Codex Energy Advanced Reactor

Advanced Reactor Model

The Advanced Reactor Model is an optional, more realistic simulation of the Fission Reactor. The server switches it on with volt.advancedReactorModel (default false). Everything else (building, fuel, cooling, alarms, SCRAM, meltdown) stays as in the standard model; what changes is how the reactivity and the power behave:

• the power does not follow the reactivity directly but grows or falls with a period, and fast when the core is prompt critical;
• where fuel, moderators and control rods stand matters: every column of the core has its own reactivity;
• xenon builds up from the power and poisons the core, also after a shutdown;
• coolant that does not flow forms voids, and voids add reactivity.

Switching the setting keeps the reactor's state. When the model is switched off again, a power above 100 % is cut back to 100 %.

Reactivity in this model

Reactivity is an excess: 0 means the power stays where it is, above 0 it grows, below 0 it falls. The values below are fractions: 0.01 is 1 %.

Term	Value	What it does
Cold excess	up to 0.06	what a well moderated column has with the rods out, cold and without xenon
Control rods	up to 0.1	taken away from a fully covered column at full insertion
Temperature	3e-05 per °C	taken away for every degree above 20 °C
Xenon	grows with the power	taken away; see below
Void	up to 0.008	added while coolant is missing
Prompt threshold	0.01	from here the power runs away

Columns

The core is divided into columns, one for every field of its footprint seen from above (the area where fuel, moderators, rods and channels stand). Each column counts its own blocks.

• Moderation of a column: its own moderators count fully, those of the four direct neighbour columns half. As in the standard model, 0.3 without moderators and full at 0.75 moderator blocks per fuel assembly of the column.
• Cold reactivity of a column = cold excess × (2 × moderation − 1). A column needs about one moderator for every five fuel assemblies (neighbours counted half) to become critical at all; without moderators it stays below zero.
• Rod coverage: a control rod column covers itself fully, its four direct neighbours half and the four diagonal ones a quarter. Coverage adds up, at most fully.
• Column reactivity = cold reactivity − rod worth × insertion × coverage.
• The core follows its strongest columns: the reactivity of the core is a soft maximum over the fuel columns. One hot spot drives the whole core.

Warning

Fuel no rod reaches A fuel column whose coverage is too small stays critical even with all rods in. The core view outlines such fuel red ("No rod can shut it down"). With full moderation a column needs at least 0.6 coverage: one rod column right beside it is not enough, two are.

Power and period

• Below the prompt threshold the power grows with a period: slowly near zero, faster the higher the reactivity. At 0.5 % it doubles in about 10 seconds, at 0.9 % in about 2 seconds.
• At the prompt threshold or above it doubles every 0.25 seconds. The core heats up faster than any cooling can follow.
• Negative reactivity lets the power fall with a period of at least 3 seconds.
• A core that becomes critical starts from a tiny source power, so a freshly withdrawn core takes a moment before the power shows.
• The relative power can climb far above 100 % (up to 50 times the rated power). Thermal output = power × loaded rods × 400 HU per tick, as in the standard model.

Temperature decides the level

Because the temperature takes reactivity away, a critical core heats up until temperature and rods balance out. The rods set the temperature the core settles at; the cooling decides how much heat it makes there. Every 1 % of excess reactivity left by the rods holds the core about 333 °C above 20 °C. In a fresh core (no xenon yet) with the rods fully out and full moderation the balance would only be reached above the failure temperature: never leave a well moderated core unattended with the rods out.

Xenon

• Iodine builds up from the power and decays into xenon; xenon decays slowly and is burned by the flux.
• At full power xenon settles at about 3 % (most of it after about 10 minutes).
• After a shutdown xenon first rises, to about 4.5 % after about 3.5 minutes, and is below 0.4 % only after about 24 minutes. A core that was shut down may not become critical again for a while, even with the rods out.
• Starting up again burns the xenon away: the reactivity rises while the power grows. Withdraw the rods slowly.

Voids

When the core asks for more coolant than it gets (no water, full Hot Coolant tank, too few channels), the missing share becomes void and adds up to 0.008 reactivity, with a short lag. A core that loses its coolant therefore gets hotter and stronger. Keep the automatic SCRAM on.

The core view

While the model runs, the reactor window shows an extra button left of the telemetry button ("Core view (advanced reactor model)"). It covers the process area with:

• The core map, one square per column: control rods grey, fuel coloured by its share of the power (bright = strongest column), moderators dark blue, coolant channels light blue. Fuel that no rod can shut down has a red frame.
• The reactivity balance: Cold excess, Control rods (with the current insertion), Temperature, Xenon, Void, and the sum, Reactivity.
• A scale from −1 % to +1.5 % with marks at 0 and at the prompt threshold of 1 %; the white mark is the current reactivity.
• The growth line: "Power doubles every … s", "Power does not grow" or "Prompt critical!".
• The xenon trend: "Xenon rising", "Xenon falling" or "Xenon steady".

The buttons on the right stay usable while the view is open. A Sensor Module and computers can also read the reactivity, the xenon, the void reactivity and the doubling time.

Designing a core for this model

• Give every fuel column moderators in or beside it; a checkerboard of fuel and moderator columns works well.
• Put control rod columns so that every fuel column touches two of them directly, one directly and one diagonally, or four diagonally.
• Keep the rod columns spread out: a cluster of rods leaves the far side of the core uncovered.
• Watch the core view while you withdraw the rods: as long as the doubling time is many seconds, you have time to react.

Tip

The example core The example reactor of the build guide works under this model too: every fuel column is fully moderated and lies between two rod columns. It goes critical with the rods about three quarters out and shuts down with all rods in. Withdraw the rods slowly: above +1 % reactivity the power doubles four times a second.