Dynamic analysis for design criteria for underground nuclear reactor containments

Abstract

This paper is based on i) the recent input of the authors for the Underground Containment Sub-section of the Seismic Task Group Report of the ASCE Committee for Nuclear Structures and Materials, and ii) parametric studies carried out by the first author on the principal underground concepts. The extensive work on aseismic design of above-ground reactors and recent studies on missile impact effects, aircraft impact, blast effects due to chemical explosions, reactor core melt-down and tornadoes indicate the advantages of underground siting with inherent general reduction to complexity of seismic amplification and benefits of structural and biological integrity. Other advantages are possibilities of urban siting, ecological considerations, reduced effects on the landscape, ability to design three-dimensionally, separation of component facilities, support capability to equipment, reduced power transmission costs, increased number of acceptable units and power capability from a single location, and reduction of decommissioning problems. In view of the limited actual experience in the structural design of underground containments (only four European reactors), the proposals are based on a) the transposition of applicable design specifications, constraints and criteria from existing surface nuclear power plants to underground, and b) the use of many years of experience in the structural design of large underground cavities and cavity complexes for other purposes such as mining, hydropower stations etc. All concept options are assumed to be similar in design criteria for structural competence to contain radioactivity and fuel heat and meet the functional, servicing, protective and aesthetic requirements. The choice of underground siting should be based on criteria developed from the sequential consideration of load-causing phenomena, concept and site characteristics. From the criteria, loads for a particular concept and site are then calculated and the design formulated. The state-of-the-art is presented and guidelines are outlined for 1) Load causing phenomena, 2) Underground siting concept considerations, 3) Siting factors and structural selection, 4) Structural types, 5) Analysis (including comparison of lumped parameter and finite element analysis), 6) Design procedures, and 7) Miscellaneous considerations (laboratory tests and field measurements, secondary equipment, faults, leakage of radioactivity, ground water control, environmental factors). Parametric studies are described for structural characteristics of the four principal underground concepts: a) Cut-and-cover in rock or soil, b) Unlined cavity in rock, c) Lined cavity in rock or soil, and d) Lined cavity in rock or soil with annular filling of soft material — with respect to shape, backfill material, cavity wall reinforcement, passive and active rock bolting, lining and annular filling. The response to a step pulse, representing a blast excitation applied horizontally, is studied. As the character, intensity, duration and frequency of earthquake and blast-induced ground motions are roughly similar, the results have practical value in studying earthquake effects.