ML20148B358

Amend to 10CFR50 to Clarify NRC Position on General Design Criteria Re Containment Design Basis & to Specify Stds for Combustible Gas Control Sys.Public Announcement Encl

ML20148B358
Person / Time
Issue date:	10/23/1978
From:	Chilk S NRC OFFICE OF THE SECRETARY (SECY)
To:
Shared Package
ML20148B355	List: ML20148B348 ML20148B358
References
FRN-41FR46167, RULE-PRM-50 NUDOCS 7810310195
Download: ML20148B358 (16)

Text

- _. . - ._ _ . _ . _ . _ .

.i 3di ,.~ -

c7sg0 013 I

/

/

Title'10 - Energy-

^

CHAPTER 1 - NUCLEAR REGULATORY COMMISSION PART 50 - DOMESTIC LICENSING'0F PRODUCTION AND UTILIZATION FACILITIES Standards for Combustible Gas Control Systems AGENCY: U.S. Nuclear Regulatory Commission ACTION: Final rule

SUMMARY

The Nuclear Regulatory Commission is amending its regula-

>tions to clarify the Commission's position on its general design i t

criterion regarding the Containment Design Basis and to provide a' new section specifying the standards for combustible gas control [

systems.  ;

i EFFECTIVE DATE: [30 days following publication in the Federal Register.] '

FOR FURTHER INFORMATION CONTACT: James A. Norberg, Office of Standards Development, U.S. Nuclear Regulatory Commission, Washington, D.C.

20553, Telephone (301-443-5921).

SUPPLEMENTARY INFORMATION: On October 21, 1976, the Nuclear Regula-tory Commission (NRC) published the notice of proposed rulemaking in the FEDERAL. REGISTER (41 FR 46167). Interested persons were 4

1 W2Y IW3l@lQ u >

+

t

[7590-01]

invited to comment on the proposed rule by December 23, 1976.- The notice concerned proposed amendments to 10 CFR.Part 50, " Domestic Licensing of Production and Utilization Facilities," to require the applicant or licensee to show that during the time period immediately following a postulated loss-of-coolant accident (LOCA) but prior to effective operation of the combustible gas control system, eithet:

(1) an uncontrolled hydrogen-oxygen recombination would not take place in the containment; or (2) the plant could withstand the

. consequences of uncontrolled hydrogen-oxygen recombir ation without l

loss of safety function. If neither of these conditions can be l l

shown, the containment shall be provided with an inerted atmosphere in order to provide protection against hydrogen burning and explo-sions during this time period. Standards for combustible gas control systems also were included in the proposed amendments.

It should be noted that under either the proposed rule or the j final rule (1) inerting of containment will not likely be required U for plants with small containment volume (Mark I and II Boiling l

Water Reactors) if the licensee or applicant can show that the calculated metal-water reaction will be well within the acceptance criteria for emergency core cooling systems and (2) inerting of containment will not likely be needed for plants with large contain-ment volume (Pressurized Water Reactors and Mark III Boiling Water Reactors) as long as ECCS acceptance criteria are met. The NRC staff has estimated that for all except two of the plants currently

I I

i

, i 1 .~ t

. [7590-0))

operating or under construction inerting containment would not be needed under this rule.

Fifteen interested persons submitted comments regarding the proposed amendments. The letters generally included numerous specific comments, but the most frequent comments could be roughly

. divided into three categories:

I

1. This inerting rule poses risks to plant personnel safety, reduces the number of unscheduled visual inspections, and 4

increases plant operating cost.

2. The rule is too conservative in defining the amount of metal-water reaction.

3. The containment air dilution system requirement of the rule should be clarified and the manner in which the NRC will implement the rule should be clarified.

Copies of the comments may be examined in the Commission's Public Document Room at 1717 H Street, N.W. , Washington, D.C. 20555. A summary of the NRC's response to these comments is presented below.

1. The risks to plant personnel and reduced inspection capability associated with inerting the containment were carefully considered and discussed in the notice of proposed rulemaking. The NRC con-cluded that the risk to plant personnel of an inerted plant would be small provided that special procedures and personnel safety 3

' [7590-01]

precautions are taken. The NRC also corcluded that, on the inspec-tion issue, inerting does not necessarily cause a reduction in the number of unscheduled visual inspections. Even assumihg that a reduction in the number of unscheduled visual inspections does occur, such reduction does not adversely affect the protection of public health and safety since an adequate program of scheduled inspections can be maintained to ensure safe operation of the plant. The NRC position on additional cost associated with requiring inerting is that the cost associated with inerting and de-inerting activities would be insignificant.

2. The NRC agrees that th'e proposed rule contains conservatisms, but disagrees that the rule is too conservative in defining the -

amount of metal-water reaction that must be assumed. It should be noted that in the proposed rule the amount of metal-water reaction assumed to occur after a LOCA is 5 times the amount calculated in accordance with 6 50.46 or is the amount that would result from cladding surfaces surrounding the fuel to a depth of 0.00023 inches (0.0058 mm), whichever amount is greater. Therefore, the amount of metal-water reaction assu,med in the proposed rule is less conservative for certain plants when compared with the 5% previously assumed by NRC. The detailed technical discusssion for making this revision was presented in the notice of proposed rulemaking (41 FR 46167).

In short, the factor of five or 0.00023-inch (0.0058 nm) depth number is based on an engineering judgment that this method provides 4

l l

[7590-01]

a sufficient margin for the purpose of containment design. It is assumed for conservatism that the emergency core cooling does not fail completely but that its performance is degraded to such an extent that a small fraction of the core becomes overheated. This could be caused by conservatively assuming the existence of a hot-spot effect of assumed coolant flow starvation resulting from (1) delivery of less-than planned cooling to a localized area and (2) local flow blockage that might be associated with excessive fuel deformation, partial core support failure, etc. While such i

conditions are not assumed to lead to core meltdown, they are assumed to result in additional hydrogen production above that calculated by Appendix K, "ECCS Evaluational Models."

Currently NRC conducts several Loss-of-Coolant Accident /

Emergency Core Cooling System (LOCA/ECCS) experimental verification and code development programs that have the potential for establish-1 ing the amount of conservatism in metal-water reaction. Pending l 1

further progress on the present LOCA/ECCS programs, the NRC has I concluded at this time that no further consideration need be given to this matter.

3. Comments regarding clarification of the rule have been incor-1 porated in the final rule but have not significantly changed the position expressed by the NRC in the proposed rule.

l One commenter opposed adoption of the rule. This commenter l believed that the change was unnecessary, because General Design 5

[7590-01] ,

Criteria 41, 42, 43, and 50 of Appendix A to 10 CFR Part 50 i'mpose r

the requirements for combustible gas control, and revised Regulatory Guide 1.7,1 " Control of Combustible Gas Concentrationsfin Contain- t ment Following a Loss-Of-Coolant Accident," 5'11 provide adequate [

i criteria and guidelines for combustible gas etntrol systems.

However, the NRC believes that the change is necessary because D ) <

General Design Criterion 41 states that containment atmosphere cleanup systems must be provided as necessary, but it is not specific in defining the types of cleanup systems (e.g., a combustible gas control system), (2) the method and basis for the calculation of i hydrogen after a LOCA should be established, (3) the degraded ECCS operation is subject to many different interpretations if not specifically defined, (4) publishing a final rule would resolve a complex technical issue that was raised on inerting by the Atomic e

Safety t.nd Licensing Appeal Board during the Vermont YanPee licensing proceeding, and (5). the rule has set forth limits of domain within f which potential hydrogen explosion is possible as a result of degrada". ion, but not total failure, of emergency core cooling functio:1ing. The rule would thus provide guidance to future designers, who could design their plants to avoid the range of hydrogen con-centrations that could have potential for explosion.

1 Copies of the Revision 2 to Regulatory Guide 1.7 may be obtained by written request to the U.S. Nuclear' Regulatory Commission, Washington,

- D.C. 20555, Attention: Director,-Division of Technical Information and i

Document Control.

6-

'\

[7590-01]

Assumptions acceptable to the NRC staff for evaluating and designing a combustible gas control system are presented in Revi-sion 2 of Regulatory Guide 1.7. Regulatory Guide 1.7 (originally Safety Guide 7) was published in March 1971. Revision 1 to this guide was published in September 1976. After consideration of the comments received on the guide, Revision 2 to this guide is now being published.

Revision 2 to Regulatory Guide 1.7 is essentially the same as

. l in Revision 1 with minor clarifying changes. Some concerns were raised in the guide about the meaning of partial burning of hydrogen in the containment. For hydrogen concentrations in the range of 4 to 6 volume percent, partial burning of the hydrogen above 4 volume percent may occur. ,

However, in this range of 4 to 6 volume percent, the rate of flame propagation is less than the rate of rise of the flammable mixture. Therefore, the flame can propagate upward, but not horizontally or downward. In this case, only a fraction of hydrogen will burn in the containment, and complete combustion will not occur until the hydrogen concentration is increased above 6 volume percent.

Interested persons were invited to submit written comments by December 20, 1976 for consideration in connection with the proposed amendments. After consideration of the comments received and other 7

[7590-01]

factors involved, the Commission has adopted the amendments as set forth below.

Pursuant to the. Atomic Energy Act of 1954, as amended, the Energy Reorganization Act of 1974, as amended, and Sections 552 and 553 of Title 5 of the United States Code, the following amendments to Title 10, Chapter I,. Code of Federal Regulations, Part 50 are published as a document subject to codification.

1. In 10 CFR Part 50, Appendix A - Criterion 50 is amended to read as follows:

Criterion 50 - Containment design basis The reactor containment structure, including access openings, penetrations, and the containment heat removal systeni, shall be designed so that the containment structure and its internal compart-ments can accommodate, without exceeding the design leakage rate and with sufficient margin, the calculated pressure and temperature conditions resulting from any loss-of-coolant accident. This margin shall reflect consideration of (1) the effects of potential energy sources that have not been included in the determination of l the peak conditions, such as energy in steam generators and, as required by 6 50.44, energy from metal-water and other chemical reactions that may result from degradation, but not total failure, of emergency core cooling functioning; (2) the limited experience and experimental data available for defining accident phenomena and 8

I

, , [7590-01] [

i containment responses; and (3) the conservatism of the calculational model and input parameters.

i;

2. A new S 50.44 is added to 10 CFR Part 50 io read as follows:

l 9 50.44. S_tandards t for combustible gas control system in liget water cooled e reactors.

(a) Each boiling or pressurized light-water nuclear power 4

reactor fueled with oxide pellets within cylindrical zircaloy cladding, shall, as provided in paragraphs (b) through (d) of this i section, include means for control of hydrogen gas that may be generated, following a postulated loss-of-coolant accident (LOCA),

by.(1) metal-water reaction involving the fuel cladding and the

- reactor coolant, (2).radiolytic decomposition of the reactor coolant, and (3) corrosion of metals.  !

(b) Each boiling or pressurized light-water nuclear power reactor fueled with oxide pellets within cylindrical zircaloy cladding shall be provided with the capability for (1) measuring

~

the' hydrogen concentration in the containment, (2) ensuring a mixed atmosphere in the containment, and (3) controlling combustible gas concentrations in the containment following a postulated LOCA.

(c) .For each boiling or pressurized l'ight-water nuclear power

!~ reactor fueled with oxide pellets within cylindrical zircaloy clad-ding it shall be shown that during the time period following a postulated LOCA but' prior to effective operation of the combustible 9

., a: . - .a . _ . w. . . - . . . _ . . ...;.--._~.

. Il

[7590-01]

3 gas control system, either: (1) an uncontrolled hydrogen-oxygen .

l-recombination.would not take place in the containment; or-(2) the

plantcouldwithstandtheconsequencesofuncontroliedhydrogen-oxygen-recombination without. loss of safety function. If neither of these conditions can be shown, the containment shall be provided with an inerted atmosphere or an oxygen deficient condition in ,

order to provide protection against hydrogen burning and explosions ,

during this time period. l (d)(1) For facilities that are in compliance with 6 50.46(b),

the' amount of hydrogen contributed by core metal-water reaction (percentage of fuel. cladding that reacts with water) as a result of degradation, but not total failure, of emergency core cooling functioning shall be assumed either to be 5 times the total amount of hydrogen calculated in demonstrating compliance with 6 50.46 (b)(3), or to be the amount that would result from reaction of all the metal in the outside surfaces of the cladding cylinders surround-ing the fuel'(excluding the cladding surrounding the plenum volume) to a depth of 0.00023 inches (0.0058 mm), whichever amount is greater. A time period of two minutes shall be used as the interval after the postulated LOCA over which the metal-water reaction occurs. (2) For facilities as to which no evaluation of compliance

'in accordance with 6 50.46(b) has been_ submitted and evaluated, the -

amounts of hydrogen so contributed shall be assumed to be that

~

amount resulting from the reaction of 5 percent of the mass of L

/

10 L

- - . , - ,u. . - . , ._...._,-.,_.-.;,,,,.c,a.,,,_.,- 1,..,, , , m...., ,

[7590-01]  ;

i a

metal in.the cladding cylinders surrounding the fuel, excluding the i cladding surrounding the plenum volume. H (e) For facilities whose notice of hearing on'ths' application ,

for a construction permit was published on or after November 5, i 1970, purging and/or repressurization shall not be the primary means for controlling combustible gases following a LOCA. However, [

B the capability for controlled purging'shall be provided. For these facilities, the primary means for controlling combustible gases following a LOCA shall consist of a combustible gas control system, such.as recombiners, that does not result in a significant release

~

from containment.

(f) For facilities with respect to which the notice of hearing on the application for a construction permit was published between December 22, 1968, and November 5,1970, if the incremental radiation dose from purging (and repressurization if a repressurization system is provided) occurring at all points beyond the exclusion area boundary after a postulated LOCA calculated in accordance with 5 .100.11(a)(2) of this chapter is less than 2.5 rem to the whole body and less than 30 rem to the thyroid, and if the combined radiation dose at the low population zone outer boundary from i 1

purging and the postulated LOCA calculated in accordance with I g 100.ll(a)(2) of this chapter,is less than 25 rem to the whole body and less than 300 rem to the thyroid, only a purging system is necessary, provided that the purging system and any filtration

!- 1 i

system associated with it are designed to conform with the general '

.I 11

-W i

i

- , -+ . -

~ , , -.

1

[7590-01]

requirements of Criteria 41, 42, and 43 of Appendix A to this part.

Otherwise the facility shall be provided with another type of combustible gas control system (a repressurization system is accept-able) designed to conform with the general requirements of Criteria 41, 42, and 43 of Appendix A to this part. If a purge system is used as part of the repressurization system, the purge system shall be designed to conform with the general requirements of Criteria 41, 42, and 43 of Appendix A to this part. The containment shall not be repressurized beyond 50 percent of the containment design pressure.

(g) For facilities with respect to which the notice of hearing on the application for a construction permit was published on or before December 22, 1968, if the combined radiation dose at the low population zone outer boundary from purging (and repressurization if a repressurization system is provided) and the postulated LOCA calculated in accordance with f 100.ll(a)(2) of this chapter is less than 25 rem to the whole body and less than 300 rem to the thyroid, only a purging system is necessary, provided that the purging system and any filtration system associated with it are 1

designed to conform with the general requirements of Criteria 41, 42, and 43 of Appendix A to this part. ' ierwise, the facility )

l shall be provided with another type r combustible gas control system (a repressurization system is acceptable) designed to conform I l

with the general requirements of Criteria 41, 42, and 43 of Appen-dix A to this part. If a purge system is used as part of the

! I 12

[7590-01]

repressurization system, it shall be designed to conform with the general requirements of Criteria 41, 42, and 43 of Appendix A to this part. The containment shall not be repressurized'beyond 50 percent of the containment design pressure.

(h) As used in this section:

(1) Degradation, but not total failure, of emergency core cooling functioning means that the performance of the energency core cooling system is postulated, for purposes of design of the combustible gas control system, not to meet the acceptance criteria in 9 50.46 and that there could be localized clad melting and metal-water reaction to the extent postulated in paragraph (d) of this section. The degree of performance degradation is not postu-lated to be sufficient to cause core meltdown.

(2) A combustible gas control system is a system that operates after a LOCA to maintain the concentrations of combustible gases within the containment, such as hydrogen, below flammability I limits. Combustible gas control systems are of two types: (i) systems that allow controlled release from containment, through filters if necessary, such as purging systems and repressurization systems, and (ii) systems that do not result in a significant l release from containment such as recombiners.

(3) A purging system is a system for the controlled release of the containment atmosphere to the environment through filters if needed.

l 13

. [7590-01]

(4) A repressurization system is a system used to dilute the concentration of combustible gas within containment by adding

~

inert gas or air to the containment. Dilution of' the combustible gas results in a delay in. time until a flammable concentration is reached and permits fission product decay. Operation is limited to a containment repressurization to 50 percent of the containment design pressure. A purging system is normally part of the repressurization system. '

(Sec. 161, as amended, Pub. L.83-703, 68 Stat. 948 (42 U.S.C. 2201);

Sec. 201, as amended, Pub. L.93-438, 88 Stat. 1242, Pub. L. 94-79, 89 Stat. 413 (42 U.S.C. 5841))

Dated at this day of 1978.

For the Nuclear Regulatory Commission.

Secretary of the Commission t

14 p

• A 4

n .

NRC CLARIFIES REQUIREMENTS FOR CONTROLLING COMBIJSTIBLE GASES IN REACTOR CONTAINMENTS FOLLOWING ACCIDENTS The Nuclear Regulatory Commission is amending its . regulations to l clarify its requirements for controlling combustible gases in the primary containment structure of light water power reactors under certain accident conditions. l I

Combustible gas control systems are provided to protect the containment against hydrogen burning or explosion following a loss of primary coolant accident. Amendments to Part 50 include standards for these systems.

1 The amendments also clarify the circumstances under which "inerting" l of the containment could be required for protection during the time period 1

immediately following the accident and just before effective operation of  !

l the combustible gas control system (a period of about two minutes),

i Inerting is a process of adding nitrogen to reduce the concentration 1 of oxygen in the containment atmosphere. This prevents any hydrogen  !

produced or accumulated immediately following a loss of coolant accident l

. from reacting with oxygen to cause;a fire or hydrogen explosion. l l

(

l l

1 l

.Z- 4 -, . . , . . . . . . , . ,u. _ _ , _ _ _ , . _ , , ___ , , _ , _ , _

_ _ _ , _ .__ _ _. ]

Inerting will be required only if a licensee is unable to show that hydrogen would not accumulate to excessive levels, or that, if such a situation should occur, it would not affect the plant's safetIy functions.

The NRC staff has estimated that for all but two of the plants now operating or under construction inerting containment would not be needed under this rule.

The amendments to Part 50 of NRC regulations are being published in the Federal Register on to be effective .

They are substantially the same as those published earlier for public comment.

2

.=,

f i _.

-)

I, : ' '> 'o : Acce53iG' Unit Re:r C50 j Chillips 2uildant, Fror- M. Fleishman, EMSB  !

Pleas: elece the attached document in the POR using the following file anf file points:

PDR File . Additional Info (Select one and enter number) (Enterifappropriate)

Proposed Rule (PR) ACRSM 'inutes lio. l Reg. Guide 1.7 s Relates to Proposed Rule (PR) {

Petition (P9M) Relates to Reg. Guide Effective Rule (RM) Relates to Petition (PRM) {

Ai!SI Relates to Effective Rule (R:i) <

IAEA Federal P,egister flotice x SD Task flo. EM 526-1

,  !!UREG Report )

Contract flo. )

Subject:

Letters to Chairmen of Committees concerning Federal Kegister iiotice and Public Keiease covering efieui.ive iule-making to amend 10~CFR Part 50 to provide cc: new 550.44 to specify standards for  !

Central Files combustible gas control systems. ,

l

}

l

.- s

\

?

. j j

I

(

l l

4