The terrible results of the earthquake and tsunami last week in Japan hold many lessons for engineers. It is of course obvious that the impact on human health, the search for and care of survivors should be the primary concern. However, much has ocurred and is occurring which can provide insight into the design of flooding control systems, earthquake-resistant building and infrstructure design, and the safety of nuclear reactor facilities.
While the most recent focus has been on the nuclear reactors and the damage to the spent fuel pool, a recent article in the New York Times (http://www.nytimes.com/2011/03/14/world/asia/14seawalls.html?_r=1&pagewanted=all) discusses the design of seawalls. As in the case of the hurricane protection system built in New Orleans (which failed in a spectacular way after Hurricane Katrina due primarily to poor design and outdated data, as well as the failure of backup pumps which should have pumped out some of the water which initially flowed over levees), the seawalls designed to protect shoreline areas including the nuclear power facilities were overwhelmed. The New York Times article (“Seawalls Offered Little Protection Against Tsunami’s Crushing Waves” by Norimitsu Onishi, 3/13/11) quotes one engineer , Peter Yanev, who points out the fatal miscalculation that “ the diesel generators [used to pump cooling water] were situated in a low spot on the assumption that the walls were high enough to protect against any likely tsunami.” While higher seawalls can be constructed, it is always possible that a wave too large even for a 40 foot high or more seawall may occur. This is not to say that seawalls are useless (and in fact have protected communities and power facilities from typhoons and smaller tsunamis). This just teaches engineers that the best “defense” against nature may be siting critical equipment (and in some cases entire facilities) in stable, protected locations, and also to use the principles of “absolute worst case design” in such cases.
Absolute worst case design (or just “worst case design”) is an important techniques (along with hazard analysis and redundancy) used to enhance reliability of complex systems. It is most often used in the case of electronics design, but also plays an importnat role in military and space systems. As you might guess, it starts with the basic idea that you design your system to withstand the worst possible operating conditions. We often note that electronics or mechanical devices designed for military use tend to be very expensive — in fact it is often a common criticism of expenditures for items built for the U.S. Department of Defense. Yet one contributing factor to this cost is the “worst case” design specifications used. A computer used in your home has far fewer requirments (in terms of reliability) than one designed to go into a tank or into a spacecraft. By developing design requirments which take into account extreme conditions coupled with the need for high reliability, engineers can create systems able to handle harsh conditions without fail. This concepts should certainly be applied to nuclear reactor components, including cooling systems.
If you are interested in reading more about the U.S. Army’s “design for reliability” practices, there is a handbook available at http://www.amsaa.army.mil/ReliabilityTechnology/RelFiles/Design%20for%20Reliability%20Handbook.pdf.
One other concept which is very important in ensuring reliability of critical systems is the use of engineering standards. Standards for nuclear power facilities (both for design as well as for operations and maintenance — including handling fuel and waste) are some of the most complex and rigorous ever developed. For example, the American Nuclear Society maintains a set of standards which consider everythign from “Nuclear Criticality Safety Training” to “Containment System Leakage testing requirements” to “Nuclear Plant Response to an Earthquake”. ANS, with the experience of many engineers and scientists to guide them, have developed standards for fuel handling, determining the impact of weather on facilities, alarm systems and reactor design. (see http://www.new.ans.org/standards/).
In 2006, ANS published a position statement on Nuclear Facility Safety Standards (http://www.ans.org/pi/ps/docs/ps24.pdf). In it, they state:
” The American Nuclear Society believes that consistent application of such standards provides a high level of safety. The ultimate responsibility for ensuring safety, however, rests with the operator of the nuclear facility in rigorously applying these standards. An effective and independent regulatory authority is also essential.”
As always, while use of standards is critical, engineering design is essentially a “human” enterprise, and it is up to those who design, operate and maintain nuclear facilities to make safety their highest priority — a lesson learned from Three Mile Island and Chernobyl as well.