This focus on “modernization” is misguided. What really matters is not how modern the technology is but whether the weapons are reliable, safe from accidental detonation and secure from unauthorized nuclear detonation.

For many years the nuclear weapons labs have assessed the reliability of U.S. weapons through a surveillance program in which they remove them from deployment, subject them to extensive tests, and then return them to the field. Currently, the labs examine 11 of each of the seven warhead types in the U.S. stockpile each year.

Public data is available for this stockpile surveillance from 1958 through 1994, during which time over 13,800 weapons were tested. For about 180 weapons (1.3 percent), the tests identified failures that would have prevented the weapon from exploding with the power it was designed to have. (This doesn’t necessarily mean the warhead was a dud or that any target would not have been destroyed.) In any case, U.S. nuclear weapons were more than 98 percent reliable—far more reliable than any other U.S. weapon system.

While the data since 1995 has not been made public, it is safe to say that the reliability has not dropped. If it had, the weapons labs would have used such information to make their case when they lobbied for a new “reliable replacement” warhead a few years ago.

U.S. nuclear weapons also meet stringent safety standards. They are designed so that the probability of an unintended nuclear explosion is less than one in a billion, and less than one in a million if there is an accident such as a lightning strike or a fire. Similarly, U.S. weapons are designed to be very difficult to detonate without authorization.

Also, the U.S. weapons labs have programs dedicated to maintaining the reliability, safety, and security of the nuclear arsenal. Under these Life Extension Programs (LEPs), all U.S. nuclear weapons either have undergone or will go through a program to extend their functional lives.

LEPs recently were completed on the B61-7 and B61-11 bombs, extending their lives for another 20 years. Currently, the W76 warhead deployed on the Trident submarine-launched ballistic missile is undergoing a LEP to extend its life for another 30 years.

These programs replace components that age in ways that could affect the performance of the weapon. Typically the replacements are identical versions that are newly-manufactured. In other cases, the replacements are based on newer technology. But the goal is not to incorporate newer technology, per se, since that could result in unanticipated problems. The old adage “If it ain’t broke, don’t fix it” applies here.

Many components do not need to be replaced because they are judged to be stable over lifetimes that exceed the age of the weapon.

A case in point is the lifetime of the plutonium “pit” that is at the core of every nuclear weapon. Until recently, the Department of Energy (DOE) believed the maximum lifetime of a pit was about 45 years, and that new pits would soon need to be produced to replace aging ones. However, based on “accelerated aging” experiments in which the aging process is artificially sped up, DOE announced in 2006 that the lifetime of pits was at least 85 to 100 years and probably longer.

Senator Kyl can get a lot of attention by raising the specter of an out-of-date nuclear arsenal.  But what we need to focus on is what really matters--the reliability, safety, and security of U.S. nuclear weapons. That means making sure that the weapons labs devote enough resources and staff to test the full complement of 11 weapons for each warhead type each year. It means that the labs continue the plutonium accelerated aging experiments, so they can learn more about the lifetime of plutonium. Rather than worry needlessly about whether our weapons are out of date, Senator Kyl should work to ensure there is adequate support for this sort of basic, perhaps even mundane, ongoing research and surveillance.

Gronlund is Senior Scientist and Co-Director of Global Security Program (GSP), Union of Concerned Scientists (UCS). Stephen Young is Senior Analyst and Washington Representative in Global Security Program.