Is a mushroom-shaped cloud about to mark the obliteration of the American banking system? Consider this recent observation by William Seidman, chairman of the Federal Deposit Insurance Corp.:

''The financial area is, like nuclear war, the kind of area that can get out of control, and once out of control cannot be contained and will probably do more to upset the civilized world than about anything you can think of.''

Seidman's gloomy prediction 10 days ago that 200 U.S. banks will fail this year, following close on Citibank's announcement of a minimum $2.5 billion loss on its foreign loans, may have led some nervous Nellies to conclude that the nation's biggest bank is on the verge of collapse. Perish the thought.

Before Citibank's depositors start a run on the bank, they should realize one all-important fact: the federal government will never permit the collapse of Citibank -- or any of the nation's top-10 banking institutions. The reason the government won't let a big bank fail is the same reason no national leader has permitted the use of nuclear weapons since World War II: the consequences are simply too awful to accept.

In fact, senior Treasury, Federal Reserve Board and FDIC officials contemplated just such a financial holocaust five years ago in a series of intense debates behind closed doors. The scenario they discussed was a hair-raising one indeed: the collapse of Citibank, Chase Manhattan and Bank of America, the nation's three biggest financial institutions.

When the officials followed the hypothetical script to its logical conclusion, what they saw was ''Son of Great Depression,'' an economic horror that no one in his right mind would let happen if it could be prevented. They realized that the collapse of even one of the top 10 would create a real crash, not just a recession with higher interest and unemployment rates.

The solution worked out by the conferees was probably inevitable, even for a conservative Republican administration ideologically dedicated to free enterprise and deregulation: the government was the only institution capable of guaranteeing against the failure of one or more of the big banks. So the federal government would have to be prepared to bail them out, whatever the cost.

Banking is essentially a game of confidence, with the perception of stability almost as important as actual stability. Most banks that fail run out of depositor confidence long before they run out of money.

So the word was spread that Uncle Sam stood ready to protect any big bank that found itself in serious difficulty. The top 10 or 15 were effectively declared fail-safe.

There would be a price for Big Brother's protection. If the government was going to step in for the rescue, it would at that time have more to say about the operations of the banks for which it had just appointed itself guardian.

Having made the basic decision, federal officials spent the next 18 months or more working out and revising contingency plans to be put into play in the event of a major banking crisis. The final result was a 40-page confidential blueprint for the emergency nationalization of the nation's biggest banks.

The premise behind this revolutionary prescription was perhaps best stated in a federal inter-agency report obtained by our associate Michael Binstein:

''It must be recognized that traditional methods of handling bank failures are probably unworkable in the case of a banking crisis. The {traditional} methodologies result in disruption of services and would fuel the public perception of financial disaster. Assuming that one or more of the banks in jeopardy is among the nation's top 10 banks, the typical . . . transaction with another operating institution would not be feasible, due to a dearth of eligible bidders, the necessity of raising a lot of money under the darkest of financial scenarios, and/or a lack of time in which to consummate a transaction.''

Subject to further revision, the contingency plans call for the Federal Reserve banks to rush currency to a big bank in the throes of a run. The reserve banks have about $19 billion worth of unissued currency at any given time.

The FDIC would let the Federal Reserve be the main fireman by making loans as needed to restore public confidence quickly. The FDIC would handle the administrative side of the crisis, assuming temporary control of the teetering bank.