ML18086B036

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Positive Moderator Temp Coefficient Rept.
ML18086B036
Person / Time
Site: Salem PSEG icon.png
Issue date: 08/07/1981
From:
Public Service Enterprise Group
To:
Shared Package
ML18086B035 List:
References
NUDOCS 8111170347
Download: ML18086B036 (9)
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Text

PUBLIC SERVICE ELECTRIC & GAS COMPANY SALEM NUCLEAR GENERATING STATION UNIT 2 POSITIVE MODERATOR TEMPERATURE COEFFICIENT REPORT (TECHNICAL SPECIFICATION 3.1.1.4)

SUPPLEMENTAL REPORT 8-7-81

  • Written by=~-~=;;:::;3-~~~'=-~~~~~~~~*-..,..._.--,-~---.-

(Reactor Engineer)

Approved by:~__,./~/~~--+tf.~*-~---.~-*-~~~--L-c...._1.....-~

(Stait.ion Manager) re111170347085010o6~11\

', ~DR ADOCK PDR I

UNIT 2 MODERATOR TEMPERATURE COEFFICIENT I. PURPOSE The main objective of this document is to establish rod withdrawal limits for SALEM UNIT 2 Reactor.

II. DISCUSSION:

A. During zero power physics testing, the All Rods Out

. (ARO) Moderator Temperature Coefficient (MTC) was measured to be +l.19pcm/°F. This measurement is in violation of Technical Specification 3.1.1.4 which requires the ARO MTC to be less.than or equal to zero. The action statement of the Technical Specifica-1 tion allows operation in Modes 2 and 1 provided Rod Withdrawal Limits are established which ensure that the MTC remains negative. The following computations establish rod withdrawal limits for Hot Zero Power (HZP), no Xenon or Samarium conditions through 50%

Rated Thermal Power where rod withdrawal limits are no longer required.

B. Computations Data which. follows was used to construct a graph of MTC versus Critical Boron Concentration . (Be) and to determine

- a maximum control bank D height to ensure a MTC less than or equal to zero for initial operation.

.. * ,\,,, L :, --~*-* **~*~***.::...*-* . . . . . . .",_ * *** ,

Table l Moderator Temperature Coefficient vs Rod Position and Boron Concentration BANK POSITION Be MTC ARO 1329 ppm +l.19 pcm/°F DIN 1197 ppm -2.48 pcm/°F A line formed by the two points in Table l crosses the Be axis (MTC-0) at Be - 1285 ppm thus defining a Be which corresponds to a Zero MTC. This line also defines a maximum control bank D rod height which calculated was as follows:

1329 ppm - 1285 ppm = 44 ppm ~ Be; ARO-~ MTC )

. 44 ppm x 10.25 pcm/ppm= 451 pcm where 1329 ppm is the ARO HZP No Xenon or Samarium Boron Concentration and 10.25 pcm/ppm is the diffe-rential Boron worth 451 pcm corresponds to*.102 Steps withdrawal on Control

~ 1 ~ - 1ti~ ~ l 1-t.,;-L,:'lS'

  • Bank D. H7C { ..1.

. "'--' 0 '*

A subsequent MTC measurement with Control Bank D = 102 Steps and Be - 1285 pcm yielded a MTC of -0.82 pcm/°F.

For initial power operations~ the maximum permissible height for Control Bank D at HZP (No Xenon or Samarium) will be 102 Steps.

The Be at which the MTC will be less.than or equal to zero with ARO can also be determined from the data and corresponding graph.

The ARO slope is obtained from Westinghouse Nuclear Design Manual.

The ARO, Zero MTC point.defines a Be= 1200 ppm. Below this Be, rod withdrawal will no longer be restricted. Using the two Be's (1285. ppm and 1200 ppm) and their corresponding rod heights that define a zero MTC, a Control Bank D withdrawal limit curve was made which ensures a negative MTC is maintained under all conditions resulting from increases in power of 3%/hr (the curve is attached).

The curve is based on reactor power defect and poison (Xenon and Samarium) buildup off setting Control Bank "D" withdrawal while the critical Boron Concentration is diluted at a rate greater than 0.7 ppm per step withdrawn to maintain ~riticality while increasing power.

The poison credit results from a ~teady power increase (Zero to 47% RTP) of 3%/hr starting with a zero poison inventory. Subsequent startup at a slower rate or with some power history would increase the Xenon and Samarium concentration; therefore, the withdrawal limit is considered conservative.

As stated earlier, a critical reactor with a Be 1200 ppm will have no withdrawal limit. A burnup has been calculated at which Be will be less than 1200 ppm. Using the 100% power, ARO, equilibrium poison critical Boron Concentration curve provided in the Westinghouse Nuclear Design Manual and using conditions which dictate the maximum critical boron (0% power, no reactivity from Xenon) the burnup was found. After a burnup of 4000 MWD/MTU there will no longer be a rod withdrawal limit. Another analysis was completed including Samarium and the burnup was found to be less than 4000 MWD/MTU. The 4000 MWD/MTU burnup will be used unless a subsequent measurement shows the ARO, Zero Power, Zero Xenon MTC to be negative before this burnup.

~* .... . ....... -*. . - . -** -* ******-**-*----***-* .

III. REACTOR ENGINEERING SUPPLIED LIMITS A. Discussi*on Startup after a trip or shutdown requires that an Estimated Critical Position (ECP) be calculated prior to the actual critical approach. The Rod Withdrawal Limlit calculated in the first section of this document (102 Steps) to ensure a non-positive Moderator Temperature Coefficient (MTC) at zero power severly limits the possible range of critical rod positions. This limitation is due to the withdrawal limit on one end, the insertion limit on the other, and the 500 pcm uncertainty imposed by procedure.

Therefore, an ECP must be picked such that with uncertainty, it would not violate the rod insertion limit (i.e. the concern is achieving criticality below the rod insertion limit). Sec-ondly, during power ascension, the fact that the rods are inserted a substantial distance into the core causes a large flux offset which exceeds the Axial Flux Difference (AFD} Limitation of Technical Specificication 3.2.l and imposes the Action Statement 3.2.1.1.C. This statement requires that the AFD be within its' band for 23 hours2.662037e-4 days <br />0.00639 hours <br />3.80291e-5 weeks <br />8.7515e-6 months <br /> of the past 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> prior to escalation of power above 50% RTP (RTP = Rated Thermal Power).

B. Computations To mitigate the effects described in Paragraph III, A, it is possible to calculate a Rod Withdrawal Limit based on the actual conditions of the reactor*core at the time of criticality, at the time of power ascension, or at the rate of power ascension.

As discussed in Section II, the Rod Withdrawal Limits assumed no poisons (Xenon and Samarium) at time of Criticality. By obtaining the poison concentrations of the core through the power history data maintained by Reactor Engineering, the

-- ..;....:.*- *- --~*;

  • . -~* ' ' ..... ~ . ... ..

projected concentrati'.ons and worths. for both. criti:ca,li.ty and power operation can be obta+/-ned through. the i;sotopi:c decay and buildup equations (See. Lamarsh.,. John R.; NUCLEAR REACTORY THEORY I Addison-:-We.sley Publishing. Co*., rkc. ,, Reading r Mass*. I 1966, pp 468.,-4 781 ~- power de.feet, differential boron worth., and rod worths are obtained from the Westinghouse Desi'.gn Report for Uni.t I:I, Cy-cle I., WCAP...-9374.. Knowing Boron Concentration, poison worth., rod worth., and power defect, a reactivity* balance can be constructed. to obta.in a rod withdrawal lim.it at zero power a.s follows:

Eq~ation 1: ..,.~ = CO-Pdref) + (_Pc-Pref)_ + (1200 ppm -*BrefJ

  • DBW- '"'"Rref Where:

/

= Rdd Worth. for Withdrawa-1* limit ..

= Power defect at conditions immediately prior to reactor trip.

p .

ref

= Poison worth. at conditions immediately prior .to reactor trip.

= Rod worth. at conditions immediately pri.or to reactor tl'i'P *.

Bref = Boron Concentration at conditions* imtnediate.ly pri.or to Rea,cto,r trip ..

DBW - Differential Boron .Concentration Pc = Poi.son worth. at criticality NOTE; *1200 ppm i.s used as the critical Boron Concentration due. to the fact that at 1200 ppm a non-positive moderator coefficient. is assured (See Section !:I.I"-

Th+/-s adds conservatis.m to the calculation-.~.

- 5 ~

-**** .... -~-' .. -- .__ -* .:-..

  • The withdrawal lim~t ca,n be calculated for any time that th.e reactor w+/-.11 90 critical *.

A s~'In+/-lar reacti'Yity balance can be. do.ne to calculate the ROD WI:THDRAWAL LI:MI.T at any time during operation after cr:i:t:i:.cal:i:.ty through. 50% RTP (See.Section II) . The reactivi.ty balance i.s as. follows:

Equation 2; ... ~ = (_Pd Ctl ... l?dref) + *(_p Ctl ~- Pref) +

  • DBW ... R ref Where:

R. Pd ref, Pref, B ref, DBW, R ref are defined

-L, the same as equation 1.

Pd (t) = Power defect at time "t" P (t) - Poison worth at time "t" t = a projected time of interest '

NOTE: 1200 ppm is the Boron Concentration at and below which a.non-positive MTC is assured (See Section II)

  • Using Equation 2, the withdrawal limit can be calculated as a function of time and rate of power increase. Therefore, from a projected power history, a minimum rod withdrawal limit can be obtained. Attached is an example calculation of ROD WITHDRAWAL.LIMITS.

It should be .noted that this method of calculation applies to specific-reactor core conditions. Any time that a result from these equations is less than the value of the Rod With-drawal Curve of Section II, the Rod Withdrawal Curve** takes precedence since it applies to all conditions.

Reactor Engineering will do calculations and supply Rod

  • Withdrawal Limits on demand.
  • ....l*.

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  • THIS LIMIT ASSURES NEGATIVE MTC IN COMPLIANCE WITH ACTION STATEMENT OF TECH. SPEC. 3.1./.4

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ROD WITHDRAWAL LIMIT CALCULATION Ca.lculci:ted: s, 10,91 for Unit 2 Reference Critical Conditions:

Date: 9, 10, 81 Ti pie: 00. 00

  • RCS Boron cone= 929PPPI Cor1t rol Bank D at 228 stePsr Worth =
  • 0PcM Rea.ct.or Power= 70% RTP Defect=-1057PcM Xenon Worth =-2655PcM Sa Ma. r i Ul"1 Worth = -498PcM Rs*a.ctor Subcri t. ic*a.i *~~t -94, 12 on 81 10, 81 Xenon Gone = .2072400*E16 atbt'ls/cc Iodine cone= .* 4~60800*E16 atoMs/cc Sa.r<ta.riuM cone= t.7729300*E16 atoMs/cc Pr0Nethi1.1f'1 cor1e= .8747300*E16 atoMs/cc

~-------------

T~e latest tir<te at 0%Power and no withdrawal liPlit is 00.34 on 8112181 Pwr Bdif' Ch<:1 Withdrawal Lif'1H DATE _____ Ti Me _Pl*.' r ___ Deft __ cb __ :..wo r ___ Rx8 ___._xe ___ s1'l--RxNeed __ Bnk-Pos it ion 8 10 81 2233 . 0. 0 +0 1200 -10, 6 -2874 -3267 -553 +2485 D 228 8 10 81 2303 0.0 +0 1200 -10.6 -2874 -3196 -554 +2415 D 228 8 10 81 2333 . 0.0 +0 1200 -10.6 -2874 -3125 -555 +2345 D 228 8 11 81 0003 0.0 +0 1200 -10.6 -2874 -3054 -557 +2275 D 228 8 11 81 0033 0.0 +0 1200 -10.6 -2874 -2984 -558 +2206 D 228 8 11 81 0103 0.0 +0 1200 -10.6 -2874 -2913 -559 +2137 D 228 8 11 81 0133 0.0 +0 1200 -10.6 -2874 -2844 -560 +2069 D 228 8 11 81 0203 0.0 +0 1200 -10.6 -2874 -2775 -561 +2001 D 228 8 11 81 0233 0.0 +0 1200 -10.6 -2874 -2707 -562 +1934 D 228 8 11 81 0303 0.~ +0 1200 -10.6 -2874 -2639 -564 +1868 D 228 8 11 81 0333 0.0 +0 1200 -10.6 -2874 -2573 -565 +1803 D 228 8 11 81 0403 0.0 +0 1200 -10.6 -2874 -2507 -566 +1738 D 22:::

8 11 81 0433 0.0 +0 1200 -10.6 -2874 -2442 -567 +1674 D 228 8 11. 81 0503 0.0 +0 1200 -10.6 -2874 -2379 -568 +1612 D 228 8 11 81 0533 0.0 +0 1200 -10.6 -2874 -2316 -569 +1550 D 228 8 11 81 0603 0.0 +0 1200 -10.6 -2874 -2254 -571 +1490 D 228 8 11 81 0633 0.0 +0 1200 -10.6 -2874 -2194 -572 +1430 D 228 8 11 81 0703 0.0 +0 1200 -10.6 -2874 -2134 -573 +1372 D 228 8 11 81 0733 0.0 +0 1200 -10.6 -2874 -2076 -574 +1314 D 228 8 11 81 0803 20.0 -280 1200 -10.6 -2874 -1964 -575 +1483 D 228 8 11,81 0833 20.0 -280 1200 -10.6 -2874 -1864 -575 +1384 D 228 8 11 81 0903 26.0 --364 1200 -10.6 -2874 -1760 -576 +1364 D 228 8 11 81 0933 26.0 -364 1200 -10.6 -2874 -1669 -576 +1274 D 228 8 11 81 1003 32.0 --449 1200 -10.6 -2874 -1577 -577 +1266 D 228 8 11 81 1033 32.0 ~~448 1200 ""'.10.6 -2874 -1498 -577 +1188 D 228 8 11 81 1103 38.0 -532 1200 -10.6 -2874 -1421 -577 +1196 D 228 8 11 81 1133 38.0 -532 1200 -10.6 -2874 -1358 -577 +1133 D 228 8 11 81 1203 44.0 -617 1200 -i0.6 -2874 -1298 -578 +1157 D 228 8 11 81 1233 44.0 -617 1200 -10.6 -2874 ~1251 -578 +1110. D 228 8 11 81 1303 50.0 -702 1200 -10.6 -2874 -1207 -578 +1152 D 228 8 11 81 1333 50.0 -702 1200 -10.6 -2874 -1176 -578 +1121 D 228 8 11 81 1403 56.0 -786 1200 -10.6 -2874 -1148 -577 +1177 D 228 8 11 81 1433 56.0 -786 1200 ~10.6 -2874 -1133 -577 +1161 D 228 8 11 81 1503 62.0. -869 1200 -10.6 -2874*-1119 -577 +1230 D 228

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