The Safety Mission
Nuclear energy is already one of the safest ways to generate electricity. The next generation of nuclear reactors incorporates major advancements that increase resilience. Although each type of advanced reactor has different safety features and characteristics, the lessons from past incidents has led to selections of designs that maximize safety outcomes.
The key factors that determine accident risk are:
- Accident probability: the likelihood of an accident
- Accident consequence: the severity of an accident, often measured in terms of contamination and exposure risk
For a nuclear power reactor, accident risk is driven primarily by the probability of excessive heat generation leading to damage to the fuel elements and subsequent atmospheric release of radioactive fission products outside the core vessel, the building housing the vessel, and the power plant site.
In the case of Chernobyl, a fundamental design flaw combined with operator errors and negative safety culture led to a loss of control over nuclear fission reactions and subsequent steam explosions. At Fukushima, the loss of off-site power and flooding of backup generators led to loss of pumping power which made it impossible to adequately cool the fuel, even after shutdown. Fuel elements melted and released radioactivity.
Advanced nuclear designs are designed to prevent these types of outcomes.
Inherent Safety by Design
Advanced reactor designs all have inherent safety features, which take advantage of principles of physics or materials, rather than relying primarily on active systems. Similar to the way that advanced body structure design in automobiles can reduce the probability and effects of a collision, these features can reduce both the likelihood and consequences of severe accidents.
Several design strategies are described below, although the list is not comprehensive.
Accident likelihood can be reduced through:
- Resilient, passive cooling and other passive systems. Most conventional Generation III+ reactors (i.e. AP1000) and advanced reactors feature passive safety systems to maintain cooling in the event of loss of off-site power and other unusual events. Such systems are typically gravity-fed or pool-based, meaning that they rely on natural phenomenon like heat convection instead of powered mechanisms.
- More robust fuels to reduce reactor damage risk. Molten-salt fuels are designed so that in the event of a loss of power, the fuel form and surface area of its container cause it to cool and eventually solidify. TRISO fuels are made of a coated material that acts as a containment system and tolerates extremely high temperatures.
- Coolants operating at or near atmospheric pressure to reduce likelihood of coolant loss. Coolant that is highly pressurized (like water) is likely to escape the cooling system if the pressure boundary is broken. Many advanced reactor designs use coolants that operate near ambient pressure, which means they would not be propelled out of the reactor if pressure was lost.
Accident consequences can be reduced through:
- Low pressure operation preventing radiation dispersal. In addition to preventing damage to the reactor core or containment, low pressure operation limits the dispersal mechanisms for radioactive materials in the event of a severe accident.
- Maintaining lower inventories of radioactive products in reactor core. By reducing “source term,” (fuel, fission products, and other byproducts) in a reactor, the potential scale of an accident is reduced. Source term can be reduced through operating more efficiently, building much smaller units, and by removing fission products during operation.
- Advanced fuels that prevent fission product release. Advanced fuels like TRISO and other ceramic fuels feature their own containment function for radioactive particles, preventing their release. Beyond the reactor itself, the use of radioactive materials poses safety challenges across the supply chain. These risks are well understood, well managed, and do not cause significant public safety hazards in the U.S. Mining, transportation, fuel fabrication, wet storage, and dry storage have been routinely managed for over fifty years with minimal safety risks.
Fusion technologies do not face the same type of accident risks as fission. Due to the physical difficulty of sustaining a fusion reaction, the process of fusion stops once magnetic or inertial confinement is broken. In very severe accidents, off-site radiological exposure is possible but would be limited.