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The following discussions address the PIRT panel’s evaluation process of the three circuit types: control, power, and instrument circuits. In addition, several ancillary circuit-related issues or concerns associated with fire-induced safe-shutdown analyses are identified by the PIRT panel for discussion.

2.3.2.1

Typical control cables used in NPPs have a voltage rating of 600 V for 120-125V range circuits. Primarily, multi-conductor cables with 3, 7, or 11 conductor configurations and 16-10 American Wire Gauge (AWG) conductor sizes are typically used in the control circuits. These cables are available armored or unarmored. Armored cable has a metallic outer sheath (or armor) made from interlocked aluminum or steel; however, unarmored control cables are more common in NPPs.

The PIRT panel used the scoring sheets to rank the influencing parameters on hot short- induced spurious operations. The PIRT panel also defined the following columns in the

MICROSOFT ® _{EXCEL scoring sheets for the likelihood of hot short-induced spurious operation}

for both intra-cable and inter-cable shorts, and its duration effect, given cable damage due to a fire:

• Effect of Parameters on the Likelihood – Intra-Cable Hot Short-Induced Spurious Operation • Effect of Parameters on the Likelihood – Inter-Cable Hot Short-Induced Spurious Operation • Effect of Parameters on the Likelihood – Inter-Cable Ground Fault Equivalent Hot Short-

Induced Spurious Operation

• Effect of Parameters on the Duration – Hot Short-Induced Spurious Operation

The hot shorts of concern are those energized source conductors coming into contact with a separate target conductor, other than neutral or ground, with the potential to provide voltage and/or current to a device such as a relay or coil. This includes an energized source conductor coming into contact with a spare conductor. The only hot short of interest is one of sufficient quality (with sufficient duration and energy) to actuate the end device.

The spurious operations of concern are those that cause an inadvertent energization of a device wherein an operational mode of the circuit is initiated. In applying their quantification method, the PRA scenario defines one or more specific spurious operation devices of interest. In the context of interpreting fire-test data, the device of interest only includes the end devices subjected to the test, which were

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• motor starter contactors • breaker close and trip coils.

The PIRT panel’s evaluation did not include the following modes as a spurious operation: • energization of spare conductors

• energization of indicating lights (burden resistors simulating light bulbs).

For the “Effect of Parameters on the Likelihood – Intra-Cable Hot Short-Induced Spurious Operation” and “Effect of Parameters on the Likelihood – Inter-Cable Hot Short-Induced Spurious Operation,” the panel made the following assumptions in ranking them:

• Fire damage has occurred, meaning in this context, that the insulation of the cable conductor was damaged to the point of functional failure.

• The hot short has occurred as per the definition above and may or may not be associated with the “device of concern”:

- solenoids

- motor starter contactors - breaker close and trip coils.

• The duration of a hot short between two conductors in a circuit is long enough to cause this device to operate spuriously.

• The duration of a spurious operation is long enough to cause change of any sort in the state of the device.

For the “Effect of Parameters on the Likelihood – Inter-Cable Ground Fault Equivalent Hot Short-Induced Spurious Operation,” the following assumptions were made for panel’s ranking: • Fire damage has occurred, i.e., insulation of the cable conductor was damaged to the point

of functional failure.

• The hot short has occurred as defined above and may or may not be associated with the “device of concern”:

- solenoids

- motor starter contactors - breaker close and trip coils.

• The duration of a hot short through a ground plane in a circuit persists long enough to cause a spurious operation of the device of concern.

• The duration of a spurious operation is long enough to cause any change in the state of the device of concern.

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For the Effect of Parameters on the Duration – Hot Short-Induced Spurious Operation,” the following assumptions were made for PIRT panel’s ranking:

• Fire damage has occurred, i.e. the cable conductor’s insulation has been damaged to the point of functional failure.

• The duration of hot short-induced spurious operation lasts long enough to cause any change in the state of the device of concern.

Cable tests indicate that spurious operations have a relatively high likelihood after fire-induced damage to a control cable. Several influencing factors were noted that could impact this behavior. Existing evidence implies that multiple concurrent spurious operations are possible and potentially likely after fire damage to multiple control cables that are susceptible to hot shorts. The PIRT panel developed scoring sheets that described the effect of each influencing parameter on the hot short-induced spurious operation, and its duration for both intra-cable and inter-cable configurations.

Using the collective expertise of the PIRT panel in analyzing fire-induced circuit failures and their level of knowledge, together with the results of industry and NRC cable-fire tests, a consensus score for each of the items was developed. This consensus was followed by an individual scoring by each of the panel members who gave comments or different views should a member’s view differ from that of the majority opinion. Specifically, each panel member evaluated and provided on MICROSOFT ® _{EXCEL scoring sheets with their independent scores}

for each influencing parameter for hot short-induced spurious operations, and their duration. If an individual score was very different from the consensus, the PIRT panel members offered comments explaining their opinion on the subject area.

2.3.2.2

Typical safety-related power cables used in NPPs are applied at nominal operating voltages from 120 V to 4.16 kV. These cables are single-conductor or multi-conductor (Triplex) cable with three spirals wound individually insulated conductors and may include an uninsulated grounded drain conductor. Each conductor’s size is 12 AWG or larger.

The most likely types of power cable faults (shorts to ground) typically lead to a loss of primary motive power to plant equipment or devices as a result of overcurrent protective device

actuation. Continuously operated components such as pumps, fans, and motors will stop and/or be unable to start. Intermittent operating devices, such as MOVs, would be unable to move or would cease moving if they were moving when the cable failed. Devices that require continuous power to maintain position, such as a solenoid operated valve, would revert to the de-energized state or maintain the de-energized state if not energized at the time of fault. Loss of primary motive power could result from several types of power cable failure modes, such as:

• Single or multiple phase-to-ground short circuits • Phase-to-phase or three-phase short circuits.

In each case, the cable failures would, for a properly designed power system, lead actuation of circuit overcurrent protective features (e.g., breakers and/or fuses opening). Given the many

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ways that power cable failures might engender an open circuit fault, the loss of motive power will be the predominant fault mode subsequent to the fire-induced failure of power cables. Finally, the PIRT panel decided not to include the power circuits in their evaluation process similar to the control circuits. Instead, they considered the following two power circuit fault modes (as described in Section 4):

• Three-phase proper-polarity hot short (ac) , and

• Multiple proper-polarity hot shorts leading to spurious operation of DC motors.

2.3.2.3

Instrument circuits (also known as instrumentation and control circuits) can typically use single twisted shielded pair conductor cables or much larger multi-conductor cables consisting of 50 or more conductors. Each instrument conductor typically is size 16 AWG or smaller. These cables frequently enclose several “twisted/shielded pairs” of conductors contained within a protective outer jacket. The shielding blocks external sources of electrical “noise” generated by other plant equipment.

Instrumentation circuits are typically low-voltage (0-5 volts) or low-current loops (typically 4-20 milliamps) connecting various types of sensors to remote monitoring equipment (e.g., control room indicators) and actuation devices (e.g., interlocks and trip units) that automatically control plant equipment.

The PIRT panel discussed instrument circuits in detail. However, due to a lack of fire test data, the parameters influencing hot short-induced spurious operations could not be identified and ranked in the same manner as for the control circuits. Therefore, the PIRT panel recommended undertaking more research and testing in this area.

Based on some limited earlier cable-failure testing of instrument circuits at SNL (Ref. 21), the following circuit failure characteristics were noted:

• Instrument cables failed earlier than co-located control cables during the tests • TP cable failed early and the instrument signal was lost abruptly

• TS cables experienced degradation and subsequent failure over a more extended period than did other cables, typically several minutes

• The behavior of an instrument circuit with specific degradation is predictable based on a simple circuit analysis.

The loss of instrument readings, and false or corrupted signals, may cause a loss of function for the system and may or may not have been identified as a part of the plant shutdown model in the Fire PRA. Therefore, the PIRT panel recommends further testing to identify the types of instrument systems that may be susceptible to fire-induced spurious operations. Section 5 reports the PIRT panel’s discussions of these issues and their recommendations for testing several analog current loop systems.

2.3.2.4

The electrical expert PIRT panel evaluated several known issues or concerns associated with deterministic fire-induced safe-shutdown analyses that often form the basis for treating issues or concerns in Fire PRAs. The evaluations for these issues were considered as part of this

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project’s ancillary objectives. The PIRT considered it appropriate to address these issues to ensure treatment within both deterministic and risk-informed analyses for the failure modes in question is consistent with the risk posed. They may or may not be associated with hot short- induced spurious operations; some have been categorized as legacy issues since the Browns Ferry fire accident in 1975, and subsequent regulatory guidance. The PIRT panel was asked to consider each of the following cases from their phenomenological- and electrical-engineering perspectives and provide their technical positions:

• Multiple high impedance fault (MHIF) on a common power supply (ac/dc) • Open circuited secondary of current transformers (CTs)

• DC control power impact on common enclosure/common power supply • Control power transformers (CPTs)

• Kapton-insulated cables • Panel wiring

• High conductor count trunk cables

Section 6 of this report discusses the PIRT panel’s technical positions on each of these issues.