Generally speaking making estimates of LOCA frequencies for non-piping components is more challenging than making estimates for piping systems. There are multiple components to consider, each having different operating requirements, design margins, materials, and inspectability. There are also widely varying failure modes and scales to consider. For PWRs for the smaller category LOCAs, one must consider SGTRs and small penetration failures. For the larger category LOCAs, common cause bolting failures and component shell failures need to be considered. For the larger components, the bigger design margins (compared to those for piping) are somewhat offset by the decreased inspection quantity and quality. Compounding all of this is the fact that there is generally not as much precursor information available for the non-piping components as there is for piping.

For the BWR plants, the three major non-piping components that were considered were the RPV, the pumps, and the valves. In general, many of the same degradation mechanisms that are of concern for BWR piping are also a concern for BWR non-piping components. Stress corrosion cracking (specifically PWSCC) is a concern for many of the smaller Alloy 600 components, such as the CRDMs and other penetrations. As with piping, multiple cracks and fast propagation rates could lead to LOCAs. While the mechanism (PWSCC) is more severe at higher temperatures associated with PWRs, this mechanism could become a more significant issue later in the life of the BWRs. Thermal fatigue is another degradation mechanism associated with BWR non-piping components that is common with BWR piping. Thermal fatigue is especially of concern at inlet nozzles and other locations that experience thermal stratification, especially at the feedwater nozzles. For the same reasons as highlighted above for BWR piping, thermal fatigue can possible lead to larger leaks or LOCAs.

Other mechanisms for non-piping components that were not of concern for BWR piping are radiation embrittlement, common cause bolting failures, and thermal aging of cast stainless steel components, such as pump and valve casings. Radiation embrittlement reduces the base metal toughness of the RPV. Fortunately, for BWRs, it is not as of much concern as it is for PWRs due to the increased shielding available with the BWRs. Common cause bolting failures are important for manways and bolted valves. The common cause mechanisms may possibly include: improper installation or maintenance of the bolts,

i. e., improper torque, external corrosion of multiple bolts, and steam cutting of bolts. One participant thought that these common cause failures will cause the greatest risk. Thermal aging of cast stainless steels can cause a significant reduction in the fracture toughness of these materials, however, fortunately to date no cracking mechanisms have emerged for these materials.

Figure L.22 shows the Category 1 LOCA frequencies for the RPV, pumps, and valves at 25 years. The RPV shows the biggest expected Category 1 LOCA frequencies. The Category 1 RPV LOCA frequencies are driven by the CRDM penetration failures. However, the severity of the CRDM failures for BWRs was reduced by about one order of magnitude with respect to PWR CRDM failures due to the BWR heads operating at a lower temperature. Other than these head penetrations, nozzle and body cracking were mentioned as possible sources of failures. A number of precursor cracking incidents have been seen in service. The valves and pumps contribute to a lesser extent. Most of the panelists generally treated these components the same. At least one panelist (F) had some experience with manufacturing
defects in valve bodies which led to some increased concern with valves. Other issues with valves, and pumps, included the potential for bolt failures for the reasons outlined above, and the fact that the material they are made of (cast stainless steel) is notoriously difficult to inspect and is subject to toughness degradation due to thermal aging.

Valves

Pumps

RPV

LOCA Frequency (CY-1)

Figure L.22 BWR Category 1 Non-Piping LOCA Frequencies by Major Component at 25 Years of

Plant Operations

Figure L.23 shows the Category 3 non-piping LOCA frequencies at 25 years of plant operations. The most noticeable difference between the Category 3 and Category 1 LOCA frequencies is the three orders of magnitude reduction in the median value of the estimated LOCA frequencies for the RPV as the CRDM concerns disappear. A single CRDM failure cannot support a Category 3 LOCA. Only about half of the participants were concerned about RPV nozzle failures, but those that were assigned comparatively high frequencies to them. For the pumps and valves, the corresponding decrease in LOCA frequency is only about one order of magnitude. Consequently, the pumps, valves, and vessel now contribute about equally to the overall LOCA frequency.

Valves

Pumps

RPV

LOCA Frequency (CY-1)

Figure L.23 BWR Category 3 Non-Piping LOCA Frequencies by Major Component at 25 Years of

Plant Operations

Figure L.24 shows the Category 5 non-piping LOCA frequencies at 25 years for the BWR non-piping components. As was the case for the Category 3 LOCAs, the pumps, valves, and vessel are all now of about equal importance. For these large LOCAs, the panelists felt that the valve, pump, and vessel bodies were the most likely subcomponents to fail. For the vessel body, the concern was with LTOP while for the valve and pump bodies, the concern was with fatigue and SCC.

Valves

Pumps

RPV

LOCA Frequency (yr-1)

Figure L.24 BWR Category 5 Non-Piping LOCA Frequencies by Major Component at 25 Years of

Plant Operations

Figure L.25 shows the cumulative LOCA frequencies for the BWR non-piping components at 25 years of plant operations. On average there is about a one order of magnitude shift in the cumulative LOCA frequency between each successive LOCA category. The median value for the estimate of the Category 1 LOCA frequency is approximately 10-4 while the median value for the Category 6 LOCA frequency is about 10-9.

Cat 6 Cat 5 Cat 4 Cat 3

Cat 2 Cat 1

10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2

LOCA Frequency (yr-1)

Figure L.25 Cumulative BWR Non-Piping LOCA Frequencies at 25 Years of Plant Operations

Figures L.26 and L.27 show the effect of time on the Category 1 and 3 cumulative LOCA frequencies, respectively, for the BWR non-piping components. For all intents and purposes there is almost no effect of time on the predicted LOCA frequencies. The median values do not change nor does the variability, i. e., the interquartile ranges remain the same. Non-piping components are affected by similar partially compensating factors as the piping components. In addition, a number of participants expressed the belief that the maintenance and mitigation issues raised for piping also apply for non-piping components. The only thing that changes is the minimum value predicted by Participant H for LOCA Category 3. Participant H foresees the non-piping LOCA frequencies increasing at both the 40 and 60 year time interval. Figure L.28 shows the effect of time on the Category 5 frequencies. In this case the median values do not vary with time, nor does the maximum values, however, a number of participants started to see the LOCA frequencies increasing near the end-of-plant license renewal (60 years) such that the lower end of the IQR (i. e., the 25th percentile) increased an order of magnitude at 60 years over what it was at 40 and 25 years. This increase in the Category 5 predictions was driven by aging concerns of a few of the panelists at 60 years.

Figure L.26 Effect of Operating Time on the Cumulative Category 1 LOCA Frequencies for BWR

Non-Piping Components

Figure L.27 Effect of Operating Time on the Cumulative Category 3 LOCA Frequencies for BWR

Non-Piping Components

———— 1———— 1———— 1———— 1———— 1———— 1———— 1————

10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6

LOCA Frequency (yr-1)

Figure L.28 Effect of Operating Time on the Cumulative Category 5 LOCA Frequencies for BWR

Non-Piping Components

Figures L.29 and L.30 show the cumulative MV estimates, along with the 5% and 95% bound values for the various participants for the BWR Category 1 and 3 non-piping LOCA frequency estimates, respectively, at 25 years of plant operating time. Of note from these figures is the fact that a number of the participants (A, E, F, and H) predicted greater uncertainty for the Category 3 LOCAs than they did for the Category 1 LOCAs. This is not unusual in that one would expect the uncertainty to increase for lower frequency events, such as larger LOCAs. It is probably somewhat more surprising that the other four participants predicted comparable uncertainty for the Category 1 and 3 LOCAs. Overall the predictions for BWR non-piping were more consistent than for PWR non-piping discussed next. For the BWRs, there are less components and failure modes to consider and the approaches used to estimate the frequencies were more closely related.

LOCA Frequency (yr-1)

Figure L.29 BWR Non-Piping Category 1 LOCA Frequencies Showing MVs, 5% LB, and 95% UB
Values for All Participants Who Responded to the BWR Non-Piping Questions

LOCA Frequency (yr-1)

Figure L.30 BWR Non-Piping Category 3 LOCA Frequencies Showing MVs, 5% LB, and 95% UB
Values for All Participants Who Responded to the BWR Non-Piping Questions