanisms are also described. Especially important factors influencing the corrosion process are listed. Detailed descriptions of each failure mode are given, along with many common, and some not-so-common, case histories. Descriptions of closely related and similarly appearing damage mechanisms allow discrimination between failure modes and avoidance of common mistakes and misconceptions. 

Failure Modes and Effects Analysis. Failure modes and effects analysis (FMEA) is applied only to equipment. It is used to determine how equipment could fail, the effect of the failure, and the likelihood of failure. There are three steps in an FMEA (4) (7) define the purpose, objectives, and scope. Large processes are broken down into smaller systems such as feed or cooling. At first, the failures are only considered to affect the system. In a more general study, the effects on a plant-wide basis can be considered. (2) Define the problem and boundary conditions. This includes identifying the system to be studied, establishing the physical boundaries, and labeling the equipment with a unique identifier for use in the FMEA procedure. (3)... 

During cool-down, the center region experiences tensile stresses, but these are less detrimental according to failure modes observed in the field. 

The causes of equipment failures are not failure modes per se. For example, fouling of the tubes of a heat exchanger is not a failure, but it leads to the failure mode of insufficient cooling. 

In addition, it should be demonstrated analytically that the mechanical systems can withstand a single active failure including failure of any auxiliary electric power source and not prevent delivery of sufficient cooling water to maintain the plant in a safe shutdown condition. A technique suitable for this analysis is a Failure, Modes, and Effects Analysis (FMEA). IEEE Std. 353-1975, "Guide for General Principles of Reliability Analysis of Nuclear Power Generating Station Protection Systems," provides additional guidance on the preparation of FMEAs. 

The model development has been carried out by a test case that was a small chiller unit for water cooling purposely developed for experimental activity. The equipment consists of two circuits the refrigerant line and the water line and it is monitored by about ten sensors that include temperatures, pressures, levels and power supplies (on/off). The sensors signals are monitored by an acquisition card and transmitted to a pc for the intelUgent elaboration. The test bed has been developed to simulate about eight failure modes hut in this work we investigated only six of them. 

Shutdown Cooling System Failure Modes and Effects Analysis... 

SHUTDOWN COOLING SYSTEM FAILURE MODES AND EFFECTS ANALYSIS... 

There was an incident of failure of freeze seal in a reactor recirculation bypass 6" (line). The incident occurred after six months shutdown. Core cooling did not pose any problem in the Emergency Core Cooling recirculation mode. Procedures were revised and successful re-freezing maintenance could be carried out subsequently. 

The plant model includes eight different safety systems that are mostly four-redundant. The safety systems are divided into two separate subsystems Reactor Protection System (RPS) and Diverse Protection System (DPS), which are implemented on different automation hardware. The RPS safety systems are automatic depressurisation system (ADS), component cooling water system (CCW), emergency core cooling system (ECC), service water system (SWS) and residual heat removal system (RHR). The DPS safety systems are emergency feed water system (EFW), and main feed water system (MFW). In addition, the AC power system belongs to both RPS and DPS. The model describes the operation logic of the safety systems, the hardware equipment used to implement each system, and the associated failure modes for each piece of equipment. 

Consequently, a single failure could reduce the flow rate of water to the reactor coolant system, but it would not disable the passive core cooling function. Table 6.2.2-3 of Reference 6.1 presents a failure modes and effects analysis of the passive containment cooling. 

The challenges that remain are in actually proving that releases absolutely cannot occur (given it is hard to prove a negative and such data are scarce) and that all potential core damage states are either avoided and/or adequately cooled. The methods used include SA analysis codes structural failure mode analysis radionuclide transport in buildings and the environment historical geotechnical, hydraulic, and seismic response analysis and PRAs. 

A potential problem in the Reactor Core Isolation Cooling (RCIC) system circuitry of a particular BWR was identified. Within this particular RCIC control system, because of the design of the RCIC steam leak detection circuit, it is possible for a sneak circuit to occur and cause an unintended, nonrecoverable isolation of the RCIC pump in conjunction with a station blackout. There are at least three subtle design aspects which lead to the occurrence of this failure mode (1) the RCIC system contains an isolation circuit, (2) the isolation circuitry is deenergized given a loss of offsite power (i.e., the circuitry is not fed by a nonintemiptible, battery-backed vital AC power supply), and (3) the isolation circuit contains a seal-in circuit. 

Figure 6 indicates the adhesion in tension of epoxy-modified mortars to ordinary cement mortar before and after warm-cool cyclings. When the polymer-cement ratio increases to 30 %, the adhesion of the epoxy-modified mortars before warm-cool cycling is about twice that of unmodified mortar, and becomes nearly constant, depending on the failure mode. Like the... 

For the loss of cooling type accidents, the requirement is to maintain a cool-able geometry, as in LWRs. The limiting failure mode is expected to be oxidation of the cladding. The criterion of the cladding temperature is set at 1,260 C for stainless steels, taken from the criterion for LOCA of early US PWRs with stainless steel cladding [8]. The criterion for Ni-alloys is also set at 1,260 C because the oxidatimi behavior of Ni-alloys is expected to be similar to that of stainless steels. 

The heat rejection system was included to show the impact of the current heat rejection system (HRS) design on different system architectures. The reference HRS design uses four separate cooling loops (see Failure Modes, Section 7,7.2.1 below, and Section 9 for details). This design minimizes the effect of HRS failures on the one and two Brayton systems because a failure in the HRS does not necessarily cause the associated Brayton to be unusable. Failures in the HRS for three or four Brayton systems require switching to standby Brayton(s) to maintain full power capability. 

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