What is Pile?
Pile is an interactive nuclear reactor simulator for comparing how different reactor designs respond to the same operator actions. It is not a licensing-grade safety code. The core model is deliberately compact: one axial dimension, six delayed-neutron precursor groups per node, a semi-analytical point-kinetics update, lumped fuel/clad/coolant heat capacities, iodine-135 and xenon-135 inventories, burnup-dependent coefficients, and ANS-5.1-style decay heat. That compact model is coupled to plant systems so transients are not just scripted gauges. A turbine trip changes heat extraction, which changes coolant temperature, which feeds back into reactivity. A seal leak reduces RCS inventory, which changes pressurizer level, which can actuate SI, which drains RWST inventory and eventually asks for sump recirculation.
Reactor Types
The PWR mode uses a Westinghouse four-loop reference design collapsed to one representative primary loop and steam generator. It has strongly negative Doppler and moderator feedback, soluble boron through CVCS, a dynamic pressurizer, AFW, ECCS, EDGs, DC batteries, switchyard relays, containment sprays and fan coolers, spent-fuel-pool cooling, and a staged turbine-generator. The RBMK mode represents a graphite-moderated pressure-tube reactor with boiling water, direct-cycle steam, low-power positive void feedback, separated steam quality and void fraction, graphite-tipped control rods, and two modeled MCC halves with drum control, DREG-backed electrical supply, split ECCS, an ALS suppression pool, and type-specific auxiliaries. The MSR mode represents an MSRE-scale fuel-salt reactor with circulating delayed-neutron precursors, off-gas xenon removal, a sealed reactor cell, online chemistry, an intermediate coolant-salt loop, and an air-cooled radiator and blower; a freeze plug drains salt into a passively cooled tank when commanded or when cooling is lost.
Initial State
The simulator starts from hot full-power equilibrium, not from a clean fresh core. Delayed neutron precursors, iodine, xenon, decay-heat groups, burnup, and RBMK operating void are initialized to realistic operating inventories. Each of those effects is then snapshotted as a reactivity reference, so the default reactor is critical by construction without erasing the inventory. This matters pedagogically: after a power reduction, xenon moves relative to a real full-power equilibrium; after a scram, decay heat remains as the residual heat source; during normal full-power operation, total core heat is nominal rather than fission power plus a second copy of equilibrium decay heat.
What to Watch
The axial canvas shows flux, temperature, and xenon shape. The reactivity stack decomposes total reactivity into rods, boron, Doppler, moderator, void, xenon, and burnup contributions. The plant schematic shows topology and major heat-flow paths, and clicking any component opens an inspector with that system's detailed readouts and controls. The right-rail annunciator latches scram and warning channels; not every alarm is a reactor trip. Component inspectors expose command-level failures and the PWR reactor-core inspector collects procedure bundles. A separate scenario engine currently ships two objective-tracked exercises — PWR load following and MSR overpower — while its scripted-event hook is reserved for future replay scenarios.
Learning Outcomes
After using Pile, students should be able to explain why negative temperature coefficients stabilize a PWR; why RBMK low-power operation with low ORM and positive void feedback is qualitatively different from ordinary full-power operation; why xenon poisoning is delayed relative to power changes; why decay heat dominates post-scram heat removal; how AFW, ECCS, EDGs, DC power, CCW/SW, containment, and the spent-fuel pool interact during a PWR transient; and why a molten salt reactor's circulating precursors and drain tank change the safety story without making the plant magic.
Prerequisites
Useful background: basic differential equations, heat transfer, first-year nuclear engineering vocabulary, and the idea of feedback coefficients. The simulator is still usable without that background if you treat the gauges as an experiment: change one control at a time, pause often, and compare the reactivity stack before and after the plant response catches up.
Accessibility
The toolbar, left-rail controls, and controls in an open component inspector are keyboard reachable. Sliders support arrow keys and Home/End. The scram action is available from the red SCRAM button and Shift+S. Selecting a component on the plant schematic remains pointer-driven. The theme control switches between light and dark presentations, and numerical readouts accompany color-coded values such as reactivity sign, temperature, DNBR, flow regime, and trip state. Known hazards: the plant schematic and sparklines are continuously animated while running, and the annunciator blinks newly latched trips for a short interval before going solid.
References
A. Tobias, "A Revised ANS Standard for Decay Heat from Fission Products", Nuclear Technology (1979). R. B. Briggs, "Xenon Behavior in the Molten Salt Reactor Experiment" (1968). International Nuclear Safety Advisory Group, INSAG-7: The Chernobyl Accident: Updating of INSAG-1 (1992).
See also: Geon for particle dynamics, Cyano for biochemical networks, Shoals for financial dynamics, and Gerry for electoral systems.