ML19351E678

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Recriticality Potential of TMI-2 Core, Submitted as Summary Paper for ANS Meeting Re Transactions for Nov 1980
ML19351E678
Person / Time
Site: Crane 
Issue date: 11/30/1980
From: Marotta C
NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:
References
NUDOCS 8012100646
Download: ML19351E678 (8)
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'$.y RECRITICALITY POTENTIAL

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The 61 movable (Ag-In-Cd) control rods successfully scrammed the TMI-2 reactor in the first few seconds of

  • e event.

Subsequent core uncovering over extended periods of time led to fuel temper-atures in excess of 1750 C and thus left uncertain the status of these control rods as well as the burnable poisons (B C-Al O )

4 2 3 contained in 68 fuel assemblies.

Hence a potential -for recriticality existed for the damaged reactor (possibly until dismantled) relying exclusively on the boron (boric acid) concentration in the moderator /

coolant to guarantee subcriticality.

This paper presents the method-ology and results of the NRC critica ity studies performed during the early stages of the accident to esfablish necessary boron concentrations for a variety of assumed damaged core conditions.

The objectives of the analyses that were successfully addressed were:

1.

Establish the minimum boron concentration for:

a) a fully intact undamaged latticed core without control rods or burnable poisons; b) the most-reactive intact damaged latticed core w.ithout control rods or burnable poisons; 8012100 [ N

2 c) a fully pelletized damaged core without control rods or burnable poisons.

2.

Establish sensitivity of reactivity to varying boron concen-trations of most reactive intact latticed core as a function s

of radial regions, i.e., consider gross boron hide,out as a function of radial core volumes.

3.

Establi-sh conditions (if any) of a possible local criticality.

The mathematical-criticality analyses were performed using the KEN 01 Monte ' Carlo computer program together with the 123 group GAM THERM 052 neutron cross section set.

Fuel pin cell k, calculations were performed to establish the most reactive lattice pitch as a function of U-235 enrichment and boron concentrations.

3-D KEND keff calculations were used to establich the reactivity of the reactor under a variety of states.

Since the TMI-2 coolant will eventually reach room temperature, all criticality analyses (with the exception of the hot, clean benchmark TMI-2 configuration) were performed at this most reactive (neutronically) temperature.

The moderator density 3 and the fuel (UO ) dens'ity was assumed as 95 was taxen as 1.0 gm/cm 2

percent theoretical.

I The core was modeled (containing only latticed fuel pellet-clad-i moderator) geometrically in 3-D, quarter symmetry, explicitly describing every fuel rod (pin diameter of 0.9398 cms. active height 366 cms, Zr clad 0.0673 cms thick) at the pitch under consideration.

3 Only fuel. rods exist in this model, all water holes contain a fuel rod.

This model assumes 36,864 (versus 36,816 actual) fuel rods, giving only 0.1 percent more fuel than actually in the core.

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

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

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

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

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

This latter modeling representing external regions of the core introduces approximately a +0.5 percent conservati k.

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

This critical configuration had a boron level-of 1500 ppm.

The k calculated for this configuration at ter erature was eff 1.050 + 0.004.

Since the

.xed B C burnable poison ' rods were 4

estimated to have worth'of 4.5 percent in k,ff and the modeling assumed these rods to be lost, the agreement can be considered excellent.

The 0.3 percent core volume occupied by stainless steel which is also neglected in the model is not expected to a

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4 change'the final k since the A k will be less than the order of eff the-uncertainty in the Monte Carlo calculations.

1 The above modeling gave the following answers to the above stated objectives.

1.

(a) k, cell ca'iculations for an infinite undamaged array of TMI-2 fuel assemblies at highest enrichment of 2.96%,

would be subtritical with 2350 ppm boron in the coolant.

(b) k, cell calculations performed as a function of rod pitch, boron concentration and U-235 enrichment indicate a-maximum reactive lattice to exist for a rod spacing of 1.26 cms (versus the 1.4,4 cms as-built).

Results are given in Figure 1 and clearly indicate the reverse trend of k vs W/F for standard LWR undermoderated fuel assemblies in unborated water.

(c) Unclad UO spheres-resulting from clad rupture of all 2

rods would be the most reactive configuration being kept subscritical with at least 3500 ppm boron.

2.

Table-1 gives k

's calculated for TMI-2 full core modeling eff under a variety of boron concentrations in the three radial core regions indicating the controlling aspects of the outer highest enriched region.

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For a local criticality, four contiguous fuel assemblies with highest enrichment at the most reactive pitch was calculated to go critical for zero boron concentration.

The' major conclusions and recommendations from the analyses undertaken are:

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1.

A 3500 ppm boron level guarantees subcriticality for all conceivable abnormal states of the TMI-2 core.

2.

The peripheral highest enriched region of the core is shown to be the most sensitive and controlling to boron concentra-tions.

3.

Regardless of the boron concentration throughout the core, a slug of completely unborated water passing threagh a minimum of four contiguous (in a square) fuel assemblies, the full length of the core, would cause a criticality.

4.

Special note should be made of reactivity increases for reduced pitches for borated lattices as indicated in Figure 1.

This would especially be applicable for guidance during core dismantling where intact pitches may have to be reduced for assembly removal.

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6 REFERENCES' l.

L. M. Petrie and N. F. Cross, " KENO IV, An Improved Monte Carlo Criticality Program," ORNL-4938, Oak Ridge National Laboratory (November 1975).

2.

~ 123 group GAM-THERMOS Neutron Cross Section Set; Available from Oak Ridge National Laboratory Shielding Information Canter.

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TABLE 1 i-keff of TMI-2 Core as Function of PPM Boron in Water (No Control Rods, Burnable Poisons, or Fission Products)

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AS BUILT PITCH MOST REAC PITCH 1.44 cms 1.26 cms PPM BORON PPM BORON U)

B(2)

C(3)l k'eff*

ZR-CLA0 A(I)

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YES (cold) 1500 1500 1500 1.040 YES 3000 3000 3000 0.944 YES (hot) 1500 1500 1500 1.050 YES 3000 3000 3000 0.883 YES 3000 3000 2000 0.954 N0 3000 5000 3000 0.857

.YES 3000 3000 1500 0.999 YES 3000 3000 1000 0.992 N0 3000 3000 3000 0.936 NO 2500' 2500 2500 0.977 NO 3000 2500 2000 1.000 (1-) Region A is central cuboid, containing 12,100 fuel rods at 2.31% enrichment (no water hole ).

(2) Region B is annular cuboid surrounding A, containing'12,Fs6 fuel rods at_2.31%

enrichment (no water holes).

(3) Region C is outer annular cuboid surrounding B, containing 12,528 fuel rods at 2.955 enrichment (no water holes).

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- Regions A, B, anc C are 12 feet in height; Regions are surrounded by 2 feet unborated water all around reflector.

  • All keff calc. by ' KENO -123 Gps, using _15,000 neutron histories and all within 10.004 in keff for i St. dev.

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