ML19345G489

Steady State Fuel Bubble Thermal Analysis

ML19345G489
Person / Time
Issue date:	09/29/1980
From:	Yackle T NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:
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ML19345G484	List: ML19345G483 ML19345G486 ML19345G489
References
NUDOCS 8104070368
Download: ML19345G489 (13)

Text

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STEADY STATE FUEL RUBBLE THERMAL ANALYSIS

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T. R. Yackle g.

I.

.0bjective: Model the temperature distribution in the ballooned region of a fuel rod during the heatup phase of a LOCA.

Include in the analysis; (1) the possibility of additional fuel in the ballooned region due to radistribution, and (2) the possibility of cladding failure and a degraded pellet-to-cladding and pellet fragment-to-pellet fragment gap conductance.

II. Model: The fuel rod was modeled by a serias of nodes shown in Figure 1.

The cracked fuel and additional rubble fuel was modeled as the series of fuel and gap nodes numbered 1 through 7.

III. List of Assumotions:

1.

Circumferential heat transfer is small.

s 2.

Axial heat transfer is also small.

H 3.

The fuel pellet has two circumferential cracks. The fuel is cracked into particles that have a dimension of about 1600 pm.

This corresponds to t:.asurements of fuel particles made from PBF and KfK (FR-2) tests.

4.

Nominal dimensions of a 0.422-inch PWR fuel rod.

2

- 5.

The cladding surface heat transfer coefficient is 60 W/m,g, This value is conservative for a stagnant steam environment.

6.

An initial rod power prior to reactor scram of 40 kW/m and 3% decay heat at 100 seconds after scram.

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Figure 1.

Steady state fuel rubble thermal analyses.

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A flat fuel radial power profile.

I-8.

The fuel rod thermal conductivity is constant at 2.6 W/m K.

9.

The gas thermal conductivity is constant end selected at an average rod temperature of 1500 K.

10.

Radiation between fuel particles is small.

IV. Fuel and Gas Properties: The fuel and gas properties are based on MATPRO, Version-11, NUREG/CR-0497, TREE-1280, Rev. 1.

1.

The fuel conductivity was assumed to be 2.6 W/mK which is conservative for temperatures about 1850 K.

2.

The conductivity of the gas within the unfailed rods was assumed to be 0.45 W/m K at an average rod temperature of 1500 K as shown in Figure 2.

3.

The conductivity.of the gas within the failed rod was assumed to be represented by oxygen as shown in Figure 3.

A value of 0.1 W/mK at an average rod temperature of 1500 K was selected.

V.

Power Generation: An initial peak rod power of 40 kW/m was assumed and the analysis completed for 100 seconds after reactor scram. Power generation at this time is 3% of the original power as shown in the Figure 4 ANS curve.

VI. Fuel Redistribution:.The amount of fuel redistribution has been determined from results of the PBF LOC-3 and LOC-5 tests. The degree

[

of redistribution was evaluated by measurements of the fission product decay along the' length of the fuel rod. An example of.the normalized decay of two fission products within Rod.04 of Test LOC-3 is provided in Figure 5.

The decay curves indicate a 30 to 50% fuel redistrib-

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uted into the ballooned region.

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0 20 30 40 50 60 70 Distance from bottom of fuel stack (cm) i Figure S.

Axial profile of fission product decay curve for LOC-3, Rod 04.

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Tt.3 percent fuel redistribution in the ballooned region of all the L

Test LOC-3 and LOC-5 rods is compared with the percent cladding volume increase in Figure 6.

The uncertainty of the fuel redis-tribution for each rod is a result of differences in the measure-ments of the decay of the two fission products. The choice of the cesium fission product as an indicator of fuel movement may be misleading due to cesium volatility and probable movement within the gas of the rod. However, tne use of cesium would be conservative as it would likely indicate more fuel relocation than expected. A line was fit through the data that represents the average fuel redistribution compared with the cladding volume increase. A nore accurate representation of this line fit, based on additional analyses, is recommended for future effort.

The calculated degree of fuel relocation for a fuel rod with 44 and 89% cladding strain is 52 and 160%, respectively.

VII. Analyses Results: The calculated temperature profiles of a fuel rod i

with 0, 44, and 89% cladding strains are provided for a number of fuel rod conditions in Figures 7 through 9.

In each figure, the gas gap l

conductivity was assumed to be that of:

(1) helium (the upper limit L

of conductivity of an unfailed rod) and (2) oxygen (the lower limit of conductivity of a failed rod). The temperatures presented in Figure 7 were calculated for a fuel rod with 0, 44, and 89% strain I

and no fuel relocation or additional fuel redistribution into the balloon. The cladding and fuel were caleclated to cool as ballooning H

occurred and temperatures were relatively low. The temperatures l

presented in Figure 8 were calculated for a fuel rod with cladding

[

strain and fuel relocation, but no additional fuel redistribution into the balloon. The cladding was again calculated to cool as ballocning occurred. The maximum fuel centerline temperature was only-200 K larger than the fuel temperature of the nominal case t

'vith no ballooning. The temperatures presented in Figure 9 were cal-ulated for a ' fuel rod with cladding strain, fuel relocation, and f

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Fi ure 9.

Fue'l rod temperatures with 0,'44, and 89'.' cladding strain, fuel 9

relocation and fuel redistribution.

A fuel redistribution into the ballooned region. The cladding temperature of the worst case (89% strain) was calculated to be about 230 K greater than the nor.inal case. The maximum fuel tem-perature ballooned region was 1300 K larger than the nominal case (no rod deformation). The calculated temperatures are well below the melting point of the respective materials, indicating that fuel redistribution into the balloon for these conditions will not pose a significant problem.

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