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 represents a Westinghouse-class four-loop pressurized water reactor. It has strongly negative Doppler and moderator feedback, soluble boron through CVCS, a dynamic pressurizer, four steam generators, 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, graphite moderator feedback, a low-power positive void coefficient, separated steam mass quality and void fraction, and graphite-tipped control rods. The MSR mode represents an MSRE-scale fuel-salt reactor with fuel dissolved in FLiBe, circulating delayed-neutron precursors, xenon off-gas removal, an intermediate salt loop, and a freeze plug that drains salt into a passively cooled drain tank when plug cooling is lost or the loop goes hot and stagnant.
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. For example, RBMK low ORM and Ledinegg-style flow excursion are warning indications, while high flux, short period, low DNBR, low flow, pressurizer level, containment pressure, seal LOCA, and turbine overspeed can trip the reactor where applicable. The component inspectors expose command-level failures and the reactor-core inspector collects the procedure bundles; it is a training layer, not a full scenario engine with scoring and replay.
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
All controls are keyboard reachable. Sliders support arrow keys and Home/End. The scram action is available from the red SCRAM button and Shift+S. Theme controls support a high-contrast presentation, 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.