The Brunswick NPP plant on the coast at North Carolina USA was operating at full capacity up to very recently. This reactor is a Westinghouse Boiling Water Reactor (BWR) Mark 1 the same as Fukushima.
The two reactor units are still hot as is the very dangerous overhead spent fuel pools located in the same containment buildings. They remain hot for years.
Reactors and Pools must have CONTINOUS POWER even when SCRAMMED (Neutron Absorbing Control Rods Inserted) to remain within a safe state. The preferred power source is from the external network if that fails the Emergency Diesel Generators (EDG) fire up within 30s of loss of the grid.
Watch this video to see how neutron absorbing control rods work:
After Fukushima the US Nuclear Regulator issued a recommendation on what changes should be made. Some of them were very expensive taking the plants offline for months. Duke Energy cherry picked the low hanging fruit (such as ensuring EDG's & rescue equipment housings were water proof and a flood wall build around the facility to handle a 22 foot flood event at the Fear Point River lock) and is still arguing about the others, to wind down the clock on the operating license when they have to scrap the plant any way.
The design is flawed 3 engineers who worked on the prototype resigned when they realized that the rush to production was over riding their requests for design changes.
Refer to this video.
The unique feature of Westinghouse Mark 1 design uses the fact that power can be fine tuned by utilizing the neutron slowing of water the cooling medium. Boiling water flattens the bell curve of neutron numbers in the critical Uranium nucleus absorbing velocity, lowering heat output. Smooth laminar flow water raises this bell curve increasing heat output.
This can create a vicious oscillation when the pump flow increases heat output jumps up exponentially backing off the pump creates steam bubbles dropping of heat output. The latent heat cannot escape quickly enough before the next pump on cycle must come to prevent the core being boiled dry and fuel rod assemblies are damaged.
This feature is what the Russians were experimenting with at Chernobyl with their latest reactor design during a so called safety test.
Dropping only a few control rods into the reactor to idle it, they were varying water flow manually outside of the normal operating mode to see how heat output could be modulated with the water flow, they tried manually to control the pumps which was completely outside of the safety protocols for this type of reactor resulting in the cycle getting 180 degrees out of phase with the heat buildup. The control rod tubes that the control rods slide through were damaged by excess heat.
When all the control rods were frantically dropped in at full SCRAM they jammed in various half in half out positions and the rest is history. The real cause was never revealed.
This information is confidential as it crosses over into bomb design and Engineers won't talk about it in detail as it jeopardizes their life confidentiality agreements.
What is the benefit of such a dangerous feature?
A quick restart from SCRAM.
Normal restarts from a SCRAM is a very time consuming process and can take days as Boron (A Neutron Absorber) is introduced into the cooling water stopping the nuclear reaction dead. This Boron must be flushed from the cooling loop and disposed of before the reactor can be restarted.
The Westinghouse design re-start can be done in hours as it is not really stopped just placed in an idle mode.
Every minute the Reactor is not generating they are losing money against the extremely high fixed costs of maintaining a Nuke plant. It is a commercial reason not a safety one.
On paper the design the Westinghouse BMR Version 1 looks safe with multiple layers of safety redundancy.
NORMAL OPERATING MODE:
Minor fluctuations in load can be smoothed out automatically by varying water flow around the core.
SCRAM MODE:
Control rods enter the steel reactor core from below the containment vessel (Major Flaw) slowing but not totally stopping fission. Pumps reduce laminar water flow and steam bubbles reduce neutron candidacy for additional fission reducing heat out put further. Cycling control valves to keep the core within a temperature range so it can quickly return to full operation. It's basically idling but not stopped like a diesel engine.
EMERGENCY LOOP MODE:
Loss Of Outside Power compressed air start Electric Diesel Generators fire up to full capacity within 30 seconds providing power to maintain SCRAM mode.
REACTOR CORE ISOLATION COOLING MODE:
Loss of EDG's places the system into a condition where small steam (from the reactor) turbine powered pumps keep circulating cooling/moderating water through the core. There are two systems a low volume pump and a high volume pump. Both systems use batteries to run the control valves which run automatically to keep the core within a temperature range. They are located deep within the reactor containment structure surrounding the steel pressure reactor vessel and cannot be swapped out when the system is running without putting the system into an unstable state. They only have enough power to last for 24 hours continuously opening and closing valves.
If power is not restored within 24 hours the water starts to boil off exposing the fuel core. It becomes overheated, the steel zirconium cladding surrounding the uranium/plutonium fuel pellets reacts with the water to scavenge Oxygen from the water molecule releasing clouds of hydrogen which if not vented to the outside mixes with the free oxygen in the air of the containment building and becomes an air fuel bomb same as the MOAB.
Blowing the concrete panels off the containment building exposing the overhead spent fuel tanks to the atmosphere which must be kept full of cool water to prevent the spent fuel assemblies from burning in the open air.
Westinghouse added a manual valve to vent the hydrogen but it is located deep in the containment building beneath the reactor and must be turned hundreds of times to fully open in a very dangerous operation beneath a malfunctioning reactor in the dark.
The Westinghouse BWS Mark 1 design is further compromised by the fact that the control rods are hydraulically driven up from the bottom not free fall from the top. The bottom of the steel pressure vessel containing the reactor core where any melted metal falls to is like a vegetable colander. The control rods are a softer heavy metal like cadmium that melts before the steel cladding of the uranium fuel assemblies so they don't have to melt through steel to escape they are basically extruded like soft spaghetti from the pressure within the vessel to the concrete floor of the containment building.
Once the control rods fall out the core goes super critical and nothing can contain the molten fuel which is also extruded out through the holes in the steel containment vessel. The geometry is not correct to permit a full release of all the energy of the Uranium/Plutonium fuel as in a nuclear weapon as there is tons of nuclear fuel in a reactor and if it all went off we would have an extinction level event. The corium as it now called a molten mixture of steel, cadmium and uranium continues to burn through the bottom of the containment building in what is known as the China Syndrome event. How far it will go before a change in geometry of the hemisphere shaped mass slows the rate of fission to naturally occurring uranium is unknown.
If the fuel rods contain a faster fission fuel like Plutonium MOX, when it goes super critical it will blow the heavy lid off the reactor vessel blasting through the roof of the containment building exposing the core of the reactor to the open air, like what happened in Reactor 3 at Fukushima.
What's happening at the Brunswick NPP in North Carolina will record flooding it turn into the Mother Of All Bombs (MOAB)?
