ML20203K854

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Rev 0 to Training Lesson Plan LO-LP-36103-00-C, Ex-Core Instrument Response to Core Damage
ML20203K854
Person / Time
Site: Vogtle  Southern Nuclear icon.png
Issue date: 04/23/1985
From: Brigdon R, Scukanec D
GEORGIA POWER CO.
To:
Shared Package
ML20203K798 List:
References
LO-LP-36103-, LO-LP-36103-00, NUDOCS 8608210382
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REASON FOR REVISION:

MAJOR REVISION DUE TO ERRORS OR OMISSIONS.

REVISION DUE TO CHANGES IN EQUIPMENT.

X REVISION DUE TO CHANGES IN FROCDURES/ OPERATING INSTRUCTIONS OR FOLICT.

DESIRE ADDITIONAL GRAPHICS /RANDOUTS FOR THIS TRAINING MATERIAL OrnER COMMITTMENTS: TES X. NO DRAFTING REQUEST FILLE OUT. TES NO X DATE SUBMITTE FOR SUPERVISOR'S REVIEW 7/1'Z.

REVIEW SAT UNSAT AFFROVAL SIGNATURE - DATE ol

          • DATE NEEDG FOR IW FROM TTFING 7 [ *****

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TRAINING LESSON PLA TITLE: EXCORE INSTRUMENT RESPONSE TO CORE DAMAGE NUMBER- 2202 u_ p-w.c3-w s PROGRAM: "'Mi E C gO - M % , % REVISION: #o  !

RICHARD D. BRIGDON AUTHOR: DATE: !23/5 APPROVED: ((Mgg DATE: //.13//f

/ i

REFERENCES:

l

l. NUREG 0737; ITEM II.B.4 -

q, MITIGATING CORE DAMAGE, " RESPONSE OF EXCORE INSTRUMENTATION " WESTINGHOUSE l ELECTRIC CORPORATION g, MITIGATING RZACTOR CORE DAMAGE, "EXCORE INSTRUMENTATION," GENERAL PHYSICS CORPORATION q.MCD TRAINING, VEGP FSAR, CHAPTER 13; ITEM 13.2.1.1.6 INSTRUCTOR GUIDELINES:

HANDOUT: "EXCORE INSTRUMENT RESPONSE TO CORE DAMAGE" ,, ,

TRANSPARENCIES:

ooi ps etose sen,,- e ust

  • e6% e s LO-TP - %%C3 -00 4 ~~

4& 4F-044-4ceLSRD RADIAL LOCATION r SR-78-969-acob SRD ELEVATION SA-TP-069--3co'4 TYPICAL SOURCE RANGE POWER DECAY CURVE SA-38-044-4 oos CORE BOILING CRISIS IHt-TP=062-Sect. PARTIAL CORE UNCOVERT 4E-TP=082=6oc7 SED RESPONSE TO HOMOGENEOUS VOIDING N eof SRD LOCATION RELATIVE TO WATER LEVEL IN DOWNCOMER AND CORE i

' 84-TP-066-4401 SRD RESPONSE TO DETECTOR LOCATION AND DOWNCOMER WATER LEVEL l gemso SRD RESPONSE TO C001. ANT DENSITY V.'JtIATIONS l

SR-TP=66t=t00*l TMI-2 ACCIDENT TRACE N188tSRD RESPONSE TO VOIDING 1

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1. PURPOSE STATEMENT:

THIS LESSON DESCRIBES THE RESPONSE OF THE EXCORE SOURCE RANGE NUCLEAR INSTRUMENTS TO VARIOUS POST ACCIDENT CORE CONDITIONS.

11. LIST OF OBJECTIVES:

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1. Describe the normal y response to post trip conditions.
2. Describe the NIS response to different core void fractions.
3. Describe the similarities and differences of SRD response to varying void fractions in the core and downconer.
4. Describe the effects of ore voiding on reactor kinetics.
5. Describe the factors that affect reactor recriticality and explain how to use the SRD to determine if recriticality is taking place.

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lil. LESSON OUTLINE: NOTES I. OVERVIEW A. Purpose of Lesson

1. Review normal shutdown power lavel indication Lc-TP-Wc3 3-C oc e
2. Discuss effects of core voiding on excore o &Itwo inscrumentation
3. Recognizing indications of return to critical conditions.

B. Reason for Lesson

1. TMI-2 accident provided information relating excore response to various void level conditions in the core.
2. TMI tentative conclusion - SR count level can be closely related to actual core water level.

C. Lesson Subje'et Ketter

1. NormalSRMIheponsefollowingtrip.
2. SRNI response for core voiding
3. SENI indications predicting core recriticality II. REVIEW: SOURCE RANGE NUCLEAR INSTRUMENTATION A. General System Description '
1. Detectors
a. Two located at o' and 180* locations outside en-9P-969-1 reactor vessel. TP - oo 4.
b. BF p tional counters located in lower SE U 000-2 y core TP- cos

,7,'-.*._

si. thermal neutrons through inter-

~.h i h BF fill gas, til b k ;.3>e.,;u+++.;e++.qa.

d. Also sensitive to gammas via photoelectric, compton scattering and pair production i

l reactions.

l 2. Instruments

a. Two independent channels 3

g P W Cl-cw C

_ _ .  : _ ee2-lil.

LESSON OUTLINE: NOTES

b. Measurepowerincpsepica11ginsixdecades of range from about 10 to 10 cps.
c. 100 cps approx. equals 10.13 percent power B. Technical Specifications -
1. Modes 3. 4. 5 - 1 instrument required for monitoring shutdown neutron level. f. 5 D/-l. 5 1
2. Mode 2 - Startup - 2 instruments required
3. Mode 1 - Instruments deenergized to protect detector from high neutron flux conditions.

C. Normal Response to reactor trip  : '" ?? ? -- 3 1.

TP-cot Immediate prompt drop ogseemt i

s. Removal of prompt neutron fraction
b. Very low delayed neutron fracti owever delayed neutrons still support the fission .: _

rate.

2. Rapid Decrease in neutron level
a. Short lived precursors die away
b. Time typically 4 minutes
3. Steady Decrease '
a. Determined by longest lived delayed neutron precursor group (80.6 sec).
b. Indication is approxia Lely equal to .33 um SUR
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4. Operators must understand normal shutdown characteristics
a. Deviations from normal indicate i

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1) Abnormal shutdown conditions OR l 2) Instrumentation problems 4

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lil. LESSON OUTLINE: NOTES

b. Remaining analysis is this lesson assumes no instrumentation problems: SRNI indicates actual core conditions.

III. SOURCE RANCE DETECTOR (SRD) RESPONSE UNDER ACCIDENT CONDITIONS A. Overview

1. SRD indications extremely useful during accident since:
a. Normal response is normally well defined.
b. Continuous SRD output recording is available
c. Detectors are located outside the core and likely to remain intact.
d. Analytical tools exist for relating SRD
response to incore conditions
2. The dis'cussion will includes u -
a. Effects of voiding on SRD response
b. Effects of no forced coolant flow
c. Differentiation of partially and fully voided core conditions.
3. Assumptions ,
s. All rods inserted on reactor trip 48-TP-064-4 TP-oo.s
b. Core pressure and temperature conditions exist to allow bulk boiling in the core.

, B. Shutdowa neutron source effects on SRD's.

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sources rc..

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2) Sb-Be
b. Intrinale sources 9 se r
1) (S n) reactions  %+9*-*4g 8
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2) Spontaneous fission 5

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111. LESSON OUTLINE: NOTES

2. Source effects under accident conditions
a. SRD more sensitive to sources on core perimeter than those inside core .
b. SRD will be more accurate for' uniform core S* " ^^ M voiding - less sensitive to localized abnormal TP-oos conditions near core center.

i C. Core Voiding Effects on Reactor Kinetics

1. Voids created during LOCA produce several competing O effects.
a. Boron displacement - contributes to increased localized fission rate
b. Water density decreases - reduces moderation
c. Increased leakage - higher flux read by the I

SRD's.

2. Subcritical Multiplication ,, _
a. Determines equilibrium shutdown neutron level following shutdown.
b. N= S 1 - K,gg
c. ,N" changes if core factors cause:
1) Core reactivity changes (K,gg)
2) Source strength changes D. Voidine Malatinnehip ta %beritical !!nitiplicatics
1. SRD response endence on core voiding Sa=33-033 ,,g'7 TP-oo7 a." terior voiding is shielded from degnetese#fno significant response)

< eg i by.

voiding causes a significant shange SRD response

c. Therefore, SED response is dependent on the degree of core voiding.
2. Void fraction effects on SRD output ON"#' d
a. Low void fractions - limited effect on SRD output

. 6

%3' a. m w . g .ch ic3 -co-C-4R= 1 r-vu2 Ill. LESSON OUTLINE: NOTES

b. Moderate to high void fractions
1) K decreases due to loss of moderation IIakageincreasesduetolowerattenuation
2) Leakage overrides loss of moderation results in overall increase in indication
c. Very high void content (more than 60 percent)
1) Loss of moderation offsets increased leakage
2) Results in decrease in indication NOTE: Actus1 response of SRD's will vary with boron concentration, detector efficiency, .

and local core effects.

3. Modification of SM equation for above effects
a. SRD = S x Attenuation factor t

1 - K,gg 4: .

b. Attenuation accounts for factors affecting leakage and neutron moderation.
4. SRD response

.; s. SR level initially increases as void fraction increases and then decreases when void fraction becomes exceptionally large. '

E. Non-Homogeneous Voiding Rffects on SRD Response i

1. RCF affects on voids N T- oole
a. RcFs remains
s. hzw- voids throughout the core and r are uniformly distributed ogqc, 3

. ous)

M %'q _ respond to changes in dowucomer 4eT A 2, density

b. RCMs stopped
1) Steam and liquid separates (non-homogeneous)
2) Upper regions of core and downcorar may fill with steam, lower cooler areas fill with water.

7

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0" '." 00 2 Ill. LESSON OUTLINE: NOTES

2. Core characteristics
a. Lower core while still covered will act as neutron source for upper core
b. Upper core reactivity very low due to low moderation and high leakage races.
c. As lower core uncovers, neutron source strength will drop.
3. SRD response is dependent upon
a. Detector location "?-T? 002 7 TP- ce B
1) In bottom plane of core (2-3 foot level)
2) Insensitive to voiding in upper core regions
b. Downconer and core water level S&-9P-60t=8-

'Tf-0o4

1) goth essentially the same* 4: *Downconer level may be slightly higher
2) Level must drop below 1/2 full to cause a significant change in SRD response
3) The most significant effect on SRD output is the degree of voiding in the downconer.
4. SRD Characteristics
s. Downconer acts as shutter
1) Its level determines the region of the core the SRD is able to see.

a) Above level 1 - SRD response changes SE N are insignificant g*M 1rf -oo9 fg-ky r low level 2 - leakage neutrons avean be seen by the SRD's therefore

";,s{ edG *^ M avel undergoes large change A

b ; ..

,n  ; buttered effect was present at 1981-2.

Example Calculation Downconer Effects on SED Response A. SRD response to neutron level in the core can be 45-33-464-9 estimated by: 'TP-OLO D=D* O

. 8

e. d- N C3 -cc - C C" * " 002-ll1. LESSON OUTLINE: NOTES where: D = SRD output (cps)

DO = Core neutron level (cps) u = pure water attenuation factor (.124 cm_l) t = width of downcomer (ca).

B. Case 1

1. Assumptions
a. Downcomer full = 31 cm
b. Upper core neutron level = 1000 eps
2. What the detector sees:
a. D=De 0

= (1000 cys) e (.124 ca-1)(31 cm)

D = 21.41 cps

b. - 47 C. Case 2 .: _
1. Assumptions
a. Downconer half full = 0 cm
b. Upper core neutros level = 1000 eps
2. What the detector sees
a. D=D* O D=D O" CPe
b. D, , 1 D

D. Concluetoms a A.

I'.

'~

, response increased by factor of downconer level decreased.

2. I completely empty
a. Factor would apply to totsi SRD response
b. *SRD output would increase by greater than
  • Conservative since 100* (with increased detector efficiency due this problem assumes to voids) pure water and not borated water.

'9

, w_u,- w o-=-u OR-Il-G62 e Ill. LESSON OUTLINE: NOTES E. SRD Response to Coolant Density Changes

1. Recall SRD = S x Attenuation Factor 1 - K,gg ,
2. Tables (Normalized to 560*F and 2200 psia)
a. Table 1 - S x Attenuation Factor SE TP 000 it TP- o s s
1) Shows effects of downconer and core coolant density on the effective source strength and attenuation factors of both fluids
2) As density decreases - SRD output increases *
3) SRD response more sensitive to downconer density changes
b. Table 2 - K eff  ?! "" 0"* 12 1)

Tt-oi4 Shows effect of downcomer and core fluid .: _

density on core K,gg

2) As voids increase, K,gg decreases.
3) K les

. cIkkges.scensitivetodownconerdensity

3. Both Tables can be used to evaluate the SRD response for all cases of non-homogeneous core voiding '

Example: Estimate the SRD response for the non-homogeneous case of f c = 0.25 and Df = 1.0 From Table 1: S x Attenuation Factor = 1.82 Table 2:. k = 0.79 f .,; W tor response is:

-[ f.g . F47.'

.?.A./ . 1' h = 6.25

} (. . s 1-0.84

. . :.i - Sd.S$C The percent chsage in SRD response would be:

1.82 SRD = 1-0.79 - 8.66 = 1.38 SRD 1 6.25 1-0.84 Or the source range detector response would increase by 38 percent. 1

)

10

w. J - 3 m c3 - co - c.

Ill. LESSON OUTLINE: NOTES Example:

Calculate the SRD response for [D = 0.0 and f C

= 1.00 Table 1: S x Attenuation Factor = 1.13 x 10 3 i Calculation left to Table 2: K,gg = .82 -

student l

3 1.13 x 10 SRD = 1-0.82 = 6.28 x 103 = 1 x 10 3 l SRD 6.25 6.25 The latter calculation clearly demonstrates the source range detector is far more responsive to downcomer conditions then core conditions. I i

IV. THREE MILE ISLAND SOURCE RANGE TRACE NOTE: Considerable effort made to relate chronological (O7t.,W' 7 ,

events to postulated core conditions by close analysis og g t of the source range instrument trace.

A. For the first 20 minutes, source range instrument behavior is consistent with a normal posttrip decay ,,

rate of about one-third decada per minute. _

B. After approximately 20 to 30 minutes, the source count rate should be decreasing through the 600-700 counts par second (cys) range. Instead, the curve has leveled out at about 5000 cps due to buildup of voids (steam bubbles) in the downconer sad core regions. This is consistent with the fact that pressure has reached saturation (approximately 6 minutes after turbine trip). '

and net outflow through the open electromatic relief valve continues to empty the system. Void formation is also consistent with the observed drop in reactor coolant flow rate because of the reduced pumping head produced by two-phase flow conditions (not shown).

C. Continued loss of coolant from the primary system leads i to increased N count rates. The recording begins l to exhibit. asfamif%h is reflective of unsteady flow l (pump surgisig) ami" phase separation characteristic of "slu~g JBewdJ C -- - m increases with time.

D. At 73 3 reactor coolant pumps are turned off by the operator.

E. At 100 minutes the A reactor coolant pumps are turned l off. This causes a flow transient and separation of voids to the upper regions of the system. Voids rising i to the top and coolant fill from the hot lege produce a " solid" water condition seen at the detector. The detector count rate abruptly drops.

11

vc - cP - h s c1 - c o - <_

R-Li-062

' ll1. LESSON OUTLINE: NOTES F. The minimum count rate is suggestive of the fact that the downconer water level is at or near the top of the active core level.

G. Continued release of fluid out of the relief valve begins to boil off inventory from the core and downcomer areas.

Make-up flow, assumed to be in the neighborhood of 18 gallons per minute, is not sufficient to maintain down-comer water level. The core mixture comes within better view of the detector as the water levels drop; the count rate increases.

H. A sharp reduction in the rate of increase is believed to be caused by the reduction of feedwater addition to the A OTSG (approximately at 124 minutes). This reduces condensation in the A loop, leaving the open relief valve as the only pathway for boil-off and the removal of core coolant. The increase in the detector count rate slows, which is believed to correspond to a reduction in the rate of the core uncovery. Further analysis is underway in this area.

I. The sienal level continues to increase slightly as core ,,

uncovery proceeds at a slower rate, approaching a near equilibrium level. It is believed that the maximum count rate coincides with loop A refill to the reactor vessel inlet level. At 142 minutes after turbine trip, the operator shuts the electromatic relief block valve. It is observed that the maximum count rate does not coincide t

with shutting the block valve. Increased make-up to the core (about 36 spa) produces gradual recovery.-

J. Over this period the count rate is decreasing as level in the core rises. The increased core mixture level is facilitated by relief block valve closure.

K. The operator starts rosetor coolant pump 25, sending a slug of cold water into the downconer and essentially filling it.

.s ;:

L. Loop f&ey i es that the pump worked effectively for a leryf . This is corroborated by the abrupq .the source range detector trace, as flow embede downconer fluid moves into the core suhd is *- __ ,"; equilibrium levels are reestablished.

M. High pressure injection flow is initiated at 200 minutes, 8 minutes after the electromatic relief valve is opened by the operator. Coolant passes into the downcomer, filling it. Detector count rates drop sharply.

l l

l 12 i

l l

uc cP - h o p m -C.

L" L" 032 Ill. LESSON OUTLINE: NOTES N. Continued addition of high pressure injection flow begins to quench the core. It is conjectured that the coolant first re-wets the outer region of the core, bypassing the hot center. ,

O. Water entering the core eventually leads to an unstable thermal-hydraulic condition. It is speculated that major portions of the core may have been suddenly quenched with a resulting large amount of coolant flashing to steam, accompanied by possible core and fuel rearrangement. The jump in detector counts may be due to the displacement of fuel and/or sustained voiding of peripheral fuel regions. This phenomenon is under-going further analysis.

General Conclusions

1. SR and IR detector traces were virtually the same.
a. Only difference - trace below 2000 cps be.cause of differences in readout ranges.
b. SR and IR level can be used to monitor "

reactor core and downconer levels. ~

2. Downcomer and core level change characteristics
a. Slowly varying SR level changes correspond to core level changes.
b. Rapidly varying SR level changes correspond to downconer level changes. '

V. RECRITICALITY ANALYSIS AND INDICATIONS A. Core Parameters

1. ARI worth approximately -9000 pcm
2. Yamam ah --

tely -2500 pcm over next 9 to gy .

3.

_ we .. ~

. e 7 to 200*P adds approximately Ts h pen)

'. % _ M M B. Recall from SR-206-TP-3.10

1. Actual power deviated from normal after 30 minutes i 2. Operator initiated scrse to ensure all rods on l bottom. l
3. Boron samples indicated decreasing C "'"

(R0 established immediate boration) B " ""*"

13 l

1

w_cp-wicheo-c.

C-LT-0^2 lil. LESSON OUTLINE: NOTES

a. Core would have still been shutdown by -3%

delta K/K

b. Low boron due to plating out C. Recriticality and void plant
1. Difficult to answer if core geometry changes
2. Keep in mind with high void content positive reactivity addition from cooldown is not credible.
3. Arguments against recriticality -
a. Core already in optimum geometry so any degradation will not result in critical mass
b. Analysis shows recriticality is highly unlikely for substantial geometry changes during fuel slump damage.

D. Recriticalit'y and Non-Void Plant Even with excessive cooldown, trip of reactor with 1.

subsequent addition of Xenon should not result in recriticality.

2. Most probable cause would most likely be boron dilution coupled with cooldown.

VI.

SUMMARY

A. Voiding of core and downconer having differing effects on sorsrce neutron population and attenuation and core  ;

aff*

1. Source and Attamustion l
a. . fact much larger than core

- &.Q facts leakage, not moderation

[" , ?

'^

1 is combination of decrease in

, . and increase in leakage.

a. Core density slightly more effective than downconer.
b. Neutron moderation by downconer is small .

fraction of moderation performed in core.

14

w - c P-wic1-co--c SR-L" 082- i i

lil. LESSON OUTLINE: NOTES B. SRD are more sensitive to downcomer conditions than in-core conditions.

C. Due to location of detectors, core water level cannot be reliably estimated until a large part of the core is uncovered.

D. A qualitative idea of how in-core conditions and down-comer conditions affect the excore instruments can be made.

Figure 11 illustrates the expected effects in the SR S" *" 002-11 for the accident conditions discussed. r f- o a t E. Recriticality must be addressed during accident response

~but it may not be a credible assumption based on conditions shown at TMI.

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AUTHOR / REVISOR S . Srsp ed ( g e rs u ca ,4, 3 ,s u.g

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REASON FOR REVISION:

MAJOR REVISION DUE TO ERRORS OR OMISSIONS.

j REVISION DUE TO CHANGES IN EQUIPMENT.

7C REVISION DUE TO CHANGES IN PROCEDURES /0PERATING INSTRUCTIOES OR POLICY.

DESIRE ADDITIONAL GRAPHICS /EANDOUTS FOR THIS TRAINING MATERIAL OTHER COMMITTHENTS: TES X NO DRAFIING REQUEST FILLED OUT. TES NO X DATE SUBMITTED FOR SUPERVISOR'S REVIEW 7/rZ.

REVIEW SAT UNSAT APFROVAL SIGNATURE - DATE ol

          • DATE NE E ED FOR IV FROM TYPING 7 [ *****

LIBRARY CLERK: M ' -

DATE TO TYFING 7 N DATE TO DIAFTING 4/4 DATE FROM TYPING DATE FROM DRAFIING #/8 COMPILED MATnmJR TO INSTEDCTOR FOR REVIW. DATE l

l INSTEDCTOR REVIEUs TYPING SAT UNSAT i DRAFTING SAT UNSAT

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DATE NEEDED BY

'8UPERVISOR SIGNATURE DATE LIBRARIAN RECEIFT FOR INCLUSION /0UTDATING OF FILES. DATE FILE WORK DONE DATE

e van-Georgia Power Ed.:,%

powen oewenmon osameswr g VOGTLE ELECTRIC GENERATIN TRAINING LESSON PLA TITLE. EXCORE INSTRUMENT RESPONSE TO CORE DAMAGE NUMBER- < 2 2 ^S2-u- p -w.c3 +s PROGRAM: Mw m REVISION: #o AUTHOR: RICHARD D. BRIGDON '23 'Oi DATE: -

APPROVED: ((d ,,, DATE: $/J3//f

REFERENCES:

1. NUREG 0737; ITEM II.B.4 t, MITIGATING CORE DAMAGE, " RESPONSE OF EXCORE INSTRUMENTATION," WESTINGHOUSE ELECTRIC CORPORATION g, MITIGATING REACTOR CORE DAMAGE, "EXCORE INSTRUMENTATION," GENERAL PHYSICS CORPORATION q,MCD TRAINING, VEGP PSAR, CHAPTER 13; ITEM 13.2.1.1.6 INSTRUCTOR GUIDELINES:

HANDOUT: "EXCORE INSTRUMENT RESPONSE TO CORE DAMAGE"

, peerose sv* e w a.ao utt ov- o6sE4cas LO-TP- %\03-00 4-4&-9-063-4 cet SED RADIAL LOCATION r S& N b SRD ELEVATION Sa-T!H HFE-3co9 TYPICAL SOURCE RANGE POWER DECAY CURVE 88-D-062-4oos CORE BOILING CRISIS SR=TP-099-Soct. PARTIAL CORE UNCOVERY 4R-TP=08T=6o07 SED RESPONSE TO HOMOGENEOUS VOIDING

-SR-TEHM>2 +oot SED LOCATION mATIVE TO WATER LEVEL IN DOWNCOMER AND CORE SR-TP=66G-4801 SED RESPONSE TO DETECTOR LOCATION AND DOWNCOMER WATER LEVEL GR-TP=664-9880 SED RESPONSE TO COOIANT DENSITY VARIATIONS GR=TP=00t=2006 TMI-2 ACCIDENT TRACE N 188&SRD RESPONSE TO VOIDING TABLE 1 - SX ATTENUATION FACTOR TABLE 2 - E ER y . ic q Sw:

mwa&.

mg MASTER COPY 1

. t.o - L. e - wc3-co -c.

n '.? ^et I 1. PURPOSE STATEMENT:

THIS LESSON DESCRIBES THE RESPONSE OF THE EXCORE SOURCE RANGE NUCLEAR INSTRUMENTS TO VARIOUS POST ACCIDENT CORE CONDITIONS.

l 1

II. LIST OF OBJECTIVES:

y ..i n,.3-_.1 e g ti nA nf ekt- 1;;;;;, ek. .enA. e v411_ i; ;ti; i.u J;; rdk- eha y core inatrument r- ;:::: E r4=- - ;;;id.-6 situacion unoer alzzarens cuesu.1-LyJ..uiirconditions in the cor= ==^ ^ = a~~-. -

Eu Hiium ^Lj;;;i...

sace M oessant. GAD)

1. Describe the normal g etfr response to post trip conditions.
2. Describe the NIS response to different core void fractions.
3. Describe the similarities and differences of SRD response to varying void fractions in i

the core and downcomer.

~

4. Describe the effects of core voiding on reactor kinetics.
5. Describe the factors that affect reactor recriticality and explain how to use the SRD to determine if recriticality is taking place.

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6 c --P - % es3 co-c CD_TD_nOS lit. LESSON OUTLINE: NOTES I. OVERVIEW A. Purpose of Lesson

1. Review normal shutdown power level indication t.c -TP-Wc3 -cc-C - oc s
2. o &IEm
  • D Discuss effects of core voiding on excore instrumentation
3. Recognizing indications of return to critical conditions.

B. Reason for Lesson

1. TMI-2 accident provided information relating excore response to various void level conditions in the core.
2. TMI tentative conclusion - SR count level can be closely related to actual core water level.

C. Lesson Subject Matter

1. Normal SENI hsponse following trip.

~

2. SRNI response for core voiding
3. SRNI indications predicting core recriticality II. REVIEW: SOURCE RANGE NUCLEAR INSTRUMENTATION A. General System Description '
1. Detectors
a. Two located at 0* sad 180* locations outside C IT 002 i reactor vessel. TP- oo z.
b. BF rtional counters located in lower S"-N 002-2 f.,

core TP-oos A.4;g eYg ,

~ {sWps thermal neutrona through inter-k BF fill gas.

3

.N . ,

, d I B) *- , Li* + fHe+* + 5,0, ,

d. Also sensitive to gesumas via photoelectric, I compton scattering and pair production  !

reactions.

2. Instruments
a. Two independent channels 3

i

l

, e se - wici,- cc -(

~

"" L" 0S2-111. LESSON OUTLINE: NOTES I

b. Measure power in cps typicalig in six decades i of range from about 10 to 10 cps.

10 cps approx. equals 10-13 0

c. percent power B. Technical Specifications *
1. Modes 3. 4, 5 - 1 instrument required for monitoring shut'own d neutron level. f5 3/4.3 1
2. Mode 2 - Startup - 2 instruments required  !
3. Mode 1 - Instruments deenergized to protect l detector from high neutron flux conditions. I l

C. Normal Response to reactor trip " " ' " ^ " "

rP-oo+ \

1. Isumediate prompt drop l ogmot \ '
a. Removal of prompt neutron fraction
b. Very low delayed neutron fractio owever delayed neutrons still support the fission .. ..

rate.

2. Rapid Decrease in neutron level
a. Short lived precursors die away
b. Time typically 4 minutes
3. Steady Decrease '
a. Determined by longest lived delayed neutron precursor group (80.6 sec).
b. Indication is approximately equal to .33 DPM SUR

".' N l g-

_ _r + . 9000 pcm l th C "-' .0125

'7 f;=.0068

4. Operators must understand normal shutdown characteristics l
s. Deviations from normal indicate l

l 1) Abnormal shutdown conditions OR

2) Instrumentation problems l

l 4 l

l

a-a u.a- -

~ ===

lli. LESSON OUTLINE: NOTES

b. Remaining analysis is this lesson assumes no instrumentatic.- problems: SRNI indicates actual core conditions.

III. SOURCE RANGE DETECTOR (SRD) RESPONSE UNDER ACCIDENT CONDlIIONS A. Overview

1. SRD indications extremely useful during accident since:
a. Normal response is normally well defined.
b. Continuous SRD output recording is available i
c. Detectors are located outside the core and  ;

likely to remain intact.

l

d. Analytical tools exist for relating SRD  !

response to incore conditions

2. The dis'cussion will include:

a.

Effects of voiding on SRD response

b. Effects of no forced coolant flow h
c. Differentiation of partially and fully voided  ;

core conditions.

]

3. Assumptions l t

)

a. All rods inserted on reactor trip  !! U 002 i l TP - oo.s I
b. Core pressure and ' temperature conditions exist to allow bulk boiling in the core.

B. Shutdown neutron source effects on SRD's.

1.

M M sources

,= gig.7

=-a w ., sources Ty % #R 1

s ..  ; \

2) _ Sb-Be
b. Intrinsic sources 9 It r
1) ( c4, n) reactiona y + 9*

8

  • 4( +o^

1

2) Spontaneous fission l

l

\

5

~

e -cP - h c3 - c o - c.

C" LT-Go2 -

lit. LESSON OUTLINE: NOTES

2. Source effects under accident conditions
a. SRD more sensitive to sources on core perimeter than those inside core
b. SRD will be more accurate for' uniform core SR P ^ 2-1 voiding - less sensitive to localized abnormal rP - oo S conditions near core center.

C. Core Voiding Effects on Reactor Kinetics  !

1. Voids created during LOCA produce several competing O effects.
s. Boron displacement - contributes to increased localized fission rate l
b. Water density decreases - reduces moderation
c. Increased leakage - higher flux read by the SRD's.
2. Subcritical Multiplication .. _
a. Determines equilibrium shutdown neutron level following shutdown.
b. N= S 1 - K,gg
c. ,N* changes if core factors cause:
1) Core reactivity changes (K,gg)
2) Source strength changes i

D. Voiding Relationship to Subcritical Multiplication

1. SRD responsa endence on core voiding " " " ^ " ' l

..p'.. "W- co7 l

a. $nterior voiding is shielded from 1

$ tersi(no significant response) l 4'fg;;s _5 by ~'

voiding causes a significant l change SRD response '

1

c. Therefore, SRD response is dependent on the 1

l degree of core voiding.

2. Void fraction effects on SRD output OG UE"J" d
s. Low void fractions - limited effect on SRD l output  !

6

w _ g . ch . c3 -co -C.

41b-L r-vc2 111. LESSON OUTLINE: NOTES

b. Moderate to high void fractions
1) K decreases due to loss of moderation leakageincreasesduetolowerattenuation
2) Leakage overrides loss of moderation results in overall increase in indication
c. Very high void content (more than 60 percent)
1) Loss of moderation offsets increased leakage
2) Results in decrease in indication NOTE: Actual response of SRD's will vary with boron concentration, detector efficiency, .

and local core effects.

3. Modification of SM equation for above effects
a. SRD g

= S x Attenuation factor 1 - K,gg ,, _

b. Attenuation accounts for factors affecting leakage and neutron moderation.
4. SRD response
a. SR level initially increases as void fraction increases and then decreases when void fraction becomes exceptionally large. '

E. Non-Homogeneous Voiding Rffects on SED Response

1. RCP effects on voids "O T7-062 ,

T- oo(a i

s. RCPs running y..g1). voids throughout the core and ogqc, 3 ggg_3 ,

r are uniformly distributed

, s y.

je. #& ifbu W oous)

Yi" 2) respond to changes in downconer

/A'. i? f3spd density

b. RCP's stopped
1) Steam and liquid separates (non-homogeneous) 2)' Upper regions of core and downconer may fill with steam, lower cooler areas fill with water.

7

W - LP lei c3 -cc -C.

S" '." 00 2 ill. LESSON OUTLINE: NOTES

2. Core characteristics
a. Lower core while still covered will act as neutron source for upper core
b. Upper core reactivity very low due to low moderation and high leakage rates.
c. As lower core uncovers, neutron source strength will drop.
3. SRD response is dependent upon
a. Detector location e= '_"_ ^^* '

_ TP- oc fo

1) In bottom plane of core (2-3 foot level)
2) Insensitive to voiding in upper core regions
b. Downconer and core water level SR-WP-00t=tr-Tf- oc 1
1) Both essentially the same* ,. *Downconer level may be slightly higher
2) Level must drop below 1/2 full to cause a significant change in SED response
3) The most significant effect on SRD output is the degree of voiding in the downconer.
4. SRD Characteristics
a. Downconer acts as shutter
1) Its level determines the region of the core the SRD is able to see.

a) Above level 1 - SED response changes SA N are insignificant )

T'F -oo 9 i low level 2 - leakage neutrons T,e rA' wncan be seen by the SRD's therefore T j dr.f ** Y ' level undergoes large change

  • e' Y*s..
  1. ng. ~

uttered effect was present at  ;

THI-2.  !

l Example Calculation: Downconer Effects on SED Response A. SRD response to neutron level in the core can be S&-35-069-9 estimated by: TP-O t o l D=D' 0 l

8

e- t.P- Kic3 -ec - c C " '." O S 2 -

Ill. LESSON OUTLINE: NOTES where: D = SRD output (cps)

DO = Core neutron level (cps) u = pure water attenuation factor (.124 cm_y) t = width of downcomer (cm).

B. Case 1 -

1. Assumptions
a. Downconer full = 31 cm
b. Upper core neutron level = 1000 cps
2. What the detector sees:
a. D = D0 e' = (1000 cys) e (.124 co-1)(31 cm)

D = 21.41 cps

b. = 47 C. Case 2 .. _
1. Assumptions
a. Downconer half full = 0 cm

)

b. Upper core f.eutros level = 1000 cps l l
2. What the detector sees '

l

a. D=D*

O D=D cps O"

b. =1 D. Conclusism c;;

l'. M thiis ' ' ~ b response increased by factor of 1 am. downconer level decreased.

, py?. -

p

2. I e completely empty
a. Factor would apply to total SRD response
b. *SRD output would increase by greater than
  • Conservative since 100* (with increased detector efficiency due this problem assumes to voids) pure water and not borated water.

9

. t.c LS - 3 u c T-- cc -u OR-I.F-052 l

.- 111. LESSON OUTLINE: NOTES l

E. SRD Response to Coolant Density Changes

1. Recall SRD = S x Attenuation Factor 1 - K,gg ,
2. Tables (Normalized to 560*F and 2200 psia)
a. Table 1 - S x Attenuation Factor S" *" ^02 it i

TP-o s 3

1) Shows effects of downconer and core coolant density on the effective source strength and attenuation factors of both ituids
2) As density decreases - SRD output increases *
3) SRD response more sensitive to downconer density changes
b. Table 2 - K,gg SS "" ^02 12 1)

Tt - ci 4 Shows effect of downcomer and core fluid .

density on core K,gg

2) As voids increase, K,gg decreases.
3) K les

. cIbses.s sensitive to downconer density

3. Both Tables can be used to evaluate the SRD response for all cases of non-homogeneous core voiding
  • Exsuple: Estimate the SRD response for the non-homogeneous case of J C = 0.25 and f = .0 D

From Table 1: S x Attenuation Factor = 1.82 Table 2: k = 0.79

. aym

'the moreafidttiestor response is:

Aph r "

4.-

ND' =. 1 = 6.25 2

.. 7dv1-0.84

,gy. ,. FC '

N percent chantge in SRD response would be:

1.82 SRD = 1-0.79 = 8 A6 = 1.38 SRD 1 6.25 1-0.84 Or the source range detector response would increase by 38 percent.

t l 10 l -

W J - 3 6-i c3- co - C-ll1. LESSON OUTLINE: NOTES Example:

Calculate the SRD response for .[D = 0.0 and f =C 1.00 Table 1: S x Attenuation Factor = 1.13 x 10 Calculation left to Table 2: K,gg = .82 student 3

1.13 x 10 SRD = 1-0.82 = 6.28 x 103 = 1 x 10 3 SRD nos 6.25 6.25 The latter calculation clearly demonstrates the source range detector is far more responsive to downconer conditions then core conditions.

IV. THREE MILE ISLAND SOURCE RANGE TRACE NOTE: Considerable effort made to relate chronological ( R D- @== ,D events to postulated core conditions by close analysis T-P- m \

of the source range instrument trace.

A. For the first 20 minutes, source range instrument behavior is consistent with a normal posterip decay a rate of about one-third decada per minute. _

B. After approximately 20 to 30 minutes, the source count rate should be decreasing through the 600-700 counts per second (eps) range. Instead, the curve has leveled out at about 5000 cps due to buildup of voids (steam bubbles) in the downcomer and core regions. This is consistent with the fact that pressure has reached saturation (approximately 6 minutes after turbine trip),

  • and not outflow through the open electromatic relief valve continues to empty the system. Void formation is also consistent with the observed drop in reactor coolant flow rate because of the reduced pumping head produced by two-phase flow conditions (not shown).

C. Continued loss of coolant from the primary system leads to incrossed deedger count rates. The recording begins ta enh1bhasiasip'W4ch is reflective of unsteady flow (pump surgids) a g phase separation characteristic of

__ increases with time.

" slugy ~.; K,.,[ g, 2 y D. At 73-7 & B reactor coolant pumps are turned l off by the operator.

E. At 100 minutes the A reactor coolant pumps are turned off. This causes a flow transient and separation of voids to the upper regions of the system. Voids rising to the top and coolant fill from the hot legs produce a " solid" water condition seen at the detector. The detector count rate abruptly drops.

11

^-6

. uc -i_P - w i cT - co - C SR-LF-Go2. l Ill. LESSON OUTLINE: NOTES F. The minimum count rate is suggestive of the fact that the downconer water level is at or near the top of the active core level. ,

i G. Continued release of fluid out of the relief valve begins to boil off inventory from the core and downconer areas.

Make-up flow, assumed to be in the neighborhood of 18 gallons per minute, is not sufficient to maintain down-comer water level. The core mixture comen within better view of the detector as the water levels drop; the count rate increases.  !

H. A sharp reduction in the rate of increase is believed to be caused by the reduction of feedwater addition to the A OTSG (approximately at 124 minutes). This reduces i condensation in the A loop, leaving the open relief valve as the only pathway for boil-off and the removal of core coolant. The increase in the detector count rate slows, which is believed to correspond to a i reduction in the rate of the core uncovery. Further analysis is, underway in this area.

I. The signal level continues to increase slightly as core ,

uncovery proceeds at a slower rate, approaching a near i

equilibrium level.

It is believed that the maximum count race coincides with loop A refill to the reactor vessel inlet level. At 142 minutes after turbine trip, the operator shuts the electromatic relief block valve. It is observed that the maximum count rate does not coincide with shutting the block valve. Increased aska-up to the core (about 36 spa) produces gradual recovery.-

J. Over this period the count rate is decreasing as level in the core rises. The increased core mixture level is facilitated by relief block valve closure.

K. The operator starts reactor coolant pump 2B, sending a slug of cold water into the downconer sad essentially filling it. .

L. Loop t h gat , ' es that the pump worked effectively for a 1sery brief . This is corroborated by the abruptp - _ ' ' $ het source range detector trace, as l flow sensee .

downconer fluid moves into the

core and is boisi6 f; equilibrium levels are reestablished.

M. High pressure injection flow is initiated at 200 minutes, 8 minutes after the electromatic relief valve is opened by the operator. Coolant passes into the downconer, filling it. Detector count rates drop sharply.

12

.' uc - P - W o 'k - w 'C-

'" L" ^"2 ll1. LESSON OUTLINE: NOTES N. Continued addition of high pressure injection flow begins to quench the core. It is conjectured that the coolant first re-wets the outer region of the core, bypassing the hot center. .

O. Water entering the core eventually leads to an unstable thermal-hydraulic condition. It is speculated that major portions of the core may have been suddenly quenched with a resulting large amount of coolant flashing to steam, accompanied by possible core and fuel rearrangement. The jump in detector counts may be due to the displacement of fuel and/or sustained voiding of peripheral fuel regions. This phenomenon is under-going further analysis.

General Conclusions

1. SR and IR detector traces were virtually the same.
a. Only difference - trace below 2000 cps because of differences in readout ranges.
b. SR and IR level can be used to monitor , ,

reactor core and downcomer levels.

2. Downconer and core level change charscteristics i a. Slowly varying SR level changes correspond <

to core level changes.

b. Rapidly varying SR level changes correspond to downconer level changes.
  • V. RECRITICALITY ANALYSIS AND INDICATIONS l A. Core Parameters i
1. ARI worth approximately -9000 pcm
2. Isasm adestaggeoximately -2500 pcm over next 9 to 3.

Wf

'esotesun-free 557 to 200'F adds approximately

, - q M (-11000 pen)

,gyv4. ,

B. Recall from SR-206-TF-3.10

1. Actual power deviated from normal after 30 minutes
2. Operator initiated scram to ensure all rods on bottom.
3. Boron samples indicated decreasing C # ""*"****' "

(R0 established immediate boration) B 13 i

e L.o - c-P - 3 u eb c c - c.

OLLT-062 lil. LESSON OUTLINE: NOTES

a. Core would have still been shutdown by -3%

delta K/K

b. Lov boron due to plating out C. Recriticality and void plant -
1. Difficult to answer if core geometry changes
2. Keep in mind with high void content positive reactivity addition from cooldown is not credible.
3. Arguments against recriticality -
a. Core already in optimum geometry so any degradation will not result in critical mass
b. Analysis shows recriticality is highly unlikely for substantial geometry changes during fuel slump damage.

D. Recriticality and Non-Void Plant

1. Even with excessive cooldown, trip of reactor with subsequent addition of Xenon should not result in recriticality.
2. Most probable cause would most likely be boron dilution coupled with cooldown.

VI.

SUMMARY

i A. Voiding of core and downconer having differing effects on source neutron population and attenuation and core E

eff*

1. Source and Attenuation
a. .

effect much larger than core Y ?l.,

6 2=8 - ~ ~ effects leakage, not moderation

. NTyb *%C;.1 is combination of decrease in and increase in leakage.

@. - ( c', V

a. Core deusf.ty slightly more effective than downconer.
b. Neutron moderation by downconer is small .

fraction of moderation performed in core.

14

J w _ a-$_.cs-w-c sa-te cea-lli. LESSON OUTLINE: NOTES B. SRD are more sensitive to downcomer conditions than in-core conditions.

C. Due to location of detectors, core water level cannot be reliably estimated until a large part of the core is uncovered.

D. A qualitative idea of how in-core conditions and down-comer conditions affect the excore instruments can be made.

Figure 11 illustrates the expected effects in the SR Sa !" 002 for the accident conditions discussed. rf- o i s-E. Recriticality must be addressed during accident response

'but it may not be a credible assumption based on conditions shown at TMI.

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

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