Reactor Operations and Safety
How is the NuScale reactor simplified compared to current generation reactors?
The SMR’s cooling systems are vastly simplified compared to traditional reactors and require no pumps, power, or human intervention to keep people safe during unplanned events or conditions. As a result, it requires a fraction of the engineered safety systems that traditional reactors need to protect the plant, the environment, and the public.
What kind of fuel does this SMR use?
The NuScale reactor will use the same type of standard fuel as in today’s commercial light water reactors. It will be in a 17 x 17 configuration (each fuel assembly contains 289 rods); each assembly is about 6.5 feet (2 meters) long. The fuel will be enriched to 3.7 percent U-235 which is typical of most commercial power reactors.
How often will it the reactor power modules require refueling?
The reactor modules would require refueling approximately every two years. In practice, it is expected that every other month one of the 12 modules would have about 1/3 of its core replaced.
Inherent or “passive” safety features rely on the natural laws of physics and do not require the action of operators or machines. Heat removal achieved by natural circulation of cooling water without the use of pumps is an example of an inherent safety feature. Inherent safety features are important because they are designed to keep the reactor and the public safe without any operator action and even if all other safety systems fail.
What are “inherent safety features”?
The NuScale design shuts down and self-cools indefinitely with no operator action, no AC or DC power, and no additional water. It replaces many engineered backup systems (such as motors, pumps and valves) with features that operate automatically by relying on natural phenomena such as gravity, convection, and conduction.
The reactor operates using the principles of gravity and natural circulation, so pumps aren’t needed to circulate water through the reactor. Instead, the system relies on a natural convection process. Water at the bottom of the reactor vessel heats up as it passes over the core and pulls heat from the fuel. The hot water then rises within the reactor vessel. Once the heated water reaches the top, it passes over hundreds of tubes in the steam generator, heat is transferred through the tube walls and water inside the generator tubes turns to steam.
When water at the top of the reactor vessel has transferred its heat through the generator tubes, it becomes cooler and denser. Gravity pulls the cooler water back down to the bottom of the vessel where it is again drawn over the core, beginning the cycle again.
The plant does not rely on AC or DC power for safety, which simplifies the electrical systems. In normal operation, the plant relies upon electric power to maintain motors, pumps, valves and control rods in the operating position. In the case of an accident that disrupts the plant electricity, the valves and control rods move to their safe configuration by gravity or pressure differentials, and core heat exchange is achieved by convection, conduction, and gravity.
What are some of the NuScale reactor’s key inherent safety features?
The United States average yearly normal background exposure to an individual is around 300 mRem, while the average in Salt Lake City is around 600 mRem, and in Denver, CO it is around 1200 mRem. For comparison, the annual exposure at the site boundary for a NuScale Power plant has been calculated to be less than 100 mRem/year for normal operations, per 10 CFR 20.1301(a)(1). Additionally, the maximum exposure in the worst case accident has been calculated to be less than 1 mRem which will meet the limits of 10 CFR 20.1301(e) and 40 CFR 190. Assumptions pertaining to the location of the nearest offsite individual are site specific and will be included in the combined Construction and Operating Licensing Application (COLA), not yet commenced.
What is the expected site public exposure to the nearest exposed individual for normal operations and in the worst case accident?
The reactor vessel sits inside a larger containment vessel, which itself resides in an underground stainless steel-lined concrete pool of water. Shield covers and the reactor building itself reside above the reactor components. The reactor building will be a Seismic Category 1 reinforced concrete structure designed to withstand the effects of aircraft impact, environmental conditions, natural phenomena, and postulated credible accidents and threats.
The reduced amount of fuel, inherent safety features, and natural physics of the reactor’s design eliminate the possibility of fuel damage in all design basis events (events that a nuclear facility must be designed and built to withstand). For “beyond design-basis” events (accident sequences that were not fully considered in the design process because they were judged to be too unlikely - on the order of once in every 10 billion years), the radiation from fuel damage would not exceed US EPA established limits at the plant site boundary.
What happens if key safety systems fail?
Yes. Accident analysis and response studies will be part of the design certification application submitted by NuScale, and the construction and operating license application submitted by UAMPS. The main bodies and non-confidential supporting reports of both those applications will be available to the public.
Will accident analysis and response studies conducted as part of the NuScale or UAMPS licensing process be publicly available?
The NuScale Nuclear Power Modules sit submerged in the reactor pool. The spent nuclear fuel is submerged in the spent fuel pool, which is fluid and thermally connected to the reactor pool. Upon loss of power, passive cooling systems on the NuScale Power Modules are automatically placed in service to transfer core decay heat to the reactor pool via natural circulation. No operator action, AC or DC power or additional water is required. Assuming conservative initial conditions, adequate water will remain in the pool to cool all the Nuclear Power Modules and to keep the spent fuel pool covered in excess of 100 days with no operator actions. The NuScale plant response to an extended loss of AC power will be detailed in the Design Certification Application and supporting technical documentation.
What happens if all off-site power is lost for an extended period of time? Are the spent fuel and the reactor core protected from overheating?
The reactor’s inherent and redundant safety features drastically minimize the size of the evacuation zone required to protect public safety. The NRC is currently evaluating the proposed emergency planning zone sizing methodology. NuScale expects that the application of that methodology to the CFPP will demonstrate radiation levels within U.S. EPA safe limits at the CFPP site boundary.
How large is the evacuation zone in case of an accident?
With the amount and type of inherent safety features built into the design of the plant the possibility of a serious accident having any off-site radiological consequences is extremely remote. Current core damage frequencies for the NuScale plant indicate a predicted likelihood of such an event to be less than one in 100 million operating reactor years and the damage would be limited to the plant. The postulated beyond design basis event for core damage and radiation leakage from the plant will be within safe limits at the CFPP site boundary, and rapidly decrease with distance. Therefore, there are no expected health impacts to the public or damage to public property. However, the CFPP, like all commercial nuclear facilities, will be covered by the Price-Anderson Act. This Act ensures the availability of a large pool of funds (currently about $10 billion) to provide prompt and orderly compensation of members of the public who incur damages from a nuclear or radiological incident no matter who might be liable.
What are the potential liabilities associated with a serious reactor accident or radiological event?
No. NuScale has conducted testing of various reactor systems since 2003 at NuScale’s integrated testing facility in Corvallis, Oregon. This facility is a one-third scale prototype of a NuScale SMR. The selected Idaho location for the SMR will NOT be a demonstration site. The design of the SMR will be thoroughly reviewed and certified by the U.S. Nuclear Regulatory Commission, which sets the world standard for defining rigorous safety requirements.