Efficient Time-Independent Method for Conceptual Design Optimization of the National Ignition Facility Primary Shield

Minimum-cost design concepts of the primary shield for the National (laser fusion) Ignition Facility are sought with the help of the SWAN optimization code. The computational method developed for this search involves incorporating the time dependence of the delayed photon field within effective delayed photon production cross sections. This method enables the time-dependent problem to be addressed using time-independent transport calculations, thus significantly simplifying and accelerating the design process. The search for constituents that will minimize the shield cost is guided by the newly defined equal cost replacement effectiveness functions. The minimum-cost shield design concept consists of a mixture of polyethylene and low-cost, low-activation materials, such as CaCO₃ or silicon carbide, with boron added near the shield boundaries. An alternative approach to the target chamber design is analyzed. It involves placing the shield interior, rather than exterior to the main aluminum structural wall of the target chamber. The resulting inner shield design approach was found to be more expensive but inherently safer; the overall inventory of radioactive activation products it contains is one to two orders of magnitude lower than in the conventional design approach.