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.
References
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.