Hiddencamper

Shutdown cooling doesn't auto start. It's one of the RHR modes that has no automatic function.

The shutdown cooling interlocks don't clear until reactor pressure drops low enough that Tsat is less than 330 degF (approximately). Usually this is set less than 100 psig. And both main EHC and the relief valves won't automatically depressurize the reactor, that takes manual operator action. Once you get below the RHR interlock, it takes manual operator action to reset the interlocks, then you need to flush the SDC loop with condensate, and perform manual warming or the loop, before finally placing RHR in service on the vessel. It's a drawn out process.

Other potentially complicating issues: unless you installed a digital feedwater control system, main feedwater has a tendency to overfeed after a scram and trip all your feed pumps off. HPCI and RCIC will cycle along with relief valves, but it's kind of a mess running those systems cycling on level 2/8, and puts a lot of fatigue on their associated injection spargers. My plant tries as hard as possible to never start RCIC after we allowed it to cycle for days following a LOOP in the 90s. We heavily fatigued the injection nozzles and our steam dryer because of the thermal shock every time it started up.

The ESBWR, if it ever gets built, will automatically go into SDC. It uses the reactor water cleanup system for SDC. The ESBWR has an oversized non regen hx with variable speed pumps to automatically control the cooldown rate. But that's the exception to the rule, and doesn't exist yet.

Sorry! I was talking to an industry guy I didn't mentally shift gears. I'm going to go with a more entry level description below!

RHR = Residual Heat Removal. The RHR system for Boiling Water Reactors (BWRs) is a multiple mode safety system that uses pumps and heat exchangers to cool the reactor, containment, and spent fuel pool during emergencies. Some of these modes, like the Low Pressure Coolant Injection mode, have automatic startup signals. Others, like shutdown cooling, suppression pool cooling, do not and require manual operator action.

After a scram, the reactor still boils water due to decay heat. It doesn't automatically cool down (unless you have insufficient decay heat, and the steam plant is allowed to overdraw steam). The EHC (Electro-Hydraulic Control) system is what normally is used to control steam pressure in the reactor. EHC automatically opens and closes the turbine valves to control pressure when the turbine is online, and when the turbine is off it will open and close condenser steam dump valves. EHC does not automatically depressurize the core, you need to manually dial down the pressure setpoint to do that. EHC does automatically maintain pressure at the setpoint.

The RHR system is only rated for about 330 degF temperature (maximum designed containment temperature during accidents). For a BWR, this corresponds to around 104 PSIG of pressure. To prevent overheating the RHR system, there is an interlock that automatically isolates the shutdown cooling system above this pressure. So before you can put SDC in service, you need to depressurize and cool down to less than 104 PSIG and manually reset the interlocks.

The SDC piping is filled with condensate water (ultra pure), but it has no flow through it for months or years at a time, so unless there is an accident, you flush the piping with clean condensate for a few hours to improve the water quality in the piping. This is to make sure you don't put nasty high conductivity water in the reactor. Once the piping is flushed, you can open the SDC valves that draw suction on the reactor, and slowly flush with hot reactor water to warm the pipes and heat exchanger up. Finally, with all of this done, you can stop flushing, and you can open up the shutdown cooling return line to the reactor to directly cool it.

Other issues you run into, is that after a scram, the old feedwater system in BWRs has a tendency to drastically overfeed the vessel (this will need another massive post to explain why if you want to know). On high reactor water level, the feedwater pumps will automatically trip offline, to prevent overfeed and filling the steamlines with cold water (this can cause severe stress or damage to the piping). The feedwater pumps do not automatically restart. With no feedwater, you are now left to your auxiliary and emergency feed systems. With no operator action, at reactor water level 2 (around 7 feet below normal level, 13 feet above the fuel), High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) auto start and feed the vessel. HPCI is an ECCS pump that uses steam to operate, and pumps over 5000 gpm to the reactor. RCIC is a small steam driven auxiliary feed pump that runs around 600 gpm. They will start up, and inject cold water (40-80 degF) into the reactor, until water level hits the high level shutoff. They will automatically start and stop every time you hit level 2 and level 8 (high water level). Every time they do this, it causes thermal shock to the injection nozzles, and they are only rated for so many shocks before needing replacement.

My plant had a Loss of Offsite Power (LOOP) in the 90s for over 2 days, and they decided to keep the reactor hot so that when they got offsite power back, they could immediately restart. Well, the only feed system they had was RCIC, and they were letting it start on low level, feed up to level 8, and shut down. We shocked our nozzles hard, and now want to minimize the number of starts/stops of RCIC injections. They aren't past their allowable thermal shock limit yet, but we are trying not to get there.

The Economic Simplified Boiling Water Reactor (ESBWR) is General Electric's Generation 3+ reactor with passive safety systems and full plant automation. It will go into shutdown cooling automatically, because instead of using a separate RHR system to do it, it uses the in-service reactor water cleanup (RWCU) system. A normal BWR's RWCU can remove around 0.1%-0.2% of heat from the reactor, not enough for shutdown cooling. The ESBWR uses higher speed pumps and larger heat exchangers, which in combination with the isolation condensers, allows the unit to automatically go to a cold shutdown state in a controlled fashion without violating cooldown limits, with NO operator action.

Current SRO. Also have a nuclear engineering degree. Never finished RE training because I got into license class. Also a huge nuclear nerd : )

Reactor engineers are focused entirely on the behavior of the reactor. They do calculations and reactor physics tests to verify the reactor core behaves as it was designed for that cycle. During operation, they plan out power maneuvers and use computer modeling to make recommendations to the operators for power adjustments while also ensuring margin to thermal limits.

The REs are advisors to the control room. They cannot direct operation, but they will run predictor models and make recommendations for the operators to perform.

RE training takes about a year. You get a crash course in plant systems and operations, but the rest is all about reactor design, theory, and operation.

The licensed operators are the only ones with the ability to operate the reactor controls, or direct operation of the facility. It takes 2 years to get a license, on top of previous experience requirements. You get a ~2 month crash course in power plant engineering/operation, thermodynamics, and nuclear physics. Then a ~4 month course in all the systems of the plant, ~6 months in the simulator, ~3 months of on the job training. You get exams in the simulator and written exams every week or two, some taking up to 8 hours to complete. The ROs and SROs actually run the plant. Operators have to go back for continuing training 1 week out of every 6.

Not OP, but I am a nuclear engineer.

From a public health/safety perspective, there were no meaningful consequences. The radioactive material release was very small. The core was severely damaged, but all maintained in the reactor vessel.

From a plant and industry perspective, this was huge. There were multiple precursor events where an identical or almost identical sequence of events occurred at similar PWR plants both in and out of the US, and it was even being discussed by the Atomic Energy Commission/NRC and the PWR vendors that the training and safety systems would not prevent this type of event from occurring as currently written. This led to a huge overhaul in training, emergency response teams, emergency procedure changes, regulatory changes, the formation of INPO (Institute of Nuclear Power Operators, the industry's private internal regulator), and what has ultimately led to nuclear plants having massive staffs of 700-1000 people per reactor.