Washington — While President Reagan was offering a carrot of arms control flexibility at the United Nations Monday, the nuclear weapons stick that he feels is equally necessary to preserve peace continued to become more hefty.
The President a few days ago signed into law a defense authorization bill that includes $2.1 billion for the first 21 MX intercontinental ballistic missiles. In essence, this is a relatively minor (although symbolically significant) down payment on what could be a $450 billion nuclear offensive and defensive budget over the next six years, if arms-reduction agreements are not reached between the United States and the Soviet Union.
And much of this money will be spent in any case, if Pentagon plans are carried out. It includes thousands of new nuclear weapons and represents a significant increase in such capability, according to government projections. This means that, with or without a significant breakthrough at Geneva, spending for nuclear forces will likely continue to grow more rapidly than the rest of the Pentagon budget.
Administration officials say this is needed for two reasons: first, to increase US deterrent capability in light of a Soviet buildup in nuclear arms (the Soviets apparently ran a new series of nuclear tests over the weekend); and second, to convince Moscow that it ought to negotiate a reduction rather than keep up with this costly escalation on both sides.
''If the MX is killed, there's no incentive whatever for the Soviets to agree to any kind of reduction on the intercontinental (missiles) except a reduction that would leave them with the margins they now have,'' Defense Secretary Caspar Weinberger said in a recent Monitor interview. Congress has yet to actually appropriate the funds for the MX.
''It's fine to talk about whether these weapons are as militarily useful as some others,'' Mr. Weinberger said. ''But the simple fact is that if you contribute to or create a situation in which only one side has nuclear weapons, I think you've made the position of the ultimate survival of the United States and, indeed, of freedom very much more questionable than it is with the deterrent capability that we try to keep in rough balance.''
Almost as an aside in his speech to the UN Monday, President Reagan noted that ''replacement of older (nuclear) weapons is unavoidable.''
Over the next few years, this is scheduled to include major advances in US land-, air-, and sea-based nuclear systems. When all nuclear-related spending is included (military support and operations as well as energy, intelligence, and civil defense spending directed to nuclear matters), the total is much higher than the $180 billion nuclear modernization program outlined by the President two years ago.
Using government data, the Center for Defense Information (an organization run by retired military officers) puts the total at $450 billion between 1983 and 1988. It reported Monday that while some 11,000 older bombs, missiles, and artillery shells will be retired, 17,000 new nuclear weapons will be built over the next 10 years for a net gain of at least 6,000.
''The American people should know exactly where their tax dollars are going, '' said Gene La Roque, a retired Navy rear admiral and director of the center.
According to another recent report, the MX missile alone will cost each American family $400 and result in net economic losses to 90 percent of all congressional districts, if all 100 missiles sought by the Pentagon are bought.
Economists with Employment Research Associates in Michigan calculated the amount of tax dollars necessary to pay for the MX with the amount that is to be spent on development and procurement to arrive at figures on regional economic impact in a given area. They find that the typical congressional district will be taxed more than $63 million for the controversial missile.
Unlike many major weapons systems which are spread around many states, this group finds in the MX a ''pattern of concentration that is so extreme, it can only be regarded as extraordinary.'' Of MX contracts totaling $27.5 billion, nearly $13 billion will go to California, $4.7 billion to Colorado, and lesser amounts to Massachusetts, Utah, Washington, and Arizona.