For personnel in nuclear power plants, time and distance are critical measurements when it comes to safe inspections. When workers need to enter a high-radiation area to examine a potential problem or make a routine inspection, proximity to radiation and the length of time the inspection takes can increase their effective radiation dose.
Suspected corrosion, or blocked conduits or vessels in the containment area, need to be handled immediately to help avoid accidental radiation leakage. In some cases, the reactor may need to be shut down so that workers outfitted in full-body protective gear can carry out the inspection. However, remote visual inspection (RVI) offers an alternative to this method, helping avoid costly shutdowns, saving time and operational costs, and reducing the risk of a harmful radiation dose for workers.
Radiation Exposure, Limits, and Risks
Exposure to radiation in the workplace is strictly regulated by safety standards, with the annual effective dose limit set at 5 rem (0.05 sievert [Sv]). For comparison, the average person is exposed to less than 0.003 Sv of naturally occurring radiation in a year. Workers in nuclear plants are generally exposed to less than 0.01 Sv annually. According to the standards, this level is considered reasonably safe.
Despite the general safety of working in a nuclear power plant, the “as low as reasonably achievable (ALARA)” principle dictates that workplace radiation safety programs should minimize employee exposure as much as possible. RVI methods can enable these workers to make their inspections from a sufficient distance to help decrease their annual effective dose level.
What RVI Can Do to Reduce Radiation Exposure
Water is essential to nuclear energy production. How the water is used can vary depending on the plant and type of technology. In a pressurized water reactor, water is generally used in three stages. They are:
■ Stage 1. In the radiation containment building, water helps cool the uranium rods in the nuclear reactor; during this process, the water is heated by the reactor.
■ Stage 2. The radioactive (or “dirty”) water from the reactor is sent in a loop to heat a reservoir of fresh, clean water.
■ Stage 3. Once the clean water is heated, it turns into steam, which powers the generator’s turbine.
1. Pipes and vessels located within the containment structure at a nuclear power plant are in a high-radiation area. Workers in these areas are exposed to this radiation and must wear full-body protective gear. Remote visual inspection is a way to enhance worker safety by keeping workers farther away from high-radiation areas as inspections are performed. Courtesy: Olympus
The pipes and vessels located within the containment structure are in a high-radiation area (Figure 1). Even with anticontamination suits and other personal protective equipment (PPE), workers who perform inspections in this area will inevitably be exposed to higher doses of radiation.
RVI offers a way for workers to carry out inspections in hazardous locations without physically entering the area. As well as letting workers maintain a safe distance from high-radiation areas, a videoscope equipped with a long insertion tube can enable the inspection of difficult-to-access locations, such as water conduits. The longer the insertion tube, the farther away the workers can be from the radiation.
Videoscope Durability in Radioactive Conditions
Unfortunately, even the best videoscope equipment (Figure 2) doesn’t come out completely unscathed when exposed to radiation. If the insertion tube is used to inspect dirty-water-filled pipes, contamination is inevitable, and damage is also a possibility.
2. Videoscope equipment can be damaged when exposed to radiation. Insertion tubes used to inspect water-filled pipes can become contaminated, and decontamination can be considered too costly. A plant operator may opt to sacrifice the tube, leaving it permanently in the radiation area. Courtesy: Olympus
For example, with long-term exposure, the clear fiberoptic material used in insertion tubes for illumination can begin to turn yellow. This yellowing causes it to absorb light, reducing the intensity at the tip. The tube’s image sensor is more susceptible to damage from short, high doses of radiation, which can result in a noisy, or milky, image on the videoscope screen.
When the equipment is used in high-radiation areas, in some instances, decontaminating the equipment may be considered too costly and risky for the health of the workers. The plant may opt to sacrifice the insertion tube, leaving it permanently in the radiation area. Regardless, the insertion tube and the videoscope need to be robust enough to satisfy the requirements and expectations of the nuclear plant’s safety inspection and maintenance programs. That’s why manufacturers build systems with features that enable them to survive longer in radiation areas.
Effective Nuclear Power Plant Videoscopes
There are five features that make videoscopes more effective for use in nuclear power plants. The attributes and a brief explanation of each follows.
Resistant to Radiation Damage. Even after being exposed to 200,000 rad (a unit that measures an absorbed radiation dose), the laser illumination and CCD (charged coupled device) image sensor should still work. Depending on the type of radiation, 1 rad is roughly equal to between 0.01 to 0.2 Sv. That means the insertion tube can withstand many times more radiation than the established safe limits for workers—about 40,000 to 800,000 times a person’s annual exposure limit.
Inspect from a Safe Distance. Extra-long insertion tubes enable workers to make inspections from a safe distance. A tube up to 30 meters (100 feet) can be fed into dirty water pipes in the radiation containment area, and workers can control and maneuver it from afar.
Tough Tube. With such a long insertion tube, there’s a lot of potential for damage. However, the videoscope’s insertion tube should be engineered to resist abrasion with a protective tungsten braided outer layer.
See the Light. A powerful light source that provides enough illumination to inspect the inside of a pipe or vessel from far away is essential. A powerful laser illumination system, coupled with wide, dynamic extended range image processing will deliver bright, contrast-balanced images across the entire depth of field.
Keep It Clean. Imagine inspecting the inside of a vessel 100 feet away. You navigate the insertion tube into the vessel and are about to start your inspection when, suddenly, a piece of dust lands on the lens and obscures your view. Instead of starting all over again, an air-powered lens cleaning system can be used to blow away dust and drip residue from the tip, so operators can have a clear view, even in dirty pipes.
—Charles Janecka is a sales engineer with Olympus, specializing in remote visual inspection.