APR: your source for nuclear news and analysis since April 16, 2010

Tuesday, March 15, 2011

FACTS: Hydrogen at nuclear power plants

--It is now known that there have been hydrogen explosions at three of the four seriously affected reactor plants at Fukushima Daiichi. Let's take a look at this gas in some further detail than previously.

HYDROGEN is present in some amounts at most nuclear plants, both inside the reactor coolant system and outside of it, for two reasons:

1. Hydrogen is a natural product of the radiolytic dissociation of the primary coolant water. This is the separation of water into hydrogen and oxygen by action of the radiation.

2. Hydrogen is made and stored outside the reactor plant. The presence of oxygen in the primary coolant system in some plants is intolerable, and so hydrogen is actually injected into the primary to suppress the oxygen by making enough hydrogen available that the oxygen will recombine and form water again before it can combine with metal which is corrosion

Hydrogen is normally not permitted to accumulate in large spaces at the plant and many plants have equipment in place in their containment structures to burn the hydrogen before it can explode

A mixture of hydrogen gas in air will burn at a concentration of 4% by volume or greater, and will explode if the concentration is greater than 19% by volume. Of course, this is not automatic and some form of ignition is required -- like a spark generated in an electric motor, a controller cabinet, a heater unit or any type of failing or damaged electric cable or connection

Some plants use an inert gas to fill the containment volume, thus making no oxygen available for any hydrogen that enters and accumulates

The exact method of hydrogen gas entering the refueling floor areas of the No. 1 and No. 3 plants is not known; it is probable that the use of primary relief valves to reduce primary pressure allowed primary coolant to enter surge tanks, after which the entrained radiolytic hydrogen exited vents and entered the buildings.. although there are a number of other scenarios, this is essentially what happened at Three Mile Island which led to a hydrogen burn at about 9-1/2 hours into the accident sequence. Hydrogen was reportedly generated by spent fuel cooling pool water dissociation at No. 4 plant according to sketchy reports.

ATOMIC POWER REVIEW 7:00 AM Eastern - Tuesday 3/14

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