The nation's top nuclear weapons plants are caught in a colossal foul-up that might interfere with America's national security objectives. Three reactors in South Carolina have been shut down because they pose serious risks to people and the environment.

The complex once produced tritium, a radioactive gas that increases the power of nuclear weapons. The reactors were closed after cracks were found in their cooling systems. That posed risks of serious nuclear accident. Some new cracks have been found since the original discoveries were made.The reactors cannot be repaired or reopened for at least a year - if then. The Defense Department has a nine-month supply of tritium. The United States thus faces a material shortage in keeping its nuclear weapons stock up to date.

That is, however, only the surface of a deeper problem. Even if the South Carolina facilities are fixed and returned to production, all must be replaced soon because of their age. However, it takes about a decade to build a new plant. In the meantime, the United States faces a tritium shortage.

The United States got into this mess because nuclear weapons plants are exempt from the stiffer regulation that covers the nuclear-powered production of electricity. weapons plants are not supervised by the Nuclear Regulatory Commission, which regulates civilian facilities. Much of today's trouble could have been avoided had civilian-type rules been enforced.

The U.S. Department of Energy, which is charged with such responsibilities, estimates it will cost between $92 billion and $120 billion to eliminate the present environmental and human risks. The department has an annual budget of about $1 billion.

There are few if any immediate answers to these serious problems. But the old days of indefinite postponements are no longer tolerable. Energy Secretary-designate James Watkins needs a lot help - soon. Even if he receives support, some risks remain.