South Ukraine - Nuclear Plant Accident 3.Dez 2014

[FleaFlyFloh]

We have just been informed by media in europe that an accident had happened (today?) at the south ukrainian nuclear plant Saporoschje (warzone). Ukrainian Energy Minister will inform the public asap. First rumors say it's not a security issue, it's a technical issue, whatever that means.

Let's hope for the best
Rainer

 

[russ_watters]

Welcome to PF. Do you have a source for this?

 

[zapperzero]

http://www.reuters.com/article/2014/12/03/us-ukraine-crisis-power-idUSKCN0JH0ZV20141203

 

[zapperzero]

some more detail here.
http://www.24heures.ch/monde/europe/accident-nucleaire-sudest-ukraine/story/12186992
apparently some sort of fire in the "electricity output section" whatever the heck that means. One of the six reactors is in a state of shutdown.

 

[Doug Huffman]

some more detail here.
http://www.24heures.ch/monde/europe/accident-nucleaire-sudest-ukraine/story/12186992
apparently some sort of fire in the "electricity output section" whatever the heck that means. One of the six reactors is in a state of shutdown.

"This is a short-circuit"
"There is no threat. The incident occurred in the third reactor at the Zaporizhia nuclear power plant within the electrical output system. This is in no way linked to the reactor. This is a short circuit, "said the Ukrainian Energy Minister Volodymyr Demtchichine.

 

[FleaFlyFloh]

thx for adding links. First news today were a bit 'overrated' maybe due to a translation problem (accident/incident). Jazenjuks statement was translated first with the term 'atomic-accident', might be he didn't even say that. Apparently everything is under control an nobody was hurt.
have phun
Rainer

 

[e.bar.goum]

More details from World Nuclear News:

http://www.world-nuclear-news.org/R...-no-threat-says-energy-minister-03121401.html

 

[Hiddencamper]

Looks like a main power transformer failure caused a generator lockout and a load reject trip of the reactor.

Main power transformers are used to step the generator's output voltage up to the power grid. If they fail, they lockout all electrical equipment next to them, this causes the plant's electrical output breakers to trip and causes the generator to lockout. Generator lockouts in turn tell the main turbine to trip and stop expecting steam. Without the power grid available if the turbine keeps trying to turn the generator, it will damage itself badly.

Turbine trips in most commercial power reactors while the reactor is at high power result in the reactor automatically scramming. This occurs because the turbine rapidly stops accepting steam, so he reactor needs to stop producing it as there is nowhere for it to go and it can cause pressure spikes in equipment.

All of this happens in under a second. Turbine trip scrams are very straight forward transients to deal with. Usually there are no relief valve or safety valve actuations, but they may occur to help reduce steam pressure during the 2-3 seconds the whole event takes. Emergency core cooling system is not needed, however aux feedwater may start if necessary to help stabilize water level the first few minutes after the scram.

 

[Doug Huffman]

Thank you. I learned something today.

My 100 MWth class plants were required to cycle from full power to minimum to full power at maximum rate a number of times without alarms or protective actions.

 

[Astronuc]

it's not a security issue, it's a technical issue, whatever that means.

The industry would consider this a technical/industrial issue, rather than a safety issue, i.e., even when a major component failed, the system worked as designed to safely bring the reactor to shutdown and remove residual heat. They do need the electrical power though.

A safety issue implies that there is a potential threat of release of fission products within the plant or to the environment. To maintain safety margins a NPP operator is required to maintain control of the reactor by ensuring that when necessary, the reactor is made subcritical (reactivity control) and the fuel/core is cooled (maintain coolability) such that there is no breach of cladding boundary, or otherwise, no excessive release of fission products.

The legal requirements are rather elaborate.

 

[jim hardy]

My 100 MWth class plants were required to cycle from full power to minimum to full power at maximum rate a number of times without alarms or protective actions.

Propulsion plant ?

Looks like a main power transformer failure caused a generator lockout and a load reject trip of the reactor.

Main power transformers are used to step the generator's output voltage up to the power grid. If they fail, they lockout all electrical equipment next to them, this causes the plant's electrical output breakers to trip and causes the generator to lockout. Generator lockouts in turn tell the main turbine to trip and stop expecting accepting?..jh steam.

The automatic controls then tell the reactor to quit producing steam... and so on for a hopefully orderly shutdown.

This one sounds like the transformer that supplies internal power for the plant equipment suffered the initial failure. That one provides what is called "Onsite Power", because the power for all those pumps and equipment comes from the plant's own generator through that transformer. There'll be another source for that plant internal power, ours was called "Offsite Power" because it originated from the switchyard (grid).
I'd expect transfer from onsite source to offsite source to be automatic and instantaneous. If it wasn't, it should have been a simple thing to do manually.
Nothing nuclear-significant or unanticipated is indicated by those reports.

As Hiddencamper said, the turbine is placed at risk by generator lockout.
Put yourself in Mr Turbine's shoes for an instant... there you are, happily converting over a million horsepower from thermal energy of steam into Torque X RPM and handing it to Mr Generator.
Suddenly Mr Generator quits accepting energy - all that torque changes now from making electricity into accelerating turbine-generator as a pair.
The steam inlet valves must be snapped shut and steam bled out of Mr Turbine and all his pipes before he overspeeds. It's quite a transient to witness.

hope above helps... old jim

 

[Hiddencamper]

Power source transfers should be fast. Usually fast transfer logic is tied to the 86 device or a differential fault logic, and I've also seen automatic transfers off of house power to the power grid tied off of main generator breaker positions. Fast transfer logic will synchronize both off site sources for a split second before dumping the faulted bus and prevents a loss of voltage.

Even with a slow transfer, a typical 4160+ voltage switchgear should transfer and close in on the alternate source before the switchgear's undervoltage relay detects it and does a load shed.

 

[jim hardy]

Power source transfers should be fast. Usually fast transfer logic is tied to the 86 device or a differential fault logic, and I've also seen automatic transfers off of house power to the power grid tied off of main generator breaker positions. Fast transfer logic will synchronize both off site sources for a split second before dumping the faulted bus and prevents a loss of voltage.

Hmm well enough.. makes perfect sense for a normal operation . By 'synchronize' do you mean parallel?
The case above though had a generator lockout... paralleling the onsite and offsite would have backfed the generator zone through the (faulted) aux transformer for duration of that parallel. Our lockout scheme would have blocked that.

Even with a slow transfer, a typical 4160+ voltage switchgear should transfer and close in on the alternate source before the switchgear's undervoltage relay detects it and does a load shed.

Indeed speed is of the essence. Obviously you are well versed, but if anybody else is curious...
Speed up your thinking to millisecond steps. We're going to transfer from onsite to offsite power.
First, onsite feed breaker opens. That disconnects the bus from the onsite power transformer that's (usually) wired straight to the generator.
Bus voltage does not drop to zero because bus has a lot of huge induction motors connected to it. And induction motors will act like induction generators until their internal field dies down which takes a small number of seconds.
However - all those motors do slow down because there's no more energy being put into the bus. So the bus voltage sinewave starts to both get smaller, and slide out of phase with and behind the offsite voltage sinewave.
You cannot close the offsite breaker when there's a large phase difference because excessive currents will flow and wreck something.
So the transfer from onsite to offsite MUST be completed either:
1. BEFORE the bus slows down and gets more than a few degrees behind offsite ,
2. or, AFTER the bus voltage has decayed to near zero

Our plant's switchgear allowed (iirc) 12 line cycles, 1/5 second for that automatic transfer, at end of that time it was blocked by a timer relay.. It actually made the transfer in 5 cycles.
Should transfer fail, diesels would start and restore power in about ten seconds.
Our switchgear had a special "early" aux contact that granted permission for offsite breaker to start closing while onsite breaker was still traveling open. Sort of a "head start", if you will. "GE Magneblast" and it was great stuff in its day...

We had high speed oscillographic recorders on bus voltages , part of post trip analysis was verifying smooth transfers.
Ohhhh nostalgia...

Thanks for your input. You seem to have seen a lot more plants than i.
old jim

 

[Hiddencamper]

Our fast transfer closes the oncoming breaker immediately even if the faulted source isn't fully open yet. Our slow transfer waits for the faulted breaker to fully open first. I think we are like 6-7 cycles (16.7ms/cycle) maximum allowable time for a slow transfer. We have this huge analysis done with computer models that shows even if we close in with degraded voltage out of phase, we won't damage our large motors which are backfeeding the bus. But this analysis says we can only do that ~10 times in the life of the plant.

We are primarily Westinghouse DHPs. We have some GE Mangablast. They are extremely robust, but ours have an arc flash distance that forces us to keep people over 100 feet away (basically 2 rooms away) when racking it with a robot.

 

[jim hardy]

Do you have oscillographs on bus voltage?
Ours showed discernible phase shift at 5 cycles but just discernible, so i'd have no worry about your 6-7 cycle window. For us that was fast.From the World Nuclear News link that zz posted

Energoatom, the country's state-owned nuclear power plant operator, said today that the unit had disconnected from the grid automatically as "protection against internal damage to the generator.

The short circuit was caused by damage to the "winding voltage section of the auxiliary transformer and switch block transformer,...
...The incident occurred at 19:24 on 28 November and was caused by damage of the house load voltage transformer coil that led to disconnection of the house load transformer and unit transformer breakers," it said..
...Work is underway to restore the auxiliary power supply unit, and the reactor is expected to return to normal service on 5 December.

They didn't say which side of the transformer, high or low.

Good luck over there, guys !