Power Optimization Method for Land-Transportable Fully Passive Lead-Bismuth Cooled Small Modular Reactor Systems

Abstract

no production hydrogen, no reaction with water and air, high capability of natural circulation, and negative void coefficients for small size core.

On the other hand, demand of fully passive cooling SMRs is increasing worldwide because they are simpler, standardized, and safer modular design by being factory built, requiring smaller initial capital investment, and having shorter construction times. They could be small enough to be transportable used in isolated locations without advanced infrastructure and without power grid, or could be clustered in a single site to provide a multi-module, large capacity power plant. Also fully passive cooling without pump even in normal operation enhances the passive safety of nuclear power plants. Thus, the solution integrated by long burning technology with LBE coolant and fully passive SMRs could solve the current weaknesses of PWRs by burning the nuclear waste, enhancing the nuclear safety, and increasing the nuclear economy. For the reactor power size, power of SMRs should be maximized to maximize the economy of SMRs, having modularization fabricated remotely and transported to the site. Moreover, it is needed to have a specific power level matching the specific demand of towns or sites that are either off-grid or on immature local grids, being right-sized for growing economies and infrastructures of developing nations. Thus, the maximized power level of SMRs should be estimated. However, flexibility of the power is limited by land-transportable shipping size, materials endurance, long burning core neutronics, and accidents conditions. The dissertation is aiming at developing the power maximization method for LBE natural circulation cooled SMRs satisfying the constraints shipping size, materials endurance, neutronics as well as safety under beyond Design Basis Events (DBEs).

To achieve the goal of dissertation, three research questions are coming up: 1) what are limiting factors to design LBE natural circulation cooled SMRs, 2) what are design tools to design LBE natural circulation cooled SMRs and how are they validated, and 3) How to develop a power optimization method.

Design limitations are divided by limitations for steady state and accidents conditions. Steady state limitations including land-transportable shipping size limits, materials limits, neutronics limits are determined. The quasi-static reactivity balance equation is used to obtain the limitations of reactivity in accidents conditions for selected Beyond DBEs

nuclear waste, nuclear safety, and nuclear economy. The spent nuclear fuels (SNFs) accumulated by operation of PWRs during last 50 years are over the 200,000 tons worldwide without any solution. From several nuclear accidents including Fukushima accident (March, 2011), nuclear energy is confronted with criticism about safety issues. Also, large amount of initial investment of current PWRs is chronic problem with blocking the activated investment of private companies.

As a future nuclear energy solution, long-burning technology and fully passive cooling Small Modular Reactors (SMRs) are emerging nuclear concepts. In order to overcome SNFs problem, long burning reactors using fast neutron spectrum could transmute high level waste into low or intermediate level waste. Also, utilization of 238U in fast reactor is more efficient than it of thermal reactor. As a coolant for fast reactors, Lead-Bismuth Eutectic (LBE) coolant has many advantages

Unprotected Transient OverPower, and Unprotected Loss Of Heat Sink. Void coefficients should be negative because steam could be penetrated into core in unprotected Steam Generator Tube Rupture.

For the design tools and validations, LBE coolant experiments using HELIOS (Heavy Eutectic liquid metal Loop for Integral test of Operability and Safety of PEACER) facility are conducted. In the forced convection test, pressure losses of core, orifice, gate valve, and expansion tank are obtained. In the natural circulation test, temperature distribution and mass flow rate are obtained for specific core heat. Also, long-term stability of LBE natural circulation is confirmed by 600-hours experimental test. Predictions for hydraulic-resistance and natural circulation behavior of experimental results are conducted by MARS-LBE and CFD (Computational Fluid Dynamics). Pressure loss coefficients of measured data are good agreement with CFD results and natural circulation experimental results are good agreement with MARS-LBE predictions when MARS-LBE uses the recommended pressure loss coefficients from CFD simulations. With comparison between measured data and natural circulation governing equations, natural circulation SMRs design equation is derived and validated and could be used for the optimization method.

Based on the previous answers including design limitations and design tools, power optimization method is developed with the flow chart. Using the power optimization method, natural circulation cooling capacity and neutronics maximized power could be calculated. Natural circulation cooling capacity is the capability of core power cooled by only natural circulation with fixed land-transportable shipping size limit and within the corrosion, erosion and DBTT limits. Neutronics maximized power is the capability of core power within materials Displacement Per Atoms limit, reactivity swing and excess reactivity swing limitations. From the comparison between these two power capacities, smaller power for each core height is the maximized reactor power for each core height. Then, one specific maximized reactor power that the largest value with the core height is determined. Case study for 20 years long-burning small modular reactor with LBE natural circulation using the power optimization method shows the maximized power is 206MWt.

In order to take a good position in future energy spectrum, nuclear energy should be satisfied with future energy demand by overcoming the weaknesses of current Pressurized Water Reactors (PWRs)