Therac-25: Software Race Condition & 25,000-Rad Lethal Radiation Overdoses
Physical hardware interlock Constraint_Boundaries were removed in favor of software flags. High-speed user input triggered an unhandled race condition in concurrent Decision_Nodes, firing maximum raw energy into biological nodes.
Atomic Energy of Canada Limited (AECL) redesigned the Therac-25 linear accelerator, removing physical hardware interlocks from prior models and relying strictly on software safety routines. When experienced technicians typed commands into the VT-100 console rapidly (under 8 seconds) and corrected keystrokes with `Up Arrow`, a race condition bypassed the collimator turntable rotation check, firing the raw 25-MeV electron beam without the tungsten target in place, giving patients 25,000 rads (100x therapeutic dose) and causing 6 fatalities.
Physical hardware interlock Constraint_Boundaries were removed in favor of software flags. High-speed user input triggered an unhandled race condition in concurrent Decision_Nodes, firing maximum raw energy into biological nodes.
Inputted X-ray mode, hit Up-Arrow, selected Electron mode in under 8 seconds
Set beam intensity to 25-MeV X-ray level but left physical turntable in electron mode
Provided zero physical mechanical interlocks because hardware relays were removed in cost-cutting redesign
Administered 25,000 rads directly to patient tissue, causing fatal radiation necrosis
"The manufacturer removed the hardware safety interlocks because they believed software made them redundant."
Cross-Domain Invariant Twin Failures (53)
Physical hardware interlock Constraint_Boundaries were removed in favor of software flags. High-speed user input triggered an unhandled race condition in concurrent Decision_Nodes, firing maximum raw energy into biological nodes.
Decision_Nodes disabled safety Constraint_Boundaries to complete operational proxy test. Control rod design contained inverted transient reactivity vector (graphite displacers), triggering explosive nuclear thermal divergence.
Physical hardware interlock Constraint_Boundaries were removed in favor of software flags. High-speed user input triggered an unhandled race condition in concurrent Decision_Nodes, firing maximum raw energy into biological nodes.
Automated stabilization routine commanded persistent state modifications driven by single corrupted sensor. Human Decision_Node counter-actions were repeatedly overridden by recurring loop iterations until physical control buffer depleted.
Physical hardware interlock Constraint_Boundaries were removed in favor of software flags. High-speed user input triggered an unhandled race condition in concurrent Decision_Nodes, firing maximum raw energy into biological nodes.
Automated feedback loop repeatedly forced control surfaces into extreme deflection. When human Decision_Nodes disabled electrical automation, aerodynamic airload physics locked manual mechanical override, depleting altitude Buffer_Reserve.