ML20062G924

Forwards Lead Items for Review of ThO2 & UC2 Topical Repts to Expedite Review Scheduled for Completion by 790629

ML20062G924
Person / Time
Site:	Fort Saint Vrain
Issue date:	02/16/1979
From:	Speis T Office of Nuclear Reactor Regulation
To:	Wessman G GENERAL ATOMICS (FORMERLY GA TECHNOLOGIES, INC./GENER
References
NUDOCS 7903080345
Download: ML20062G924 (7)

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FEB 1 E 1979 i

Mr. George L. Wessman, Director Plant Licensing Division General Atomic Company P. O. Box 81603 San Diego, California. 92133

SUBJECT:

REVIEW OF THORIUM-0XIDE AND URANIUM CARBIDE FUELS We have completed our initial review of your document, " Safety Analysis Report: Use of UC2 Fissile Fuel Particles in Fort St.

.Vrain Fuel Elements," Document Number GA-LTR-23, September 1978.

We find this document acceptable for review and plan a review schedule parallel to that for the review of thorium oxide fuel, Document No.

GLP-5640, June 1978. These reviews are now scheduled for completion by June 29, 1979.

The uranium carbide review will be administrative 1y similar to the thorium oxide review and will require a fee based on actual cost up to a limit of $20,000. The fee will be payable at the completion of the review as described in my letter to you of January 3,1979, pertaining to thorium oxide fuel. We will provide the Public Service Company of Colorario and the public document room in Greely, Colorado with a full set of documentation.

It will be Public Service of Colorado's responsibility to make a separate application to us for loading the uranium carbide fuel described ily your documentation into the Fort St. Vrain reactor.

In order to expedite the reviews for both fuels, this letter transmits the enclosure, " Lead Items for Review of Th02 and UC2 Topical Reports."

We have arranged with David Alperstein to discuss, this material with you and General Atomic staff members on February 28 and March 1,1979, prior to our development of first round questions.

7903080.395'

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Mr. George L. Wessman,,

1 Please let us know if you have any questions.

Sincerely Themis P. Speis, Chief Advanced Reactors Branch Division of Project Management Office of Nuclear Reactor Regulation

Enclosure:

As stated cc: Mr. J. K. Fuller, Vice President Engineering and Planning Public Service Company of Colorado P. O. Box 361 Platteville, Colorado 80651 Bryant O'Donnell, Esquire Kelly, Stansfield & 0'Donnell 990 Public Service Company Building Denver, Colorado 80201 James B. Graham, Manager Licensing and Regulation East Coast Office General Atomic Company 2021 K Street, NW.

Suite 709 Washington, D. C.

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Leau Items fo* Review of Thin and UC9 Tooical Recorts I.

Th02 Fuel Performance Data Base

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The discussion of (a) irradiation test results and (b) fuel per-formance during a potential core heatup event (sections 4.2.3 and 4.4, respectively, of GLP-5640) is, in each case, rather brief. Perhaps the several references listed in GLP-5640 contain all that is needed as supplementary information. On the other hand, it might be possible to expedite the review if separate and more detailed discussion and analysis were provided of the overall data base for Th02 TRISO particle performance under normal and off-normal conditions. Thus, it would be helpful to have a summary presentation of the details of the irradiation tests and performance analyses, which would include the following:

1.

Fuel test particle material paraneters, such as kernel diameter, coating thicknesses, densities, anisotropy factors, etc.; di.fferences between test material parameters and potential reload material parameters should be highlighted and discussed in terms of possible effects on performance.

2.

Fabrication parameters should be discussed in terms of the precursors, methods, etc. that were used to produce the test particles as compared with the future reload particles and the effects of these fabrication parameters on fuel particle component structure and performance.

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Test conditions, including numbers of particles testec versus number failed, method of failure detection, statistical significance of the test results, etc.

4.

The analysis of the test results should include a discussion of the significance of the apparent lack of data on high exposure Th02 TRISO fuel particles (taking into account that the fraction of fissions in the fertile particles increases with time as more Th-232 is converted to U-233).

5.

An attempt should be made to indicate the expected failure fraction for each potential failure mechanism, as was attempted for UC2 TRISO and Th02 BISO particles in GA report GA-12971. The supporting data should be correlated with the predictions.

6.

The relationship of the sic tensile stress model to the irradiation test data base should be discussed.

c For example, for a given test sample, what was the calculated distribution of sic tensile stresses, how many specimens (particles) failed in the sample, and what were the calculated tensile stresses in the failed particles? As a tie-in to the que.itions on fabrication parameters and particle coating structure l

l and properties, what effect does sic microstructure (grain size) and density have on strength, how are l

l these parameters affected by fabrication, and what i

quality assurance procedures are used to ensure that the desired sic structure and strength are obtained?

l

. !!. Gaseous Fission Product Release From Th09TRISO Particles The statement in GLP-5640 (p. 4-9) that gaseous fission product during cpra heatup is less than FSAR release from TRISO coated Th02 predicted values for ThC2 particles seem inconsistent with the res-pective release-to-birth ratios (R/B) on page 4-7.

Further infor-mation should be presented on the data base for the R/B values for each type of fuel particle. The way these values are used in the dose calculations should be discussed also.

III. Effect of Kernel Tyce on Steam-Graphite Reactions It is indicated in GLP-5640 that, since the retention of barium and strontium (both known to catalyze steam-graphite reaction) is kernels, the reaction rate of the expected to be improved with Th02 t

core graphite will be reduced. There are no numerical values pro-vided in the report discussion, however, and so it is not possible to determine the full ramification of this potential change in reactivity. For example, if less core graphite is reacted per unit time, does this result in greater oxidation of the core supports?

If so, what is the potential safety significance? Moreover, within the core itself, a reduction in reactivity of the core graphite will not necessarily result in a reduction in total oxidation, but may merely shift the extent of oxidation from the top (colder) regions of the core to the bottom (hotter) regions. In addition, if the reactivity of the core graphite is, in fact, reduced significantly, there is a greater potential for hydrolysis of the carbide fissile k ernels. These concerns should be addressed in sufficient detail to permit an assessment of their safety significance.

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.1-IV.

Carbothermic Reduction of Thfb Although it is asserted in GLP-5640 that test results indicate that "the carbonaceous reduction of Th0 is not a performance con-2 sideration up to 2000*C, tests conducted at Brookhaven National Laboratory (reported in BNL-NUREG-50785) indicate that the " rate of reaction of UO2 and Th02 with graphite becomes appreciable at 800*C to 1000*C and is very rapid above 2000*C." Some analysis and discussion of these apparently centradictory data or interpre-tation of data is desirable, particularly regarding its potential safety significance for the six events identified in Section 3 of GLP-5640.

VI. Th0, Kernel Migration The most recent work on Th02 kernel migration (C. L. Smith, Nucl. Tech. E, pp. 403-412) indicate that the migration is characterized by an incubation period that decreases with increasing temperature and burnup. What assurances are there then that the out-of-pile Th02 kernel migration measurements are not influenced (to a unknown degree) by removal from an irradiation out-of-pile and that the measurements out-of-pile do not provide false (low) indications of the rate in-reactor? Are there any decay processes that might affect the out-of-pile measurements, considering the period of time between the irradiation of the particles and the testing of the particles in an out-of-pile facility?

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VII. sic-Fission Product Interaction in TRISO-coated Th07 Particles l

l A fairly extensive data base exists which shows that the

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. s pressure vessel, sic-tensile stress correlation breaks doun at temperatures > 1500 - 1600*C. This is believed to be due to a chemical interaction between the sic layer and fission products that have migrated out of the kernel to the inner surface of the sic layer. The data that address this phenomenon in TRISO Th02 particles should be discussed, and the fraction of failures due i

to this mechanism should be predicted as a function of burnup and temperature.

VIII. Earlier review efforts (circa 1976) on HTGR fuel particle failure models and data culminated in an evaluation report NUREG-0111.

In that report the TRISO UC2 particle ' irradiation test data base, analyses, and coating failure models were critiqued, weaknesses were identified, and model modifications were sugge.ited (see, in particular, section III of NUREG-0111).

It would be helpful, therefore, to have a summary of the advances in the state-of-the-art regarding TRISO UC2 particle testing and fuel failure model development that have been made since issuance of NUREG-0111.

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For example, what new evidence has been generated in support of s

i (1) the sic tensile stress and pressure vessel failure correlation i

and (2) the sic fission-product-interaction failure rate at temperatures above 1500*C? What are the predicted failure rates for each identified potential failure mechanism as a function i

of particle age and temperature, and what specific data support the predictions?

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