Mushrooming demand for nuclear disarmament; UN conference: substance or posturing?

''What we're telling Reagan and Brezhnev, essentially, is 'cool it,' '' says a senior third-world representative to the second Special Session on Disarmament here.

But, whatever its outcome, this conference on disarmament will probably disappoint participants in the peace movement in the United States and abroad.

In all likelihood a comprehensive disarmament program will be adopted. But according to informed sources, it will not be binding, nor will it determine concrete disarmament measures, specific cuts in weaponry, or deadlines.

The superpowers don't want to be told how to deal with their strategic relationship, when and how to negotiate, what type and how many weapons to store in their arsenals.

However, the governments of the major and medium powers do have to take the snowballing peace movement into account. By announcing its decision never to be the first one to use nuclear weapons, the Soviet Union has shown that it is eager to play up to public opinion. Vietnam has ratified the nuclear Nonproliferation Treaty. The Soviet Union has offereda program to ban chemical weapons in its proposals to declare nuclear war a ''crime against humanity.''

Italy, Mexico, Brazil, Sri Lanka, and the Netherlands, among others, are pitching vigorously for various disarmament measures (such as more nuclear-free zones, the setting up of an independent world disarmament agency, and a ban on the use of outer space for military purposes).

Some Western officials privately take a cynical view of the special session, seeing it largely as an exercise in posturing. Other officials feel that ignoring the growing concern by a large middle-class body of opinion in Japan, in Europe, and in North America concerning the arms race could be politically costly.

But analysts say neither the big powers nor the nonaligned nations will want to be responsible for the failure of the session.