Resurrecting Nuclear: "We Have to Get It Right"

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).

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 is thousands of times less than the background radiation humans receive each year. Source: NRC

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 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.