Rightsizing the EPZ
With creating the new generation of nuclear power comes the responsibility of ensuring safety for all humankind. The Emergency Planning Zone (EPZ) is the area surrounding the nuclear power plant within which special considerations and management practices are pre-planned and exercised in case of emergency.
In practice there are two EPZs surrounding the plant site. The first, called a Plume Exposure Pathway, is traditionally at a 10 miles radius for conventional nuclear plants, and is designed to avoid or reduce the dose from potential exposure of radioactive materials from the plant. The second, called the Ingestion Exposure Pathway, is about 50 miles in radius for conventional nuclear plants, to avoid or reduce the dose from potential ingestion of food contaminated by radioactive materials. For both zones specific emergency procedures are in effect.
The exact size and shape of each EPZ is determined by careful consideration of the following factors:
- The operating characteristics of the plant
- The geographical features where the plant is located
- The population areas surrounding the plant
Under an emergency, managing the EPZ becomes very important for the plant itself and the areas beyond the EPZ.
As the NuScale SMR is so much smaller and safer than large nuclear reactors, rightsizing the EPZ is important. The EPZ is expected to be smaller since our safe design means there is less likelihood that something unexpected, such as severe accidents, would occur. Also a smaller reactor core means less radioactive material that could be released in an accident.
NuScale Power worked with the Nuclear Energy Institute (NEI) to develop methodologies to support smaller EPZs for SMRs for discussion with the NRC. In December 2015, NuScale submitted a topical report to the NRC detailing its proposed methodology for NuScale Plant licensees to determine the appropriate EPZ.
Why is the NuScale SMR safer? Our safety features make the difference.
Low Core Damage Frequency (CDF)
Accidents that cause damage to the nuclear core of a reactor are serious because they may result in damaged fuel or a core meltdown, which could result in large releases of radioactive materials. Core damage frequency (CDF) is a term used in probabilistic risk assessment to express the likelihood of an accident that would cause damage to a nuclear reactor core. The lower the CDF, the lower the chance that an accident will occur that would result in core damage and potential release of radioactivity.
NuScale has worked hard to achieve a reactor design that reduces core damage frequency: NuScale’s simple design eliminates numerous systems and components whose failures contribute to core damage in conventional reactors. The likelihood of core damage due to NuScale reactor equipment failures while at full power conditions is 1 event per module every ~3 Billion Years.
NuScale’s design is informed by our study of over sixty years of operating experience in the nuclear industry, lowering the frequency of safety systems and component failures to very small values.
NuScale’s approach to safety has been to build features, layer upon layer, into our design that prevent accidents and mitigate the consequences of events at NuScale plants. There are now seven barriers to the release of radioactivity from the fuel in a NuScale power plant. Individually, these features provide a high degree of protection. As a whole they take safety to a new level.
For any potential hazard that extends beyond a facility boundary, the goal of all responsible parties is to minimize the potential danger to the public and the environment. NuScale drives these risks down to the greatest extent possible.
Through our layered approach to safety, we have minimized the number, the frequency, and the consequences of these events.
Features like the cladding on the fuel, the reactor vessel, and the containment have been used for years. NuScale has optimized the design of these features and added layers of protection to retain and inhibit the release of radioactivity.
The NuScale SMR design significantly reduces the risk of a radiological release and offsite consequences. The postulated release of radioactive material from a nuclear power plant to the environment (the source term) depends on the amount of fission products and other radionuclides in the reactor core.
The NuScale reactor core is only 5% of the size of large, conventional reactor cores. The smaller inventory of radioactive material in the NuScale core ultimately yields a smaller source term. This reduces the Emergency Planning Zone (EPZ) size required to provide the same level of protection to the public and the environment as currently operating nuclear power plants.
Slow (Accident) Events
The NuScale plant design includes features such as low power density, high coolant-to-power ratio, extremely reliable passive safety systems, robust containment, and immersion within the reactor building pool (ultimate heat sink). Large break loss-of-coolant-accidents have been eliminated by design. These features cause postulated core damage events to progress more slowly than in large, conventional nuclear reactors.
Slow accident progression provides more time for the short-lived radioactive isotopes to decay. Decay of short-lived radionuclides is therefore more likely to occur within the multiple boundaries incorporated into the NuScale power plant design.
The slow development of potentially damaging events allows greater time for an operator to respond effectively and take mitigating actions.