EH-9406 June 1994 Occupational Safety Observer JUNE 1994

The Loss of the USS Thresher

Thirty-one years ago, the nuclear submarine USS Thresher failed to surface from a test dive and was lost at sea.

On the morning of April 10, the Thresher proceeded to conduct sea trials about 200 miles off the coast of Cape Cod. At 9:13 a.m., the USS Skylark received a signal indicating that the submarine was experiencing "minor difficulties." Shortly afterward, the Skylark received a series of garbled, undecipherable message fragments from the Thresher. At 9:18 a.m., the Skylark's sonar picked up the sounds of the submarine breaking apart. All hands were lost--129 lives.

The Investigation

The subsequent investigation of the disaster by the Navy identified a leak in an engine room seawater system as the most probable cause of the tragedy. Further, both the Navy's investigation and a Congressional inquiry identified several additional probable causes linked to management, communication, and the practices and procedures employed by the Navy and the shipyards. These findings suggest a number of lessons applicable to the Department of Energy.

The Thresher was the first of a new class of nuclear submarine designed to dive significantly deeper than its predecessors. After nearly a year of record-breaking operations, the submarine underwent a scheduled shipyard overhaul that entailed significant alterations to its hydraulic power plant. Because of Fleet operational requirements and competition for resources with four other submarines under construction in the same shipyard, the overhaul was conducted under tight schedule constraints.

The Navy's investigation concluded that while the Thresher was operating at test depth, a leak had developed at a silver-brazed joint in an engine room seawater system, and water from the leak may have short-circuited electrical equipment, causing a reactor shutdown and leaving the submarine without primary and secondary propulsion systems. The submarine was unable to blow its main ballast tanks, and because of the boat's weight and depth, the power available from the emergency propulsion motor was insufficient to propel the submarine to the surface.

Practices and Procedures

After the investigation, the Navy embarked on an extensive review of practices and procedures in effect during the Thresher's overhaul. The reviewers determined that existing standards at the time were not followed throughout the re-fit to ensure safe operation of the submarine. Four issues were of particular concern:

Design and Construction. The submarine was designed and built to meet two sets of standards. Because the submarine's nuclear power plant was the focus of the engineers, the standards used for the nuclear power plant were more stringent than those for the rest of the submarine. As a result of the emphasis placed on nuclear-related aspects of the design, builders assigned less importance to the steam and saltwater systems, even though those systems were crucial to the operation and safety of the vessel.

Brazing. Two standards for silver-brazing pipe joints were used during the Thresher's construction and overhaul. Brazing is a process that joins metal parts by heating them to a temperature sufficient to melt a filler material, which then flows into the space between the closely fitted parts by capillary action. Induction heating, which provides better joint integrity, was used for easily accessible joints. Where accessibility was restricted, hand-held torches were used. Reviewers determined that hand-held torches were used to heat many of the Thresher's crucial, but less accessible, pipe joints.

Quality Assurance. A newly accepted nondestructive testing technology for quality assurance was not implemented for the Thresher's overhaul. The Navy had experienced a series of failures with silver-brazing, which resulted in several near misses, indicating that the traditional quality assurance method, hydrostatic testing, was inadequate. Therefore, the Navy directed the shipyard to use ultrasonic testing, a method newly accepted by industry, on the Thresher's silver-brazed joints. However, the Navy failed to specify the extent of the testing required and did not confirm that the testing program was properly implemented. When ultrasonic testing proved burdensome and time consuming, and when the pressures of the schedule became significant, the shipyard discontinued its use in favor of the traditional method. This action was taken despite the fact that 20 of 145 joints passing hydrostatic testing failed to meet minimum bonding specifications when subjected to ultrasonic testing.

Procurement. Finally, specifications for Government procurement were not strictly enforced. The Navy found that the reducing valve components installed in the pressurized air systems used to blow the main ballast tanks of the submarine did not meet design specifications. Because of the magnitude of the pressures anticipated, the valve manufacturer had added a strainer feature upstream of the reducing valves to protect the sensitive valves from particulate matter. When the Navy conducted tests on another Thresher-class vessel, it found that the pressure drop across the component at high flow rates caused entrained moisture to accumulate on the strainers and form enough ice to block the air flow. Venturi cooling, as this phenomenon is called, was thought to be the reason that the Thresher's attempts to blow its main ballast tanks were ineffective.

Lessons Learned

The lessons learned by the Navy are still applicable and should be applied by the Department of Energy.

(1) Engineering, design, and construction must place equal weight on nuclear and nonnuclear systems when the operation of either system can affect the safety or integrity of an overall system.

(2) In selecting the standard for which a task is performed, the pressures of time and resources should not override the safe and continued performance of the result. Selecting the easy standard to save time and money increased the probability of a failed weld.

(3) Communication of near-miss events by management to various departments, or feedback, helps to resolve weaknesses or flaws that in future events could prove tragic.

(4) Procurement of equipment and components must be checked upon receipt as well as tested under operating conditions to verify its suitability. Valves or other parts could be assembled with counterfeit bolts, which will fail when stressed.


Krahn, S., "The Loss of USS Thresher (SSN 593): Lessons for the Development, Implementation and Use of Standards," Defense Nuclear Facilities Safety Board presentation, undated.

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