Severe Accident Simulation

Due to events in Japan in the wake of the 11 March 2011 earthquake and tsunami, nuclear regulators now more closely scrutinize the capabilities of nuclear power plant operators to manage situations caused by beyond design basis and severe accidents. This close scrutiny requires that operators are trained in handling situations involving severe accidents, and that their training includes knowledge on and experience in performing recovery operations safely.

A severe accident can occur if a loss of fuel cooling is not mitigated in time. L3Harris' thermal-hydraulic model simulates various effects related to loss of cooling such as steam formation, core heat-up, fuel failure and hydrogen generation. However, the primary assumption in ANTHEM™ is that there is no change in the geometry of the fuel or the surrounding structures. This is also true for all other thermal-hydraulic models used for power plant simulation. If such conditions must be simulated, then additional codes are required. Therefore, for the purposes of simulation, a severe accident is defined as any initiating event whose evolution results in a change of geometry of the fuel or the surrounding structure.

Highly sophisticated computer codes have long existed to analyze plant designs. They are designed to be used in an off-line, stand-alone, non-continuous and non-real-time application. An example of such a design code is MAAP (Modular Accident Analysis Program). MAAP is an internationally recognized model used to analyze severe accidents and provided under license from EPRI. We support both MAAP4 and MAAP5. The code considers a number of processes that occur during severe accident conditions such as:

• Steam formation
• Core heat-up
• Cladding oxidation and H2 evolution
• Vessel failure
• Core debris-concrete interactions
• Ignitions of combustible gases
• Fluid entrainment by high-velocity gases
• Fission product releases transport and deposition

The plant systems included in the scope of MAAP models are:

• Reactor Coolant System
• Steam Generators and Main Steam Header
• Reactor Core
• Containment
• Spent Fuel Pool

While the models for these systems are sophisticated, they are designed specifically to simulate severe accident conditions. Other plant systems, such as core cooling systems, balance of plant systems and control systems, are treated as boundary conditions or functionally simulated. For this reason, MAAP is used for severe accident simulation and cannot be considered a model that can be used for all plant operations that are typically considered in a real-time simulator.