Changelog

What has changed in the simulator, newest first.

Older releases are collapsed — click a version to open it.

A locked walkthrough can no longer be started from the one before it, and fast-forward carries across steps

Alpha 1.7.5 17 September 2026

FixedFinishing a walkthrough started the next one even when that one was locked. The finished-walkthrough card’s next-leg button was the one route of six that did not check whether the walkthrough it was opening is released.
ChangedFast-forward now carries from one walkthrough step to the next. Every step advance used to drop the clock back to 1×, so selecting a higher speed bought nothing. Measured on the Mode 3 to Mode 1 startup: at 10×, 48.4 minutes of real time becomes 5.0; at 60×, 0.9. Seventeen involuntary re-presses a leg are gone. A reactor trip, a new failure, a new alarm on a quiet board and a step’s own declared hold all still drop the clock.
FixedThe pressurizer spray cycled on instrument noise. The spray ladder read the raw indicated pressure error rather than the lagged one the heaters use: 100.8 open/shut cycles a minute at the 1700 psi step, now 7.4, and 15.2 across the whole heatup, now 0.0. Heatup rate and the relief valves are unchanged.
FixedPressurizer pressure control did nothing in Mode 5. The cold plant loaded with the operator’s pressure setpoint at 363 psia, below the 1700 psig floor the box itself enforces, so the board showed a number it would refuse and HEATER AUTO raised 0.0 psi in 45 plant-minutes. The setpoint now loads at the floor, and the heatup’s two pressure steps arm the spray and let the heaters climb instead of asking for a number.
ChangedThe 1/M startup rungs wait on the rods, not on a settling reading. Each rung now waits until the control bank has not moved for a minute; both previous checks were proxies for “the operator has stopped pulling”, and a slow dribble of single taps satisfied them with the bank still moving. It costs 49 seconds of plant time across the ladder.
ChangedA setpoint box flashes when it clamps your entry. Typing a value outside a control’s range used to be corrected silently.
FixedWalkthrough text corrected. A claim that Plot point had done nothing when it had, a ring called green that is cyan, an ACKNOWLEDGE that reads ACK on the board, an undefined unit, four steps whose speed advice contradicted itself, and a missing remedy for a startup rate that is too low.

Rod control has authority at power, the board warns when the plant runs off its program, and the checklists are now Walkthroughs

Alpha 1.7.4 15 September 2026

FixedThe At Power starting condition loaded with the control bank already on its stop. Loading the plant at power — rather than climbing to it — put the bank at 627 of 627, so WITHDRAW had nowhere to go: measured, commanding it out moved AVG COOLANT TEMPERATURE by 0.01 °F. It now loads at 606 of 627, the position a Westinghouse plant runs at, worth +4.7 °F on the same command. Climbing to power from Mode 5 was never affected — that route leaves the bank near 351 of 627. Boron moves 621 → 612 ppm and nothing else about the full-power plant changes. Rod drive speeds are now the real plant’s 8, 48 and 72 steps/min, and the trend chart, the System Scanner cards, the 1/M startup plot and the board’s own rod position readouts take the bank’s scale from the plant instead of the previous engine’s 912 steps — the plot’s steps axis, its predicted criticality and its “critical near step N” label used to be drawn to 912 from the moment you opened the tool until you plotted your first point. The startup also plots one point fewer: the last one used to be taken after the reactor was already critical, which is the wrong side of the approach, and the ladder now finishes short of criticality and creeps up to it. The overshoot that caused is gone — power arrests on its own near 4 % instead of running on through Mode 1 — at the cost of a longer wait at the criticality step, which the step now names and tells you to spend at 10×. The at-power permissive is 8 % as well, matching the plant it is modelled on; at 10 % it sat inside the meter’s own noise and two trips armed and disarmed 92 times in ten minutes on a steady plant.
AddedThe board warns when the plant runs off its program. PZR LVL DEV HI comes in 10 points above the level the plant is aiming for, mirroring PZR LVL DEV LO; the PRESSURIZER LEVEL gauge’s amber edge now follows that target instead of a fixed 25 %, which read amber for the whole run in the three states every startup begins in; and AVG COOLANT TEMPERATURE gains a cold edge 20 °F below program. Measured: a plant 105 °F cold at 96.5 % power previously gave no warning at all until the reactor tripped. A new ROD BANK FULL OUT alarm says when the control bank has reached the top of its travel and there is no withdraw authority left — the condition a plant trimming rods against xenon arrives at after about a day, previously with nothing on the panel to say so. It stays dark at the normal full-power position: measured, a plant trimmed through a 100 → 70 → 100 MWe swing never lit it in 14 hours, peaking at 616 of 627.
FixedFinishing the power ascension could leave a plant that cooled itself down for hours. The boron figure was set but never checked, and the closing trim was a written-out instruction with nothing to press. Measured: 105 °F down over sixteen hours and a reactor trip, with PZR LEVEL sinking to 25 % because the target follows temperature. The boron steps are now graded, the closing step dials 617 ppm and checks it, and the four stage temperature checks have a lower bound as well as an upper one — a band is no longer satisfied by passing through it. The ascension also re-latches the turbine before the first rod pull. Dialling the closing boron figure straight in overheated the plant and tripped the reactor on Overtemperature Delta-T a quarter of an hour after the step had already checked itself off; the step now walks boron down and pulls rods as xenon builds, which is what the plant needs — measured, the control bank is worth 56 °F over its travel against about 82 °F of xenon, so neither lever does it alone. The ascension now ends with a boron step you actually perform and that is checked off, instead of a paragraph describing one: without it the walkthrough finished with the plant 250 rod steps and 47 ppm short of where a full-power plant sits, and trimming with rods alone ran the bank out of travel a day later, 24 °F below programme for good. What happens if the trim is skipped is now measured rather than guessed: PZR LEVEL on its floor in 7 hours and a reactor trip on STEAM GENERATOR LEVEL LO-LO in 17, with REACTOR POWER reading 100 % the whole way down — the text used to say “about 40 °F cold”, and it is 100 °F.
ChangedThe live checklists are now Walkthroughs, rewritten. The name is the same everywhere the sim says it — the tab, the panel, the Scanner cards, the tour, the Instructor’s own messages and the operator manuals — instead of the feature being called one thing in the menu and another in the text. Every step names the control and tile as the board prints them, starts with the action or “Nothing to press”, says what done looks like, and uses the board’s units throughout — no SI. One heatup step is graded on Net reactivity, which no board gauge shows, and now says to read it on the Indications tab. They run in the Instructor tab one step at a time; Continue lights when the instruments say the step is done, and Rewind step takes the plant back one step. Plant & Mission starts them — a green NEW badge marks its Walkthroughs tab, the plant column is gone, and a Main Menu button beside Settings reopens that window in place of the SELECT PLANT, MISSION & RESET bar under the speed controls; the Campaign and Scenarios tabs are gone. In the panel the redundant Walkthrough heading is gone, Continue and Rewind step sit under the step text, End walkthrough sits at the bottom, and the header clock no longer pulses. Reset, switching plant or starting a different walkthrough now releases a pause a walkthrough step asked for, instead of leaving the clock frozen with nothing on screen saying why. The startup checklist uses the 1/M plot, which is now a floating window you can drag where you want it, with its buttons in a row above the plot and the predicted criticality step printed large. The plotted area is 80 % bigger than it was and the alarm panel takes its width back (392 → 652 px), because a floating window takes its height from the screen instead of from somebody else’s column. Its highlight rings the button the step actually asks for — the plot-point button inside the window, the rod speed button, INSERT — instead of the button that opens the panel, and the walkthrough text can be selected and copied. The highlights are now cyan rather than green, and pulsing or steady says which kind they are: pulsing marks a control to press, steady marks an indication to watch. Every rod number in the startup now names the readout it belongs to and says it is the total on CONTROL ROD POSITION, not a further count of steps to add on — read as an increment, step 6 alone took the rods 100 steps past where the counts reached its target. Two steps disagreed about which way the 1/M plot errs, and the plant settles the argument: it goes critical at 208 of 627 at 719 ppm, about five steps BELOW the plot’s prediction, so both now say stop short of it and walk up in single taps. Six startup steps rang SOURCE RANGE and STARTUP RATE with the pulsing ring that means “press this”, though they are numbers to read; thirteen such rings are now steady. The Mode 5 to Mode 3 heatup highlights SHUTDOWN ROD POSITION on the shutdown-bank step, and eleven steps that named two controls were drawing one ring between them — three on the heatup, and eight more across the startup, the power changes and the cooldown, where the control the step told you to press glowed not at all. Where a step sets a number, the ring is now on the setpoint box it asks you to type in rather than on the card around it. A step that asks for more than one action now shows them as lettered rows — 3a, 3b — each with its own instruction and its own tick, and you still confirm the step once at the end. The explanatory text has moved into its own labelled box headed “Background — not an action”, set apart from the action rows, so it reads as background rather than as something to do, and the short operating tip under a step is now italic so it reads as a note rather than as another instruction. On the cooldown, turning the pressurizer heaters off and putting the spray in hand are now one step with two lettered parts. The speed bar is one row with a status line saying why the clock last dropped or what WARP is achieving, and pausing and resuming no longer wipes that line while the hold it reports still stands. When a waiting step suggests a speed, only that rung is highlighted rather than the whole bar, and it stops pulsing once you press it. Pausing clears the selected speed button and it now stays cleared — it used to light again the moment you touched any control while paused. Only the Mode 5 to Mode 3 heatup walkthrough is offered while the other five are in testing. The pulsing “press this” ring is now thinner and dimmer than the steady “watch this” ring at every point of its cycle — it used to reach the same width and beat it on brightness, so twice a second the two cues meant the opposite of what they say. A step taller than the panel now opens at its own instruction rather than scrolled past it. Pressing Plot point out of turn used to bank the point silently while the card claimed the button had done nothing; the card now names the step that comes first and no longer says something untrue. The speed status line states the actual reason the clock last dropped instead of one of five, and a step can declare the alarms its own evolution causes so they no longer stop fast-forward. Each rung of the 1/M ladder is now four ordered lettered parts, and the approach to criticality is read on INTER RANGE and STARTUP RATE rather than REACTOR POWER, which sits at 0.0 % for about twenty-five plant-minutes of it. Walkthrough text corrected: a ring called green that is cyan, an ACKNOWLEDGE that reads ACK on the board, an undefined unit, four steps whose speed advice contradicted itself, and a missing remedy for a startup rate that is too low.
FixedThe cooldown and shutdown walkthroughs could not be finished as written, and the power reduction could be finished without doing it. The dump setpoint now walks 50 psi at a time (typing 640 straight in cooled the plant 50 °F in a minute and emptied the pressurizer), the spray step asks for 50 %, the DUMP SETPOINT stages press AUTO first, and a 1/M step no longer locks the Mode 3 to Mode 1 run if you overshoot the source range. A boron dose now counts what the plant delivers rather than what was asked for, and BORON ON stops a running dose; a startup step no longer says BORON CHEM updates only after a SAMPLE — that control is gone and the reading is live. The power reduction’s four load drops now also check that the generator is still carrying load, so a tripped reactor no longer satisfies them for free, and the completion note reports that the board does not match the leg’s expected finish instead of describing a plant you do not have. The cooldown no longer strands the plant: it used to tell you to shut the pressurizer spray on the grounds that it does nothing once the pumps stop, which is not true here — the spray is the only pressure control left at that point, and shutting it reversed pressure from −92 to +33 psi a minute, tripped RHR closed at 610 psig and refused to let it back in. The spray now stays in until the plant is cold, and the cooldown rate step asks for 12 % rather than a split that ran at 154 °F an hour against a 100 °F limit. Trip block steps are also graded on the lineup rather than the button press, so a block placed earlier checks the step off and UNBLOCKING one no longer does — and pressing START no longer ticks a step that asked you to press STOP. Six steps told you to do something the sim never checked and now check it: the rod trim that brings AVG COOLANT TEMPERATURE back into band on each of the three power reductions, ECCS STOP, the spray’s 50 % setting on the cooldown (at 100 % the pressurizer goes solid, which the step warns about and nothing was grading), and the RHR heat-exchanger split. The heatup’s Mode 3 confirmation was checking nothing at all.
FixedCharging and letdown rates were about 15 % too high, and several manual figures were stale. The pressurizer was counted on one side of the RCS volume comparison and not the other; charging maximum is now 26 gpm (was 30), normal letdown 12 gpm (was 13). The manuals also stopped quoting figures this plant does not have — the atmospheric dump valve’s capacity (about 5 % of rated steam flow, not 10), the steam dump’s Tavg mode during a heatup, a source-range high-flux trip that is not modelled, and stale LO TAVG alarm and quick-reference AVG COOLANT TEMPERATURE setpoints — and stopped teaching PRESSURIZER LEVEL as 55 % at full power in nine places: this plant’s program runs 25 % at no load to 61.5 % at full power.
FixedBug reports send again. The attached recording wrote every reading to seventeen digits, so a four-hour session came to 2.9 MB against a 2 MB limit and the form refused it; the same report is now 646 KB with every reading kept, and an over-long one drops its oldest readings rather than being refused. The Send report button no longer sticks on “Sending…” — it says what went wrong and lets you press it again.
FixedBoard and alarm text corrected, and panels that could be scrolled off screen are pinned. A mouse wheel over the reactor diagram used to pan it out of view, with no scrollbar and no way back short of reloading the page; the diagram, the System Scanner’s description line and the Instructor and Walkthroughs panes now refuse the scroll, and on a window narrower than 1200 px those panes get a minimum height instead of being squeezed to 20 px against 62 px of content. The Reactor Trip alarm names this plant’s own causes (Overtemperature Delta-T, Lo-Lo Steam Generator Level, Turbine Trip P-9) instead of translating the previous engine’s; SUBCOOLING MARGIN no longer prints “-0”; the PORV block valve’s card no longer claims a two-press confirm and the relief line’s seated tailpipe reads 122 °F; the TRIP BLOCKS panel no longer grows over the board with its release button on top of the block valve, closes when you press outside it, opens clear of the nuclear instruments instead of on top of them, highlights the row a step asks for, and its own highlight ring now fits the button; the NUC INSTR card’s gap is closed; five board captions (SOURCE RANGE, STARTUP RATE, SHUTDOWN ROD, CONDENSATE, RCP FLOW) drop a point so they clear their card edge where the default font is wider; the TMI-2 walkthrough’s step 8 highlights the gauge it names; and the TRIP BLOCKS button now flashes amber when the plant takes a block away by itself, with the card saying which row went and why — open it and close it to stop the flashing. The card also shows the whole lineup: which trips are blocked, which are waiting on their permissive, and which you released yourself. If your system is set to reduce motion, the board no longer flashes at you: six signals that ignored that setting now hold still, and the three that told themselves apart by colour and blinking — critical alarm, a protection latch, and a message — now carry distinct outlines as well, so they stay distinguishable without either. The rod controls now answer back: press WITHDRAW or INSERT when the bank is already at that end of its travel and the button and the step count flash red, so a press that does nothing no longer looks like one that worked, and both step counts turn amber while their bank is actually moving. The amber cue on the TRIP BLOCKS button and its rows now clears once you have looked at it, instead of standing after it has been read; a fresh release by the plant lights it again. The power meter’s block window also opens where the plant opens it, at 8 % rather than 10 %. A standing trip block could be taken away by one noisy instrument sample; the plant now needs the condition to hold for 2 seconds before it releases one.

Faster waiting steps, and a card that printed over itself

Alpha 1.7.3 5 September 2026

FixedThe ECCS card's MODE reading was printed on top of its own caption. The word and the caption shared a line, which only fitted the shortest words; STANDBY — the reading on a quiet plant — overprinted MODE. The card's three readings are re-spaced and the mode word now sits below its caption.
ChangedA waiting checklist step suggests a faster speed. The rung it names now clears the wait in about half a minute of real time rather than a minute: 16 of the 24 waiting steps read 60×, and the whole Mode 5 → power → Mode 5 chain spends 5.6 minutes of real time waiting instead of 12.4.

Cold shutdown, and the board stops lying about a correct plant

Alpha 1.7.2 5 September 2026

AddedMode 5, Cold Shutdown is a Free Play starting condition. The plant boots cold (122 °F) and depressurized, and heats up to Hot Standby on reactor-coolant-pump work with the reactor never critical.
AddedTime acceleration has two tiers. 1×–60× (a 5× rung is new; keys 1–6) run the full physics; 600× and 3600× are WARP — the same physics at a 0.5 s step, for xenon, decay heat and long heatups. WARP refuses a plant in a transient, drops itself to 60× the moment one begins, and a readout beside the buttons shows the rate actually achieved. A per-broadcast step budget ends the freeze at high speed, and WARP gets a larger share of it than the lower rungs: measured here, 3600× achieves 931× at power and 1,082× in Cold Shutdown, up from 671× and 765×.
FixedThe board stops lying about a correct plant. RCS flow now reads like the elbow-tap instrument a real plant uses, so cold water no longer pegs the gauge at 120 % — the flow itself was always right. Safety injection could actuate during a heatup with nothing annunciating it, and tile bands were fixed at full-power values — the pressurizer level band now follows the live level program, so an on-program Mode 5 level no longer paints 15 points below normal. A standing 17 % letdown isolation now darkens the four orifice lineup lamps and reads ISOLATED on the card, instead of lighting the lineup you picked over two shut valves. Two boxes would not accept typing — the Pressurizer Pressure Setpoint and the Report a Problem form. A click that missed the field left the keyboard driving time acceleration instead.
ChangedProtection and starting conditions match this plant. The intermediate-range flux trip is built and TRIP BLOCKS no longer offers two blocks no operator has; the control bank is 627 steps on the sourced four-bank overlap program; and every mode that carries no load — Hot Standby, Hot Shutdown, Cold Shutdown — boots with the turbine tripped, so latching it is a real step instead of one the plant had already done for you. Cold Shutdown and Hot Shutdown now boot with the letdown orifices out, letdown running on the residual heat removal cross-connect, so the LETDOWN selector has a job and the heatup has a step for it — and the 17 % low-level letdown isolation no longer erases the orifice selection you made. Cold Shutdown and Hot Shutdown also boot with the pressurizer heaters off and the spray in hand, where the cooldown leaves them, so putting pressure control back in service is a heatup step. STEAM DUMP AUTO now selects the controller the plant conditions call for — steam-pressure control with the turbine tripped, holding the 1020 psi no-load anchor; average-temperature control on line. Pressing it during a heatup used to do nothing, so the plant heated up on the atmospheric dump valve and parked at 551.6 °F / 1042 psig instead of 547.2 °F / 1005 psig; the heatup checklist has a step for it.
FixedThe checklists and manuals were quoting the retired plant. All six live checklists are rewritten: one action per step in the board's own words, the reasoning one click below, the SOURCE RANGE named as the indication you read counts from, burst sizes in steps, and criticality declared the way a real procedure declares it — startup rate positive with the rods stopped. Rod tables across five manual chapters were re-measured off this plant, the accumulator cover gas is 665 psia, and P-10 is 8 %. New: a Help button on the 1/M startup plot.
FixedThe power ascension checklist now starts where a startup leaves you — at low power with the control bank part-way out, not at 50 % with it already fully withdrawn. Its “withdraw the bank” steps had nothing left to withdraw, so they did nothing. The boron target moves 705 to 660 ppm, which lands average coolant temperature on its program instead of 34 °F (19 °C) below it, and the checklist now explains why the bank sits low until xenon builds in.
ChangedTime acceleration and the checklists now know about each other. The clock drops back to real time when a step checks off, and the speed buttons flash so you can see why. Steps that ask nothing of you wait for an Acknowledge press instead of racing ahead, waiting steps say roughly how long they take in plant time and which speed to set, and the six vital gauges keep a three-minute trace at every speed. Every step card now leads with its number, with the active step’s criteria and controls below it. The accumulator arming window can no longer be missed at speed: it opens at 665 psia, nothing announces it, and the pressure dial cannot take you back — so the clock now drops out there and refuses to accelerate again until the accumulators are armed, the checklist shows the accumulator step while it holds, and a refused speed click stays on the checklist.
ChangedRaising turbine load now ramps instead of stepping. Dial a higher load and the board shows it at once while the machine walks up at 5 % of rated per minute — 5 MWe/min, this plant's sourced load-following envelope. A load increase delivered instantly pulled average coolant temperature down 15 °F and took pressurizer level from 27 % to 16 % in 44 seconds, through the 17 % low-level isolation; on the ramp it bottoms three points clear of it. Lowering load is unchanged and takes effect at once.

Playtest fixes and a faster board

Alpha 1.7.1 31 August 2026

FixedThe condenser cooling-water temperature box works. Vacuum follows the water temperature, and a warm enough lake now rings COND VAC LO (crossing ~76 °F).
FixedFrame rate and flicker under load. The strip chart no longer redraws unchanged frames and board animations run on a 12 Hz clock — render load cut roughly in half.
FixedThe pressurizer spray box shows the demand you set, not delivered flow — it no longer zeroes on a solid pressurizer or reads 100 % on a stuck valve.
FixedTrip-block rows match this plant's protection — dead rows removed, the safety-injection block added.
FixedThe Failures tab lists named failures for the PWR (was raw internal ids), and a refused injection is no longer shown as active.
FixedRod stop and turbine runback are indicated on the ΔT power tile, and the period readout no longer overlaps it.
ChangedAuxiliary feedwater manual START and throttle removed — the automatic level control runs auxiliary feed.
ChangedThe AUX SPRAY box is removed from the pressurizer card.

A new physics engine

Alpha 1.7.0 30 August 2026

AddedThe PWR runs on a rebuilt physics engine. Derived design basis, steam-table water properties throughout, and conservation laws the plant can be measured against.
ChangedThe plant now starts at Hot Shutdown (Mode 4, 250 °F) — Cold Shutdown is gone for now. Mode 5 is not modelled on the new engine. Four starting states: Hot Full Power, 50 % Power, Hot Standby, Hot Shutdown.
AddedAuxiliary pressurizer spray, on the pressurizer card. It lowers pressure with the reactor coolant pumps stopped, which the normal spray cannot do.
ChangedThe Three Mile Island level deception is emergent. Leave a relief valve stuck open and pressurizer level rises while the plant loses water — produced by the model, not scripted.
FixedEvery row of the instrument-failure panel works. 35 of its 50 instruments previously refused to fail.
FixedA tripped turbine can be re-latched. One scram used to end electrical generation for the rest of the session.
FixedBoard indications, alarm setpoints and control ranges read this plant's numbers rather than the previous engine's.
ChangedOperator manuals at Revision 17.

Cold shutdown starts with the rods in

Alpha 1.6.1 18 August 2026

ChangedCold shutdown now starts with the shutdown bank inserted. Withdrawing it is a step in the startup procedure. Loading Mode 5 and driving to Mode 5 now give the same plant.
ChangedThe turbine rolls at 10–15 % power, not below 5 %. The steam dumps already pass that flow, so total steam flow barely changes as the governor valves open.
FixedShutdown cooling can no longer be aligned during a safety injection. The same pumps do both jobs, and aligning mid-accident gave the full shutdown-cooling heat sink through heat exchangers that have no cooling in that lineup.
ChangedManuals at Revision 16. Shutdown margin is defined, and pressures are marked psia or psig.

The control room, rebuilt

Alpha 1.6.0 12 August 2026

ChangedThe strip chart is a stack of lanes. One lane per indication, each with its own scale, name and value, and a shared cursor that reads every lane at the same instant. A dashed line marks where the run began.
AddedSequence of events. A ribbon under the chart marks what happened and when — cyan for your actions, amber for the plant's. Trips and safety injections cross every lane; component events cluster into counted badges.
AddedChart settings. Every channel the plant publishes, listed with its current reading, and a choice per channel: trace the instrument, the physics, or both. Both draws them together in one lane so you can see how far the gauge is from the plant.
ChangedThe Instructor is a tab, and it keeps a transcript. Guidance and walkthrough steps accumulate and scroll instead of overwriting one another.
ChangedPlant & Mission opens on load and the plant is running when you close it. Checklists are ordered by relevance, launch from free play, and link to the manual section they came from.
ChangedIndications has Plant and Physics columns, and the disagreement flag now measures each channel's own noise instead of a fixed band — so a gauge is flagged when it is further out than it normally wanders.
AddedReport a Problem from the header, in one action, with the diagnostics it will send shown before you send them.
FixedFour plant behaviours. Safety injection now sheds the pressurizer heaters, ending a slow pressure oscillation after every loss-of-coolant accident; shutdown cooling no longer aligns itself into an unisolated break; full auxiliary feedwater cools the plant instead of banking the heat; free play starts with rod control in manual.

A protection function that never fired

Alpha 1.5.2 10 August 2026

FixedAutomatic steam line isolation never fired. On a steam line break the valves stayed open and you had to isolate by hand. It now closes on the rate of pressure drop, as the setpoint specifies, within a few seconds of the break.
FixedBug reports recorded almost nothing during fast transients. A report sent from an accident at high time acceleration captured a handful of readings instead of the event; it now records the peaks and troughs of every channel.
AddedContact offers GitHub issues alongside email. It opens in a new tab and the plant keeps running.

Minor fixes

Alpha 1.5.1 9 August 2026

ChangedUsage data. The first-launch prompt is gone — anonymous usage data is on by default and can be switched off under Settings → Share usage data. What is and is not collected is on the privacy page.
FixedMinor bug fixes.

Every reading the plant produces, on one tab

Alpha 1.5.0 9 August 2026

AddedIndications tab, replacing Graph. All 84 of the plant's channels with their live values and a checkbox to trend any of them, grouped like the Physics tab. Status channels plot as on/off traces, and every row explains what it is on hover.
AddedSimulator performance readout on the Physics tab — where each update's time goes, physics against rendering, and whether the sim is keeping up. It is included automatically in bug reports.
ChangedFree play starts at the 50 % power preset, not Hot Full Power — there is somewhere to go in both directions.
AddedPhysics tab gained a plot column and 12 more rows; leak flow now reads in gpm.
ChangedThe PORV reads its demand signal, not the valve. Under a stuck-open relief valve, Indications shows it shut while Physics shows OPEN · STUCK — the Three Mile Island control room, on two tabs.
ChangedCondenser cooling water inlet: 35–85 °F (1.7–29.4 °C), default 60 °F (15.6 °C) — was 40–100 °F on an 80 °F day. A default day still makes rated vacuum and 100 MWe.
ChangedTurbine load raises are rate-limited to 30 %/min.
FixedThe strip chart's x-axis no longer jumps as it scrolls.

Hydrogen is real, and the containment fights back

Alpha 1.4.0 8 August 2026

AddedHydrogen. Overheated cladding generates it, it collects in containment, recombiners work the tail, and past 8 % it burns once — a TMI-2-style pressure spike. New H₂ gauge and annunciators (H2 HI, H2 BURN, RECOMB ON).
AddedContainment protection and heat removal. Spray at 30 psig, fan-cooler realign on safety injection, an unblockable 3.5 psig SI backup, and steam-line isolation — all automatic; board controls come with a later board redesign.
ChangedContainment pressure response re-worked. A station-blackout boil-off now pressurizes the building slowly and stays under the spray point — it used to pass design pressure on relief steam alone.
ChangedPressurizer level is a real inventory node. The gauge tracks the vessel's own liquid: large breaks empty it honestly, and the small-break level lift is now the documented real-plant class (IE Bulletin 79-06A).
ChangedAuxiliary feedwater starts on the sourced single signal — the 17 % low-low level that also trips the reactor; the invented 3-point offset is retired.
ChangedAuto steam-generator feed regulates to the programmed 65 % level, and the feed demand box reads NO FLOW when commanded feed is not being delivered.
FixedThe ADV valve's noisy-instrument failure did nothing; 14 instruments missing from the in-app manual data are restored.

Accidents run on a real clock

Alpha 1.3.0 7 August 2026

ChangedLoss-of-coolant events run at real rates. Break flow, safety injection, accumulators, relief and charging all move true fractions of reactor coolant inventory per second, so the gallons-per-minute readouts are literal. A stuck-open pressurizer relief valve with injection secured now reaches core damage in about 2 hours 20 minutes — the Three Mile Island timeline, where it used to take minutes.
ChangedThe relief valves are sized for this plant. The pressurizer PORV passes about 112 gpm and the code safeties 3.2 times that. Feed-and-bleed cooling is viable, and full safety injection out-runs one wide-open valve — which is why securing injection in 1979 was the decision that mattered.
ChangedA large break blows down toward the containment building instead of stalling at the remnant's boiling pressure: a full-size break bottoms at 116 psi (0.80 MPa) where it floored at 170 psi (1.17 MPa). Smaller breaks are unchanged.
FixedThe pressurizer level gauge no longer argues against a large break. It read 100 % at the moment the core top uncovered; with a hole in the loop the pressurizer now drains, while the Three Mile Island stuck-valve deception keeps its full strength on the paths where it is historical fact.
AddedCore-exit thermocouple. The subcooling margin reads the hotter of the loop bulk and a new core-exit channel, so it goes hard negative over an uncovering core instead of reporting comfort from injection-chilled water.
AddedChecklists check their prerequisites. A live checklist grades its procedure's preconditions against the plant and shows what the procedure expects against what the plant reads. It warns and never blocks, and clears itself when the plant is brought to where the procedure assumes.
ChangedA reflooded core can recover. Core damage is no longer latched as terminal while water is coming back — a partially melted core that is re-covered stabilises, as Three Mile Island's did. Unmitigated accidents still end the same way.
ChangedAtmospheric dump valve setpoint 1247 → 1272 psi (8.60 → 8.77 MPa), onto the sourced placement rule. A bottled steam generator now holds at 1276 psi (8.80 MPa) / 576.5 °F (302.5 °C).

The board stops flickering during transients

Alpha 1.2.2 6 August 2026

FixedElements blinking during large transients. Gauges, the trend chart, the alarm list and the clock were being rebuilt many times a second whether or not their values had changed, and could be caught mid-rebuild on screen.
FixedBubbles in the reactor, steam generator and pressurizer restarting. The bubble fields are reused instead of rebuilt, so they keep rising instead of snapping back to the start.
FixedVital gauge trends drawing outside their card, across the gauge band and onto the board, when a reading sat outside that gauge's scale. An off-scale reading now pins to the edge of its trace box.

A steadier board, and the atmospheric dumps start in auto

Alpha 1.2.1 6 August 2026

ChangedThe atmospheric dump valves start in AUTO. They hold a bottled steam generator at 1247 psi (8.60 MPa) instead of leaving it on its code safety valves. They still do not cool the plant on their own — lower the setpoint or open them for that.
FixedFlickering during large transients. Gauges, the trend chart, the alarm list and the clock no longer strobe when a lot is happening at once.
FixedAlarm acknowledgements that did not register. During an alarm flood a click could land on nothing. The alarm list can also be scrolled while alarms are coming in.
FixedThe six vital gauge trends were blank above 600× speed. Their window now follows the speed setting and they sample between updates.
ChangedThe vital gauge trends match the strip chart. They show a min/max band, hold a steady scale instead of rescaling every frame, and no longer drop brief spikes.
FixedThe turbine exhaust pipe ran underneath the turbine control card.
FixedSteam generator bubbles stopped partway up the water. They now rise to the surface and fade there.
ChangedManual corrections. The full-open cooldown rate is re-measured at about 630 °F/hr, and the steam dump section carries current numbers.

Containment, atmospheric dumps, and automatic steam line isolation

Alpha 1.2.0 6 August 2026

AddedA containment building. It has pressure, temperature and a sump, and a break or relief discharge pressurizes it. A steam generator tube rupture does not — that break bypasses containment.
AddedAtmospheric dump valves. You can cool the plant down with the condenser lost. They ship shut, and at full open they cool about three times faster than the 100 °F/hr limit.
AddedAutomatic main steam line isolation on high steam flow together with low steam pressure. It cannot be reopened until the generator re-pressurizes.
ChangedDecay heat is refitted to the published standard. Post-trip timings are longer: a station blackout reaches core damage at 2.6 hours, against under 2 before.
ChangedThe graph shows the full range a value reached, its time windows follow the speed setting, and its resolution no longer depends on speed.
FixedA leaking plant that goes water-solid now reaches its relief valve instead of filling to the model ceiling.
FixedA steam line break no longer empties the generator to the model floor. It is a mass flow now, so the generator recovers to its safety band.
AddedThe release number is shown beside the ALPHA badge on every page.

Cooldown rate indication, turbine stop valves, and a run of physics fixes

Alpha 1.1.0 5 August 2026

AddedRCS cooldown and heatup rate meter, with high-rate annunciators at ±100 °F/hr.
AddedTurbine stop valves. A trip now shuts steam off in a fraction of a second, separately from the governor.
ChangedFeedwater carries heat. Overfeeding overcools the plant, and auxiliary feedwater is a real heat sink.
FixedSteam generator code safety valves act on steam pressure itself. A failed pressure transmitter can no longer stop them lifting.
FixedPressurizer level no longer freezes at a normal reading as the plant heats. It rises to the top and the relief valve lifts.
FixedA break stops cooling the plant once the coolant stops boiling.
FixedA stopped reactor coolant pump no longer heats the coolant.
ChangedTwo limits are stated in the manual: steam line isolation is a manual action, and the steam dump goes to the condenser only.

Pressurizer, relief and the control board

Alpha 1.0.1 5 August 2026

FixedA water-solid reactor coolant system now repressurizes. Emergency injection into a full plant lifts the relief valve instead of vanishing.
FixedLosing coolant now moves pressure and subcooling margin, not just pressurizer level.
FixedA break weakens as the plant depressurizes. Closing the pressure difference reduces leak flow, and an empty vessel stops discharging.
FixedThe Three Mile Island scenarios and campaign play through again, and the decision point is now the one the 1979 crew faced: restore the injection that was just secured.
ChangedThe tube-rupture procedure secures injection before depressurizing — without it, walking the pressure setpoint down does nothing.
ChangedThe diagram shows delivered flow, not commanded flow. Pumps, valves and pipes stop animating when nothing is moving.
ChangedPipe colours read the plant. Letdown, charging, emergency injection, accumulator and auxiliary feed lines take live temperatures; coolant green and orange are darker.
AddedPressurizer spray flow indication and a main feedwater RESTORE control on the steam generator feed card.

Initial Alpha release

Alpha 1.0.0 4 August 2026

AddedInitial Alpha release. Pressurised water reactor.