Reactor Control System
Beryllium Oxide Reflector Reactivity Control Assembly
The beryllium oxide reflector reactivity control assembly is a 60-kilogram mechanism that regulates reactor criticality from full shutdown to 110% nominal power and executes emergency scrams within one second. Surrounding the core with a sliding beryllium oxide reflector and a boron carbide absorber rod, it adjusts neutron leakage with 0.1-millimeter positional accuracy. Operating on the lunar surface requires the system to execute autonomous startups without human oversight and actuate mechanical components reliably in vacuum across wide temperature ranges. Furthermore, its actuator mechanisms and neutron instrumentation must endure continuous radiation exposure over a 15-year operational lifetime without recalibration or maintenance.
Reactivity control system using a sliding beryllium oxide neutron reflector and B4C absorber rod to manage reactor power level, startup, shutdown, and emergency scram.
Purpose
Provide autonomous, reliable control of reactor criticality for startup, steady-state power regulation, load following, and emergency shutdown (scram), ensuring the core operates safely within thermal and neutronic design limits.
Context
Wraps around the reactor core (L3-PWR-NUC-CORE) with the BeO reflector controlling neutron economy. Receives control commands from the power management system (L2-PWR-MGMT) and provides reactor status telemetry. The KRUSTY design uses a single sliding reflector for simplicity and reliability.
Principles
- ▸Beryllium oxide reflects neutrons back into the core, increasing criticality when fully engaged
- ▸Sliding the reflector away from the core increases neutron leakage, reducing reactivity
- ▸Boron carbide (B4C) absorber rod provides independent emergency shutdown capability
- ▸Negative temperature coefficient provides passive self-regulation (higher temp reduces reactivity)
- ▸Reactor period and power monitored by neutron detectors at multiple positions
Typical implementations
- ▸KRUSTY: single BeO radial reflector with linear actuator, B4C safety rod
- ▸SNAP-10A: movable reflector drums with neutron absorber segments
- ▸TOPAZ: rotatable control drums around the reactor periphery
- ▸Commercial PWR analogy: control rods, but space reactors favor reflector control for compact geometry
Lunar considerations
- ▸Fully autonomous startup sequence without human operator oversight
- ▸Actuator mechanism must function reliably in vacuum and wide temperature range
- ▸Scram mechanism must be fail-safe (spring-driven or gravity-assisted reflector drop)
- ▸Neutron instrumentation must survive radiation field for 15+ years without recalibration
- ▸Seismic loads from moonquakes must not inadvertently shift reflector position
Specifications
Functional
| primary function | Control reactor criticality via reflector positioning and provide emergency shutdown |
| inputs | Power level setpoint commands from L2-PWR-MGMT, Neutron flux measurements from in-core and ex-core detectors, Core temperature measurements from thermocouples, Emergency scram signal from safety system |
| outputs | Reflector position (controls reactor power from 0 to 100%), Scram actuation (rapid reactor shutdown in < 1 second), Reactor status telemetry to L2-PWR-MGMT |
| reactivity control range | Full shutdown to 110% nominal power |
| scram time seconds | 1 |
| position accuracy mm | 0.1 |
| load following rate | 10% per minute |
| fail safe | Loss of power causes reflector to drop to shutdown position |
Physical
| mass kg | 60 |
| dimensions | Annular reflector ~15 cm OD around core, ~25 cm height, plus actuator assembly |
| materials | Beryllium oxide (BeO) neutron reflector segments, Boron carbide (B4C) absorber rod, Stainless steel actuator mechanism, High-temperature stepper motor or linear actuator, Fission chambers and ion chambers for neutron detection |
| operating temperature c | 200-600°C (reflector surface) |
| radiation field | High neutron and gamma flux at reflector surface |
| vacuum | True |
Operational
| power consumption w | 30 |
| thermal range c | 20, 600 |
| lifetime years | 15 |
| mtbf hours | 200000 |
Interfaces
Provides
- Neutron reflector positioning controlling core reactivity and power level
- Reactor status: neutron flux, core temperature, reflector position, criticality state
Requires
- Power level setpoints, startup/shutdown commands, scram authorization
- Core temperature and neutron flux measurements for feedback control
- Electrical power for actuator motor and instrumentation electronics (~30W)
- Structural support and alignment for reflector actuator mechanism
Decomposes into
Cite this entry
Lunar Ark Codex. "Reactor Control System" (L3-PWR-NUC-CTRL). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-PWR-NUC-CTRL
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.