hydrotests are used to validate models simulating the implosion of a weapon primary.

Input Parameters Input parameters are the physical data that characterize the behavior of the materials used in a simulation. Examples are equations of state, opacity, and neutron cross sections. Input parameters can be selected for best fit to integral data such as that from underground nuclear tests. Input data are not knobs. Once selected for optimizing a baseline calculation, they remain fixed until the model is changed.

Knobs Knobs are a resort to ad hoc normalization to integral data. There is no firm physics in a knob. If knobs are used in a baseline calculation, the knobs should remain unchanged from one calculation in a baseline suite of data to another. The degree to which knobs are used in a simulation weakens the ability of that simulation to model similar data.

Nuclear Explosive Package The nuclear explosive package—also called the physics package—is the portion of a nuclear weapon that contains all of the components that generate the actual nuclear explosion; specifically, the fission primary—with its plutonium pit—and the thermonuclear secondary device.

Performance Gate A performance gate is a range of acceptable values, defined by subsystem margins and uncertainties, for the performance of each of many subsystems in the chain of events occurring in a nuclear explosion. It is a range of values for some performance metric that must be achieved for success. These values are associated with the key components and operating characteristics of the weapon; their failure would severely compromise the overall performance of the weapon.

Performance gates vary in importance and type. All involve a performance threshold, expected performance variations, and performance margins. The nature of the margin depends on the gate. Examples include shape, timing, neutron fluence, criticality, temperature, yield, and functional mode.

The understanding of performance gates is incomplete.

Physical Inputs Physical inputs define the problem to be simulated. Size, shape, thickness, mass, material, and density are examples. These are measurements and subject to random uncertainties.

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