atw 2018-07
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<strong>atw</strong> Vol. 63 (<strong>2018</strong>) | Issue 6/7 ı June/July<br />
374<br />
AMNT <strong>2018</strong><br />
TESPA-ROD Code Prediction of the Fuel<br />
Rod Behaviour During Long-term Storage<br />
Heinz G. Sonnenburg<br />
| | AMNT <strong>2018</strong>: Best Paper Award, awarded by Dr. Erwin Fischer (left) to<br />
Dr. Heinz G. Sonnenburg (right).<br />
| | Fig. 1.<br />
Crystallographic length change of UO 2 /PuO 2 -fuel relative to displacement per atom (dpa) [RAY 15].<br />
The paper “TESPA-<br />
ROD Code Prediction<br />
of the Fuel Rod<br />
Behaviour During<br />
Long-term Storage” by<br />
Heinz G. Sonnenburg<br />
has been awarded as<br />
Best Paper of the<br />
49 th Annual Meeting<br />
on Nuclear Technology<br />
(AMNT <strong>2018</strong>), Berlin,<br />
29 and 30 May <strong>2018</strong>.<br />
Introduction The TESPA-ROD code is applicable to both LOCA and RIA transients. Recently, the code’s models<br />
have been extended in order to predict the transitional fuel rod behaviour during long-term storage [SON 17].<br />
Due to permanent α-decay of actinides<br />
in the fuel during long-term storage,<br />
both fuel swelling and helium release<br />
continue and generate an impact on<br />
the fuel rod behaviour. Therefore, the<br />
TESPA-ROD code extension requires<br />
particular modelling of fuel swelling<br />
and modelling of the associated<br />
helium gas release. These processes<br />
and their modelling have significant<br />
impact on the prediction of cladding’s<br />
stress level.<br />
Continued fuel swelling reduces<br />
the gap between fuel and cladding<br />
which reduces the fuel rod fission<br />
gas volume and might increase<br />
the fuel rod inner pressure by that.<br />
Simultaneously, the release of<br />
helium tends to keep the rod<br />
internal pressure high, thus the<br />
gap between fuel and cladding could<br />
be enlarged. If fuel swelling is the<br />
dominating process, as in case of<br />
MOX fuel, even gap closure might<br />
occur which leads to pellet- cladding<br />
interaction which finally enhances<br />
significantly the stress level in the<br />
cladding. A priori, which effect<br />
dominates cannot be estimated with<br />
simple engineering judgment. Therefore,<br />
a code prediction is inevitable<br />
in order to get reliable estimates about<br />
dominating processes.<br />
Fuel swelling<br />
Fuel in a fuel rod accumulates fission<br />
gases in the fuel matrix during normal<br />
operation. E.g., small gas bubbles of<br />
micrometer size appear within the<br />
fuel grain at higher burn-up levels.<br />
Because the accumulation of fission<br />
gas in the fuel matrix is limited, some<br />
quantity of fission gas will get released<br />
from fuel.<br />
There is a well-known interlinkage<br />
between the accumulation of fission<br />
gas and swelling of the pellet. The<br />
more the fuel accumulates fission gas,<br />
the more the fuel swells.<br />
The same mechanism is true for<br />
the long-term storage, but here helium<br />
is accumulated instead of fission<br />
gases. This helium stems from the<br />
decay of α-emitting actinides.<br />
Patrick Raynaud [RAY 15] from<br />
US.NRC has compiled fuel swelling<br />
correlations for UO 2 fuel and PuO 2<br />
fuel which refer to the α-decay in the<br />
fuel (Figure 1). Correlating parameter<br />
is dpa (displacement per atom). This<br />
compilation reveals a swelling mechanism<br />
which saturates at a certain<br />
maximal swelling level. Consequently,<br />
the swelling can be expressed as<br />
exponential function:<br />
upper bounding values (1)<br />
and<br />
mean values (2)<br />
where ∆a is the change of lattice<br />
parameter, a 0 is the undeformed<br />
lattice parameter.<br />
The parameter dpa correlates with<br />
time. Raynaud /RAY 15/ provides for<br />
60 GWd/t UO 2 fuel the relation<br />
dpa(t) =0.01172 t 0.72246 , where t is<br />
measured in years. In case of MOX<br />
fuel, this relation can be multiplied by<br />
3, because MOX fuel has 3-times more<br />
α-decays, see figure 5.3 on page 54 in<br />
[SON 17].<br />
The swelling mechanism, as correlated<br />
above, refers mainly to the production<br />
of Frenkel pairs and helium<br />
atoms at interstitial positions in the<br />
crystal structure of UO 2 . The effect<br />
AMNT <strong>2018</strong><br />
TESPA-ROD Code Prediction of the Fuel Rod Behaviour During Long-term Storage ı Heinz G. Sonnenburg