Going Nuclear: criticality, SUR, xenon and decay heat
Handbook · Operation · Next: RBMK
REALISTIC mode calculates how power develops after a change. The rod slider is not a direct power percentage. Heat, water, fuel and fission products also affect the reaction.
Criticality and reactivity
k-effective describes whether one neutron generation sustains the next. Below 1, the chain reaction declines; at 1 it is self-sustaining; above 1 it grows. Criticality therefore means a sustained reaction, not necessarily an accident. Prompt criticality is dangerous: growth no longer relies on the slower response of delayed neutrons.
Reactivity measures departure from equilibrium. The calculator uses pcm or dollars. Dollars compare reactivity with the delayed-neutron fraction; they are not currency. A larger reading is not simply a larger usable SU reserve.
Control rods absorb neutrons. Insertion reduces reactivity, withdrawal raises it. Graphite moderates neutrons and changes the core. Extra graphite is not merely decoration and does not automatically make a design safer.
Reactor period and startup rate
The period is the time for power to grow by a factor of e, approximately 2.718, at a constant trend. It is not the doubling time. A short positive period means rapid growth.
SUR, startup rate, is measured in decades per minute, dpm. At a constant
SUR of 1, power increases tenfold in a minute. For positive period T in seconds,
SUR ≈ 26.06 / T. A 10-second period is approximately 2.6 dpm.
With standard protection enabled, a period below 10 seconds triggers a trip. The configuration describes rod withdrawal being held below 20 seconds and rods driven back in below 15 seconds. Settings and the current model state remain decisive. Low present power does not protect against rapid growth.
Physics Desk calculations
| Calculate input | Decision aid |
|---|---|
| Rods in at | Reactivity, k, period and SUR for the proposed insertion |
| Hours after trip | Xenon worth, restart availability and decay heat after that time |
| Power at | Steam, feedwater and turbine requirements |
| Short period limit | Reactivity corresponding to the short-period protection threshold |
Measure supplies the current context. Some older text describes values that newer views expect you to calculate. Calculate a position, adjust cautiously and check the actual trend rather than looking for a permanently displayed perfect rod setting.
Xenon and restarting
Fission creates products including iodine, which decays into neutron-absorbing xenon-135. Xenon can continue building after shutdown even while fission power is falling. Available reactivity may temporarily be insufficient for restart: this is xenon dead time.
Wait until the calculator indicates restart is possible, and recheck cooling and fuel. A heavily withdrawn setting used to overcome xenon may become excessive once xenon decays or burns away during operation. Large load changes therefore need continuing adjustment.
Decay heat and time compression
Fission products keep generating heat after shutdown. The core and spent-fuel pool can overheat without water even when normal power operation has ended. Heat depends on operating history and elapsed time: inserted rods do not mean zero heat.
timeCompression accelerates slow processes. The checked default and local
BuildUp value are 60. Do not equate calculator hours directly with wall-clock
hours. Older releases use different factors. Use current measurements and
calculations for the actual plant.
REALISTIC versus SIMPLE
REALISTIC requires attention to reactivity and growth. SIMPLE makes power follow rods with a configurable delay. Its configuration says criticality, reactor period and short-period trips do not apply. Heat, steam, cooling and meltdowns still do.
A SIMPLE startup recipe can therefore be unsuitable for REALISTIC. Mode is a pack/server choice, not a substitute for fixing a broken cooling system.