POWER May 2010 - 124

CONFERENCE REPORT
Should We Fear Water Leaked from Buried Pipes?
The revelation of leaking pipes at Vermont Yankee is not the first
time the Nuclear Regulatory Commission (NRC) has investigated
leaking underground pipes. In fact, the NRC expects buried pipes
to leak. One of the safety functions performed by the NRC is to
review every plant's groundwater monitoring programs and to determine
if a leak poses a threat to the public health, safety, or the
environment. Every safety system with buried piping is subject to
inspection and testing per NRC rules and ASME standards, which
require routine flow testing and surveillance, often several times
a year (Figure 2).
Other industry organizations are also deeply involved in buried
pipe testing and assessment. According to the NRC, NACE International
(formerly the National Association of Corrosion Engineers)
has formed a Nuclear Buried Piping group to examine how its standards
can be optimized for use at nuclear power plants. EPRI has
produced recommendations for controlling buried pipe corrosion
and other degradation, based on existing NACE standards. The
nuclear industry is also working with EPRI to develop additional
procedures for buried piping maintenance. Several recent nuclear
operating license renewal applications have also included the
requirement for added buried pipe inspections and replacement.
NRC's Buried Piping Integrity Initiative will soon roll up all these
inputs into new rules.
The Source of Tritium
Public concern really isn't about the leaking pipes but rather about
the radioactive water that can enter the local groundwater. Tritium,
an isotope of hydrogen, is often cited as present in the leaking water.
That's not surprising, as tritium originates from the boron used
as a neutron absorber to help control the nuclear chain reaction.
Tritium emits a weak form of radiation, a low-energy beta particle
similar to an electron. The tritium radiation does not travel very
far in air and cannot penetrate the skin. When tritium bonds with
oxygen it forms radioactive " tritiated " water. The tritiated water is
chemically identical to water, odorless, tasteless, and the tritium
can't be filtered out.
For John Q. Public, any amount of radioactive groundwater contamination
is unsatisfactory. Fortunately, the amount of radiation
that is harmful is determined based on a half-century of nuclear
experience and not on perceptions. Nuclear power plants routinely
release dilute solutions of tritiated water every day. Tritium also
naturally occurs in groundwater by the collision of cosmic rays
with air molecules. In fact, the tritium radiation dose from working
in a nuclear plant is less than what is received by exposure
to natural background radiation. According to the NRC, tritiated
water accounts for less than 0.1% of the total background dose we
each receive each year.
Little Danger from Tritium
According to the NRC, the amount of radioactive materials involved
in the most recent underground pipe leaks that have " above-normal "
amounts of tritium do not present a public health hazard.
As of March 25, the Vermont Department of Health reported that
samples of drinking water well samples continued to show no tritium
in excess of the lower limit of detection. Tritium levels in
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2. Leaky pipes usually pose no danger. The NRC has safety
programs in place to closely monitor and test underground pipes.
Critics point to the radioactive contamination of groundwater with tritiated
water at Vermont Yankee. A closer analysis shows that the radioactive
levels of tritiated water are thousands of times less than the
background radiation humans receive each year. Source: NRC
Drinking
Nuclear
water well
power plant
Monitoring
well
Sand & stone layer
Upper clay
Damaged pipe
Tritium plume
Lower clay
Aquifer
drinking water are measured in picocuries per liter (trillionths of a
curie per liter, pCi/L) in drinking water with 1,600 pCi/L being a
fairly normal level. Using Environmental Protection Agency (EPA)
consumption assumptions, consuming this water over a year would
provide a total radiation dose of 0.3 millirem (mrem). This dose
is ten thousand times less than that received from a full-body CT
scan (3,000 to 10,000 mrem), one thousand times less than the
average U.S. natural background radiation each year (300 to 360
mrem), one hundred times less than a dental x-ray (30 mrem), and
10 times lower than the radiation dose received during a round-tip
cross-country airplane flight (3 mrem).
For those who work at or visit nuclear plants, the NRC has set
a 500-mrem dose limit for pregnant nuclear plant workers, 5,000
mrem for other workers, and 100 mrem annual dose limit for members
of the general public. Another interesting statistic: Radon gas
accounts for two-thirds of the 300 mrem normal background radiation
we receive each year, with cosmic, terrestrial, and internal
radiation accounting for the remainder. No adverse health effects
have ever been observed from doses arising from these levels of
natural radiation exposure nor from levels found in drinking water.
Another useful comparison is the EPA 1976 drinking water standard.
The EPA set the limit of tritium radiation allowable in drinking
water at 4 mrem per year. The calculation methods at the time
conservatively equated the standard to a maximum tritium level of
20,000 pCi/L. In 1991, improved calculation methods determined
that 4 mrem was actually equivalent to a tritium dose of 60,900
pCi/L, three times higher. The EPA elected to leave the drinking
water tritium limits at the 1976 levels in its current regulations.
A final comment: The total amount of radiation emitted by the
average painted tritium watch (which enables reading the time
in low-light environments) is a maximum of 25 to 26 millicuries,
roughly ten thousand times more than the level found in typical
drinking water.
-Dr. Robert Peltier, PE is POWER's editor-in-chief.
www.powermag.com
POWER | May 2010
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POWER May 2010

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