ML19341A424

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Proposed Tech Specs Sections 3 & 4 Changing Explosive Gas Mixtures in Tanks W/Radioactive Effluents & Deleting Requirement for Hydrogen Monitor in Waste Gas Holdup Sys
ML19341A424
Person / Time
Site: McGuire, Mcguire  Duke Energy icon.png
Issue date: 01/12/1981
From:
DUKE POWER CO.
To:
Shared Package
ML19341A423 List:
References
TAC-46812, NUDOCS 8101230496
Download: ML19341A424 (5)
v • d • e


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PROPOSED TECHNICAL SPECIFICATION REVISION A. RADIOACTIVE EFFLUENTS EXPLOSIVE GAS MIXTURE LIMITING C0ffDITION FOR OPERATION (McGuire Unit I)

Technical Specification 3.11.2.6 now reads as follows:

3.11.2.6 "The concentration of hydrogen and/or oxygen in the waste gas holdup system shall be limited to 1 2% by volume."

Technical Specification 3.11.2p should be changed to read as follows:

3.11.2.6 The concentration of oxygen in the vaste gas holdup system shan ba limited to 13% by volume.

APPLICABILITY: At all times.

ACTION:

a. With the concentration of oxygen in the waste gas holdup system

> 3% by volume but 14% by volume, restore the concentration of oxygen to within the limit in 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.

b. With the concentration of oxygen in the waste gas holdup system

> 4% by volume, immediately suspend all additions of waste gases to the system and reduce the concentration of oxygen to 1 3% within 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.

SURVEILLANCE REQLIREMENTS The Surveillance Requirements should be changed to read: s 4.11.2.6 The concentration of oxygen in the waste gas holdup system ,

shall be determined to be within the above limits by continuously moni-toring the waste gases in the waste gas holdup system with the oxygen monitors required OPERABLE by Table 3.3-13 of Specification 3.3.3.10.

810123 04{ta .. -.

REASONS AND JUSTIFICATION FOR CHANGE

1) Below 5.0% 0xygen, no mixture of hydrogen and oxygen is flammable with nitrogen acting as the diluent in this mixture.1 This f a the case for the McGuire Waste Gas System.
2) "The fact that a combustible gas, vapor, or mist will not burn or explode when the oxygen content of the atmosphere is reduced belew a certain definite value, varying with the combustible material under consideration, enables one to definitely control and in many cases actually eliminate explosion hazards."2
3) McGuire's Waste Gas System is designed to operate at low oxygen concentrations (below 15 ppm in the system, excluding portions of the recombiner tkid) with a hydrogen recombiner in operation. The oxygen feed valve to the hydrogen recombiner can never pass more than 3.0% oxygen without causing an alarm which trips the oxygen feed valve closed. An inlet oxygen concentration to the hydrogen recombiner of > 3.5% oxygen will cause both the oxygen feed valve and the Volume Control Tank hydrogen purge isolation valve (Unic I and II) to closa.
4) The Waste Gas System is designed for operation in one of the two following modes:

(A) Compressor - Recombiner - Decay Tank (B) Compressor - Decay Tank - Recombiner Design of the system is such that operation begins in Mode (A) until approximately 25 psig is reached in the decay tank. At this point the valve alignment should be switched to Mode (B) . The hydrogen purge from each VCT will normally be approximately 0.75 SCFM. With a combined VCT purge (both units) of 1.5 SCFM there is 3.75% hydro-gen in the WG System at the point where this purge mixes with the WG loop flow (approximately 40 SCFM). The recembiner is designed to remove all but approximately 0.1% 3H which will pass to the decay tank. When the decay tank reaches 20 - 25 psig, the valve alignment of Compressor - Decay Tank - Recombiner must be made due to the backpressure on the recombiner which exists with approximately 20 psig in the tank. (Maximum recombiner design pressure is 30 psig.)

Mode B would normally be the means by which the tank in Mode A is used and filled to its allowable limit (approximately 100 psig -140 psig). However, as the Mode (3) diagram below shows, this would not be possible with the demands of the 3.11.2.6 Technical Specification.

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MODE (A)

UNIT I & UNIT II VCT PURGE % 100% H2 , DECAY %30 psig T i TANK a s ' RECOM-

- 02 '

1.4-2.4 SCDi BINER

\/

40 SCFM (In Loop)

COMPRESSOR }

40 SCFM (N2 , H2, Fission Cases)

MODE (B)

UNIT I & UNIT II VCT PURGE % 100% H2 / %30 osist ' -

RECOMBINER s a

e

/ DECAY

) { TANK 1.4-2.4 SCFM  ;

%d 40 SCFM (In Loop)

) COMPRESSOR )

40 SCFM (N2 , H2, Fission Gases)

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Example: Flow from Units I and II = 1.4 SCFM (100% H2)

This would give a 3.5? H e ncentration in the Waste Gas System 2

loop, and eventually a 2.5% H., concentration in the WG Decay Tank in j service. If ene Decay Tank 13 at 25 psig (2.7 atmosphere), it would 1 take approximately 40.5 minutes to reach this 3.5% H concentration i 2

. in the tank, based on the following estimate: l 100% (1.4 f t3 2H /"i") " e (x %/ min)(2.7 atm)

(tank volume) t H

= 100% (1.4min 2-) = 1620 f t3 (x %/ min) ,

x = 0.0864 %/ min increase in tank .*. = 40.5 min 0.086 %/ min With 0 psig (1 atm) in decay tank it would take approximately 8.5 minutes to reach a 2% H concentration which is the present Techni-3 cal Specification limit. At 100 psig in the tank, within 60 minutes

this 2% concentration would be reached.

This points to either a flaw in the normal modes of operation of the WG System or, more likely, a discrepancy between the Technical Specification and reasonable limits for operating the system in Mode (B).

5) Westinghouse Standard Technical Specifications (STS) word their specification for the explosive gas mixture in the Waste Gas holdup system similar to this:

The concentration of oxygen in the waste gas holdup system shall be limited to < 7.4 by volume.

Note that no mention of hydrogen concentrations exists in this statement.

6) "Before an explosive mixture can propagate flame a portion of the mixture must be heated to its ignition temperature."3 The minimum ignition temperature of hydrogen vapors in oxygen is 1067 F."

The normal operating temperature of the hydrogen recombiners is approximately 850 F. An alarm and corresponding shutdown of the oxygen feed to the hydrogen recombiner occurs when the temperature in the recombiner catalyst bed reaches 1050 F. This high tempera-ture can be detected by any one or more of the six thermocouples which are positioned at various levels throughout the recombiner catalyst bed. These thermocouples, along with five others located at specific point between the recombiner skid inlet and outlet valves, are wired to a chart recorder thereby providing the operator with continuous temperature monitoring capability. Additionally ,

there are six backup thermocouples already installed in the recom-biner catalyst bed which can be used in the event that one of the i normally used thermocouples fails.

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7) "The usual problem in dealing with combustible mixtures is to con-trol the atmosphere so that the composition at a.1 times is outside the flammable range."5 Because the Waste Gas System is designed to operate within limits to prevent combustible mixtures, the above condition of operating out-side of-the flammable range is met. Only if there is a break in the Waste Gas piping could the potential for an explosive H -O mixture 2

exist and even then the ignition temperature of 1050 F must he present.

In summary, a Technical Specification of < 3% oxygen concentration in the Waste Gas holdup system is quite conservative, leaving a 40 margin of error between this limit and the limit of 5% oxygen for nonflansnab .e oxygen-hydrogen mixtures.

Moreover, the Waste Gas System is designed with built-in safety shutdown features which prevent the oxygen-hydrogen concentrations from reaching flammable proportions. This is evidenced by the fact that the analyzers which measure oxygen inlet and outlet concentra-tions across the recombiner bed possess features which will shut off the oxygen supply to the system before flammable oxygen-hydrogen concentrations develop. If for some reason the inlet oxygen concen-tration to the recombiner reaches 3.5%, the inlet oxygen analyzer will not only close the oxygen supply valve but will close valve IWG3 and 1WG14 which are the hydrogen purge isolation valves from Unic I and Unit II Volume Control Tanks respectively.

The hydrogen analyzers in the Wasta Gas System are also designed with alarm and/or shutdown features. The hydrogen analyzer which monitors the inlet hydrogen concentration to the recombiner will trigger an alarm at 6.0% hydrogen and shut off the oxygen supply to th_e recombiner at concentrations of > 9.0% hydrogen. The other hydrogen analyzer monitors the concentration of hydrogen as it

leaves the recombiner and modulates the oxygen addition valve to l maintain proper oxygen-hydrogen mixtures for safe, automatic opera-tion.

Inputs to the Waste Gas System from sources other than the VCT Purge header have been determined to be of low enough oxygen or hydrogen concentration as to minimize interference with the normal operation of the Waste Gas System. The Pressurizer Relief Tank appears to be the source which could contribute the largest volume of gas at one time to the Waste Gas System; however, this is mostly nitrogen and should only impact the system gas composition for short periods of eine, such as during plant startup.

References 1 Patty, Frank A. Industrial Hygiene and Toxicology. Volume I. General Principles. Interscience Publishers, Inc. New York. Pgs. 339-540.

2 ibid, P. 538.

3 ibid, P. 524.

i 4 ibid, P. 534 5 ibid ,' ? . 5 41.

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