WASHINGTON — IN the spring of 1987 a United States Air Force helicopter dropped a series of special dummy bombs at remote Alaskan military ranges. The purpose of the secret tests was to see how far nuclear warheads could bury themselves in frozen soil and ice before detonation. High winds troubled the helicopter crew. Digging bombs out of the cement-hard impact areas was a trying task. But eventually the joint Air Force-Energy Department team found that a strengthened B83 nuclear bomb casing could burrow 11 to 13 feet into frozen strata.
As these tests show, the Pentagon is hard at work on a Strategic Earth Penetrator warhead capable of threatening hardened Soviet bunkers - including those in frozen Siberia. ``A great deal of emphasis'' has been placed on this new weapon, says a Lawrence Livermore National Lab report.
And that's not the only next-generation nuclear warhead now in US plans. Among other things, the Department of Energy (DOE) is working on several new short-range nuclear attack missiles. It is supposed to begin work on a new Stealth nuclear bomb in 1991.
Perhaps it's ironic that in this period of warming superpower relations, production of current-generation US nuclear warheads is scheduled to surge, at least for the short term. ``Total weapons deliveries increase by 27 percent between [fiscal] 1989 and [fiscal] 1990,'' the Energy Department stated in a recently declassified congressional hearing record.
Much of this jump is driven by ``the steep ramp-up in production for the W88,'' according to the Energy Department statement. W88 is the designation for the warhead atop the new Trident II submarine-launched missile.
Ever since the atomic age began, continual modernization of superpower nuclear arsenals has been a way of life. Since 1960, the US has spent about $142 billion in today's dollars on nuclear-weapons development and production.
To critics this process is the definition of overkill, as the point of more weapons can only be ``to make the rubble bounce,'' in Winston Churchill's classic phrase. To strategic planners, modernization is necessary to keep the nuclear superpower balance stable, as the US tries to maintain a nuclear arsenal that would still be fearsome after any kind of Soviet attack.
Consider the rationale behind the Strategic Earth Penetrator. In recent years the Soviet Union has increasingly buried underground key military facilities, such as command and control centers, and hardened the silos holding heavy nuclear missiles.
Defense Department officials say the US needs a way to threaten these targets so Soviet leaders know they might lose them if they were to start a war. And the blast of a nuclear weapon that penetrated 20 meters deep before exploding would be up to 50 times more effective against a buried target than an air burst would.
Intercontinental ballistic-missile reentry vehicles travel so fast they are not promising as earth-penetrating weapons - the warhead's internal mechanism would just shatter when it hit the ground. The Energy Department, which controls nuclear weapons development and production, is thus probably focusing on modification of existing weapons. These would include cruise-missile warheads or free-fall bombs for an interim earth-penetrating weapon, which enters the design and cost-study phase this year.
A follow-on specially designed penetrating warhead is still under study. And in 1991, according to Energy Department documents, weapons designers are also scheduled to begin concept definition of another type of specialized weapon - the low observable bomb for aircraft, or LOBFA.
Stealth bomber, sure - but why a Stealth bomb? A Pentagon weapons consultant theorizes that such a bomb would not give away the position of a stealthy plane to enemy radar when dropped. It might also be intended for external mounting on low-observable fighters or bombers.
Other new nuclear weapons listed as entering full-scale development in the next two years include several versions of a new air-launched short-range attack missile, and the follow-on to Lance, the new short-range nuclear weapon whose utility was recently the cause of a serious dispute between the US and West Germany. For possible application to the Strategic Defense Initiative, the DOE is also looking at three types of nuclear-driven directed-energy weapons: the X-ray laser, the optical-frequency laser, and the hypervelocity projectile project.
Currently there are 28 types of nuclear weapons in the US arsenal. Of these, eight are scheduled to be in production in 1990, according to budget documents prepared by the DOE for Congress. They range from two types of tactical bombs to the new Trident warhead.
The proposed 1990 DOE budget earmarks $4.6 billion for weapons research and production. That would represent a rise of about 9.5 percent over 1989 funds to support ``a greatly increasing weapons production workload,'' Army Brig. Gen. Paul Kavanaugh, a DOE deputy assistant secretary, told Congress in March.
But although production is increasing in the short term, the long-term trend is for the number of nuclear warheads in the US arsenal to stay about the same, as old warheads are retired on almost a one-for-one basis with new weapons, DOE officials said. Since 1984, the megatonnage, or total explosive power, of the US nuclear stockpile has continued to decline.
Of course, for at least the next several years, if not several decades, the primary nuclear weapons problem of DOE officials is not going to be assembly of new warheads.
With the US nuclear weapons production complex a worn-out, environmentally hazardous shambles after years of neglect, ``cleanup is the No. 1 priority,'' Troy E. Wade II, acting assistant secretary of energy for defense programs, told Congress. The proposed 1990 DOE budget includes $1.3 billion for defense waste cleanup and environmental restoration, and $304 million for new reactors to produce the radioactive material necessary in nuclear weapons building.