A CLOUD OF unresolved doubts continues to hang over the nuclear cooperation agreement signed last summer between this country and China. Congress has the power to reject the agreement, but should not do so. China has possessed nuclear weapons for two decades, and if it chose could greatly increase the risks to the world's security by disseminating this technology to the countries that seek it. In the past China has occasionally seemed inclined to do that. But more recently it has taken several steps indicating suppport of the regimen in which most of the world's governments have joined to prevent further proliferation of these weapons. The agreement between the United States and China could strongly reinforce that commitment.

The agreement is a bargain. The United States is to give the Chinese access to its reactor technology for peaceful purposes, allowing American companies to sell the Chinese equipment, fuel and engineering advice. China, for its part, has agreed to divert none of this technology to its own military uses and to give no further assistance to any other country's attempts to build weapons. The questions about the agreement begin with the absence of provisions for verification and extend to several areas of ambiguity on other crucial points. But it's not a subject to be left to vague good will.

Sen. John Glenn pointed out earlier this month that on two occasions this year U.S. authorities have intercepted what were evidently attempts to smuggle advanced electronic equipment to China. Last week Sen. Alan Cranston charged that China has recently held nuclear trade talks with, or actually sent nuclear exports to, five countries with nuclear ambitions -- Iran, Pakistan, South Africa, Brazil and Argentina.

The history of this agreement is similarly troubling. It was first initialed a year and a half ago, during President Reagan's trip to China. But instead of forwarding it routinely to Congress, the administration put it in the deep freeze. Although the reason was never publicly stated, there had been intelligence reports of Chinese technicians working at a uranium enrichment plant in Pakistan. The agreement was finally signed last July. The State Department, evasive and secretive throughout this process, has not succeeded in persuading many senators that its current information is adequate to support the assurances it is giving them.

Sen. Glenn has drafted an ingenious remedy. He does not want to see the agreement rejected. But he has introduced a bill providing that, before this country licenses any nuclear exports to China, the president would have to certify that the key questions have been settled with the Chinese. The State Department opposes the Glenn bill, arguing that it demands a degree of legalistic detail to which the Chinese will never consent. But that's a decision for the Chinese to make, not the State Department.

This American technology has an immense capacity for good or, unfortunately, evil. Mr. Glenn and the growing number of senators of both parties who join him are right to want a more precise agreement on China's intentions.