The Gen4 Module has incorporated advanced safety features:
LBE Coolant: The key attribute that enhances the safety of the Gen4 Module is the LBE liquid metal coolant. These characteristics radically reduce the potential energy available for dispersion of radioactivity in a worst-case accident.
- Potential radioactivity releases from vaporization of the coolant are eliminated because of the high boiling temperature of LBE
- Potential radioactivity releases due to pressurized release of the coolant are eliminated as LBE is at near-atmospheric pressure
- Because LBE is non-reactive with air, water, fuel, metal, and concrete, rapid exothermic reactions with associated radioactivity releases are eliminated. In addition, reactions that produce flammable hydrogen (a concern in LWRs and sodium reactors) are eliminated.
Underground Vault: The Gen4 Module is sited in an underground containment vault. The hardened containment vault is designed to protect against worst-case natural and man-made events.
- Any radioactivity release from the Gen4 Module, however unlikely, would be contained in the vault and isolated from the environment.
- The underground vault eliminates or minimizes the potential impact of natural disasters such as floods, hurricanes and tornados by preventing any contact of wind, water, or debris with the reactor. The vault is designed to survive a worst-case seismic event.
- The vault prevents the possibility of intrusion or tampering, as well as minimizing the impact of security threat scenarios such as an airplane impact or an explosive device.
- Because the Gen4 Module is factory sealed, never opened on site, and located in an underground vault, the potential for any radioactive contamination on the site is virtually eliminated.
Decay Heat Removal: U.S. Nuclear Regulatory Commission (U.S. NRC) General Design Criteria requires redundant, independent, and diverse means to reliably remove decay heat under all plant shutdown conditions. The Gen4 Energy design has incorporated two extremely simple systems:
- During operational shutdowns, decay heat is removed from the core through the normal coolant pathway by dumping steam to the condenser.
- Backup decay heat removal system provides natural circulation of LBE through a fixed bypass path in the core. Water from an emergency cooling tank is gravity-sprayed onto the exterior surface of the Gen4 Module, and heat is removed by passive vaporization of water. The backup decay heat removal system can provide adequate cooling for up to two weeks without power or operator action.
Proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as “Nuclear Weapon States” by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the Nuclear Nonproliferation Treaty or NPT. As a fissile material, the fuel is the only part of the Gen4 Module for which this could be a legitimate concern. However, there are a number of features of the Gen4 Module design that make it inappropriate for proliferation activities.
- The Gen4 Module uses fuel that is more difficult to separate than fuel used in production reactors. Reactors that are specifically designed for plutonium production typically use uranium metal or uranium alloy fuel. Use of metallic fuel provides for a relatively simple chemical separation process during the reprocessing operation that is necessary to separate the plutonium from the spent fuel.
In contrast, the Gen4 Module uses a fuel that is more difficult to separate than uranium metal or alloy. The fuel in the Gen4 Module system is uranium nitride, a high-temperature ceramic material that is not used in production reactors. Separation of plutonium from uranium nitride fuel would require new, sophisticated chemical processing capabilities and significant new infrastructure and equipment. Reprocessing plants for this type of material could cost hundreds of millions of dollars to build and operate. This is very similar to the complex reprocessing associated with conventional large light water reactors.
- There is no onsite access to plutonium in the Gen4 Module. Reactors that are designed for plutonium production provide for easy onsite refueling to allow frequent removal of the fuel for separation of plutonium and replacement with new fertile material. Consistent with that purpose, a production reactor would have built-in refueling capability, including lifting devices, crane, transfer facilities, a refueling deck, and pool for temporary storage of the irradiated fuel before it is shipped to a reprocessing facility.
The Gen4 Module is designed to be fueled at the assembly facility and shipped sealed to the site. No provision is made for onsite refueling or on-site access to reactor internals. At the site, the system would be operated for ten years, allowed to cool for one to four years, and then returned sealed to the assembly facility. Since the Gen4 Module is a sealed system, no material can be inserted into or removed from the reactor without extensive modifications to the entire system and significant exposure to anyone trying to access the module after use.
- The Gen4 Module does not incorporate processes or shielding to allow for the safe removal of plutonium. As in a traditional large light water reactor, the plutonium that is produced as a byproduct in the Gen4 Module is contained within the fuel and is part of the highly radioactive spent fuel. The spent fuel cannot be handled directly and is considered “self-protecting” due to the high dose rates that would be experienced unless proper facilities for handling and shielding are present. Even if the Gen4 Module could be unsealed by the operator, the removal of fuel would require shielding and remotely operated equipment to avoid lethal radiation doses. These elements are not part of the Gen4 Module design nor can the design be readily adapted to allow for such activities.
In conclusion, the Gen4 Module is an inappropriate design for production of plutonium and is much more proliferation resistant than a typical LWR.
Following the tsunami and nuclear incident near Fukushima, Japan, there has been worldwide concern for prevention of similar events in the future. The technology differences between the Fukushima reactors and the Gen4 Module are considerable.
|Fukushima||Gen4 Module||Positive Impact|
|Design Era||1950s-1960s||2000s-2010s||Incorporates 50 years of reactor operating experience|
(boils at 212F)
(boils at >3000F)
|Coolant is highly unlikely to ever evaporate|
|Containment||Aboveground structure||Underground silo||Better environmental isolation|
|Fuel Cladding||Zirconium||Stainless Steel||Non-reactive with coolant or environment|
|Decay Heat Removal||Active, electric power needed||Passively safe for more than 14 days||Less susceptible in accident scenarios|
|Size||Large||Small||Simplified earthquake resistance|
Factory Assembly, Standard Design
Because the Gen4 Module is produced in a factory using a single design, Gen4 Modules will be nearly identical. This leads to several advantages:
- Manufacturing process controls will be uniform and will not vary between units.
- Nuclear fabrication and assembly will be completed at the factory before the unit is shipped, minimizing the nuclear construction capabilities that are necessary on site.
- On site construction activities will be limited to the reactor vault, the non-nuclear systems, placement of the Gen4 Module in the vault, and connection to the Gen4 Module to non-nuclear systems and controls. This will significantly reduce the on-site construction complexity and result in a faster construction schedule.
- Gen4 Energy will provide standard operating procedures, operator training, licensing support, technical support, in-service engineering, and safety analysis, significantly reducing the nuclear expertise and staffing that is required of the owner/operator.