ML19338E103
| ML19338E103 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 08/29/1980 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Hendrie J NRC COMMISSION (OCM) |
| Shared Package | |
| ML19338E104 | List: |
| References | |
| NUDOCS 8009240560 | |
| Download: ML19338E103 (9) | |
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UNITED STATES ND
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NUCLEAR REGULATORY COMMISSION N
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AUG 2 91980 3M MEMORANDUM FOR: Comissioner Joseph M. Hendrie THROUGH:
William J. Dircks Ee:D T. A.Rehm Acting Executive DiFeftor for Operations FROM:
Harold R. Denton, Director Office of Nuclear Reactor Regulation
SUBJECT:
USE OF HALON TO SUPPRESS HYDR 0 GEN IGNITION Enclosed for your consideration is a preliminary assessment by the staff on the use of halon as a hydrogen combustion control measure.
The paper calls attention to at least five questiens regarding halon's use in the Sequoyah Nuclear Facility:
1.
Selection and design of a halon delivery system, i.e., should halon be introduced through the containment sprays?
2.
Uniform distributions of halon in amounts in the containment approaching 20 volume percent is required for effective hydrogen combustion inhibi-tion.
How will this mixing be achieved?
3.
What are the effects of inadvertent actuation of the halon delivery system?
4.
What are the chemical stability requirements and reactivity and comp 3t-ibility consequences of halon injection?
5.
How will the hydrogen be removed from the halon mixture? How will it then be chemically deactivated?
TVA and RES will be addressing these questions and others through their research programs.
M sn Harold R. Denton, Director Office of Nuclear Reactor Regulation
Enclosure:
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Comissioner Joseph M. Hendrie
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- Discussion on Halon I.
Introduction Halon is a chemical compound'made up of four halogen (fluorine, chlorine, bromine and iodine) atoms for each carbon atom.
Chemical formulas for some Halon compounds include CBrF and CBrClF '
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The accident at Three Mile Island, Unit 2 (TMI-2) involved a large amount of metal-water reaction in the core with resulting hydrogen generation well in excess of the amounts specified in 10 CFR 50.44 of the Commission's regulations.
Large amounts of hydrogen released inside of containment can result in detonations thut are damaging to safety equipment.
If Halon gas were properly mixed with the containment atmosphere, neither combustion nor detonation of hydrogen gas would occur.
Halon prevents' detonation of hydrogen by a. process known as chemical fiame inhibition.
This method is only partially understood and is the subject of current research.
The outstanding effect of this method is the extreme rapidity and the high relative efficiency with which flames can be extin-guished.
Moreover, Halon systems are the only means by which an explosion can be prevented in a flammable gas / air mixture after ignition has occurred.
The combustion reactions proceed ir a complex series of steps having a chain reaction characteristic.
For the hydrogen-oxygen reaction, the process oc-curs as follows:
H2 + e -q> 2H*
H* + 02 -=9> 0H * + 0*
0* + H2 --1> 0H * + H
- OH * + H 2 --9> H 2 O + H
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Following the initial splitting of the hydrogen molecule', the individual I
hydrogen atoms (active H* species) interact with oxygen molecules to pro-duce active OH* and 0* species. As the result of much research, it has been found that the flame velocity is dependent upon the concentration of the active OH* species and upon the pressure at which the reaction proceeds.
Halon prevents detonations by not allowing the active species OH*, H* and 0* to fulfill their role in sustaining the flame. The exact manner in which the active species are interferred with in achieving flame extinguish-ment is, at present, not certain.
II.
Discussion In proper concentrations, halon can prevent the detonation of hydrogen
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inside containment.
The amount of halon required to prevent detonations in the containment atmosphere is about twen'ty volume percent.
With stored halon systems that can be automatically actuated, there exists the possibility of inadvertent operation when plant personnel are in the discharge area.
Halon 1301 is classified by Underwriters Laboratory in the least toxic group of gases.2 Halon 1301 is in the toxic Group 6 which is composed of gases which in concentrations up to at least 20 percent by volume for durations of exposure of the order of two hours do not appear to produce injury. This classification is based on animal tests.
Halon 1301 appears to be the ~least todr agent that could be chosen for the containment application, which would meet all 'the requirements of the total,, problem.
However, in spite of the low toxicity of 1301, personnel would have to be warned to leave the l
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imediate vicinity in the event of an accidental discharge in a con-f fined space. Thiscaneasilybeaccomplishedbyinstallingaudioalarms that actuate when Halon system discharges.
In general, it can be as-sumed that 1301 toxicity would present no hindrance to choosing this type of inerting system.
In order to get thorough mixing of Halon and hydrogen, Halon is normally delivered by discharge nozzles that are located throughout the discharge area.
In a containment application it may be possible to discharge Halon through the containment spray system and use the hydrogen mixing i
system to get the distribution of Halon that prevents hydrogen detonations inside of containment.
This would eliminate the need for a new system dedicated to dispersing Halon in the containment.
Further study is re-quired to determine the efficiency of dispersing Halon in the contain-ment with the containment sprays and hydrogen mixing system.
Inadvertent operation of Halon systems also leads to exposure of equip-ment inside containment to Halon. The chemical properties of Halon 1301 of interest concern its compatibility with materials of construction, its reactivity and thennal stability.
Fluorinated hydrocarbons a,re among the most inert and stable chemicals known.
As a general rule, except for reactivity with certain active metals such as alkali metals, 1301 can be considered compatible and non-reactive with almost all materials, and storable-indefinitely even at elevated temperature.
For example, DuPont D
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conducted tests wherein liquid 1301 was sealed into glas's tubes contain-ing test strips of common metals for a 44-month period at tem'peratures of
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130*F and 250*F. As a result of these tests they report that the following
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metals are suitable-for use with 1301:
Stainless Steel 302 Steel 1020 CR Stainless Steel 321 Aluminum 1103 Aluminum 2024 Aluminum 6061 Yellow Brass Magnesium AZ-91C Commercial Titanium Titanium A110 AT Thus, in the event of inadvertent operation of the Halon system, Halon 1301 I
co;1d be removed from the containment with no immediately apparent adverse effects on the equipment in the containment, This matter should be con-firmed by appropriate testing.
Because Halon is a non-condensible gas, its discharge into the containment during an accident results in an increase in the containment pressure, Preliminary calculations indicate the increase in containment pressure due to Halon is approximately 5.3 psi. This can result in the reduction j
of design margin of the containment, especially in ice condenser contain-t l'
ment where the design pressure is 12 tc 15 psia, a
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h Also, preliminary calculations indicate that an inadvertent actuation of a Halon injection system during normal plant operation could implorie the con-
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tainment vessel.
Liquid Halon, in a quantity sufficient to result in a 20%
concentration, would upon vaporization absorb enough heat from the air and surroundirig structures and equipment to drop the air temperature down to
-72*F (boiling point of Halon) and air pressure from standard atraospheric conditions to about negative 4.3 psig.
If Halon were mixed in twenty percent concentrations with hydrogen, the disposal of the hydrogen becomes a problem.
Presently, hydrogen inside containment is eliminated by passing it through hydrogen recombiners which convert hydrogen and oxygen to water by burning the hydrogen.
Hydrogen does not burn in the recombiner with Halon in twenty percent concentratioris.
v In this situation the hydrogen would have to be continuously mixed with Halon to prevent explosions until it could be removed from containment i
by some means other than burning, e.g,, diffusing membranes.
Further study on the removal of hydrogen-halon, mixtures from containment is re-
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quired to conclude on the advantages of mixing hydrogen with Halon.
III.
Conclusion Halon in proper mixtures with the containment atmosphere, prevents com-
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bustion or detonation of hydrogen gas.
Its low toxicity level allows ample time to evacuate areas where halon systems inadvertently discharge, The chemical properties of halon do not appear to result in corrosion of i-metals such as stainless steel, steel or aluminum at temperatures below x
6-250*F; therefore, inadve'rtent halon system operation should not adversely F
affect equipment inside the reactor.
I Discharging Halon into containment during an accident will increase the containment pressure resulting from the accident, causing a reduction in the design margin of the containment.
Also, preliminary calculations in-dicate that an inadvertent ~ actuation of a halon injection system during normal plant operation could damage the containment vessel.
Further study is required to determine the feasibility of using present plant systems to distribute and mix the halon with hydrogen and to devise methods of removing the halon-hydrogen mixture from containment after an accident.
Presently, we do not recommend the use of halon to suppress hydrogen detonations inside containment.
Further study is required to better determine the characteristics of its use to suppress hydrogen detone-tions inside containment.
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References E
1.)
Memorandum for the Consnissioners from H. R. Denton, Director'of Nuclear E
Reactor Regulation, dated February 22, 1980 (SECY-80-107).
2.)
Halon 1301 Explosion Suppression System for CNSG Containment Vessel by 1
Engineering Research Department of Atlantic Research Corporation,.
October 24,1973 (ARC No. 41-5381).
3.)
Fire Protection Handbook, Fourteenth Edition, January 1976, 75-34683.
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