ML22182A405

All Creatures Great & Small: a Brief Survey of the Impact of Flora/Fauna on Nuclear Power Plants

ML22182A405
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Issue date:	07/01/2022
From:	John Hanna NRC/RGN-III
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All Creatures Great & Small (1): A Brief Survey of the Impact of Flora/Fauna on Nuclear Power Plants John David Hanna Region III Office, US Nuclear Regulatory Commission, USA. E-mail: john.hanna@nrc.gov The US Nuclear Regulatory Commission licenses and regulates the nations civilian use of radioactive materials to provide reasonable assurance of adequate protection of public health and safety, promote the common defense and security, and protect the environment. The impacts of nuclear power plants on the environment and specifically on neighboring flora and fauna are considered in the design and licensing processes for these facilities. Some of these impacts have been analyzed in scientific articles, (e.g., service water cooling systems affecting fish populations, seaweed, etc.) But the vector/threat also goes in the opposite direction and the environment can pose a threat to the safety of nuclear power plants. Flora and fauna have caused a number of safety significant events and/or conditions at these facilities. This paper surveys the wide variety of biological challenges and describes, where possible, the risk significance of those events and/or conditions. The current state-of-the-art of probabilistic risk assessment modeling is briefly described and potential modeling improvements are broached. Potential operational and design enhancements that may mitigate these risk impacts---which are described in other scientific papers---are referenced.

Keywords: nuclear power, external event, PRA, flora, fauna, environment.

1. Introduction As analysts, regulators, and operators of nuclear power plants (NPPs) we need to be sensitive to the impact of our facilities on the environment. That focus is written into the mission statement of the US Nuclear Regulatory Commission (NRC) and is a goal the agency takes very seriously. (2) However, we also need to be similarly concerned when the direction of the arrow is reversed, namely when flora and fauna are the threat vector to the safety of NPPs. These environmental threats have been evaluated through the licensing process and have been considered in various scientific articles, however this paper will attempt to advance additional risk-informed perspectives.

The NRC has evaluated the impact of biota through safety-system functional inspections, analyzed the generic safety implications, (3) required our licensees to take actions or evaluate specific failure mechanisms, e.g., via bulletins or generic letters. (4) The NRC has engaged the industry on biological impacts in the past, but these interactions heretofore typically were:

confined to reviews of a specific system or biological threat, evaluated the risk of an individual event, outside of a greater context, and rarely evaluated the state-of-the-art of probabilistic modeling the biological/environmental impact.

Disclaimer: the events/conditions described in this paper occurred at NPPs in the United States, and hence may not be representative of all the different potential impacts from biological sources worldwide. However, given the size of the US, the number of operating nuclear units (93 as of the time this paper was submitted), and the diversity in wildlife and the design/manufacture of NPPs in the US, it is believed that the insights presented in this paper may be useful outside of the US.

The reason we as analysts, regulators, and operators should consider these vectors/threats is because they are:

1) creating actual events and demanding front-line safety systems, and hence 2) challenging the stability, reliability, and sustainability of the electric grid. Given the need for a resilient, reliable, and stable electrical grid and the potential for increasing frequency/severity of natural phenomenon we need to understand and manage these risks.

2. Survey of Biological Impacts on NPPs Biological impacts, whether from flora or fauna, are a subset of all potential environmental hazards that could affect NPPs. The greater population of environmental events includes tornadoes/high

winds, extreme heat/cold, external flooding, and seismic threats. Types of vectors/threats that are environmental and arguably within the definition of biota, but are not characterized in this paper include:

Sedimentation deposition (silt, mud, and clay) in cooling systems, Non-living biological fouling (e.g., detritus such as twigs, leaves deposited in cooling water ultimate heat-sinks, or larger branches, trees, etc. thrown by tornadoes or high wind events), and Microbiologically induced corrosion. While this last vector is a living one-celled organism that has posed challenges to NPPs, it is not included because the threat is a slowly evolving condition, as opposed to an event, and has been effectively mitigated by the nuclear industry. (5)

2 John David Hanna Table 1. Sample of events that have occurred at NPPs in the US, including the flora/fauna that created the event, a (very) brief description of the impact and the calculated risk.

a The common and scientific name of the flora/fauna is provided. When the exact genus and species is uncertain, the known order, suborder or family is shown.

b For the calculated risk, when there was a multi-unit impact, the higher of the calculated risk values is shown. Per the guidance for evaluating the risk significance of a given event, the conditional core damage probability (CCDP) is shown. (6) When the risk values are not already available from a historical source (e.g., the Accident Sequence Precursor analysis), the CCDP values were calculated based on the description in the event report.

Site & Unit Affected Event Date Flora/Faunaa Impact Risk (CCDP)b Oyster Creek Nuclear Generating Station (7) 15 April 1981 Sea lettuce (Genus = ulva)

Sea lettuce caused decreasing levels in the intake structure, and one loop of containment spray was declared inoperable.

Minimal change above baseline Brunswick Steam Electric Plant, Unit 1 (8) 19 April 1981 American oysters (Crassostrea virginica)

Loss of shutdown cooling due oyster shell buildup in the residual heat removal heat exchanger.

7E-3 Oyster Creek Nuclear Generating Station (9) 8 June 1981 Sea grass (Order

= Alismatales)

Two emergency service water pumps inoperable and potentially non-function; possible common cause failure (CCF) for other pumps.

Minimal change above baseline Salem Nuclear Generating Station, Unit 1 (10) 11 August 1983 American oysters (Crassostrea virginica)

Various single systems and component adversely affected (8 events in 1980s). The August 1983 event was a loss of condenser heat sink with a consequential loss of offsite power (LOOP) &

failure of an auxiliary feedwater pump.

1.2E-4 Pilgrim Nuclear Power Station (11) 28 August 1981 Blue mussels (Mytilus edulis)

Bivalves at heat exchangers created blockage &

high differential pressure resulting in flow bypass.

Potential loss of service water at system level.

1.3E-4 La Crosse Boiling Water Reactor (12) 16 July 1984 Mayflies (Order =

Ephemeroptera)

LOOP, failure of an emergency diesel generator (EDG) breaker to close & unavailability of both trains of high-pressure core spray.

9E-4 Catawba Nuclear Station, Unit 2 (13) 9 March 1988 Asiatic clams (Corbicula fluminea)

Asiatic clams caused CCF failure to two of four auxiliary feedwater flow control valves and subsequent reactor trip.

3E-4 Zion Nuclear Power Station, Unit 2 (14) 7 March 1994 Zebra mussels (Dreissena polymorpha)

With plant in hot shutdown, an auxiliary feedwater pump failed with an accompanying EDG trip (heat exchangers blocked by zebra mussels).

2.3E-5 Salem Nuclear Generating Station, Unit 1 (15) 7 April 1994 Marsh grass (Spartina alterniflora)

Automatic reactor trip and loss of condenser heat sink, multiple safety injections, multiple cycles of primary power operated reliefs with damage and an alert declaration.

2.6E-6 Wolf Creek Generating Station (16) 4 September 2000 Squirrel (Genus =

Sciurus)

Automatic reactor trip with a fire in a unit auxiliary power transformer.

7.2E-5 Donald C. Cook Nuclear Plant, Units 1 & 2 (17) 29 August 2001 Zebra mussels (Dreissena polymorpha)

Bivalves overwhelmed the service water (SW) strainers and led to the inoperability of all four EDGs, auxiliary feedwater room coolers, and a component cooling water (CCW) train in each unit.

1E-5 Donald C. Cook Nuclear Plant, Units 1 & 2 (18) 24 April 2003 Alewife fish (Alosa pseudoharengus)

Manual reactor trip of both units, all EDGs inoperable (but ultimately determined to be functional), multiple heat exchangers and traveling screens damaged resulting in an alert declaration.

4E-4

All Creatures Great & Small: A Brief Survey of the Impact of Flora/Fauna on Nuclear Power Plants 3 Table 1. (Continued)

The events from Table 1 above are merely a sample of actual events that have occurred in the US nuclear industry. A study covering the time period from 1980-1987 identified 980 operational events involving service water, of which 276 were determined to have generic applicability. Of those 276 events twenty-eight involved biofouling, and while the number of these events has decreased significantly over the following three decades, they are still occurring with some periodicity (26).

However, despite the limited sample size, the following insights can be drawn from Table 1.

(i) A wide variety of plant designs in various locales have been susceptible to the impact of flora/fauna.

(ii) The diversity of biological impacts on NPPs (and in all likelihood non-nuclear sources of electrical power, such as gas and coal plants) is large, ranging from single cell organisms which foul a heat exchanger to medium/large birds, reptiles, or mammals which challenge electrical systems and cause reactor trips and LOOPs.

(iii) The risk impact of the flora/fauna can range from negligible to substantially high (i.e., two to three orders distant from core damage).

As will be discussed in the next section of this paper, the risk impacts are larger when an initiating event (IE) and degradation/loss of mitigating system(s) occur simultaneously. Additionally, it is important to note that the magnitude of these events may be increasing as a result of climate change, exacerbating the frequency and/or intensity of these events and, hence affecting a sustainable electric grid. (27)

3. Risk Modeling of Biological Impacts 3.1. Description of the Current State of Risk Modeling The NRCs Standard Plant Analysis Risk (SPAR) models are plant-specific probabilistic risk assessments (PRAs) maintained, frequently exercised by analysts within the agency, and are used to inform regulatory decisions. The NRCs 1995 PRA policy statement specified that PRA evaluations supporting regulatory decisions should be as realistic as practicable. (28)

Consistent with this realism principle, the impacts of biota can be further developed in the SPAR models and thus captured in subsequent risk assessments.

PRAs, including the SPAR models already account for some biological impacts implicitly via IE frequencies and component failure probabilities. For example, the IEs for partial or complete LOOPs, losses of service water, and losses of CCW are included in the calculation of initiating event frequencies used in PRA models regardless if those events were caused by say a jellyfish, or due to a low water level in the ultimate heat sink.

However, this data-driven approach does have limitations in that it requires actual events (or near-events) to occur before being factored into a PRA.

Probabilistic risk assessments also explicitly address biological impacts through the support system initiating Palo Verde Nuclear Generating Station Units 1, 2 & 3 (19) 14 June 2004 Scientific name =

Aves (animal class) excreta Bird caused a ground fault on a transmission line and failure in protective relaying which then caused a three-unit NPP trip with six additional generation units lost; several components failed on Unit-2; short duration LOOP but extensive regional impact up to Canada.

4E-5 Edwin I. Hatch Nuclear Plant, Unit 2 (20) 1 August 2007 Snake (Suborder

= Serpentes)

Snake ascended power lines, shorted and required a power reduction due to loss of cooling towers; fire caused an unusual event declaration.

Minimal change above baseline St. Lucie Plant, Units 1 & 2 (21) 22 August 2011 Jellyfish (Class =

scyphozoa)

Manual reactor trip of Unit 1 and power reduction on Unit 2 due to jellyfish & associated fish kill causing loss of condenser backpressure.

8E-7 R.E. Ginna Nuclear Power Plant (22) 3 June 2012 Raccoon (Procyon lotor)

Partial LOOP, loss of two safety-related electrical buses and automatic EDG start; unit stayed online.

Minimal change above baseline Surry Power Station, Unit 2 (23) 29 December 2012 Brown Pelican (Pelecanus occidentalis)

Bird contact with power lines resulted in a partial LOOP, loss of an electrical bus & auto-start of an EDG; unit stayed online.

Minimal change above baseline Fermi, Unit 2 (24) 1 July 2020 Mayflies (Order =

Ephemeroptera)

During shutdown conditions a partial LOOP occurred with a valid actuation of an EDG.

3.3E-7 Browns Ferry Nuclear Plant, Units 1, 2 & 3 (25) 20 July 2020 Eel grass (Zostera marina)

Units 1 and 2 manually scrammed, with Unit 3 down-powered due to loss of condenser vacuum.

3E-6

4 John David Hanna eventc (SSIE) modeling as described in the industry recommendations. (29) The guidance states:

Any historical loss of support systems that resulted in a plant trip or significant transient should be considered as a potential support system initiating event. As an example, consider the potential for seasonal influx of materials causing blockage of the plants service water intake structure. This has been a historical occurrence at several plants and can be caused by debris from flooding or storms, or environmental sources such as mussels, marsh grass, or frazzle ice.

The SPAR models typically include loss of service water, loss of component cooling water or other support cooling water system (e.g., reactor building cooling water system) via the IE fault trees. (30) The SSIE modeling explicitly describes the support system design aspects of the plant (e.g., number of pumps, strainers, etc.) while incorporating actual plant-specific events that have occurred. The goal of this modeling is to more accurately reflect the potential impact of environmental effects (of which biologics are a subset) than could be achieved with a

single probability distribution representing the IE frequency.

3.2. Characteristics of Biota Threats There are several aspects to the flora/fauna threat to consider when developing and maintaining PRAs.

Design Aspects Characteristics such as independence, physical separation (e.g., closed cooling water loops) and defense-in-depth (e.g.,

number of offsite power lines or service water pumps) can significantly affect the PRA modelling.

Initiating Event Frequencies - The frequencies of service water system failures and degradations as observed in the operating experience, are relatively high: 1.2E-2 per reactor year for system failure and 4.1E-1 per reactor year for system degradation. (31)

Also, as can be seen in Table 2 below, an SSIE with an expected frequency of 5E-3 per year is likely to have occurred in the US nuclear industry, even if the entire plant population is not susceptible to the SSIE.

Table 2: Expected Occurrence Rate and Probability of Observation of SSIEs c SSIEs are defined as: Any event such as a component, train, or complete system failure (or causing the failure of a component, train, or system) that:

• Challenges a reactor safety function, then Seasonal Aspects - Biological events typically occur at a specific time of year, especially when flora/fauna populations fluctuate with growth and/or breeding.

Plant (or Location) Specific - As described in reference 32: Degraded water quality is plant-specific. Reports of system failure due to degraded water quality are included in the generic data bases but are commonly averaged in with other nominal operating data. Service water degradation has been shown to occur due to ice, seaweed, sea grass, and fish runs. Because these phenomena are plant-specific, they should be quantified on a plant-specific basis. Because of their short duration and severe effects, they should be modeled with separate common cause factors and split fractions for yearly exposure times.

Potential Effects from Climate Change - Variations in biological blooms driven by random fluctuations or due to climate change can overwhelm the defenses put in place at NPPs which had/have historically been effective, (e.g., the mayfly event at Fermi listed in Table 1).

3.3. Areas for Possible PRA Development (i) If the PRA and the larger scientific community believes that there has been a potential shift in external hazards on biota (e.g., due to climate impacts) then the PRA may not be as static as previously thought, and events not seen in some locations may become more common. Hence, prior experience may not predict future hazards, which could be an area worthy of investigation.

(ii) In general practice, the SSIEs included in PRAs have been limited to those types of events and systems described above, and this modeling has a technical basis due to relying on actual events or near misses. However, there are other potential impacts of biological (or more broadly environmental) vectors. These might include losses of instrument air or ventilation systems, (e.g., EDG room cooling or main control room habitability).

(iii) The potential failure of service water intake structures would likely result in the complete failure of the service water system and potentially lead to loss of the ultimate heat sink, but the frequency of failures may be affected more by

• Leads to a reactor trip, and also

• Fails a train or complete front-line system normally available to respond to the reactor trip or reactor shutdown and successfully mitigate the loss of the critical safety function.

Annual Frequency of Initiator (Per reactor year)

Expected Number of Occurrences in 2000 reactor critical years Chance of observing one or more events in 2000 reactor years 1E-4 0.2 18%

5E-4 1

63%

1E-3 2

86%

5E-3 10 99.9%

1E-2 20 99.9%

All Creatures Great & Small: A Brief Survey of the Impact of Flora/Fauna on Nuclear Power Plants 5 environmental conditions such as detritus or frazzle ice than by hardware failures. The collection of data (i.e., separating out the biological impacts and considering them apart from other CCFs) is an open issue for consideration in future research.

(iv) Additionally, modeling the impact of biota in a seasonal manner in that IE frequencies are not static throughout the year, could improve licensee decision-making on scheduling maintenance activities.

(v) Development of Dynamic PRAs may better capture biological impacts through explicitly modeling timing dependencies. Flora/fauna events happen quickly, and the recovery probabilities change with time, and thus are the types of events that Dynamic PRAs capture more effectively than traditional PRAs. (33)

It is in these interstitial spaces where one finds rich terrain for PRA model development.

4. Mitigation Processes or Techniques The following is a summary list of engineering, maintenance and operational practices that can help mitigate the threat from flora and fauna:

For various CCW and SW systems:

o Well-designed service water strainers (e.g.,

duplex, triplex, self-cleaning),

traveling screens, trash racks, interception nets in front of intake structure. (34) o Thermal backwashing for bivalves. (35) o Periodic checking of flow rates and/or flow balancing. (36) o Periodic flushing of system heat exchangers.

o Visual examinations and routine testing of heat exchangers for heat transfer capabilities.

o Chlorination of systems, timed to coincide with when systems are laid up.

o Routine maintenance (e.g., upkeep for traveling screens) to prepare systems prior to an event. (37) o Fish sonar or other detection methods. (38) o Seasonal inspections of service water/circulating water intake bays.

o Operational responses during an actual event:

Staggering trains of traveling screens or SW systems to improve survivability for the plant for a longer period.

Considering use of Mitigating Strategies equipment (commonly known as FLEX) during an event; this equipment was designed, licensed & built for extended loss-of-AC-power coincident with a loss of the ultimate heat sink.

Carefully consider the cross train/unit operation when there is biofouling present due to the loss of defense-in-depth & potential common cause impacts created. (39)

For electrical buses, transformers, offsite power lines, and other electrical equipment:

o Using systems to draw biologics away from important equipment e.g., lighting away from important

breakers, transformers, disconnects, etc. such that the biota, e.g.,

insects, will not cause an event. (40) o Routine inspection and cleaning, especially for animal excreta.

o Ensuring electrical equipment is in fact well sealed, i.e., maintenance checks, to ensure small animals cannot get within.

o Noise or visual cues to repel birds or mammals from important equipment e.g.,

sounds of predatory birds to scare smaller

birds, and prevent them from nesting/congregating.

5. Conclusions A wide variety of plant designs in numerous locales have been susceptible to the impacts posed by various types of flora/fauna. The diversity of biological impacts on NPPs (and in all likelihood non-nuclear sources of electrical power) is large, ranging from single cell organisms which foul a heat exchanger to large birds, reptiles, or mammals which challenge electrical systems and cause reactor trips and LOOPs.

The risk impact of the flora/fauna have ranged from negligible to substantially high (i.e., three orders distant from core damage.) And in terms of PRA modeling, the nuclear risk community has tackled issues of CCF, fire modeling, incorporation of mitigating strategies equipment (commonly known as FLEX) and others. Though progress has been made in modeling biological (and the larger environmental) impacts at NPPs, there is room for improvement. Are environmental/biological threats the next horizon for PRA?

The industry needs to recognize the near misses and remember the lessons-learned from past events where biological (or other environmental) threats to the NPPs posed a challenge to a resilient, reliable, stable, and sustainable electrical grid.

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Alexander, Cecil Frances. 1848. Lyrics to the hymn, All Things Bright and Beautiful

2.

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25. US NRC Licensee Event Report for Browns Ferry Nuclear Plant 05000259/2020-001-00 Manual Reactor Shutdown of Two Units on Decreasing Condenser Vacuum due to Eel Grass Intrusion, 16 September 2020, ML20260H103

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37. See reference 18 above.

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40. See reference 24 above.