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Forschungszentrum Jitlich-GmbH FZJ
FUEL PEBBLES OPERATIONAL EXPERIENCES IRRADIATION AND POSTIRRADIATION EXAMINATION
G. Pott
EL Nabielek
JMilich, 09.July 01
* Reference fuel ,TRISO coated particles * Irradiation tests in research reactors * PIE, heating tests
/,-IPBrMR
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Executive Statements Summary
1. The design of modem HTRs is based on high qualified fuel. This fuel designed in the
1960s and 1970s had been perfected for steam cycle applications in the 1980s and early
1990s enabling the design of small inherently safe modular HTRs with self-limiting
temperatures of < 1600 °C.
2. In the past for normal reactor conditions, irradiation testing has been performed in
material test reactors and in the AVR- Parameters such as bum up, operating temperature
and fast neutron fluence are varied to assess fuel performance. Continuous monitoring of
released fission gas during irradiation tests gave a direct indication of the integrity of fuel
coatings.
3. In the German program, relevant irradiation tests with more than 2x10s particles
were performed without a single coated particle failure during irradiation.
Statistically, this result corresponds to a 95% confidence level that the coating -5
failure fraction is less than 2x10
4. Postirradiation examinations had been carried out in the FZJ - Hot Cell Laboratories. One
of the most important examination method are the heating tests for simulating accident
conditions in special designed and constructed furnaces.(e.g. KUFA coldfmger furnace)
These tests under off-normal conditions has provided fuel performance information as a
function of burn up, fast neutron fluence, heating time and temperature up to 2500 0C.
5. Kr 85 gas release fractions during accident condition testing up to 1600 'C were low
at <10 6 , even at 1800 'C for 50-100 h. With >11% FIMA fuel, release remains at this
low level throughout a 350 h test at 1600 OC. At 1800 (C, 10-3 release fractions are
reached as a consequence of diffusion through degraded SiC.
6. At 1600 OC the fuel does not suffer irreversible changes and continues to retain all
safety- relevant fission products ( e.g. Cs, 1, Sr ). Ag 110m diffuses at 1200- 1600 OC
through intact SiC, but the amount of the generated silver is low.
7. Know how transfer with ESCOM representatives is going on by the author. Additional
experiments should be performed with higher temperatures, longer heating time and with
fuel from accelerated tests to establish the performance margins under accident conditions
of new designed reactors. This means also to irradiate actual fuel produced for the new
ESCOM reactors.
PBMR
SPHR"AL. FUEL EIL W
Fuel Free Zone
Graphite Matrix
Coated Particle
Fuel Element
Kernel
Silicon carbide layer
Pyrocarbon layers
Section of a Coated Particle
60 mm
±L
1 mM
_L
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IRRADIATION QUALIFICATION OF HTR FUEL ELEMENTS
* TEST FOR DETERMINATION OF PARTICLE
DEFECT RATES UNDER CONDITIONES
EXCEEDING NORMAL OPERATING
CONDITIONS 800-12000 C
* IRRADIATIONS OF FUEL PARTICLES WITH
KNOWN FAILURE FRACTION 800-13000 C
* TEST FOR DETERMINATION OF BURN UP
INFLUENCES ON DEFECT RATES
1000-12000 C
* FUEL ELEMENT REFERENCE TESTS
Sweep Gas Inlet
Thermocouple
075
FULL SIZE SPHERE RIG FRJ-2
Outlet
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Release Rate R/B Kr 85m
0 200 400 600
Experiment FRJ 2-K15 - Fission Gas Release
KFA-HZ-3/94
1
10-7
200 400
- f�i� �
600
HTR REFERENCE TEST , FISSION GAS RELEASE
(HFR-K6)
KFA-HZ-3/94
Experiment Specimens Number Irrad. Temp. Burnup Fluence In-pile Release per of time centre (1025 M-2 R/B