ML20044G741

SER Re Mod of Post Accident Sampling Sys Requirements.Mod to Consider Time Requirements for Performing PASS Analyses to Be Obtained Any Time During Accident Unacceptable

ML20044G741
Person / Time
Issue date:	04/12/1993
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20044C666	List: ML20044G741
References
NUDOCS 9306040181
Download: ML20044G741 (7)

Text

_

i 1

i l

l l

SAFETY EVALUATION REPORT BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO MODIFICATION OF POST ACCIDENT SAMPLING SYSTEM RE0VIREMENTS MATERIALS AND CHEMICAL ENGINEERING BRANCH TAC NO. M82498 l.0 INTRODUCTION The objective of the NRC staff review of the report in Reference 1 is to evaluate the proposed clarifications and modifications and, if appropriate, to i

approve the departures from the current NRC requirements.

The initial NRC requirements for the Post-Accident Sampling System (PASS) were specified in Reference 2.

However, some of them were subsequently revised or are in the process of being revised. The modifications to the original requirements in Reference 2 are described in References 3 and 4.

~

l The report in Reference I contains the following clarifications and.

j modifications of the existing requirements:

o Clarification of the requirement for measurement of-sump pH.

o Deletion of the requirement to measure containment hydrogen and-

~

l relying on the safety-grade containment hydrogen monitors.

o Clarification of the requirement for heat tracing of the containment atmosphere sample lines and modification of the Core Damage Assessment Procedure to base the core damage predictions on noble gas concentrations.

Use of hydrogen / total gas measurements in the ' reactor coolant, samples o

only as a backup method to the Reactor Vessel Level Monitoring System and include an option for using either PASS or the normal sampling system for performing these measurements.

o Clarification of the requirements for oxygen analysis in the reactor coolant.

o Clarification of the requirement for sample points in PASS.

o Modification of the time limit for the determination of dissolved gases, activity and boron concentrations in the reactor coolant.

2.0 EVALUATION i

The above listed clarifications and modifications were reviewed and evaluated by the NRC staff, relative to the criteria specified in the referenced documents and the current NRC staff interpretation of these criteria. The results of these evaluations are presented below:

1 9306040181 930412 4

PDR TOPRP EUTFPL i

C ppg J

4 t

2 2.1 Measurement of oH of Reactor Coolant There is no specific requirement in the PASS specifications for measuring pH in the sump. However, Criterion 10 in Reference 2 implies that appropriate parameters should be measured in order to describe radiological and chemical status of the reactor coolant system. Also, for the same reasons, Regulatory Guide 1.97 recommends to measure pH of the primary coolant and in the containment sump. The justification provided in the report (Reference 1) for. deleting of pH measurement is based on the fact that in the post-accident environment pH in the sump is controlled at a value of 27 either by the addition of sodium i

hydroxide to the containment spray solution or by a passive pH control achieved by buffering action of the chemicals stored in the sump (usually trisodium phosphate dodecahydrate). The reliability of this control is sufficiently high to ensure that neither corrosion of reactor components nor reevolution of dissolved iodine will occur.

In view of the fact that the current specifications for PASS do not specifically require pH measurement in the sump water, and the existence of the reliable methods for pH control, we consider the proposed deletion of pH measurement in the sump water is acceptable.

2.2 Measurement of Containment Hydrocen Concentration Criteria 2b and 8 in Reference 2 require PASS to have a capability to measure hydrogen levels in the containment and, if inline monitoring instrumentation is used, to provide a grab samples backup. However, the requirement in Reference 2 specifies that hydrogen in the containment be 4

monitored by the safety-grade hydrogen monitors which would be capable to provide hydrogen concentrations after an accident. In the report (Reference 1) it is requested, therefore, that the requirement for containment hydrogen measurement by PASS be deleted. The staff concludes that because the hydrogen monitors in the containment provide adequate capability for monitoring post-accident hydrogen, there is acceptable justification for this deletion and finds-the requested modification acceptable.

2.3 Heat Tracina of Samole Lines The purpose of heat tracing of PASS sample lines is to prevent iodine plateout which would result in incorrect measurements of iodine concentration in the containment and could constitute an important source of errors, if iodine concentrations were used for assessing degree of core degradation. Criterion 11 in Reference 2 specifies, therefore, that provisions should bo made for reducing this plateout.

However, if iodine is not used for determination of the degree of core degradation, the information about its concentration is not needed and its plateout in the PASS sample lines does not constitute a problem.

This issue was discussed in Reference 3, and it was determined that when the core damage assessment procedures are based on representative containment atmosphere xenon and or krypton activities, there is no need to sample iodine in order to meet the requirements of_ Reference 2.

The

3

)

modification proposed in the report (Reference 1) would modify the Core i

Damage Assessment procedure for the CE plants-to base.the assessment on i

noble gas concentrations. This modification.is consistent with the i

current NRC policy and the staff find it, therefore, acceptable.

1 4

2.4 Measurement of Hydroaen/ Total Gas in Reactor Coolant 1

Measurement of hydrogen or total gas in the coolant water is required so that the operators would know about potential problems which may occur, if during a depressurization dissolved gas is released. into the reactor l

vessel and forms a bubble which could interfere with heat removal by natural circulation. Also, since most of the hydrogen in the reactor vessel is formed by oxidation of fuel cladding by steam, information.on the amount of hydrogen dissolved can give some indication on the degree of core damage. Criterion 2c in Reference 2 requires.that PASS should i

have a capability to provide within 3-hour time frame quantitative determination of dissolved gases in the' reactor coolant, and Criterion 4 J

specifies that this measurement should not be necessarily performed on pressurized sample, if the licensee can quantify the amount of dissolved i

gases with unpressurized reactor coolant samples. In addition, Criterion 4 states that measurement of either total dissolved gases or hydrogen gas is considered adequate.

The modification proposed in the report (Reference 1) consists of replacing dissolved gas ~ measurements by PASS as a primary method for i

monitoring gas release in the reactor vessel _by the procedures relying on the Reactor Vessel Level Monitoring System and the Emergency Procedures for maintaining natural circulation in the reactor vessel.' A core damage assessment will be performed using coolant radionuclide concentration whose determination is required by Criterion 2a in Reference 2.

Dissolved gas determination by PASS will be used only as a confirmatory method and as such would not be needed to meet the stringent requirements specified in Reference 2.

Dissolved hydrogen could be measured by an inline hydrogen monitor _ for which dissolved gas results are easily obtainable. The grab samples backup will be of a secondary importance and will not be subject to 3-hour time requirements for taking the first sample. Also, there will be an option of using either PASS or normal process sampling system for performing this measurement, depending on activity levels.

The report claims that using the process sampling system would improve the accuracy of measurements of low concentrations of dissolved gases, because the inherently small volumes of PASS samples make this determination very difficult. The report (Reference 1) claims that the requirements in Reference 2 are applicable to the plants similar to TMI-?_ in design. In the existing CE plants, the information for which gas concentration in reactor coolant is needed could be obtained by alternative means. Combustion Engineering has developed the Emergency Planning Procedure to assist the licensees in developing plant-specific procedures to account for various accident scenarios, including recovery from loss of natural circulation.

i j

4 o

The staff considers concentration of dissolved. gases in the' reactor i

coolant to be one of. the most direct parameter in diagnosing of the j

. problems related to the interruption of natural circulation caused'by' i

release of noncondensible gases. in the reactor vessel.

It also constitutes an important parameter in determining the degree of core damage from the estimate of the amount of fuel cladding oxidation.

Although, as suggested in the report, other sources could be used for.

obtaining this-information, the direct determination of the amount of i

dissolved. gas in the reactor coolant still provides. the most-reliable-i evidence of the potential problems caused by the presence of dissolved.

gases in the reactors vessel. This is _especially applicable to some postulated accident sequences"in which the reactor coolant system is '

i intact at reduced pressure,'and heat is removed by subcooled decay heat-removal. For these cases, it will not be possible.to' evaluate'-

t concentrations'of.the dissolved gases in reactor coolant by measuring.

1 their concentrations in the containment by the inline hydrogen monitor.

For PWRs exposed to.these conditions, information on the amounts'of dissolved hydrogen in the reactor coolant is an important factor in 6

evaluating post-accident conditions in the. reactor vessel..Therefore, the staff concludes that the modification proposed in the report is not 1

acceptable and~ PASS in the Combustion Engineering plants.should. meet the requirements of Criterion 2c of Reference l.. However,.the option.for 1

using either PASS or the normal sampling system for taking coolant' samples is acceptable provided the radiation exposure limits.of General Design Criterion 19 are not exceeded.

2.5 0xvaen Analysis of Reactor Coolant -

{

i There is no_ specific requirement in Reference 2 for measuring

~

concentration of oxygen in the reactor coolant, although Criterion 4 of j

this reference recommends that oxygen concentration be determined. The change proposed in the report (Reference 1) deletes the requirements for i

oxygen analysis'and instead specifies that it be obtained.by the-I calculation based on the oxygen concentrations in the Reactor Water Tank or containment atmosphere. Since Reference 2 does not mandate for PASS-to have the capability for measuring oxygen concentration ~1n the reactor-

.j coolant, and there is a provision for obtaining this information by i

indirect means, the change proposed in the report is acceptable.

2.6

-Samole Points Reauirement Specifications concerning sampling by PASS are contained in_ Criteria 1 y

and lla of Reference 2.

Criterion 1 requires for PASS to have a j

capability for obtaining reactor coolant and containment atmosphere samples and Criterion lla specifies that these samples be representative I

of the reactor coolant in the core area and'the containment atmosphere i

following a transient or accident. The report i

e

-i

5 i

(Reference 1) clarified this requirement stating that the required sample points in PASS.are only those needed for taking containment i

atmosphere and reactor coolant samples. This statement does not contradict the requirements of Reference 2 and is, therefore, acceptable.

i 2.7 Samolino Time Reauirement Criteria 1 and 2 in Reference 2 specify that PASS should have a capability to obtain and analyze reactor coolant and containment samples within three hours from the time a decision is made to take a sample.

As the decision could be made anytime during the accident management.

phase, PASS should be designed for obtaining analytical results within three hours from the beginning of the accident.

Reference 3 provides clarification that the three hour time limit is not a rigorous requirement and a small departure from this value is permissible. Also, the specified time period applies to the first sample only and it does l

not include the time required for sampling preparation (i.e., suiting up, radiation measurements, etc.).

)

In the report (Reference 1) a statement is made that the time requirement is only a recommendation applying to the first sample only.

The statement implies that the licensee is at liberty to provide PASS sampling results at any time during the accident. This was not the intention of the clarification in Reference 3.

A specified time limit for providing PASS sampling results is still a valid requirement.

However, the NRC is in process of revising some of the time limits specified in Reference 2.

For PWRs, the new time limits for taking the samples and analyzing them for dissolved gas and activity could be extended to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, and for boron to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, after the end of power i

operation. Should the Commission approve-this revision, the change in sampling times stated above would be acceptable.

The reason for the proposed extensions is the availability of other sources of post accident information which may satisfy immediate needs of the operators.

In the case of dissolved gas analysis, the PASS results are needed to provide only a confirmatory evidence on the potential for formation of bubbles of noncondensible gases in the reactor vessel that could interfere with core cooling. The information on core cooling could be obtained by other instrumentation, e.g. reactor vessel water level, which although less reliable, could provide, in a much shorter time, an estimate of the thermal-hydraulic conditions in the reactor vessel.

Similarly, the analyses of PASS water samples for activity are needed j

only to confirm the information on the degree of core degradation obtainable by other means, such as high-range containment radiation

)

monitors, or core-exit thermocouple readings. These instruments give the operators usually enough information to manage the accident in its initial stage.

- - - _ ~

S 1

6 Information on accurate boron concentration in the reactor coolant-is essential in preventing criticality during.the degraded core accident.

It is needed relatively soon (during accident management phase) to

-verify the operators'. estimates.of boron. concentration that are based on mixing ratios.- After reactor' isolation this Linformation could not be obtained from the process sampling system and PASS becomes.the only means of providing direct' boron concentration..However.for the plants with the neutron flux instrumentation complying with the Category 1

+

criteria of Regulatory Guide 1.97, i.e. having fully qualified,=- _

1 redundant channels that have capability to monitor.the power. range of.

10E-6 percent to full' power,.enough information'could be gained from the

. instrumentation, and for these plants there -is a good justificationL for!

postponing measurement of boron concentration to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> after the end' of power. operation.

In view of considerations discussed above, the modifications to thel time i

requirements-proposed in the -report (Reference' 1) are not' acceptable.

l The time requirements fnr. making dissolved gases, activity and boron:

concentration determinations should remain unchar.ged from being those.-

specified in Reference 2.

However, as-. discussed in. Reference 3, ; slight departure from the specified time requirements. is permissible... Future revision and relaxation of.these. requirements would have to be= approved by the NRC.

3.0

SUMMARY

AND CONCLUSIONS The intentions of the clarification and modifications presented in this topical report is to ideitify alternative methods of meeting the intent i

of the requirements in Reference l2.

In general, the proposed changes represent departure from using LASS as the-primary means of obtaining the post accident information on reactor coolant and containment atmosphere and relying on the information provided ' by other safety.

grade equipment. The report provides clarifications in the following areas: pH measurement.of reactor coolant, heat; tracing of-sample lines, oxygen analysis of reactor coolant and. sample' points requirement. These clarifications provide an interpretation of the requirements presented in Reference 2.

They do not depart from these requirements and are, therefore, acceptable. On the other hand,ithe modifications proposed-in the report (Reference 1) are not acceptable because they include the changes which in two cases significantly alter the-intent of the original requirements.

The modification consisting.of deletion of measurement of hydrogen concentration in the containment was= justified by existence of containment monitoring system and it was accepted by.the staff in the previous licensing action.

i i

-....~

...---,.:.~..

J.-

-~..L.,...,.

,,,,. i

S

'7 Use of hydrogen / total gas measurements asia backup to other methods =for.

determining conditions in the reactor vessel was found not acceptable because reliable estimate of gas release in the reactor vessel could'be provided only by direct measurements of disscived gas concentration..

However, it is acceptable to take coolants samples ;using normal l

sampling system, provided radiation exposure'requireme,ts are met.

The modification to consider time requirements for performing PASS analyses as only a recommendation which would allow PASS sampling results to be obtained any, time.during the accident is'not: acceptable.-

l The time requirement-for taking first: sample should be in conformance with the requirements specified in Reference 2. However,cin the future, subject to the Commission approval,'this requirement may undergo some-changes.

4.0 REFERENCES

1)

Report CEN-415, Revision 1,." Modification.of' Post Accident Sampling?

System Requirements", Prepared for C-E Owners Group by ABB Combustion 1

Engineering, Inc., December 1991.

2)

NUREG-0737, " Clarification of TMI Action Plan Requirements", November 1980,-Items.II.B.3 and II.F.1.

3)

Letter from C. Y. Cheng (NRC) to W. T. Wagner'(PASS Owners'. Group),

dated May 3, 1990.

4)

Draft Commission Paper, " Issues Pertaining to Evolutionary and Passive

)

Light Water Reactors and Their Relationship to Current Regulatory i

Requirement", Item I, Attachment to the Letter from.D. M. Crutchfield (NRC) to E. E. Kintner ALWR Steering Committee), dated February 27, 1992.

• y w

y

,<n-E w,

.--we, 4

m.'

. - -.r,

= -- 42

-.4o e -

.