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Author Topic: Has there ever been a serious accident with a Heavy water moderated reactor?  (Read 2629 times)

Offline Pecos_Bill

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Someone brought up Fukushima in another thread.

As far as I recall, there has never been an emergency with a heavy water reactor like the Canadians have used. Is this so?

Would such a reactor have melted down at fukushima?


 

Offline syhprum

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I recall German researchers had an accident due to Hydrogen evolution during WWII Which probably reduced their chances of producing a weapon
 

Offline CliffordK

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I believe 3-mile island was a pressurized "light water" reactor which is common in the USA.

Fukushima apparently used a boiling water reactor. 

My question is whether there is significant difference between the 3-mile pressurized light water reactor, and the Canadian pressurized heavy water systems that would have prevented a similar incident from occurring in Canada.

There apparently have been several nuclear accidents in Canada including the release of tritium water on multiple occasions.

And another commentary about "minor" accidents in Canada including groundwater contamination, and apparently decades of failure to report spills.


 

Offline Pecos_Bill

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There <<is>> a significant difference in reactor design. I refer you to the safety features section of the Wikipedia article.

Here is a pertinent passage...

"There are two independent, fast-acting safety shutdown systems as well. Shutoff rods are held above the reactor by electromagnets, and drop under gravity into the core to quickly end criticality. This system works even in the event of a complete power failure, as the electromagnets only hold the rods out of the reactor when power is available. A secondary system injects a high-pressure gadolinium nitrate neutron absorber solution into the calandria."

and..

"It is important to remember that the fuel is not critical in light water.[2] This means that cooling the core with water from nearby sources will not add to the reactivity of the fuel mass."

Therefore, I reckon that this design -- as opposed to Fukushima's conventional light water designs, loss of power would have scrammed the reactor immediately and you would not have seen the horrific mess that now prevails.

As to leaks of tritium, I seriously doubt they would approach the level of contamination (or risk) with long half life isotopes now seen at either Chernobyl or Fukushima.

So .. Anybody know of <<serious>> problems during the 40 plus years this design has been operating
 


Offline evan_au

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Regardless of the cooling method, uranium reactors continue to produce considerable heat after the chain reaction has ceased.

This is because most of the heat is produced immediately by the fission of uranium, but a fraction of the heat is produced by breakdown of the radioactive daughter nuclei, and occurs over an extended period of time. For a reactor producing 1000 MW of heat when active, it will continue generating somewhere around 50MW of heat in the first hour; this dissipation will need to continue for longer if the reactor is to be brought from operating temperature down to "room temperature". The reactor will continue to generate >1MW long after it is shut down.

So nuclear reactors require continued cooling after the control rods are inserted; if the cooling system fails, the water eventually turns into steam, and when the steam reacts with the hot metal surrounding the fuel pellets, it results in a hydrogen explosion (chemical reaction, not a nuclear explosion).

The unexpectedly large tidal wave at Fukushima took out the emergency power supply in the basement, destroyed the public electrical grid and eliminated any possibility of shipping in additional generators. So even though the reactor was shut down as soon as the earthquake was detected, several hydrogen explosions ensued.

There were multiple almost-simultaneous failures at Fukushima; with 20/20 hindsight, some of them could have been prevented (like putting emergency generators on a higher floor, and better historical analysis of tsunami heights). I think that a similar number of concurrent failures would be disastrous for most of the older reactor designs.

The major improvement in new reactor designs is to ensure that the residual heat of the reactor can be channeled to drive a cooling system which is able to remove this residual heat even without the use of external electrical generators.
« Last Edit: 10/01/2015 06:48:22 by evan_au »
 

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