And so Mark I, although located almost as far from sea water as possible in the North American continent, was a true seagoing power plant—no shore-based engineering short cuts were allowed in its construction. As the Naval Reactors Branch engineers put it, “Mark I equals Mark II.” This meant that while they were designing the world’s first nuclear power plant, they also would have to meet the special problems of seagoing submarines. Some of these were:

1. At operating depths, a submarine experiences hundreds of pounds of sea pressure on each square inch of its surface—hundreds of thousands of tons on the entire vessel. This pressure must be resisted by the hull which is in contact with sea water. Mark I and all its components could withstand very high sea pressures.

2. A submarine and its machinery must be able to continue operation after enemy depth charges have exploded just outside the hull. Mark I was built to the high mechanical shock standards which resulted from the Navy’s World War II experiences. Some of its important units were shock tested to destruction in an actual submarine submerged in Chesapeake Bay, and then redesigned to strengthen the failures.

3. The Nautilus would need to take air into hr hull while submerged to refresh the atmosphere after long cruises under water and to provide oxygen to her stand-by diesel engines if her reactor failed in enemy waters. Pressure variations due to this “snorkeling” might disturb sensitive instrumentation systems. Mark I could snorkel.

4. When a submarine is submerged, the sea surrounding it tends to reflect dangerous atomic radiation back into spaces occupied by the crew. Mark I was placed in a large tank of water to test the atomic radiation problems of a submerged sub.

In short, Mark I was built to reproduce the conditions of an actual submarine power plant in every respect save one: it could not be tested in the motion of the open sea. The ability to withstand such motion was designed into the Mark I systems and components, however, and these items were individually tested for thousands of hours ashore under the conditions of ship motion experienced at sea.

As time went on and as the problems of reactor plant design became more forbidding, it became increasingly difficult for Rickover to hold the designers rigidly to the concept that the submarine problems must be faced simultaneously with the reactor problems. There were bitter technical debates between Rickover and his engineers, already hard-pressed by a tight schedule and almost insurmountable difficulties of simply obtaining atomic power for the first time and hoping to postpone the submarine problems until the Mark II design stage. An example was the discussion as to whether air conditioning should be provided in Mark I.

Surface ships ventilate heat out of boiler and engine rooms by blowing large quantities of air through them. The Nautilus, as the first vessel to be driven submerged by steam propulsion, could not be ventilated—at least not while she was submerged. All the heat escaping from her machinery would have to be pumped overboard by large air conditioning sets. This problem did not have to be settled for Mark I, which could have been adequately ventilated with cool Idaho air. But the Captain insisted that Mark I be air conditioned, and in a typical example of his uncanny engineering insight, he ordered three times as much air conditioning capacity as was required. “In 1917 the British built two submarines which used steam for surface operations,” he said, “and they were failures because they became too hot when they submerged. The Nautilus will not be a failure for such a reason.” Operating experience has proved that the Nautilus would have been unsuccessful without the extra air conditioning.