Diablo Canyon Nuclear Power Plant: The WikiBook/Overview of DCPP earthquake issues

Diablo Canyon (Nuclear) Power Plant, located on the water's edge in San Luis Obispo County California, was originally designed to withstand a 6.75 magnitude earthquake from four faults, including the nearby San Andreas and Hosgri faults, but was later upgraded to withstand a 7.5 magnitude quake. It has redundant seismic monitoring and a safety system designed to shut it down promptly in the event of significant ground motion. ]]

Introduction
Diablo Canyon (Nuclear) Power Plant (DCNPP/DCPP)is located proximal to the Los Osos, Hosgri, San Andreas and Shoreline earthquake faults. Many of these were discovered late in the process of design and construction, necessitating redesign. Construction errors compounded the delays and expense before the plant went online and began producing. Thereafter, discovery of the Shoreline fault caused a round of controversy leading Assemblyman Sam Blakeslee to introduce legislation mandating 3-D seismic studies. The plant is up for relicensing and, in the Fukushima aftermath, there is renewed opposition due in part to public perception that the risk of earthquake or tsunami make the plant unsafe. On the other hand, company officials contend that the plant is inherently safe from tsunami due its situation on an 85 foot coastal bluff. Re-licensing is contingent upon consistency with the Coastal Act and thus review by the California Coastal Commission, however seismic issues are more properly within the purview of the Nuclear Regulatory Commission. Much to the chagrin of some parties, such as US Senator Markey, the NRC in June 2011 announced that it had already completed its Safety Evaluation Report (SER) for the plant.

Public controversy
Critics contend that the Diablo Canyon (Nuclear) Power Plant was built so close to a set of geological fault lines that it is for all practical purposes to be regarded as built "directly over" an earthquake fault. This is the Hosgri fault which was discovered while the plant was under construction, and led to massive redesign. More recently, the Shoreline fault was discovered, a few hundred meters offshor, sparking renewed interest in the controversy.

Early geologist perspectives
From a more scientific perspective, the geology is as follows:" Three Pliocene-Miocene marine sedimentary units dominate the geology of Diablo Canyon: the Pismo Formation, the Monterey Formation, and the Obispo Formation. According to a Lawrence-Berkely report entitled Geologic Investigation of a Potential Site for a Next-Generation Reactor Neutrino Oscillation Experiment – Diablo Canyon, San Luis Obispo County, CA the area is tectonically active, located east of the active Hosgri Fault and in the southern limb of the northwest trending Pismo Syncline." The Obispo Formation is made up of marine and volcaniclastic sedimentary rocks. In 1985, the USGS of the Department of Interior published a report by Kenneth Campbell, at that point in the history of earthquake science probably a "deterministic study", entitled "Empirical prediction of near-source ground motion for the Diablo Canyon power plant site, San Luis Obispo County, California". More recent studies include probabilitist seismic hazard assessment.

seismic source characterization studies
Various techniques are used to identify faults. These include:


 * aeromagnetic survey


 * marine magnetic survey


 * seismic reflection profiling


 * seismicity


 * sub-bottom profiling


 * geologic mapping


 * geodetic survey


 * gravity survey

Fuel rod assembly vulnerability
In response to concern that ground acceleration, or shaking, could cause spillage of submerged fuel rod assemblies which, upon exposure to air, could ignite,  PG&E and NRC regulators insist that the foregoing scenario is anticipated and controlled for, and that there is no basis to anticipate spillage. Additional seismic studies are in process, however completion of those studies is not a condition precedent to reissuance of the operating licenses for the two onsite units. In 1994, a probablistic seismic report was released Additional seismic studies are in process, however completion of those studies is not a condition precedent to re-issuance of the operating licenses for the two onsite units.

Risk estimates
The Nuclear Regulatory Commission's estimate of the risk each year of an earthquake intense enough to cause core damage to the reactor at Diablo Canyon was 1 in 23,810, according to an NRC study published in August 2010. At a community forum in San Luis Obispo, NRC, PG&E and academic experts presented information on seismic risks to the general public. This included a review of both older deterministic and more contemporary probablistic studies.

Post-Fukushima developments
In April 2011, in the wake of the Fukushima nuclear incident in Japan, PG&E asked the NRC not to issue license renewals until PG&E can complete new seismic studies, which are expected to take at least three years. The ongoing (as of 6/2011) seismic studies were recommended by the California Energy Commission and are approved and funded by the California Public Utilities Commission. Coomunity based organizations and activists contend that the plant is inherently unsafe due to the earthquake faults, the lack of knowledge about them, and a putatively inadequate track record of government and private industry in the anticipation and control of such hazards.