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USS Thresher (SSN-593)

The keel of USS Thresher (SSN-593) was laid down by Portsmouth Naval Shipyard, Kittery, ME, 28 May 1958. Launched: 9 July 1960, Thresher was sponsored by Mrs. Frederick B. Warder. Thresher was commissioned 03 Agust 1961 with Cdr Dean L. Axene in command. Following trials, the nuclear fast attack submarine, USS THRESHER (SSN-593), took part in Nuclear Submarine Exercise (NUSUBEX) 3-61 off the northeastern coast of the U.S. from 18 to 24 September 1961.

On 18 October, she headed south along the east coast. After calling in at San Juan, Puerto Rice, she conducted further trials and test-fired her torpedo system before returning to Portsmouth on 29 November. The boat remained in port through the end of the year and spent the first two months of 1962 evaluating her sonar system and her Submarine Rocket (SUBROC) system. In March, the submarine participated in NUSUBEX 2-62, an exercise designed to improve the tactical capabilities of nuclear submarines, and in antisubmarine warfare training with Task Group ALPHA.

Off Charleston, the boat undertook operations observed by the Naval Antisubmarine Warfare Council, before she returned briefly to New England waters from whence she proceeded to Florida for SUBROC tests. However, while mooring at Port Canaveral, the submarine was accidentally struck by a tug which damaged one of her ballast tanks. After repairs at Groton, CT., by the Electric Boat Company, the boat returned south for more tests and trials off Key West. THRESHER then returned northward and remained in dockyard hands through the early spring of 1963.

In company with USS SKYLARK (ASR-20), THRESHER put to sea on 10 April 1963 for deep-diving exercises. In addition to her 16 officers and 96 enlisted men, the submarine carried 17 civilian technicians to observe her performance during the deep-diving tests.

Fifteen minutes after reaching her assigned test depth, the submarine communicated with SKYLARK by underwater telephone, apprising the submarine rescue ship of difficulties. Garbled transmissions indicated that -- far below the surface -- things were going wrong. Suddenly, listeners in SKYLARK heard a noise "like air rushing into an air tank," -- then, silence.

Then just 5 minutes later, the Skylark's sonar picked up the sounds of the submarine breaking apart under the tremendous sea pressure. Efforts to reestablish contact with Thresher failed, and a search group was formed in an attempt to locate the submarine. Rescue ship Recovery (ASR-43) subsequently recovered bits of debris, including gloves and bits of internal insulation.

There had been some concern over the Navy's method of performing UnderWater Explosion Shock and Vibration Testing [UNDEX]. Traditionally, explosives are detonated in the vicinity of the ship. Measurements are taken before and after the test to determine any damage to the piping and equipment in the engineering spaces. This test is obviously intended to show the ship's ability to withstand wartime activity. The Thresher was UNDEX tested before her last overhaul. Mis-alligned equipment continued to be found months later, even as late as April 10, the morning she set sail for her last deep dive.

Photographs taken by bathyscaph TRIESTE proved that the boat had broken up, taking all hands on board to their deaths in 1,400 fathoms of water (approximately 8,500 feet), some 220 miles east of Boston. The photos indicate she is in six major sections on the ocean floor, with the majority of the debris in an area about 400 yards square. The major sections are: the Sail; Sonar Dome; Bow; Engineering; Operations and Tail sections.

The submarine community, the Navy and the nation were stunned. Two days after the disaster President Kennedy issued Executive Order 11104, ordering US Flags to "be flown at half-staff on all buildings, grounds and naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions," from April 12th to 15th. Thresher was the best of the newest. To the Navy, the disaster meant more than the loss of 129 crewmembers and civilians. Thresher had been the most advanced submarine in the world, capable of reaching depths and speeds unimaginable a decade before.

A Navy board of inquiry concluded that the most likely cause of the sinking was a failure in either a pipe, pipe valve, or hull weld which cause flooding near the engine room. According to investigators, a silver-brazed joint in a seawater pipe in the aft engine spaces broke, spraying water into the engine room and shorting one of the main electrical bus boards. The flooding probably short-circuited an electrical system related to the main engine causing the reactor to shut down.

The reactor scram (emergency shut down) resulted in a loss of steam power to the primary propulsion system. Submarines rely on power to push them to the surface. Without this power, and taking on water from the leak, the Thresher had negative buoyancy and she began sinking. The aft part of the sub filled up with water and tilted down. Attempts were surely made to stop the leak, but the isolation valve was not nearby and did not have a remote operator. Darkness, a sea mist, and sheer terror may have inhibited the crew from manually actuating the valves.

It is suspected that the Thresher started her emergency propulsion system, but this was not able to push the submarine to the surface from this depth. The submarine apparently managed to point her nose upward in preparation to emergency blow air into her ballast tanks. Some sources say that the Thresher actually began to surface, but the lines to the air pressure-reducing manifold valves iced over and froze shut, an action enhanced by the recent addition of a filter on the air inlet to the manifolds. This frozen pipeline formed an impenetrable barrier between the high-pressure storage air flasks and the manifold valves leading to the Thresher's ballast tanks.

So instead of rising to the surface, the Thresher kept sinking. Having a "positive up-angle" with no power or air providing forward momentum, the Thresher slipped backward into the depths of the ocean. Without power, Thresher was not able to surface and the continued flooding caused Thresher to drop below her crush depth where the pressure of the ocean destroyed her. The entire crew of 129 were lost. A ghastly death for an entire crew, and one the US Navy vowed never to allow happen again.

Thresher is in six major sections on the ocean floor, with the majority in a single debris field about 400 yards square. The major sections are the sail, sonar dome, bow section, engineering spaces, operations spaces, and the tail section. Owing to the pressurized-water nuclear reactor in the engine room, deep ocean radiological monitoring operations were conducted in August 1983 and August 1986. The site had been previously monitored in 1965 and 1977 and none of the samples obtained showed any evidence of release of radioactivity from the reactor fuel elements. Fission products were not detected above concentrations typical of worldwide background levels in sediment, water, or marine life samples.

Comprehensive deep ocean radiological monitoring operations were conducted in August 1983 and August 1986 at the THRESHER site. The THRESHER site had been previously monitored in 1965 and 1977 and none of the samples obtained showed any evidence of release of radioactivity from the reactor fuel elements.

Very low concentrations of cobalt 60 in the form of corrosion products from THRESHER piping systems were detected in sediment. Cobalt 60 is the predominant activated corrosion product found in the reactor coolant piping system on U.S. nuclear powered warships. Therefore, it was the primary radio-nuclide released when the coolant piping system aboard THRESHER was breached. The conclusion of the earlier surveys was that THRESHER had not had a significant effect on the radioactivity in the environment. The purpose of the monitoring in 1983 and 1986 was to identify whether radiological conditions had changed and to demonstrate the use of improved sampling and navigation equipment deployed from both a surface ship and a deep ocean submersible.

The 1983 and 1986 surveys confirmed the conclusion of earlier surveys. Fission products were not detected above concentrations typical of world wide fallout levels in sediment, water, or marine life samples. Thus, there continues to be no evidence of release of radioactivity from the reactor fuel elements. Cobalt 60 concentrations in the sediment were generally lower than those found in 1977 as would be expected due to radioactive decay. No cobalt 60 was detected in the large number of fish and other marine life specimens or in undisturbed water samples collected at the THRESHER site. This confirmed that cobalt 60 in the form of insoluble corrosion products is not concentrated in the deep sea food chain. No Uranium was detected above background levels from natural radioactivity and world wide fallout from past atmospheric weapons testing.

The maximum cobalt 60 concentration detected in the sediment was 0.32 pCi/gm and most samples contained much less. This is approximately a factor of 100 lower than the concentration of naturally occurring radioactivity in sediment. For perspective, if a person's entire diet contained Cobalt 60 at the maximum concentration detected in the sediment at the THRESHER site, that person would receive less than five percent of the radiation exposure typically received from natural background radioactivity.

The 1983 and 1986 survey results confirm that THRESHER had not had a significant effect on the radioactivity in the environment.

The reactors used in all U.S. Naval submarines and surface ships are designed to minimize potential hazards to the environment even under the most severe casualty conditions such as the actual sinking of the ship. First, the reactor core is so designed that it is physically impossible for it to explode like a bomb. Second, the reactor fuel elements are made of materials that are extremely corrosion resistant, even in sea water. The reactor core could remain submerged in sea water for centuries without releases of fission products while the radioactivity decays, since the protective cladding on the fuel elements corrodes only a few millionths of an inch per year.

Thus, in the event of a serious accident where the reactor is completely submerged in sea water, the fuel elements will remain intact for an indefinite period of time, and the radioactive material contained in these fuel elements should not be released. The maximum rate of release and dispersal of the radioactivity in the ocean, even if the protective cladding on the fuel were destroyed, would be so low as to be insignificant.

Radioactive material could be released from this type of reactor only if the fuel elements were actually to melt and, in addition, the high-strength, all-welded reactor system boundary were to rupture. The reactor's many protective devices and inherent self-regulating features are designed to prevent any melting of the fuel elements. Flooding of a reactor with sea water furnishes additional cooling for the fuel elements and so provides added protection against the release of radioactive fission products.

A report of the 1983/1986 environmental monitoring expeditions to the THRESHER site is available and provides details of the environmental sampling of sediment, water and marine life which were taken to ascertain whether THRESHER has had a significant effect on the deep ocean environment. It also explains in detail the methodology for conducting deep sea monitoring at the THRESHER site from both surface vessels and submersibles.



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