3
PIRT PANEL EVALUATION OF CONTROL CIRCUITS
One mode of failure of control circuits is hot short-induced spurious operation the behavior of which may be dynamic and transient, changing its state throughout the course of a fire event. As the fire continues, multiple cables may fail at discrete times, and multiple circuit hot shorts may become part of the circuit failure. This pattern could entail the simultaneous behavior of two or more circuits experiencing concurrent spurious operations. With an adequately protracted fire exposure, conductor-to-conductor shorts are likely to transition into shorts to ground that eventually actuate an overcurrent protective device (i.e., blow fuse or trip breaker). During the first PIRT meeting, the PIRT panel developed ranking worksheets in MICROSOFT ®
EXCEL involving all the influencing parameters listed in Section 2.3.1 for all three circuit types (i.e., control, power, and instrument), with separate worksheets for AC- and DC-power supply (i.e., six worksheets ). The panel also agreed upon a scoring scheme commensurate with the objectives of this project and the figures-of-merit. Thereafter, the PIRT panel initially ranked the influencing parameters and then forwarded the worksheets to BNL for analysis. The analyses of these independent panel-member’s rankings using the “Number Scheme” revealed that each person had considerable difficulty in interpreting the ranking scheme. Also, most encountered problems in filling out these worksheets for power and instrumentation circuits due to lack of adequate test data, and the inapplicability of many parameters to power and instrumentation cabling. During the second PIRT meeting, the panel discussed these drawbacks, and agreed to abandon the PIRT process for power and instrument circuits. For control circuits, the PIRT panel applied the PIRT process. However, because of the extreme difficulty in analyzing the raw data in the test reports, they decided that both the CAROLFIRE and DESIREE-Fire test results must be analyzed and organized such as to allow a clearer interpretation of the effects of each influencing parameter. They also combined the results of the EPRI/NEI tests in the overall data analysis; this action is documented in NUREG-2128 (Ref. 20).
As shown in Figure 2-1, the PIRT panel followed Steps 7 through 9 to identify those control circuits that would be vulnerable to hot short-induced spurious operations. To identify the most significant influencing parameters, the members first highlighted all plausible influencing parameters and ranked them to assess their impact on hot short-induced spurious operation and duration. During meetings, the panel discussed each influencing parameter in great detail and selected some of the significant ones to include in the control circuit characterization. Furthermore, they derived various circuit characteristics (such as the power source variations, hot short modes of failure) from fire tests (in Step 4) and from their past experience on the subject. Finally, the panel identified those simplified control circuit electrical diagrams typically used in NPPs to control certain end devices, specifically, the MOV, SOV and circuit-breaker circuits. The final control circuit configurations they selected included six with a single contact (or break) and seven with double contacts (or breaks).
3.1 Impact Assessment of Influencing Parameters
The PIRT panel spent a considerable time debating the impact of the influencing parameters on hot short-induced spurious operation of control circuits. Tables 3-1 and 3-2, respectively, present their consensus ranking of all influencing parameters for AC- and DC-control circuits.
PIRT PANEL EVALUATION OF CONTROL CIRCUITS
3-2
The panel made the baseline assumption that AC control circuits are grounded, and DC control circuits are ungrounded (see footnote 7 on page 2-3). In addition to individual scores for the subgroups associated with each influencing parameter, an overall score (in bold) is shown for the impacts of this parameter on the likelihood of spurious operation and its duration.
The following is a summary explanation of the PIRT panel’s ranking given Tables 3-1 and 3-2. The column headings are described in Section 2.4. The following apply to both tables.
1. Conductor Count
Control cables typically contain 3, 7, or 11 conductors. Some trunk cables that are used for multiple control circuits contain as many as 37 or more conductors. While the number of
conductors can influence the likelihood of spurious operation, the PIRT panel did not consider it to be a primary influencing parameter because the wiring configuration has a higher influence on the likelihood of spurious operation. Therefore, the number of conductors was considered a secondary factor in the wiring configuration parameter because of its effects on some of the wiring configuration’s sub-categories (i.e., the number of sources, targets, and mitigating
conductors (e.g. grounded or return)). Hence, the PIRT panel determined that conductor count could be ranked “Low” as an influencing factor, but its overall importance ultimately would be reflected in the wiring-configuration parameter (see item 12 below).
2. Fire Exposure Condition
A range of data was reviewed for direct flame impingement, plume, and hot gas layer (HGL) exposures. All Penlight10 tests performed at SNL were subjected to radiant heating, a usage
that closely simulated the presumed HGL behavior of the fire. However, the Penlight temperature profiles were designed to assure cable damage and the conditions for thermal exposure were significantly higher than that would be expected for an actual HGL in an NPP. Often the test temperature of the Penlight shroud was high enough to exhibit thermal conditions similar to plume- or flame-fire zones.
One surrogate for the exposure conditions explored in the data analysis was time to damage. Since the initial cable damage and subsequent degradation of the conductor’s insulation occurs more rapidly for cable in the flame and more slowly for a cable in the HGL, the potential was evaluated of fire exposure conditions to affect the duration of hot short-induced spurious operation. Plots illustrating their effects are included in the data analysis NUREG-2128 (Ref. 20). The assumption is that the cable’s behavior under fire more closely simulates that in the HGL condition may be accurate for comparing the thermal conditions, but the failure-mode data in Figures 2-5 and 4-9 of Ref. 20 does not show this relation. Rather, the data indicates that modes of fire-induced cable failure do not follow this assumption of simulating the HGL condition, and specifically, that the likelihood is that the radiant-failure mode is similar to the conditions of exposure to the plume and flame conditions. The PIRT panel determined that the fire exposure conditions seemingly play some role in the likelihood of spurious operation
(ranked Medium), and a relatively strong role in the duration of spurious operation (ranked “High”).
10 Penlight are small-scale tests involving one to six lengths of cable exposed to grey-body radiant heating in a cylindrical exposure chamber in the CAROLFIRE and DESIREE-Fire test programs (Refs. 9 and 10) performed at SNL.
PIRT PANE L EVALUAT ION OF CONTROL CIRCUITS 3-3
Table 3-1. Panel Con
sen sus Ra nking of Infl uenci ng Paramete rs for AC Co ntrol Circui ts Identificatio n Influencing Parameter Parameter Applicability Researc h Ease Effect of Par ame ters on the Likelihood - INTRA -CA BLE HO T SHORT-I ND
UCED SPURIOUS OPERATION
Parameter Importan ce Sta te of K nowle dg e Effect of Par ame
ters on the Likelihood -
INTER-