Text

Recriticality Potential of TMI-2 Core
	ML19224A810
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Site:	Crane
Issue date:	05/14/1979
From:	Marotta C; NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
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,

w RECRITICALITY POTENTIAL OF TMI-2 CORE u

C. R. Marotta I.

INTRODUCTION AND CCNCLUSIONS This memorandum considers the conditions under which the TMI-2 core may achieve an unplanned criticality (before final dismantling) by analyses of mathematical models of various TMI-2 core configurations and boron concen-trations using reasonable conservative scenarios reflecti.ig the recent NRC I

assessments of core damage.

Common to all models in this analysis are the assumpticas of comolete loss of both ALL movable control rods (Ag-In-Cd; total worth s 10% in k) and ALL fixed burnable poison rods (Al 0 ~

23 8 C; total worth

4.4% in k).

The above assumptions are reasonable 4

w (although highly conservative) since from Reference (1) it appears that the temperature of unfueled components lagged the temperature of fuel rods (exceeded 1750 C) by only about 20 F.

Consequently, in the hot region of the core, Zr components should have oxidized, and components with Inconel, stainless steel and Ag-In-Cd should have melted.

The poison, baron, of the fixed burnable rods is probably also lost since baron is known to leach out of B C-Al 02 3 pellets when exposed to water in a radiation 4

environment.

Boric acid in the moderator / coolant is assumed as the only poison keeping the reactor in a subcritical state for all calculations.

168 156 I Internal NRC memo from R. O. Meyer to R. J. Mattson, April 13, 1979,

Subject:

CORE DAMAGE ASSESSMENT FOR TMI-2.

/,

J.;;

7905300 7 3 O t

e

.i 2

The mathematical-criticality analyses were performed using the KENO Monte Carlo ccmputer program together with the 123 group GAM-THERMOS neutron s

crocs section set.

The core was modeled (containing only latticed fuel cM pellet-clad-moderator) geometrically in 3-D, quarter symmetry, explicitly describing every fuel rod at the pitch under consideration.

The centiral

" checkerboard mixture" of 1.98% and 2.64% enriched fuel assemblies were modeled as assemblies having an effective enrichment of 2.31%, occupying two distinct regions:

a central square (33% of core) surrounded by a square annulus (33% of core) of identical rods.

The outer portion, containing 2.96% enriched fuel assemblies, forms the last square annulus (34% of core).

A different borati concentration can be specified for each of these three regions.

A two-foot unborated water, all around reflector surrounds the above-described core.

The most reactive core configuration established was that of all 36,816 UO

-r- :..

2 (model assumed 36,864) fuel rods with clad intact and all rods taken at a reduced pitch (from "as built" 1.44 cms to 1.26 cms).

This " worst case" of reactivity was arrived at by KEN 0 cell parametric studies of k, versus enrichment, boron concentration and pitch spacing.

Since the TMI-2 coolant will eventually reach room temperature, all criticality analyses were performed at this most reactive (neutronically) 3 temperature.

The moderator density was taken as 1.0 gm/cm and the fuel (UO ) density was assumed as 95% theoretical.

2 168~157

.x

3 Some confidence is established in the "above calculational procedure for the configurations of interest noting the successful KEh0 run for the critical TMI-2 core (zero power, hot 530 F, clean, all rods out).

This critical had a boron level of 1500 ppm.

The K calculated for this configuration at eff rocm temperature was 1.040 + 0.004.

Since the fixed 8 C burnable poison 4

rods were estimated to have worth of 4.5% in k and the modeling assumed eff these rods to be lost, the agreement can be considered excellent.

We note here that the 0.3% core volume occupied by stainless steel which is also neglected in the model is not expected to change the final k as is the eff contribution from the moderator temperature coefficient of reactivity +

~4 0.10 x 10

, in going'from hot ($30 F) to room temperature (70 F),

i.e.,

the a k s.0046 is of the order of the uncertainty in the Monte Carlo calculations.

Conclusions from the above analyses are:

.., ~

l.

A 3500 ppm boron level guarantees subcriticality for all conceivable S

abnormal states of the TMI-2 core; however 3000 ppm boron i# the realistic conservative concentration.

It is strongly recommended that this latter concentration be maintained uniformly throughout the core until dismantled.

2.

The peripher&l highest enriched region of the core is shown to be the most sensitive to boron concentrations.

Special boron concentration monitoring (if possibile) of at least this region would be prudent.

168 158 m

~

3.

Recardless of the baron concentration throughcut the core, a slug of e mpletely unborated water passing through a minimum of four contiguous (in a square) fuel assemblies, the full length of the core would cause t.w a criticality.

Four fuel assemblies corresponds to s 2% of the core volume.

II.

DESCRIPTION OF CALCULATICNS AND

SUMMARY

OF RESULTS a)

Praliminary Parametric Studv Figure 1 shows the k, results as a function of boron concentration for an infinite array of TMI-2 fuel assemblies - as built with 208 fuel rods and 17 water holes at the highest TMI-2 enrichment of 2.96% in the U-235 isotopc.

Since neutron leakage is relatively small in the TMI-2 core, this curve can be interpreted as the maximum k of the core without control rods, burnable eff poisons or fission products.

A7 proximately 2350 ppm baron would guarantee subcriticality for the "as built" TMI-2 care if all fuel reis remain intact and maintain the "as built" pitch.

Since the effective enrichment of the core is 2.57%, the above b::ron level can be considered quite conservative.

~

s b)

Pellet-Clad-Moderator Cell Calculations

~

Figure 2 shows k, results for KENO infinite cylindrical cell calculations for two enrichments (2.31% and 2.96%) as a function of water to fuel (W/F) ratio in the cell for 1000, 2000 and 3000 ppm baron in full density water.

Examination of this Figure shows clearly that the 3000 ppm boron would guarantee subcriticality for an infinite system at the as-built pitch, i.e., k, is less than unity for the as-built W/F s 1.69 for both enrichments, w

168 159

5 All the curves of Figure 2 clearly indicate the reverse trend of k vs W/F for standard LWR undermoderated fuel assemblies in M borated water.

This is d e to t.he heavy absorption in the moderator (when boron is present) giving a positive effect when pitch spacing is reduced.

Figure 3 shows the usual trend for the TMI-2 rod (2.96% enriched) in unborated water.

Figure 2 formed the basis in establishing the most reactive lattice pitch.

Examination of all the curves show that a reasonable average value of a W/F 1.0 would give a maximum k, for the two enrichments over the range of s

2000 to 3000 ppm boron.

This W/F of unity translates to a pitch spacing of 1.26 cms frcm the 1.44 cm as built pitch.

c) 3-Region Modeling of TMI-2 Core The objective in these calculations was to get some handle on the relative importance of the core regions (radial only),to criticality as a function e

of the boron concentration.

This is the classic boron hide out problem considered for control systems.

Figure 4 is a plan view of the quarter-symmetry of the geometry used in the KEN 0 calculations for all TMI-2 core calculations.

There are a total of 55 x 55 (or 3025) fuel rods in region A (s 14 fuel assemblies of 2.31% enrich-ment); a total of 78 x 78 - 55 x 55 (or 3059) fuel rods in region 8 (s 14 3/4 fuel assemblies of 2.31% enrichment); and a total of 96 x 96 - 78 x 78 (or 3132) fuel rods in region C (s 15 fuel assemblies of 2.95% enrichment).

In this matrix of 9216 rods no water lattices are modeled; in other words, 3

there is slightly more U-235 per cm of core, however, the total masc of 168 160

6 fuel in the core is greater by only 0.1% than actually exists in the TMI-2 core. Since the borated water is replaced by a fuel rod (i.e., the 17 water holes now hold a fuel rod), this represents a conservatism. A mirror boundary condition is applied along the &Y axis and the 5X axis giving a total parallelepiped core. A two foot unborated all 'around water ref. lector surrounds the core.

Table 1 gives k calculated for the TMI-2 core at the as-built and eff's the most reactive pitch spacing for a variety of ppm boren in the three separate regions A, B and C.

Included in the list is the initial critical configuration achieved by TMI-2 core, all reds out, with a 1500 ppm boron level.

Results frcm Table 1 indicate that:

(1)

For an intact core at the most reactive, pitch, 3000 ppm baron uniformily throughout the entire core will guarantee subcriticality.

(2) The presence of Zr clad gives a higher keff, since it would be replaced (if lost) by borated water - a much stronger neutron absorber.

(3) The outer highest enriched (2.96%) region is most sensitive to boron concentration and appears that maintaining 3000 ppm in 66% of the core but lowering the outer 34% of the core to boron concentrations lower than 1500 ppm can cause the core to become critical.

168 161

.~- '

7 d)

Scherical Pellet Pile These calculations were performed to estimate the reactivity effects if all the fuel rods were to rupture emptying all their pellets (H/D s 1 gives a spherical pellet radius of.53 -m.s) into a pile of bare UO 2 spheres with barated water in between.

Two KEN 0 k,c'ases were run us.ing an effective core enrichment of 2.57%.

It appears from the results of Table 2 that slightly more than 3500 ppm boron will be. 2aded to avoid criticality here.

e) local Criticality (4 FA in contact)

These calculations were undertaken to estimate a minimum ppm borcn dilution needed for a local isolated criticality in the TMI-2 core.

A system composed of four 2.96% enriched fuel assemblies, contact in a square array (represents 21/4% of core volume) with unborated water reflector was analyzed for a variety of boron concentrations.

Results are given in Table 3 and show that regardless of the amount of boron concentration throughout the core, if s 2% of the core volume receives a slug of unborated water in a localized region a criticality would occur.

GlauS>

168 162

8 m

TABLE 1 K

of TMI-2 Core As Function.0f PPM Boren in Water eff (flo Control Rods or Burnable Poisons)

(Room Temp)

AS BUILT PITCH MOST REAC PITCH 1.44 cms 1.26 cms PFM BORON l

PPM BORON ZR-CLA0 l[

l A

+

A B

C K

l ZR-CLADj A

B C

,Kgff eff it 0.944 YES 1500 1500 1500 1.040ll YES 3000 3000 3000 li I

YES 3000 3000 3000 0.883 $ YES 3000 3000 2000 0.954

[l t:0 3000 3000 3000 0.857 YES 3000 2000 1500,

0.989 YES 3000 3000 1000 0.992 N0 3000 2000 3000 0.936 '

fl0 2500 2500 2500 0.977 14 0 3000 2500 2000 1.000 "All K calc. by KEfl0-123 Gps, using 15,000 neutron histories and all within

+0.00$,in K f r 1 St.dev.

7 eff

,f conTAm s to,to o est;_ noe s;;.3i+/, s ; 23 y, cy, j

cooras i2, n ve' Ro35 pai%E 33% cou i

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12 FT ALTsVG tJ!qI,4' f:3 Aste, L ' '-

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TAe W 1, A3cVE

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9 TABLE 2 K+ for Bare UO Spheres, 2.57% ENR.

2

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In Contact as Function PPM-Boron'~'

Pellet H/D = 1, Rg = 0.538 cas.

s PPM k,

~

3000 1.030 + 0.004 3500 0.997 + 0.004'

KENO cell calc.,' 123 gps,15,000 neuts hist.

TABLE 3 K*ff f Four, 2.96% ENR. Fuel Assy's e

in Contact Sauare Array

~

As Function PPM Boron PPM K'

2LL 2500 0.839 + 0.004 2000 J.866 + 0.004

~'

1500 0.836 7 0.004 I".00 0.924 + 0.004 500 0.953 + 0.004 0

.l.000[0.004

KENO calc.-123 Gps; Explicit Desc. of 900 Fuel Rods in Borated Water surrounded by 1 foot unborated water reflector l68 164

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Vessel ecclant cutlet te=perature, F 607.7 h.

Core ecclant cutlet te=perature, F 610.6 5

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Fuel red cutside dia:eter, in.

0.h30 6.

Cladding thickness, in.

0.0265 7

Fuel red pitch, in.

0.568 8.

Fuel assembly pitch spacing, in.

8.587 9

Unit cell =etalfwater ratic 0.82 i

(volume basis) 10.

Cladiing =aterial Zirealcy h (ccid verked)

C.

Fuel 1.

Material UO s

2.

For:

Dish-end, cylindrical pellets 3.

Pellet dia:eter, in.

0.370 h.

Active length, in.

Ikh 5

Censity, % of theeretical E'. 5 0

f,,.{ O h.1.'O,

13 TABLE h.3-2.

NUCLEAR DESIGN DATA Fuel Asse=bly Volt =e Fractions _

0.303 Puel 0.580 Moderator 0.102 Zirealey 0.003 s

Stainless steel 0.012 Void 1.000 Total UO2 (3OL)_

93 1 First core, =tUO2 Core Di=ensions_

128.9 Equivalent diameter, in.

lhh.0 Active height, in.

Unit Cell H20/U Atc=ic Ratic, Fuel Asse=bly 2.88/2.06 Cold /het Pull-Pcver Lifeti=e s

h21 First cycle, days 284 Each succeeding cycle, days Fuel Irradiation 1h,220 First cycle avg, M'Jd/=tU 9,600 Each succeeding cycle, KJd/ tU Fuel Leading Core avg first cycle, vt% 23"U 2.57 s

Centrol Data As-In-Cd Centrol rod material 61 No. of full-length CRAs 8

No. of APSEAs

'Jorth cf 61 full-length CRAs, (ak/k)%

11.1 SS30k Centrol red cladding material 68 ( first cycle enly)

No. of EPRAs Zirealcy h, EPRA cladding =aterial cold-verked B C in Al 03 S

2 3?? poisen caterial lfg j7 h. 3-20

/s

(

TA2LE h. 3-5 MOEERATCR TEMPERA"'JP.E COEFFICII'C 1

No. of fuel asse=blies in core 177 Core avg enrich ent, t% 235U 2.57 Pcver density, MWt/asse:bly 15.661 s

Initial critical conditiene (hot, full payer, clean)

Ecron conc, pp=

15hD CRA inserted verth, %ak/k 0.7 EPR poison verth, %4k/k k.h Moderater te=p coeff, 10 '(ak/k)/F E)

+0.10 I

Threshcid value of =cderator temo coeff for a inuthal instability,10-"(ak/k')/ y(b )

Reference value, 50% flatness

+1 5 Assu=ing ec pound errers, 50% flatness

+0.7 Reference value, 25% flatness

+2.2 Assuming ccepcund errors, 25% fle.tness

+1.2 Mcderator te=p coeff at end of equil fuel cycle, 10-4( ak/k)/ F

-3.0 Moderator temp coeff at end of first cycle, 10-4(ik/k)/F 7,6 Pcver coeff at BOL with 1230 pp: boren, 10-6(ak/k)/MWt

-4.3L (a)?.to-di=ensicnal isethemal calculations.

(b) Values frc= =cdal analysis, three-dimensional calcu-lations shev ruch greater stability; reference 3AW-10010.

s eusse m e

li' TA312 h.3-8.

CC..,,. w.e.S

,X, e_c : n.,,.r,._y

.w i i

A..

._ v.

Reacter core ecndtien(E) k rr e

Cold, TCT, clean 1.252

'~

Ect, 532F, clean.

ero Pcver 1.205 Het, 58k?, clean, full power 1.182 s

Ect, SELF, full pcVer, equilibriu:

.senen and sa=arium 1.133 Single fuel assenbly(b) (ve )

, 0.70 Two fuel assenblies(b) (yet) 1,g1g Single fuel asse:bly(D) (dry) 0.03 Tvo fuel asse=blies('")( dry) 0.0L Cold array (c) 0 90

" First cycle at 30L, 68 3? ras in cere.

(b) Eased en highest probable enrich =ent of 3 5 vt%.

(-)A center-to-center assembly pitch of

~

(

21 inches is required fer this k,,

in cold, unbcrated vater with no

xenen er samariu=.

i.

. x. r_ - *. L.9

_C a' 7 ' r. _e 'A CV.C7 ?

-.,7 ' u~~. ~/ 2'. v.

2_n CCUTEOL DISTRI2 C CN Reactivity,

%ik/k r

Centrolled by Soluble Scrcn Moderater te=p deficit (TO to 532F) 3.h Equil Xe and S:

3.5 Fuel turnup and fissien product buildup 10.5 Transient Xe 1.0 Controlled' by 3FRAs Fuel burnup and fissicn product buildup h.h Centro 11ed by Mc table CEAs Scppler deficit (0 to 2772 '5't) 1.2 Moderatcr temp deficit 0.0 (532 to 58kF) 0.2 Dilutien centrel 1.0 Shutdcun =argin 0.h Xenon undershoct 168 172.

u.3-25

/f

(

TA3LE h.3-11.

SCLU3LE 3CRCN IZTEM CD

'n'CRTH - FIRST CYCLE

-SQL bcron Cere conditices level, 5:=

Tor, k

= 0 99 No CRAs in 1582' All CRAs in 1,057 One stuck CRA ("ull cut) 1327 532F, 0 pcuer, k,,= 0 99 e..

No CRAs in 1710 All CFAs in Thl One stuck CFA (Full cut) 1C83 58hF, rated pcVer, k,, = 1.00 e..

(

No CRAs in 15h0 5 ELF, rated power, equil Xe and Sm, k

= 1.00 g

No CRAs in 1175 f

3eren verth. (5Ak/k)/tr=

58LF, rated power 1/1C0 TOF, zero pcVer 1/75 F

6 e

168 173 4.3-2T

If 3

3 3

i 3

3 3

2 3

3 3

(,

3 3

1 2

1 3

3 3

3 1

2 1

3 3

3 1

2 1

3 3

3 2

1 2

3 3

3 2

1 2

3 3

3 2

1 2

2 2

1 2

3 3

3 2

1 2

3 J

3.

3 2

1 2

1 1

1 2

3 1

2 1

3 3

3 1

2 1

3 3

1 2

1 3

3 l1 3

3 2

3 3

3.,

3 C

Batch No.

W/0 U235 1

1.98 2

2.64 3

2.96 FIRST CYCLE CORE TMI-2 168 174

~

po

(

L X

X X

X X,

X X

X 168 175 t

LCCATICH OF FUEL ASSEMBLIES CCNTA!NING BURNABLE P0150H RODS T11REE MILE ISLAND NUCLEAR STA W "3";'

r1 CED

' ~ ~~~

I,#

FIGURE 4.3-2

a

>I

{

I 6

7 t

6 4,

2 l2 4

5 8

5

\\

5 8

i 1

4 7

3 3

7 4

6 8

5 S

5 8

5 2

3 1

1 3

2 7

5 6l

'7 6

5' 7

2 3

1l 1

3 2

5 8

5l 6

5 8

6 4

7 3

3 7

4 5

8 5'

I 8

,5

)

4 l2 2

4 6

7 6

i g

Sank No. Rods Purpose 1

4 Safety 2

S Safety 3

8 Safety 4

8 Safety 5

12 Regulating S

12.

Regulating 7

9 Regulating 8

8 APSR 168 176 I

ROD LCCATICHS,0 200 FFD THREE MILE LSLAND NL' CLEAR STATION L' NIT 2

(

2=-

7 Q W.%q....'..." 1 4

FIGUR E 4.3-25

~

0[

5 4

'5 6

2 2

g3 g

5 6

7 I8'

, i 6

4 3

3 4I lE, 5

8 7

5 7

BI 5_'

~

2 3

1 1

3 2

l 4

7 Sl 4

S 7l 4

2 3

1 1

l3 l2l

.5 8

7 Sl 7,

8 5

6 4'

3 3

4 S

5 Sl 7

8 5

6 2,

2 l

B l

5 4

5 5

i Bank Nc. Reds Pur;:as e 1

4 Sa f ety 2

8 Safety 3

8 Safety 4

9 Safety 5

12 Regulating B

12 Regul a ting

.7 3

R t 'u l a ting 8

0 APSR 168 177 i

(

ROD LCCATICNS, 200.421 FPD THREE MILE ISLAND NL' CLEAR STATION L' NIT 2

(

k FIGUR E 4.3-26 ogpm

ML19224A810

Text

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