ML20198C101

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Discusses Analysis of Criticality Aspects of Accidental Dropping of Contents of Loaded Canister Onto Similar Loaded Stored Canister.Accident Poses No Criticality Hazard
ML20198C101
Person / Time
Site: Three Mile Island Constellation icon.png
Issue date: 10/30/1985
From: Marotta C
NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To: Weller R
Office of Nuclear Reactor Regulation
Shared Package
ML20198C081 List:
References
NUDOCS 8511110296
Download: ML20198C101 (6)
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Text

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APPENDIX II

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OCT 3 01985 MEMORANDUM FOR: Richard A. Weller, Leader Safety and Environmental Review Section Three Mile Island Program Office Office of Nuclear Reactor Regulation l T11RU: Charles E. MacDonald, Chief Transportation Certification Branch Division of Fuel Cycle and Material Safety Office of tbclear Material Safety and Safeguards FROM: Charles R. Marotta, Senior Criticality and Shielding Engineer Transportation Certification Branch 4

Division of Fuel Cycle and Material Safety j Office of Nuclear Material Safety and Safeguards SUSJECT: CRITICALITY SAFETY EVALUATION OF A LOADED CANISTER I

DROPPING ITS CONTENTS ONTO A S!!!!LAR LOADF.D CANISTER

IN A MAXIMUM VOLUME STORAGE UNIT
1. Introduction and Summary As agreed in our conference phone call with Phil Grant and John Thomas on Friday, October 18, 1985, I have analyzed the criticality aspects of the accidental dropping of the contents of a loaded canister

, onto a similar loaded stored canister. The analysis indicates that .

i for the loading limitations per canister, maxinum storage volume per canister available and 4350 ppm boron in water, such an accident '

coses no criticality hazard and under very conservative assumptions (discussed below), the keff shutdown range is between 32% (max) to 13%(min). A total of six KEND Monte Carlo (123 gps) cases were ,

analyzed and form the basis of the above conclusion. Results are given in Table 2. The computer input-output for these cases are on file in Transportation certification Branch, NMSS.

2. Problem Definition The concern of the subject accident scenario is the criticality state of a stored loaded canister when surrounded by the dropped i contents of a similar canister. The stored canister resides in a

, parallelipiped borated (4350 ppm) water region of dinensions 18 inches 1

by 18 inches by 14 feet - a volume of 892.000 cc.

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Richard A. Weller 3. Problem Solution: Assumptions and Method _s The approach in solving the above problem was to assume all canister contents to have a maximum payload of dry 900 kg U(3)0 7 pellets -

this nominal value is 4-1/2% higher than the greatest ayload (C61 kg - total) for a knock-out canister.

To understand the detailed approach taken (described below) in solving-the problem, the following criticality observations are reviewed. They were established in previous studies.

I a. The as-built pellet is the form and geometry of the fuel to affect the optimum Vol fuel to Vol water ratio (V /Vp )gboth for unborated water and borated water,

b. Unborated water; maximum reactivity exists for fuel as pellet for Vp/Vy = 30/70, water is more important than fuel.
c. Borated water; maximum reactivity for fuel pellet shif ts to Vr/Vu= 60/40 over the boration of 2500 ppm to 4500 ppm boron.

, Fuel is more important than the borated water. But the ratio goes from 58/42 to 62/38 over the boration range showing the small dependence on ppm; we have thus assumed an average value of 60/40,

d. Since the above ratios (30/70 and 60/40) represent optimum values and further increase of fuel into the system would decrease reactivity, small uranium slurry volume and/or uranium fines in the moderator region give a crude-first approximation of reactivity reduction. This is not exactly correct since 1

introducing fuel in the moderator region shifts the optimum value. This has been neglected and is considered a second order effect on the assumption the system spectrum remains constant and the shift is small.

i With the above as background, Table 1 can be constructed showing how mcny canister-full contents can be accommodated in the water storage paralleliped of 892.000 cc total volume. The canister contents are assumed to be 900 kg 00 at density 10 grams /cc. No canister structural material or cani,ter s poison material is considered present in the storage volume.

Richard A. Weller TABLE 1 Number of Canister Contents in Storage Volume U0, Volume Volume H O vp/Vp 2

j 90,000 cc U0 112 802,000 cc H 2

2 180,000 cc U0,, "253

'712,000 cc H O 2

270,000 cc U0 9 3

622,000 cc H O .434ch=.428) 70 2

360,000 cc UO 532,000ccH6 2

5 450,000 cc 00, 442,000 cc H D '

2 6

540,000 cc U0

.,52,000ccHf 1.538cS(-1.500) 40 2

This Table 1 shows that it will take about six canister contents to approach the optimum 60/40 ratio for borated systems and about only three canister contents to approach the optimum 30/70 for unborated systems.

The criticality analysis of the cases specified in Table 2 were modeled as cells as a discrete pellet region surrounded by its associated moderator close-fitting into the 18" x 18" cross-sectional area. This gave a UO, mass loading of 2764 kg (vs 2700 = 3 x 900) for the 30/70 ratio and 5678 kg U0 7 (vs 5400 6 x 900) for the 60/40 ratio due to the arithmeticaT discrepencies of fitting prescribed volume fractions into a fixed region. The 30/70 case is very slightly non-conservative, whereas, the 60/40 is quite conservative since more fuel is a more reactive situation here.

i Richard A. Weller 4. Discussion of Results and Conservatisms Comparison of Cases 1 and 4 show that keff will decrease by 0.14 for the unborated case by increasing the fuel by a factor of 2 in line with neximum reactivity for the 30/70 mixture. For the horated cases, a comparison of Cases 2 and 5 and Cases 3 and 6, an increase in keff of 0.14 and 0.19 results respectively by increasing the fuel by a factor of 2 in line with maximum reactivity for the 60/40 mixture.

Case 6 represents approximately six canister-fulls filling the storage volume at the most reactive mixture 60/40 for 4350 ppm boron in the storage water. If one considers the canister poisons and structural materials as well as the core (canister contents) material to contain control-rod poisons, fixed poisons, core structure material, fission products and lower average core enrichment, all the tabulated keffs of Table 2 can be decreased by at least 0.10.

Since only 2 canister contents represent the accident conditions, subcriticality is assured by a large margin.

In addition, Case 7 represents a 14 foot deep infinite slab of Case 6 contents with a resulting keff of 1.085.

Case 3 of Table 2 rerun as an infinite systen in the X-Y-Z direction, gave a koo of 0.8021.

Case 7 of Table 2 rerun as an infinite systen in the X-Y-Z direction, gave a koo of 1.095.

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l Richard A.- Weller l l

TABLE 2 KEN 0 K_,2s for an 18 inch x 18 inch x 14 feet Para 11elepiped Canister a 5torage Volume Containing Most Reactive U(3)0,,-li.,0 tiixture (for baron concentrations of zero, 3000 ppm and 4350' ppm in water)

. KEN 0 PPf1 Vfuel Contents of Storagg Volume5 (c) 3 l No.

Case Boron Vwater keff(a)(b) (18"xl8"xl68"=31.5ft=8.92x10cm) j i 2764 kg U(3)02; 618 kg H 2O

.1 0 30/70 1.239 zero gas boron i2 3000 30/70 0.775 1893 gas boron 3 4350 30/70 0.677 2746 gms boron i 5678 kg U(3)0 ; 362 kg H gG 2

4 0 l 60/40 1.099 zero gas boron I

5 3000 j60/40 0.918 1113 gms boron 6 4350 , 60/40 0.871 1614 gas boron

~

K-INF(X-Y) 5678 kg O[3T6 ; 362 kg H 0 2 z i7 4350 ,

60/40 1.085 1614 gms boron i

(a)to within 10.003 for 1 std. dev.

(b)all cases (except No. 7) reflected by 1 foot all around appropriate borated-water reflector.

(c) storage volume does not contain any structural (internal and external) canister materials or canister poisons.

(d) reflected top and bottom, 2 direction by 1 foot of borated water.

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!' . :.?, , [,,e $h . s. Charles R. Marotta l l l L .. 

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