S1W / S2W (Submarine Thermal Reactor Mark STR)
In 1948, the Atomic Energy Commission (AEC) contracted with Westinghouse Electric to design, build, operate, and test a prototype pressurized-water, Naval nuclear propulsion plant known as the Submarine Thermal Reactor Mark I or S1W. Bettis developed the original Oak Ridge National Laboratory design of the pressurized water reactor for operational naval use, and in collaboration with Argonne National Lab, developed the Submarine Thermal Reactor (STR). The fuel elements were clad in an alloy of Zirconium. Reactor physics measurements were made in Argonne National Lab’s Zero Power Reactor 1 (ZPR-1), which was built in 1950.
The Navy ordered three S1W/S2W two-loop PWRs from Westinghouse. The S1W (S for submarine, 1 for first prototype, W for Westinghouse) plant was a prototype for the first nuclear-powered submarine USS NAUTILUS (SSN 571). S1W construction was completed in 1953 at the AEC National Reactor Testing Station (NRTS) in Idaho.
The S2W (Submarine Thermal Reactor Mark II / STR MK II), wtith a power of 13,400 SHP and a core lifetime of 900 hours at full power, was used on the SSN 571 Nautilus.
The second reactor, initially named STR Mark II and then S2W, was installed in USS Nautilus (SSN-671) and became the reactor that made the world’s 1st “underway on nuclear power.” The propulsion system consisted of 1 x S2W Westinghouse PWR rated @ 70 MWt (est); 2 x main steam turbines delivering a total of 13,400 shp (10 MW) to 2 x screws. The 1st refueling was Feb – Apr 57. Core life was about 2 years; ship traveled 62,000 miles.
The “spare” (third) S2W nuclear plant was installed on USS Seawolf (SSN-575) in 1958 after removal of its original S2G liquid metal-cooled core and conversion of its secondary propulsion plant to operate with saturated steam.
The fuel elements were sandwich plates of U and Zr and clad in Zr. The maximum temperature in the fuel was 645°F and the sheath temperature was 551°F with an average cycle time of 600 hours or just 600 / 24 = 25 days. The reactor temperature was limited by the pressure needed to prevent boiling, necessitating high pressure vessels, piping and heat exchangers. The steam was generated at a relatively low pressure. A high level of pumping power was required, and the fuel was costly. However this design presented few hazards, was proven in service, and an expensive moderator was not needed.
S1W reached criticality on March 30, 1953 becoming the first reactor to produce significant quantities of useful nuclear power in the world. Later in 1953, S1W achieved full design power and commenced a successful 96-hour sustained full-power run that simulated a submerged crossing of the Atlantic Ocean. During operations of the second S1W reactor core in 1955, a 66-day continuous full power run was performed, which could have propelled a submarine at high speed twice around the globe.
Over its lifetime, S1W was used to train over 13,000 Navy officers, enlisted operators, and civilian students. S1W was permanently shutdown in October 1989 after 36 years of operation of which the last 22 years were performed with a single core establishing a longevity record.
Rickover’s approach to milestone moments was rather calculating. He felt the occasion to be of extreme importance. Certain people who should have been there were not. Some of those who were present should not have been. So he postponed the procedure for a day and adjusted the mix of people in the control room.
Rickover was in the control room, along with AEC commissioner Thomas Murray. At the epochal moment, Rickover instructed Murray to open a certain valve. John Simpson later wrote: "The commissioner was thrilled at the chance to play an active role in making history. Murray stepped forward, grasped the valve handle, and slowly turned it. In the adjoining area, inside the hull of this submarine-on-land, steam hissed against the turbine blades. A propeller shaft began to turn."
The heat of the reactor was doing muscle work. A hasty champagne made of alcohol from the chemistry lab and soda water from a soft drink dispenser was mixed for a toast.
Charts went up on the wall of the reactor control room for plotting a 2,500-mile route from Nova Scotia to Ireland, and the Navy men who happened to be there for training began four-hour watches. For sixty hours, the run went well. Then a condenser tube began to leak, and radiation was soon detected near it. Next a steam generator sprung a few small leaks. The Westinghouse people argued among themselves: should they shut down the reactor? Given the location of the leaks, neither public nor personnel safety was at stake. The run continued.
Then the control for one of the steam generators failed, causing the water level to drop and the reactor power level to become erratic. Debate about shut-down became more heated. The crew reduced the power level to half and restored the water level. In the end, the reactor “crossed the Atlantic” in ninety-six hours, but not entirely at full power. The crew had resourcefully fixed emerging problems and kept the reactor running.
Admiral Rickover sent the first crew of the Nautilus to train at S1W in Idaho. After that, training seemed an obvious mission for the reactor, to the lifelong gratitude of generations of young women in the neighboring towns. When each trainee arrived in Idaho, he had a few days to find and settle a place to live. One in four of the trainees married before his six months of training were over, and it wasn’t unheard of for a man to meet, court, and marry, all in his first two weeks.
Rickover’s ideas about the training of nuclear plant operators were controversial within the Navy. He wanted to train a new type of naval officer, unfettered by what Rickover saw as the useless traditions embedded in regular Navy training. On the basis that assuring safety aboard ship required that all ship personnel be able to evaluate potential hazards, Rickover had his way. He established a system of nuclear training schools, and the desert prototype was an essential part of it.
Rickover supervised the preparation of textbooks and ordered that no examinations contain multiple-choice or true-false questions. Tests required essays, definitions, statements of fact, or calculations. Homework was required, and since it often involved classified material, trainees had to do it on the premises, not at home.
The controlling philosophy was self-responsibility. Rickover rejected simulations in favor of real reactors. “You have to train people to react to the real situation at all times. But if they are trained with a simulator, they tend to expect there will be no consequences,” he said. Rickover didn’t want to train the wrong instincts by using a machine that could not mimic a real nuclear power plant under real-time conditions, including casualties. Computers capable of doing this were not available at the time.
Cross-training also was important. Electricians should know mechanical systems, for example. Trainees came to the desert after six months of theoretical instruction from a specialty school elsewhere in the system. In Idaho a trainee began by picking up — or trying to — a “triple-hernia-sized” crate of operating manuals, instructions, and schematics.
Using the books and seeking the instructors he needed, the trainee traced every system, component by component. Enlisted men, no less than officers, learned and used technically accurate vocabulary, no nicknames or shortcuts. Common language tended to level everyone; it wasn’t unusual for a petty officer 3rd class to be instructing an officer. Due to the prevailing Idaho practice — at least in the early years — of wearing civilian clothes, visiting Navy brass from the regular Navy were in for new experiences. An admiral once toured the S1W prototype and then stayed for lunch with his guides. Later he learned who the men were. “Enlisted men! I thought they were college physicists!”
When he felt competent in a system, the trainee sought an instructor to examine him and sign his checklist. Mastery gradually produced a long list of signatures. The trainee then stood watch at one of the operating stations in the hull. At first, he was paired with a more experienced mate, but then he himself was in charge. Learn one station, move to the next. The trainees started the reactor plant, took it up to full power, maneuvered, shut it down, repaired it, maintained it. Although the nuclear program attracted the top two percent of the Navy’s enlistees, some men wiped out, usually because of a failure of self-initiative, not academic insufficiency. There were few second chances.
The training program grew more complicated as nuclear fuel evolved and as the Navy adapted nuclear power to surface vessels. In 1958 a second prototype, A1W (A for aircraft carrier), went critical in a new building west of S1W.
Westinghouse physicists had determined that the predominant source of gamma radiation from the reactor was nitrogen-16, which was generated in the reactor cooling water when a stray neutron struck an atom of oxygen-16. What they didn’t know was how likely it was for any given number of nitrogen atoms to capture neutrons and therefore how big a problem nitrogen-16 would be. Dr. John Taylor, responsible for the Nautilus shielding studies, recalled: "Because the half-life of nitrogen-16 is only 7.35 seconds, the capture cross- section [probability of capture] could not be measured by the normal accelerator methods. Without that information there was no way to design the major components of the shielding for the submarine."
Thus, a closed loop was installed in the MTR to circulate water through the MTR neutron flux so as to be able to measure the nitrogen-16 activity generated in the water. The inferred cross section was used to design the shielding. The facilities at the NRTS were essential to solving this key question bearing on the radiation safety of the Navy crew.
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