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SATURDAY, 19 MARCH 2011 18:50 ANJO C. ALIMARIO / RESEARCHER/WRITER
WITH the current nuclear crisis in Japan, global debate on the issue of nuclear power has been raised among experts and various stakeholders. In the Philippines alone, the revival of the mothballed Bataan Nuclear Power Plant (BNPP) has stirred mixed reactions.And for Dr. Alfredo Mahar Lagmay of the University of the Philippines (UP) National Institute of Geological Sciences (NIGS) and a team of geologists, the BNPP site is not suitable as validated by their study that dealt with geohazards of a volcano near the plant based on international guidelines set by the International Atomic Energy Agency (IAEA).
The 16-man team, led by Lagmay, conducted a study on the geological hazards of southwest Natib Volcano, the site of the BNPP.
According to the authors of the study, considerable geological data has been collected to assess the geological hazards of Natib Volcano and serve as scientific basis for the evaluation of the proposed recommissioning of the BNPP.
“The results presented in the study can be used for general hazards preparedness of communities in the area and by the government in their decision to recommission or condemn the nuclear power plant,” the abstract of the unpublished study read, a copy of which was obtained by the BusinessMirror.Pyroclastic density current deposits present
THE assessment of volcanic hazards at the site of the BNPP began with the identification of field deposits.
Mapping of the southwest sector of Natib Volcano, where the BNPP is located, revealed lahar and lava flow fields and at least six pyroclastic density current (PDC) deposits. At least 5 PDC deposits were identified in the southwest sector of Natib Volcano with three of the units directly underlying the site of the plant.
Lagmay noted that PDC is a hot volcanic flow ranging from 400-600 degrees Celsius and can travel up to 100 kilometer (km) per hour.
“The PDC deposit, which is adobe in layman’s term, suggests that a volcano nearby erupted violently in the past,” Lagmay explained to the BusinessMirror on Wednesday.
He added that portions of the deposits are pinkish in color with flattened pumice, which indicates that it was very hot when emplaced.
Moreover, a previously unidentified eruptive center located 5.5 km from the main building of the BNPP was mapped.
The study noted that the explosive pyroclastic deposits were identified as early as the late ’70s, collectively described as the Napot Point tephra. In their study, four of the PDCs are massive while two are stratified in character.
Mount Pinatubo, located more than 50 km away from the BNPP site and with topographic barriers in-between, may be excluded from the list of possible source volcanoes for the PDC deposits found in the study area.
Lahar deposits are widespread along the coastline of Napot Point where the BNPP is located. These deposits are found on top, below and in fault contact with other volcanic deposits.
This is not unexpected as the widespread occurrence of lahar in a seismogenic region can easily bring the lahar in contact with various deposits in different ways. Several lava flow ridges were also identified, including an eruptive center 5.5 km away from the nuclear site, the study reported.
Active faults near the site
One of the most important conditions to investigate when evaluating a site for nuclear installations is surface faulting. Capable faults are structures that are most relevant when evaluating the geologic features of the site.
The IAEA provides criteria in identifying whether a fault is capable or not. The first criterion is that the fault shows evidence of past movement or movements (significant deformations and/or dislocations) of a recurring nature within such a period that it is reasonable to infer that further movements at or near the surface could occur.
In highly active areas like the Philippines, where both earthquake data and geological data consistently reveal short earthquake recurrence intervals, periods of the order of tens of thousands of years may be appropriate.
The second criterion for a capable fault is it shows a structural relationship with a known capable fault such that movement of one may cause movement of the other at or near the surface.
Using the same criterion of the IAEA guidelines for seismic hazards, these tectonic structures in Napot Point are evaluated as capable faults because they show a structural relationship with the Lubao Fault, identified by Soria and Siringan (2009) as active.
The Lubao Fault is situated northeast of Natib Volcano.
Most of the faults identified in the southwest of Natib edifice are within 5 km of the nuclear facility with one thrust fault cutting rocks exposed on a cliff at the tip of Napot Point immediately outside the fenced perimeter of the BNPP and with its linear trace 200 meters (m) away from the nuclear reactor.
Besides Radon measurements perpendicular to the lineament trace yielding very high counts of Radon (222Rn) gas, there is yet no direct evidence of active fault movement on the southeast portion because of the absence of age dates of lithologies.
Measurements of Radon may suggest fault activity, Lagman noted.
Using the second criterion, however, there is reason to believe that the faults found in the 5-km vicinity of the BNPP are active. The active Lubao Fault, which is located northeast of the volcanic edifice of Natib has the same orientation and is colinear with the lineaments in the southeast of Natib’s edifice.
Faults are known to traverse volcanoes and many have been reported in the literature. One example is Mayon Volcano, which is traversed by the northern bounding fault of the Oas Graben.
Described as the Northern Oas Fault, this structure ends abruptly as it enters Mayon in the west and reemerges in the eastern side. The apparent disappearance of a linear fault on Mayon’s edifice is due to the conical shape of the volcano and active deposition of primary and reworked material on the edifice.
According to the IAEA guidelines, when evidence shows the existence of capable faults within 1 km of the nuclear facility, another site must be considered. Such is the case for the BNPP where capable faults associated with the Lubao Fault were identified within 1 km of the nuclear power plant.
IAEA draft guidelines
To determine whether a site should be excluded in the selection for a nuclear facility, the IAEA draft guidelines present the different phenomena associated with volcanic events that may pose potential hazards to a site.
According to these guidelines, a “Yes” in the site selection and evaluation column indicates that the presence of a significant hazard from this phenomenon generally constitutes an exclusion criterion. A “No” in the design column indicates that it is not practical to mitigate potential hazard by facility design or operational planning. (Refer to the table)
In the case of BNPP, the area is underlain by PDC deposits and lahars. Lava deposits and an eruptive center are also proximal to the BNPP site. Out of all these volcanic phenomena, the occurrence of PDCs and opening of new vents proximal to the site do not have any engineering solutions. Lava flow and lahar hazards, however, can be addressed by engineering design.
At Natib and Mariveles, there is more certainty about the physical characteristics of past events, such as their volume and spatial extent, than there is about the ages of these events.
Thus, the volcanic risk assessment for the BNPP leans more toward a deterministic analysis, focused on the geological characteristics of volcanic phenomenon and their spatial extent rather than estimation of the likelihood of the occurrence of hazardous phenomena.
For seismic hazard evaluation, the IAEA Safety Standard Series suggests relevant coverage areas for different levels of investigation. A radial extent of 150 km for regional investigation, 25 km for near regional investigation, 5 km for site vicinity and 1 km for the site area are the typical coverage of these investigations. Any geologic structure, therefore, within these coverage areas will have its corresponding effect on the nuclear power plant, the study said.
Significance of studying Natib Volcano
The BNPP was built in the late 1970s and early 1980s when safety of nuclear power-plant facilities were evaluated without the benefit of well-established, internationally accepted guidelines that set criteria and procedures for assessing potential volcanic hazards, the study said.
Permits for constructing nuclear installations were granted based on investigations carried out according to local practices prevailing at the time the site was selected and were assessed based on science that preceded many aspects of volcanology that have rapidly developed only over the past 30 years.
According to the study, there is still lack of adequate geologic maps of Natib Volcano until today, the same criticism given by experts of the IAEA, US geologists and oversight panels in the Philippines to the original hazard assessment for the BNPP.
Without detailed geological maps that identify the stratigraphy and distribution of Natib’s eruptive products, the volcanic hazards at the site cannot be assessed properly, Lagmay said.
Considering the importance of the study area not only because it is the site of a controversial nuclear power plant but also because of its proximity to Pinatubo and Metro Manila, which is only about 70 km away, it is puzzling that almost 30 years after mothballing the BNPP and nearly 20 years after Pinatubo’s cataclysmic eruption, the level of understanding of Natib Volcano is still at its present state, the study said.
Recognizing the need for better understanding volcanic hazards at Natib, a preliminary interpretation of the geology of the southwest sector of the volcano and stratigraphy of the BNPP site was provided in the paper for assessing nuclear-plant safety according to 1) the IAEA Draft Guidelines for volcanic hazards in site evaluation for nuclear installations; and 2) the recommended guidelines in a book authored by Hill in 2009 for volcanic hazards assessment for surface nuclear facilities.
According to the report, a capable volcano or volcanic field is one that may experience volcanic activity during the performance period of the nuclear installation and, as discussed by the IAEA, such an event has the potential to produce phenomena that may affect the site of the nuclear installation. The concept of capability of a volcano is introduced to define the state of a volcanic system and as a means for evaluating its potential reactivation.
Natib is considered a capable volcano based on its active hydrothermal system with a magmatic signature. Thus, the impacts of volcanic hazards to the site were assessed based on the draft guidelines provided by the IAEA.
“Although the work on Natib Volcano is still in progress, enough data has been gathered, sufficient for use as one of the scientific bases for the decision of the Philippine government to recommission or not, the mothballed Bataan nuclear facility and general hazards preparedness by communities on the slopes of the volcano,” the study said.
Lagmay pointed out that international guidelines set by international experts are not there to make exemptions for anyone.
“We must remember that Japan and the Philippines have the same geological setting.”
In Photo: (A) Faulted rocks in a cliff outside BNPP’s fenced perimeter at the tip of Napot Point. (B and C) Close-up view of the faulted rocks. (D) View of the faulted cliff relative to the BNPP property.
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