Reactivity converter
Convert reactor reactivity between pcm, dollars and k-effective, and see the period it produces.
That's about the same as a control rod step. That is about 0.15 dollars of reactivity.
What it means
How far from critical?
Dollars use β = 0.0065 for uranium-235. A plutonium-fuelled or high-burnup core has a smaller β, so the same pcm is worth more dollars.
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Learn
Three things worth knowing
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A dollar is 650 pcm
For a uranium-235 core. Below a dollar, power changes over seconds and can be controlled; above it, prompt criticality takes over.
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Delayed neutrons make reactors possible
Less than a percent of fission neutrons arrive seconds late rather than instantly. That tiny fraction is the entire margin that makes a reactor controllable by mechanical rods.
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The dollar is not a fixed number of pcm
β depends on the fuel: about 650 pcm for uranium-235 but only 200 for plutonium-239. The same pcm means three times the dollars in a plutonium core.
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Background
About reactivity
A reactor at exactly critical has a reactivity of zero: each generation of neutrons exactly replaces the last. pcm — hundred-thousandths — measures the departure from that.
The dollar is the important one: one dollar of reactivity is the delayed neutron fraction, the point at which a reactor no longer needs its delayed neutrons to keep going, and control passes from seconds to microseconds.