ML19261C423
| ML19261C423 | |
| Person / Time | |
|---|---|
| Issue date: | 02/28/1979 |
| From: | Meyer R Office of Nuclear Reactor Regulation |
| To: | Beyer C, Buescher B, Klotz R ABB COMBUSTION ENGINEERING NUCLEAR FUEL (FORMERLY, BABCOCK & WILCOX CO., HANFORD ENGINEERING DEVELOPMENT LABORATORY |
| References | |
| NUDOCS 7903220406 | |
| Download: ML19261C423 (7) | |
Text
CORRRfdT1JE CORRESPO d(R 728E C -
SOCIETY / COMMITTEE:
ADDRESS CORRESPONDENCE TO:
ANS-5.4 R. O. Meyer
SUBJECT:
U.S. NUCLEAR REGULATORY COMMISS:ON Fuel Plenum Gas Activity
.iGENDA ITEM:
FILE NO.: N/A DATE:
f[8 2g l379 TO:
C. E. Beyer L. D. f'ohle Westinghouse Hanford A/59 General Electric Comoany, M/C 138 Hanford Engineering Development Lab.
175 Curtner Avenue P. O. Box 1260 San Jose, California 95125 Richland, Washington 99352 M. J. F. Notley B. J. Buescher Atomic Energy of Canada, Ltd.
The Babcock & Wilcox Company Chalk River, Ontario P. O. Box 1260 Canada, K0J1JO Lynchburg, Virginia 24505 Chang S. Rim R. J. Klotz Korea Atomic Energy Research Institute Department 9492 P. O. Box 7, Cheong Ryang Combustion Engineering, Inc.
Seoul, Korea Windsor, Connecticut 07085 R. L. Ritzman R. A. Lorenz Science Applications, Inc.
Oak Ridge National Laboratory 2680 Hanover Street P. O. Box X Palo Alto, California 94304 Oak Ridge, Tennessee S. E. Turner W. Leech Southern Science Applications, Inc.
Nuclear Fuel Division, W Corp.
P. O. Box 10-33528 P. O. Box 355 Dunedin, Florida Pittsburgh, Pennsylvania 15230
Dear Group Members:
Enclosed is a draft of the introduction to the ANS-5.4 support document as requested at the November 8,1978 meeting.
Your comments are welcome.
Sincerely, O'#
M 790322O L/6 6 Ralph 0. 'Meyer, Leader Reactor Fuels Section Core Performance Branch snc ronMgl sure: As stated 00 h)
I.
Introduction (R.0. Meyer, NRC)
ANS Werking Group 5.4 has met a number of times since its inception in June 19/4 to examine fission product releases from U0 f"'I*
H#"#
2 calculations of reactor behavior require a knowledge of the gradual release of fission pro 6 cts from the ceran.ic U0 pellets.
This is 2
especially true in accident analysis where the inventory of radioactive volatiles ed mes released from fuel pellets, but retained by the fuel cladding (plenum), defines a source term for plant-release calculations.
The scope of ANS-5.4 is thus narrowly cer;ned to study such releases and includes the following:
1.
Review available experimental lats on release of volatile fission products from UO nd mixed-oxide fuel.
2 2.
Survey existing analytical models currently being applied to light-water reactors.
3.
Develop a standard analytical model for volatile fission product release to the fuel rod void space.
Emphasis is placed on obtaining a model for radioactive fission product releases to be used in assessing radiological consequences of postulated accidents.
The volatile and gaseous fission products of primary significance are krypton, xenon and iodine.
The radioactive isotopes of interest are, by their nature, unstable, i.e., they have finite half lives.
Ironically, for krypton, xenon and iodine there are no radioactive isotopes with half lives greater than 8 days except Kr (10.7 yr) and I (1.6x10 yr).
1-1
phenomenological model, the Booth diffusion-type model (1-5), and has fitted this model empirically to a selected data set (6), whose virtues will be described below.
The Booth model describes diffusion of fission-produc+ atoms in a sphere of fuel material. The governing equation is 3C/at = B - AC - VJ, (1) where C is the isotope concentration (atoms /cd ), B is the production or birth rate (atoms /cm3 sec), A is the decay constant (sec-l), and J is the local mass flux (atoms /cm2 sec).
This equation is fundamental and applies to isotopes of any chemical species with any half life.
It simply says that the rate of concentration change in a region is equal to the rate of production minus the rate of decay minus the rate of loss by mass flow out of the region.
Equation 1 says nothing about the mechanism of mass flow.
The apparent diffusion coefficient D is contained in the flux term, which is given by J = -D VC.
(2)
This basic diffusion equation, like Eq. 1, contains no information about the diffusion mechanism and merely assumes that a net flow of matter occurs because of the existence of a concentration gradient and that the flux is proportional to that gradient.
The production rate B and decay constant A is known for all iso-topes, but the diffusion coefficient is unknown and must be determined
.I - 3
enpirically from experimental data.
From a general knowledge of atomic migration (7_) it is known that the diffusion coefficient of a species in a host material depends on the properties of that material and its inter-action with the diffusing species. These interactions are primarily electronic in nature so that different atoms (elements) would have different diffusion coefficients.
Because the valence and ionic pro-perties of krypton and xenon are similar, it is not surprising to discover that their diffusion coefficients in UO2 are similar, but there is no reason to expect them to behave like iodine.
Therefore, it must be presumed that different elements migrate and are released at different rates.
On the other hand, the diffusion behavior of a chemical species can be expected to be the same for all isotopes of that species.
- While, strictly speaking, there is a diffusion isotope effect that is dependent on isotopic mass (8), this effect is very small, has only been detected in a few precise experiments using isotopes with large mass differences, and such small differences in diffusion behavior would be totally imperceptable in the context of fission gas release.
Admittedly, the Booth diffusion model is an over-simplification of the physical process, and the effective diffusion parameters that are determined by empirically fitting the Booth model to gas release data are not the diffusion coefficients for atomic diffusion of krypton and other chemical species in pure U0. Atomic diffusion, 2
gas bubble nucleation, bubble migration, bubble coalescence, inter-action of bubbles with structures and irradiation resolution are all I-4
mechanisms that are involved in fission gas release.
Some of these processes, like bubble migration, are relatively well understood.
The microscopic parameters that govern these mechanisms are, in turn, dependent on the atomistic materials prcperties, such as diffusion coefficient, heats of vaporization, etc., which are independent of isotopic makeup.
It, therefore, seems appropriate to assume that the overall release kinetics are the same for all isotopes of the same chemical species regardless of the complicated nature of the release mechanisms.
There is a recognized pitfall in the method chosen by the Working Group.
The Booth equations describe a smooth continuous release process and, therefore, do not show discontinuous releases or bursts. To the extent that burst releases affect the relatively small sub-population of radioactive gases, an error is incurred.
It is considered beyond the state of the art to model burst releases in a quantitative manner and, therefore, such errors must be tolerated.
Finally, a temperature--independent recoil mechanism is also expected to be important for radioactive gas releases. As with the temperature-dependent diffusion-type model, the release fraction will depend on the isotopic half life. Because of the mechanical nature of the recoil process, however, all chemical species are treated alike.
1-5
References,Section I 1.
A. H. Booth, Chalk River Report, CRDC-721 (1957).
2.
A. H. Booth, Chalk River Report, DCI-17 (1.357).
3.
A. H. Booth, and G. T. Rymer, Chalk River Report, CEDC-72D (1958).
4.
S. D. Beck, Battelle Report, BMI-1433 (1960).
5.
B. Lustman, in J. Belle (ed.), " Uranium Dioxide:
Properties and Nuclear Applications," (USAEC, Washington,1961) p. 431.
6.
C. E. Beyer and C. R. Hann, Battelle Report, BNWL-1875 (1974).
7.
P. G. Shewmon, " Diffusion in Solids," (McGraw-Hill, New York,1963).
8.
N. L. Peterson, Solid State Physics j![, 409 (1968).
j 1-6