Radiological Emergency Plan: North Anna Nuclear Power Station

On March 11, 2011, the Tōhoku-Chihou-Taiheiyou-Oki Earthquake and Tsunami struck Japan's northeastern coast with devastating consequences. Roughly 20,000 people perished that day. In addition, three reactors at the Fukushima Dai-ichi Nuclear Power Station (NPS) incurred fuel meltdowns with massive releases of radioactive material into the air and the ocean, contaminating large swaths of land. Roughly 80,000 residents had to be evacuated from the vicinity of the NPS and have not been able to return home.

About five months later, the August 23 Virginia Earthquake precipitated the emergency shutdown of the two pressurized light water reactors at North Anna Nuclear Power Station (NPS) roughly 35 miles from my home. According to the U.S. Geological Service, the station is located at a distance of only 8 miles from the quake's epicenter and 20 miles from a quaternary fault zone. Seismologist Dr. James Martin of Virginia Tech. gave a comprehensive interview on the quake to Beverly Amsler aired on WVTF Radio IQ's Evening Edition for Nov. 22, 2011. The events made me wonder what a radiological emergency similar to that in Japan would entail in the U.S.

Area of quaternary faults and liquefaction in less than 15,000 years (hatched; source: USGS). The arrow points at the location of North Anna NPS. The epicenter of last Summer's Virginia quake was located near the junction between state routes 22 and 208.

When a utility company applies to the U.S. Nuclear Regulatory Commission (NRC) for the license to build a nuclear power reactor, an emergency plan for the eventuality of a radiological accident must be submitted. The plan must conform to federal regulations (10 CFR § 50.47 Emergency Plans). Dominion, the operator of North Anna NPS, is planning to construct a third unit on the station's premises and has hence developed such plan.

Examining Dominion's "North Anna 3 Combined License Application, Part 5: Emergency Plan, Revision 0 November 2007", reveals that much of the responsibility for the immediate decisions to be taken in the course of a severe reactor accident rests with the station operator, that is, primarily on the shoulders of the station's Emergency Coordinator (page II-1 of the proposed emergency plan). This person is instructed to collect information about the radioactive contamination monitored in the surroundings of the station, estimate the source term, that is the amount of radioactivity released from the failed reactor, determine the prevailing winds and precipitation, dispersing the radioactive matter, and constantly keep the relevant local, state and federal agencies up to date about the evolution of the accident.

Map showing county lines and distance radii of Emergency Planning Zones proposed for North Anna NPS Unit 3 (source: Fig. I-2; Dominion).

Precise recommendations to the public depend on the source term and the progression of the plume in which the radioactivity is dispersed. The anticipated immediate protection measures are contingent mainly upon the distance from the power station. Radii of action are drawn (see drawing above). In addition to the protection of station personel (site emergency), the greatest concern lies with the safety of residents living within a 10-mile radius around the station, for whom precise instructions for health protection, evacuation and shelter have been planned (general emergency). This area is designated as the Plume Exposure Pathway Emergency Planning Zone. Radio stations periodically test emergency alert announcements (texts and further details can be found in the Commonwealth of Virginia Emergency Alert System Plan). Sirens alert residents to tune into local radio and TV stations for detailed announcements. Residents living within the 2 mile perimeter around the stricken NPS and within 5 miles straight downwind, including a margin on both sides (keyhole pattern), will be asked to evacuate immediately. Other residents within the 10-mile perimeter will be asked to stay indoors (NRC Backgrounder on Emergency Preparedness at Nuclear Power Plants).

The plans for residents beyond 10 miles depend on the amount and the duration of the release of radioactive matter from the reactor. The 50-mile radius encompasses the largest territory taken into consideration in the station's emergency plan. This area is designated as the Ingestion Exposure Pathway Emergency Planning Zone. Dominion set up a series of monitoring stations around the station. Near-real time ionizing radiation measurements available to the public within 100 miles from North Anna NPS, are disseminated by only two Radnet stations: one in Richmond and one in Harrisonburg, Virginia.

The color-coded map shows the magnitude of the fallout of cesium-134 and cesium-137, two major radioisotopes released after the severe nuclear reactor accidents at Fukushima Dai-ichi NPS in March last year. The deposits were mapped at the beginning of August, 2011. Radii are superimposed for orientation. Note, the plume's impression in red and yellow (source: MEXT).

Local wind and precipitation mainly dictate the shape of the plume in which the radioactivity is transported and disperses. To provide an example the color-coded radiation map above shows the overland plume-shaped radioactivity dispersal reconstructed with computer models from radiation monitor recordings after last year's severe accidents at Fukushima Dai-ichi NPS.

Average prevalence of wind directions at Louisa County Airport, Virginia, compiled from data collected between 1999 and 2011 (source: weatherspark.com).

In a hypothetical release at North Anna NPS resembling that of Fukushima, local weather conditions could produce a similar plume heading in a different direction. Wind speed and direction recorded at Louisa County Airport (see above) a few miles from North Anna NPS suggest that prevalent local winds blow on average from South/Southwest to North/Northeast. The plume would most likely progress toward Northern Virginia. Dispersal to the South toward the Richmond Metropolitan Area represents the second most probable direction.

The distance over which the plume eventually extends depends on the amounts of radioactivity released, the altitude of the release, wind speeds affecting the release, and precipitation. In Japan, the plume barely impacted Fukushima City, 35 miles from the nuclear reactor accidents. Note, however, that prevailing winds blew much of the airborne radioactive material out to sea.
Dominion's proposed emergency plan is most concerned with immediate impact after a severe radiological emergency. The long-term evolution of a wide-spread radioactive contamination can only be extrapolated from experiences after the Tchernobyl reactor disaster in 1986 and the ongoing contamination and clean-up around Fukushima Dai-ichi NPS. Regardless of the uncertainties, it seems only prudent to consult our local Radiological Emergency Plan, if we live within the 50-mile ingestion zone around a nuclear power station.

Acknowledgement
I thank the members of the simplyinfo.org discussion group for their cogent input, without which this post would not have been possible.

Relevant Sources

• Backgrounder on Emergency Preparedness at Nuclear Power Plants
• NRC Regulations (10 CFR) § 50.47 Emergency plans.
• North Anna 3 Combined License Application, Part 5: Emergency Plan, Revision 0 November 2007.
• Commonwealth of Virginia Emergency Alert System Plan.

The Value of RadNet Ionizing Radiation Detection

In this post, I discuss uses of ionizing radiation measurements made available to the public by the U.S. Environmental Protection Agency's RadNet stations. In particular, I examine the usefulness of RadNet data for the early detection of radioactive fallout from distant radiological emergencies. As example, I use the reactor accidents at the Fukushima Daiichi Nuclear Power Station in the wake of the Mar. 11, 2011, Tōhoku-chihō Taiheiyō Oki Earthquake and Tsunami.

Iodine-131 and cesium-137 are prominent radioisotopes released after severe nuclear reactor accidents with fuel meltdowns. The decay of these isotopes produces a mix of beta and gamma radiation. The gamma radiation is emitted when the atomic nuclei transmute through intermittent energy states at isotope-specific energies.

Near-real time air filter gamma gross count rates measured in nine energy ranges at the RadNet station in Harrisonburg, Virginia (courtesy EPA). Because of the disparate values, the count rates are scaled logarithmically on the ordinate, compressing the peaks at high count rates. 

I used air filter measurements from the RadNet station in Harrisonburg, Virginia. The EPA publishes graphs of hourly gamma radiation gross count rates in nine energy ranges (see graph above).

Gamma spectrum of iodine-131 (Arena, 1971). Counts/channel are plotted on a logarithmic scale versus energy [keV]. Peaks are labelled in MeV.

Iodine-131 with a physical half-life of 8.05 days and an effective half-life in the whole body of 7.6 days possesses its greatest energy peak at 360 keV, a second-ranking peak at 280 keV, a third peak at 638 keV and a fourth peak at 724 keV (Arena, 1971). The lower couple fall into range 3 (200-400 keV) , whereas the higher couple fall into range 5 (600-800 keV) in the RadNet graph.

Gamma spectrum of cesium-137 (Arena, 1971). Counts/channel are plotted on a logarithmic scale versus energy [keV]. Peaks are labelled in MeV. 

By contrast, cesium-137 with a physical half-life of 30.17 years and an effective half-life in the whole body of 70 days emits gamma radiation at 662 keV, falling into range 5 of the RadNet graph. However, backscatter (Compton effect), that is low-energy detector counts produced by incomplete energy transfer between the ionizing radiation and the detector material, contributes a considerable fraction of the total count rate to range 3. Although the precise shape of the spectral curves shown above depends on the radioactivity of the sources and the instruments used for the measurement, the energy peaks remain invariable and representative. Therefore, the spectra may can be employed for the demonstration of principles.

Partial integration of the areas under the spectral curves taking the logarithmic scale of the counts/channel into account suggests that the iodine-131 decay contributes about 91 percent of the total count rate summed over both ranges to range 3, whereas cesium-137 decay will contribute about one third to this range. Therefore, if both isotopes are present in the sample, the count rate measured in range 3 does not exclusively reflect iodine-131 decay, and iodine-131 will also contribute to the count rate measured in range 5, though to a smaller degree than cesium-137. Despite this cross-contamination, a prominent increase in range 3 suggests the presence of iodine-131 and in range 5 that of cesium-137. Furthermore, the EPA provides offline post-hoc data for identified radioisotopes.

Regardless of its low spectral resolution, the real-time RadNet graph may potentially have its uses for the identification of a radiological incident. An increase above the average counts per minute (CPM) measured in ranges 3 and 5 beyond three standard errors of the mean can be considered statistically significant with 95 percent confidence. Mean count rates measured in the past can be queried in the RadNet database. Sequences of up to 400 measurements can be downloaded in a batch.

To determine mean count rates for the two energy ranges of interest, that is ranges 3 and 5, I chose the period between Feb. 22 and Mar. 10, 2011, and subsequently compared the means averaged over this epoch to the count rates observed in the same epoch this years. Radioactive decay adheres to the Poisson distribution in which the standard error is equal to the square root of the mean count rate. If the current count rate exceeds the mean by three standard errors, the probability of the increase being random is less than five percent.

Graph of gamma gross count rates measured in nine energy ranges taken from air filter samples at the RadNet Station in Harrisonburg, Virginia. The bottom lines of the red boxes indicate three standard errors above the mean count rate of the same epoch a year ago.

The graph above shows that this year's count rates statistically significantly exceeded the mean in the examined time window on roughly a handful of occasions. However, most comprise all energy ranges except 9 (magenta). If increases were only detected in range 3 (blue), we would most likely be confronted with a statistically significant presence of iodine-131. By contrast, a statistically significant increase in ranges 3 (blue) and 5 (yellow) would most likely indicate the presence of cesium-137.

Concomitant increases in gross beta count rates would further affirm the conclusions above, because both radioisotopes also emit beta radiation. Note, however, that the above assertions are purely based on statistical probability and causal inference. Great care must be taken to establish reasonable cause.

Iodine-131 and cesium-137 can be distinguished with time, using the difference in physical half-life between the two isotopes. Iodine-131's half-life is at 8.05 days considerably shorter than that of cesium-137 at 30.17 years. Following a pulse release of both isotopes into the atmosphere, iodine-131 should mainly contribute to the count rates measured early after the release. When the samples are remeasured ten half-lives later, that is after 80 days, the contribution of iodine-131 to the count rate will have diminished to one-thousandth. By contrast, the contribution of cesium-137 with its 30-year half-life is going to persist. Therefore, if repeated measurements of the same sample ascertain a decline in count rate commensurate with iodine-131's half-life, the radioisotope was present in the sample, and the remainder ought to be cesium-137.

Though the RadNet measurements may not immediately lend themselves to the distinction between iodine-131 and cesium-137, they may prove useful for the detection of radioactive fallout from a distant radiological accident. To test this idea, I tapped into the data collected at Harrisonburg in the weeks after the catastrophic nuclear reactor failures at the Fukushima Daiichi Nuclear Power Station situated on the shores of the Pacific Ocean 160 miles north of Tokyo in early March last year, that is between Mar. 12 and Mar. 28, 2011.

The station's reactors incurred loss of cooling in the wake of a magnitude-9 earthquake on Mar. 11, 2011, followed by 15-meter high tsunami waves inundating the structures (TEPCO press release with the title "Analysis and evaluation of the operation record and accident record of Fukushima Daiichi Nuclear Power Station at the time of Tohoku-Chihou-Taiheiyou-Oki-Earthquake," dated May 23, 2011). The nuclear fuel in three operating reactors melted down, producing great amounts of hydrogen. Between Mar. 12 and Mar. 15, hydrogen that had accumulated in the reactor buildings triggered massive explosions devastating the top floors of the structures and releasing vast amounts of radioactive matter into the atmosphere.

The first unit, Unit 1, exploded in the early afternoon of Mar. 12. The second unit, Unit 3, exploded two days later, and the third unit, Unit 4, though it was shutdown for inspection at the time, incurred an explosion in the early morning of Mar. 15. The reactor building of Unit 2, which was operating, suffered minor visible damage. However, since Unit 2's fuel melted, radioactivity was released from this reactor as well.

 Gross count rates from air filter samples collected at Harrisonburg, Virginia, over roughly two weeks after the first radioactive release from Fukushima.

Iodine-131 and cesium-137 were prominent in the airborne releases from the damaged reactors. The graph above depicts the gamma gross count rates collected in nine energy ranges at Harrisonburg over roughly two weeks after the first radioactive release from Fukushima, the statistical significance thresholds (dashed black lines) in ranges 3 (dark blue line) and 5 (yellow line), as well as the gross beta count rate (dashed red line). Peaks of significant gamma gross rate increases are evident in almost all energy ranges. None was confined exclusively to ranges 3 and 5. The light-blue line above the abscissa indicates the period in which the Fukushima Daiichi Nuclear Power Station incurred hydrogen explosions.

March 14 was the first day after the Fukushima Daiichi Nuclear Power Station began to release vast amounts of radioactivity, on which Harrisonburg registered significant peaks in gamma ranges 3 and 5 concomitant with an increase in beta gross count rate (dashed red line). A second such coincidence followed on Mar. 15. Then a triplet of coincident peaks followed on March 17, 19 and 20 (light blue dots above the abscissa). Because the peaks are spaced in about the same time intervals as the three hydrogen explosions at the Fukushima Daiichi Nuclear Power Station, this triplet of peaks represents the most probable harbinger of the arrival of fallout from Fukushima in Virginia, suggesting that radioactive airborne particulate from Fukushima reached Harrisonburg in five days.

The suggested time of arrival is a day ahead of that predicted by the cesium-137 dispersion model of Winiarek and others (2011) at the Centre d'Enseignement et de Recherche en Environnement Atmosphérique (CEREA), Marne la Vallée, France, and two days in advance of that of the iodine-131 dispersion modeled by Wotawa (2011) at the Zentralanstalt für Meteorologie und Geodynamik (ZAMG), Vienna, Austria.

Global iodine-131 dispersion after the Fukushima reactor accidents, 2011, according to the simulation model by Wotawa (2011), ZAMG, Vienna, Austria. 

Taking the above findings together, RadNet air filter count rate measurements seem to provide superb sensitivity for the detection of minute traces of airborne radioactive material, permitting us to identify distant radiological accidents half ways around the globe, if examined in proper factual context. Certainly, RadNet stations represent potent tools capable of alerting us to fallout from radiological accidents closer to home.

References

• Arena V (1971) Ionizing Radiation and Life. Mosby, St. Louis.
• Winiarek V, Bocquet M, Roustan Y, Birman C, Tran P (2011) Atmospheric dispersion of radionuclides from the Fukushima- Daichii nuclear power plant.
• Wotawa G (2011) Fukushima: Resümee über bisherige Rechnungen und CTBTO Messungen.

Seismic Activity & Reactor Safety: Lessons from Japan

I worked with radioactive isotopes known as radionuclides in laboratory research for almost three decades in my professional career. Since three reactor fuel cores melted down at the Fukushima Dai-ichi (number one) Nuclear Power Station (NPS) owned by Tokyo Electric Power Company (TEPCO) in the aftermath of the Tohoku-Chihou-Taiheiyou-Oki Earthquake and Tsunami on March 11, 2011 [1], I have been concerned about the safety of nuclear power reactors.

At magnitude M_w 9.0 (I_JMA 7.0 on the Japan Meteorological Agency intensity scale), this earthquake was the strongest recorded in Japan's history, causing massive damage. Moreover, tsunami waves up to 40 meters high inflicted horrible devastation on the coast. The Government of Japan reports that 15,687 people lost their lives and 4,757 are still missing as of Aug. 9, 2011 [2].

The quake and tsunami caused a total station blackout at Fukushima Dai-ichi NPS. Reactor fuel core cooling could not be maintained, the fuel began to melt, and radioactive material could not be contained. After powerful hydrogen explosions, still unknown amounts of radioactive material were released into the air and the sea. Roughly 80,000 people living in immediate proximity of the power station were evacuated and have not been able to return home, except for brief visits [2]. Soil and sea near the NPS are highly contaminated with radioactive material. Long-lived radionuclides have been found in foods like rice [3] and beef [4] at levels too high for consumption. Hotspots of radiation are being discovered in Tokyo and further away [5]. The impact on public health is not yet fully comprehended. The costs of this crisis and its long-term consequences are unfathomable, but will be in the hundreds of billion yen.

Aerial photograph of Fukushima 1 (Dai-ichi) NPS taken March 24, 2011. Note the obliteration of the reactor buildings at Units 1 (top), 3 and 4. Fuel cores melted in Units 1, 2 and 3 because of loss of coolant. Unit 4 was shutdown for service at the time of the earthquake and tsunami (source: cryptome.org).

The islands of Japan lie in a seismically active zone near the Pacific Rim of Fire. They are situated in close proximity to the Japan Trench in the Pacific Ocean off Japan's northeast coast where two tectonic plates violently collide. No doubt Japanese engineers have collected in-depth theoretical and empirical knowledge on the effects of seismic waves on industrial systems, structures and components, particularly in nuclear engineering. All nuclear power stations in Japan are equipped with seismometers that record ground motion in the three spatial dimensions and automatically scram nuclear reactors at predetermined set-points.

Aerial photograph of Kashiwasaki-Kariwa NPS. Older Units 1 (bottom, right) to 4 (top) are in the foreground. The most recently built Units 5 to 7 are in the background (top, left) (source: TEPCO).

Kashiwasaki-Kariwa NPS
As recently as Jul. 16, 2007, a M_w 6.6-magnitude (I_JMA 6.8) earthquake known as the Niigata-ken Chuetsu-Oki Earthquake, or NCO Earthquake for short, struck Kashiwazaki-Kariwa Nuclear Power Station (KK) on Japan's northwest coast. The NPS, with seven reactors and 8.2 Gigawatt electrical power output the world's largest, is owned by the same utility as Fukushima Dai-ichi NPS, that is TEPCO. The quake automatically tripped the four reactors operating that day.

Kayen and others (2007) [6] examined the geotectal causes for the quake in great detail and compiled a comprehensive account on land failure and the damage to infrastructure in the region. I have explained fundamental wave concepts underlying earthquakes in the context of the Great Haiti Earthquake of Jan. 12, 2010, in my post with the title "Neuroanatomy of an Earthquake" published Jan. 18 of that year. In this essay, I shall attempt to explain in more detail measurements of ground motion, projections of observations, and their relationship with quake damage at nuclear power stations.

Ground Motion Acceleration Units, Dimensions, and Conversions

1 Gal (Galileo) = 1 cm/s²

1 g (fraction of gravitational acceleration) = 980 Gal

At Kashiwasaki-Kariwa NPS, operators were able to recover recordings of ground motion from only one third of roughly 100 recording locations [7]. Fortunately, recordings could be retrieved from the bottom floors of the seven reactor buildings, known as base mats.

As typical examples, the accelerogram for the east-west ground motion recorded on the base mat of Kashiwasaki-Kariwa NPS Unit 2 is shown below as well as the observed acceleration response spectrum (thick line) and the design-basis acceleration response spectrum (S2, thin line) for the same location. The latter is computed, using synthesized time histories of past earthquakes.

TEPCO [8]

Peak ground acceleration (PGA) on the reactor building base mats exceeded the design basis for all seven units. The greatest PGA was recorded at Kashiwasaki-Kariwa NPS Unit 1. The worst exceedance was 3.6-times design basis at Kashiwasaki-Kariwa NPS Unit 2 [8]. Note 5% damping is assumed in the calculations, because the structures absorb quake energy [9].

TEPCO [8]

The trip points for automatic reactor shutdown known as SCRAM were set at 100 Gal (0.10 g) in the horizontal directions and at 120 Gal (0.12 g) in the vertical direction [10]. The quake caused a massive switch yard transformer fire. Radioactive gaseous effluent was released into the atmosphere because of a malfunction of the HVAC system. The amount was deemed small enough not to pose a threat to the public.

Visual inspections were carried out during extensive post-quake walkdowns, as well as pressure and function tests, on about 20,600 components, instruments, panels and valves, and 155,000 meters of pipe [10]. Nuclear power station systems, structures and components (SSC) are classified in three groups of importance for seismic safety. SSCs in the most important group did not appear to have incurred any damage at Kashiwasaki-Kariwa NPS. This group is known as class A in Japan and seismic design category I, or SDC-I for short, in the US (Antaki and Johnson, 2011) [11]. Damage, deformation, and impeded function were uncovered only on class B (SDC-II) and C (SDC-III) systems, structures and components deemed unimportant to reactor safety.

Using advanced ultrasonic technology, inspectors detected some hidden damage. For example, cracks were found in the rotor shaft and blades of power-generating low pressure main steam turbines. TEPCO concluded that this damage was a result of fatigue rather than the quake.

All SSCs at which damage was discovered were repaired. Because of the broad exceedance of the design basis earthquake at the seven units, additional safeguards were installed, and new spectra were computed to ensure that the implemented measures strengthened the design basis.

The apparent disassociation between earthquake magnitude, observed peak ground acceleration and damage led to the search for new indicators better predicting quake damage. Apparently, the currently used measurements of magnitude and ground motion do not adequately appreciate the temblor's duration. To remedy this shortcoming, cumulative absolute velocity expressed in cm/s or g-sec, CAV for short, was introduced. CAV constitutes the sum of the ground accelerations added over the duration of the quake. Standardized cumulative absolute velocities are filtered for non-damaging ground motion frequencies.

Standardized cumulative absolute velocities [g-sec] are plotted against the magnitude of quakes that shook nuclear power stations in Japan before 2011. Magnitude is expressed as seismic intensity on the I_JMA scale used by the Japan Meteorological Agency (JMA). Historically, trained JMA agents qualitatively scored quake magnitude. Today, instruments register quake intensity automatically. Damage was detected at main turbines, that is SSCs unimportant to reactor safety. The data suggest a damage threshold for such SSCs at CAV 0.8 g-sec and I_JMA 5.5 [12]. The lowest PGA the JMA allows at this intensity is 0.25 g. 
No quake damage is anticipated for PGAs below 0.25 g.

Foreshadowing the reactor catastrophe at Fukushima, however, in tests during the restart of Kashiwasaki-Kariwa Unit 7 in May 2009 the Reactor Core Isolation Cooling system, or RCIC for short, did not perform as expected because of valve failure. RCIC steam entered a pressure control chamber at the bottom of the reactor pressure vessel, and the cooling water in the chamber rose. Operators had to intervene. At Fukushima Dai-ichi NPS, the RCIC systems of the two operating reactors outfitted with this system would fail last March, profoundly impeding the operator's efforts of preventing the reactor fuel cores from melting down. I have described the weaknesses of the RCIC system in detail in my post with the title "Fukushima: Failure by Design" published Jul. 3, 2011.

TEPCO [13]

Fukushima Dai-ichi NPS
TEPCO insists that the reactor fuel core meltdowns at Fukushima Dai-ichi NPS were precipitated by the total station blackout caused by the loss of emergency diesel power through tsunami inundation  [13]. The company has given seismic impact as cause little credence. TEPCO's interpretation is supported by the ground motion recorded on the base mats of the reactor buildings. Though PGAs broadly exceeded design basis in the horizontal east-west direction at Fukushima Dai-ichi NPS, the greatest exceedance was roughly a third of that found at Kashiwasaki-Kariwa NPS after the NCO Earthquake, that is the greatest exceedance was only 1.26-fold at Unit 2. CAV values have not yet been released.

Aerial photograph of Fukushima 1 (Dai-ichi) NPS taken Sep. 18, 2010. Units 4 (left) to 1 (right) were built in a row. Unit 1 closest to the water front is the oldest commercial nuclear power reactor of Japan completed in 1971. Units 5 and 6, set apart on the upper right, are the youngest reactors at the station (source: cryptome.org).

Of the six reactors at Fukushima Dai-ichi NPS, units 5 and 6 were least affected by the tsunami and exhibited the smallest PGA exceedance [13]. They were disconnected from the power grid when the quake struck, undergoing tests fully loaded with fuel. Combined with the findings at Kashiwasaki-Kariwa NPS after the NCO Earthquake, the seismic impact uncovered at these units may provide valuable lessons for predictions of future quake damage at nuclear power stations.

In fact, the lessons learned from both quakes in Japan stipulate that ground motion in exceedance of the design basis may impact nuclear power station systems, structures and components (SSC) in subtle ways. Although quake damage may mainly be detected on less quake-resistant SSC categorized B (SBC-II) and C (SBC-III), the subtleties warrant in-depth inspections for potential hidden damage of all SCC, regardless of class.

North Anna NPS
On Aug. 23, 2011, the Northeastern Seaboard of the U.S. was struck by a magnitude M_w 5.9 earthquake centered near Mineral, Virginia, in the Central Virginia Seismic Zone close to the North Anna Nuclear Power Station with two pressurized water reactors.

USGS [14]

Both reactors were shut down safely. The quake was felt as far as Boston, Massachusetts [15]. By and large minor damage was found at monuments, as well as governmental, residential and commercial buildings, extending over a surprisingly large area. This Washington Post picture gallery with the title "Magnitude-5.8 earthquake shakes D.C." provides a sweeping impression of the quake's impact.

The damage, combined with subjective experience, matches the criteria of a six-lower (6弱) magnitude quake on the Japan Meteorological Agency (JMA) shindo scale, pegging the quake's PGA between 0.250 g and 0.315 g. In accord, the United States Geological Service (USGS) estimates 0.26 g [14].

According to the findings in Japan discussed above, this peak ground acceleration merely touches the threshold at which damage to systems, structures and components must be expected at nuclear power stations. Based on observations after the 1995 South Hyogo Prefecture Earthquake, also known as the Kobe Earthquake, Ochiai and others (2010)[12] suggest that the potential damage at nuclear power plants with ground shaking of the magnitude of the Virginia quake may comprise: falling air ducts, tumble (with weak anchorage), failure of foundation anchorage, circular storage tank wall buckling (elephant foot), contact/hitting of pipes (insulation and grating damage), and buckling of crane basements. On occasion, pulling out/fracture of anchor bolts, overflow (sloshing), pipe support structure damage (pulling out of anchor bolts), and failure of transmission line support may be observed [12].

North Anna NPS is the first nuclear power station in the Central Eastern United States (CEUS) at which reactors scrammed because of a seismic event. The reactors at North Anna NPS lack automatic seismic scram systems. The reasons for the scrams are not yet fully understood. Offsite power was lost because of ground motion. Emergency diesel generators started up. The ensuing power fluctuations may have tripped the reactors. Seismic recordings were obtained from the Unit 1 reactor containment base mat as well as the turbine buildings and sent out for evaluation in the days after the incident by the station's operator, (Dominion) Virginia Electrical and Power Company, or VEPCO for short. In addition, VEPCO immediately carried out walkdowns, visually inspecting the station's systems, structures and components (SSC) for damage. Operability and performance was examined extensively. Underground piping was unearthed, checked, and pressure-tested. Welds were examined with ultrasonic devices.

In public meetings at Nuclear Regulatory Commission (NRC) headquarters near Washington, D.C., on Sep. 8 and Oct. 21, 2011, VEPCO reported preliminary findings. VEPCO is determined to restart the reactors as soon as the inspections are completed, and the NRC approved the results.

Webcast of the September 8, 2011, NRC meeting with VEPCO on the Aug. 23 Virginia Earthquake near North Anna NPS (Docket Nos. 50-338 and 50-339; slides).

Webcast of the October 21, 2011, NRC meeting with VEPCO on the Aug. 23 Virginia Earthquake near North Anna NPS (slides).

At the Oct. 21 meeting, VEPCO presented accelerograms retrieved from the reactor building base mat of North Anna NPS Unit 1. Note the short duration of the quake. The figure below as well as other North Anna NPS data shown in this post were released for the two NRC meetings.

Accelerograms showing ground motion on the base mat of the North Anna NPS Unit 1 reactor building in the three spatial directions (left ordinate: acceleration [Gal]; right ordinate: [g], abscissa: time [s]). Note the ground motion at the NPS lasted only 3.1 seconds (VEPCO, Oct. 21, 2011).

In addition, VEPCO provided the peak ground accelerations (PGA) the company used to calculate the design basis spectra for North Anna NPS.

Stipulated peak ground acceleration limits for rock (reactor building foundation) and soil (foundation of turbine buildings and ancillary structures) by ground motion direction for Operating Basis Earthquake and Design Basis Earthquake (VEPCO, Sep. 9, 2011).

The stipulated PGAs vary for ground motion direction and ground type, that is rock and soil on which the reactor buildings and ancillary structures are built, respectively. According to Antaki and Johnson (2011)[11], NPS systems, structures and components are designed to withstand ground motion based on the operation basis earthquake, or OBE for short, which is anchored at half the PGA used to determine the Safe Shutdown Earthquake (SSE), that is at the ground motion at which the reactors are to be shut down to protect their safety. By contrast, the design basis earthquake, or DBE for short, is calculated to envelope the expected ground motion envisaged at 80 percent of the NPS sites in the Central and Eastern U.S., assuming an OBE equal to one-third or less of the SSE. Dependent on NPS location, the PGA used to determine the SSE is set between 0.1 g and 0.3 g, which is equal to or higher than the trip points for automatic seismic scram systems used at Japanese nuclear reactors. In sum, the reactors are supposed to be shut down at peak ground accelerations much below DBE. However, note that the greatest anticipated DBE at North Anna NPS is 0.18 g, which is below the USGS estimate for the Virginia quake. Therefore, it is not surprising that recorded peak ground motions were greater than assumed in the DBE.

Recorded (ondulating curves) and computed (smooth curves) seismic response spectra at 5 % percent damping for the vertical (top) and horizontal (bottom) directions at North Anna NPS Unit 1 reactor building base mat (ordinate: acceleration [g]; abscissa: frequency [Hz]). The bottom and middle smooth curves in each graph describe OBE and DBE, respectively. Note that the recorded ground motion exceeded DBE at frequencies greater than 1.0 Hz in the vertical as well as in the north-south direction, and greater than 10.0 Hz in the east-west direction (VEPCO, Sep. 9, 2011).

The two graphs above show the seismic response spectra constructed from the data VEPCO recovered from the base mat seismometers of the North Anna NPS Unit 1 reactor building. Clearly, PGA exceeded DBE at frequencies greater than one Hertz. Damage was to be expected at class B and C systems, structures, and components, that is SSCs unimportant for reactor safety.

However, VEPCO reasoned that the anticipated damage might not have been detrimental to the safety of North Anna NPS, because quake duration and DBE exceedance were short in comparison with other known damaging quakes. The operator reasoned that cumulative absolute velocity (CAV), which takes the duration of the quake into account, represents a more accurate damage predictor than PGA, as discussed in context with the NCO Earthquake and the Tohoku Earthquake. VEPCO therefore presented standardized CAV values at the NRC meetings.

Standardized cumulative absolute velocities [g-sec] calculated by three expert companies from the accelerations detected on the base mat of North Anna NPS Unit 1's reactor building for ground motion in the three spatial directions. Note that north-south direction motion exceeded the NRC prescribed limit of 0.16 g-sec below which a nuclear power reactor can be operated safely (VEPCO, Sep. 9, 2011).

Standardized cumulative absolute velocities [g-sec] calculated from recordings and modeled for the three spatial directions of motion on the North Anna NPS Unit 1 reactor building base mat. The observed velocities did not exceed the modeled design base earthquake (DBE) and the limits modeled in the Individual Plant Examination for External Events (IPEEE) (VEPCO, Oct. 20, 2011).

The standardized cumulative absolute velocities determined for recorded north-south ground motion on the base mat of North Anna NPS Unit 1 reactor building exceeded OBE, necessitating the shutdown of both units. Yet, the values remained below the cumulative absolute velocities calculated for DBE, as well as the higher limits calculated in the Individual Plant Examination for External Events (IPEEE) stipulated by the NRC. The IPEEE, but not the DBE values, are above 0.8 g-sec, that is the CAV threshold for quake damage at nuclear power stations in Japan, according to the CAV/quake magnitude plot of Ochiaia and others (2011)[12]. The authors note only one incident with damage that occurred at a CAV below 1.5 g-sec. Main steam turbine damage was detected at Hamaoka NPS Unit 5 after the Aug. 11, 2009, Shizuoka Earthquake, which scrammed the station's units 5 and 6.

By contrast, the greatest recording-based CAV at Kashiwasaki-Kariwa NPS reached 2.8 g-sec, 16-times the highest CAV at North Anna NPS. For ground motion predictions, Campbell and Bozorgnia (2011) plotted the standardized cumulative absolute velocity against I_JMA, that is the quake intensity the Japan Meteorological Agency (JMA) uses (Fig. 4.1) [16]. The plot shows a close direct proportionality between the logarithm of cumulative absolute velocity and the JMA quake intensity scale. VEPCO's cumulative absolute velocities for recorded ground motion fall short of the range between 0.3 and 2.0 g-sec (median = 1.2 g-sec) that the plot predicts for the Virginia quake, though the qualitative descriptions of the damage and the quake experience in the region closely match the ones provided by JMA for a quake of this magnitude. Therefore, VEPCO's cumulative absolute velocities may underrate the possible extent of the damage.

North Anna NPS lost offside power because of a disruption in the switchyard, necessitating the startup of emergency diesel generators. This event arguably represents the most consequential impact of the Virginia quake on the NPS known to date. So far, the station inspection uncovered minor damage to SDC-II and SDC-III systems, structures, and components unimportant to reactor safety. Intriguingly, the inspectors noted quake impact similar to that found at Fukushima Dai-ichi Units 5 and 6, though the recorded peak ground acceleration at North Anna NPS was only roughly half of that observed at these units and, according to the Japan Meteorological Agency, the quake reached six-upper (6強) on the shindo scale in the region with seismic intensities I_JMA between 6.0 and 6.4. By contrast, the Virginia Earthquake qualifies for six-lower (6弱).

As examples for similar quake impact at Fukushima Dai-ichi and North Anna nuclear power stations, two types of visible damage are explored below:
Component Relocation: At Fukushima Dai-ichi Unit 5's turbine building, a support for the moisture separator drain pipe shifted,

Moisture separator drain pipe support moved in Fukushima Dai-ichi Unit 5 turbine building (TEPCO).

while at North Anna, 25 of 27 spent fuel dry storage casks shifted.

Dry casks for spent fuel moved at North Anna NPS storage area (VEPCO). 

Concrete damage: At Fukushima Dai-ichi Unit 6's turbine building, the concrete pedestal of the feedwater heater chipped at the edges,

Fukushima Dai-ichi Unit 6 concrete pedestal chipping at feedwater heater in the turbine building (TEPCO).

while at North Anna the concrete pedestal of a demineralizer chipped at the edges.

North Anna NPS turbine building chipping of concrete base for demineralizer (VEPCO).

These similarities suggest that it may be imprudent to underestimate the possibility of damage to North Anna NPS comparable to that observed at nuclear power stations in Japan after a 6-upper quake, because of similar differences between ground motion anticipated in design and ground motion actually occurring. Notably, differences in ground motion between the reactors built on rock and ancillary structures and components built on soil may exacerbate quake impact on their connections.

Outlook
Learning lessons from the Japanese experience, three areas of focus important to safety at North Anna Nuclear Power Station emerge:

1. Seismic risk reduction through automatic seismic scram
   Only few commercial nuclear power stations in the U.S. are outfitted with the equipment needed to automatically scram the reactors when they are subjected to ground motion reaching stipulated setpoints. The Sep. 8, 2011, meeting revealed that the control room operators at North Anna NPS lacked information on containment motion when they needed it most. At the instant of the quake, seismic instruments had lost power. The alarms that would have informed the operators whether the ground motion warranted a shutdown failed. On Sep. 28, VEPCO informed the NRC that negative power flux rates at the station caused the scrams. The precise cause of the of the negative flux rates remains to be resolved. Automatic seismic scram systems may have benefitted the operators in shutting down the station's two reactors more expediently. VEPCO should be asked to evaluate whether state-of-the-art automated seismic scram systems would reduce current levels of seismic risk. Moreover, it seems prudent to independently retrofit all seismically important instrumentation separately with 24 hour-lived batteries. Additional, solar power may extend battery life.
2. Seismic risk re-evalution of the ultimate heat sink
   The reactor core decay heat persisting after a scram must be transferred to the ultimate heat sink. For example, North Anna NPS uses Lake Anna, a dammed lake created for the purpose, as ultimate heat sink. The heat transfer to the ultimate sink is crucial to cooling the reactors, preventing fuel core meltdowns with disastrous consequences as the reactor accidents at Fukushima 1 NPS last March strikingly demonstrate. The preservation of the ultimate heat sink and its use via residual heat removal systems are of existential importance to reactor safety.
   
   
   Lake Anna's water level had fallen two feet after the Virginia Earthquake Aug. 23, 2011 (source: WTVR CBS 6 Richmond, VA) [17].
   According to CBS Channel 6 News, Richmond, VA, the water level of Lake Anna had fallen roughly two feet after the quake [17]. At the Sep. 8 meeting in Rockville, VEPCO stated that an inspection of the dam provided no evidence of quake damage. At the Oct. 21 meeting, VEPCO elaborated that Lake Anna and its main dam do not represent the ultimate heat sink for the station, but that this function is served by a smaller body of water separated from the lake by another dam. VEPCO inspected this dam and found no damage. Regardless of this barrier, the lake represents the ultimate heat sink, if only indirectly. The question what might have precipitated the reported rapid loss of water from the lake remains to be answered. Uncovering the cause for such drastic water level diminution, should it be confirmed, might be of profound importance for the seismic safety of North Anna NPS, because Lake Anna constitutes the ultimate heat sink for the decay heat of the reactor fuel cores during shutdown. According to wikipedia, earthquakes of M 4.0 and greater may trigger tsunamis in lakes. Depending on the exact ground motion and the shape and size of the lake, small transient rises on one side may be accompanied by larger drops on the other.
3. Seismic risk re-evaluation of all emergency core cooling system SSC
   All SSC of the emergency core cooling system, including condensate storage tanks, are important to seismic safety and should be tested structurally and functionally. Particular attention must be paid to SDC-II and III SSC built on soil that are connected with SDC-I SSC built on rock, because soil ground motion is commonly greater than rock ground motion. Differences in ground motion between rock and soil may result in damage to the connections between SSC built on either substrate. At Fukushima Dai-ichi NPS, efforts to prevent the meltdowns of the fuel cores of units 2 and 3 were rendered ineffective by the failure of reactor core isolation cooling (RCIC) systems and high pressure coolant injection (HPCI) systems. The meltdown at Unit 1 was precipitated by a failed isolation condenser (IC). Combined with the failure of the Residual Heat Removal (RHR) system primarily attributed to tsunami damage, these shortcomings contributed in no small part to the failure of the emergency core cooling system (ECCS). The inspections at North Anna NPS must ensure that all SSC of the ECCS conform to the highest seismic safety level and can withstand the ground motion the station is anticipated to incur by substantial margin.

We shall see how VEPCO's re-evaluation of North Anna NPS proceeds. Like at Kashiwasaki-Kariwa NPS Unit 7, function tests of ECCS components during restart will show whether the reactors can be operated safely. Decisive insights will be gained at the very end.


Acknowledgment
I am indebted to the numerous commenters on scribblelive.com Japan Earthquake5 without whose up-to-date input I could not have written this post. The information on www.simplyinfo.org was of invaluable help. I thank MJ Racer whose cogent post on physicsforums (comment number 10945) provided the lead for this essay.

References

1. Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety -The Accident at TEPCO’s Fukushima Nuclear Power Stations- submitted to the International Atomic Energy Agency Jun 7, 2011.
2. Additional Report of Japanese Government to IAEA - Accident at TEPCO's Fukushima Nuclear Power Stations Transmitted by Nuclear Emergency Response Headquarters, Government of Japan, 15 Sep 2011
3. Kimura S (Sep 24, 2011) Cesium exceeding safety threshold detected in Fukushima rice. Asahi Shimbun, Tokyo
4. NHK World News (Jul 19 ,2011) Govt bans beef cattle shipments from Fukushima. Japan Broadcasting Corporation, Tokyo.
5. The Japan Times Online (Oct 12, 2011) Yokohama finds high strontium-90 levels. The Japan Times, Kyodo.
6. Kayen R, Collins BD, Abrahamson N, Ashford S, Brandenberg SJ, Cluff L, Dickenson S, Johnson L, Kabeyasawa T, Kawamata Y, Koumoto H, Marubashi N, Pujol S, Steele C, Sun J, Tanaka Y, Tokimatsu K, Tsai B, Yanev P, Yashinsky M, Yousok K (2007) Investigation of the M6.6 Niigata-Chuetsu Oki, Japan, Earthquake of July 16, 2007: U.S. Geological Survey, Open File Report 2007-1365.
7. Measures taken by the Nuclear and Industrial Safety Agency concerning the Kashiwazaki-Kariwa Nuclear Power Station, affected by the Niigataken Chuetsu-oki Earthquake (2nd Interim Report), June 29, 2009, Nuclear and Industrial Safety Agency.
8. TEPCO Press Release (Jul 30, 2007) The (First) Report on the Analysis of Observed Seismic Data Collected in Kashiwazaki-Kariwa Nuclear Power Station on the Occasion of the Niigata-Chuetsu-Oki Earthquake in 2007. (Appendix)
9. Naeim F, Kircher CA (2001) On the damping adjustment factors for earthquake response spectra. Struct Design Tall Build 10: 361–369.
10. Gaku, S (2010) Experience of NCO Earthquake and Restart of Kashiwazaki-Kariwa NPP. 1^st Kashiwazaki International Symposium on Seismic Safety of Nuclear Installations and Embedded Topical Meetings: C-13.
11. Antaki G, Johnson J (2011) Seismic design and retrofit of essential systems in nuclear power plants. Becht Nuclear Services.
12. Ochiai, K, Kobayashi K, Chigama A (2010) Damage Indicating Parameters and Damage Modes of Mechanical Components. 1^st Kashiwazaki International Symposium on Seismic Safety of Nuclear Installations and Embedded Topical Meetings: C-24.
13. Additional Report of Japanese Government to IAEA - Accident at TEPCO's Fukushima Nuclear Power Stations Transmitted by Nuclear Emergency Response Headquarters, Government of Japan, 15 Sep. 2011.
14. Leith W (2011) Comments on the Japan Near-Term Task Force Report.
15. Joe Dwinell J, Sherman N (Aug 23 ,2011) Virginia quake shakes up Boston. The Boston Herald, Boston.
16. Campbell KW, Bozorgnia Y (2010) Analysis of cumulative absolute velocity (CAV) and JMA instrumental seismic intensity (I_JMA) using the PEER-NGA strong motion database. PEER 102.
17. CBS 6 WTVR News (Aug. 25, 2011) Mineral residents concerned about nuclear power plant. WTVR CBS 6 Richmond.

Related Posts

• Fukushima: Failure by Design
• The Enigma of 1 Fukushima 4 号機
• Fukushima: Failure of the Mind
• Ionizing Radiation & The Mind

Addenda

• In response to the comment to this essay, I provide a Google-maps image of North Anna NPS. Looking over the station's lay-out, I find it difficult to conceive that Lake Anna does not represent the ultimate heat sink for the station as the commenter claims. I leave the decision to the experts (06/12/2011).

Bird's eye view of North Anna NPS. Note Lake Anna and two separate small bodies of water on the premises of the station that could potentially serve as the ultimate heat sink (courtesy Google maps).

• Listen to WVTF Radio IQ's Evening Edition for Nov. 22, 2011. There are three contributions. In the second contribution, Beverly Amsler interviews Virginia Tech seismologist James Martin about the after-effects of the Aug. 23, 2011, Virginia Earthquake. Dr. Martin's assessment suggests that it would be only prudent to retrofit all nuclear power stations on the Northeastern Seaboard with automatic seismic scram systems like nuclear power stations in Japan. The broadcast can be heard with the embedded mp3-player below (06/01/2012):
  
  

  Hosted by kiwi6.com file hosting. Download mp3 - Free File Hosting.
• The damage rendering the emergency core cooling systems of the reactors at Fukushima Dai-ichi NPS inoperable seems to have been mainly inflicted by flooding because of the tsunami. According to wikipedia, tsunamis do not occur in oceans alone, but can also be generated in lakes by fault displacement beneath or around the body of water, resulting in earthquakes of 4.0 magnitude or greater.