atw 2018-07

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atw Vol. 63 (2018) | Issue 6/7 ı June/July

374

AMNT 2018

TESPA-ROD Code Prediction of the Fuel

Rod Behaviour During Long-term Storage

Heinz G. Sonnenburg

| | AMNT 2018: Best Paper Award, awarded by Dr. Erwin Fischer (left) to

Dr. Heinz G. Sonnenburg (right).

| | Fig. 1.

Crystallographic length change of UO 2 /PuO 2 -fuel relative to displacement per atom (dpa) [RAY 15].

The paper “TESPA-

ROD Code Prediction

of the Fuel Rod

Behaviour During

Long-term Storage” by

Heinz G. Sonnenburg

has been awarded as

Best Paper of the

49 th Annual Meeting

on Nuclear Technology

(AMNT 2018), Berlin,

29 and 30 May 2018.

Introduction The TESPA-ROD code is applicable to both LOCA and RIA transients. Recently, the code’s models

have been extended in order to predict the transitional fuel rod behaviour during long-term storage [SON 17].

Due to permanent α-decay of actinides

in the fuel during long-term storage,

both fuel swelling and helium release

continue and generate an impact on

the fuel rod behaviour. Therefore, the

TESPA-ROD code extension requires

particular modelling of fuel swelling

and modelling of the associated

helium gas release. These processes

and their modelling have significant

impact on the prediction of cladding’s

stress level.

Continued fuel swelling reduces

the gap between fuel and cladding

which reduces the fuel rod fission

gas volume and might increase

the fuel rod inner pressure by that.

Simultaneously, the release of

helium tends to keep the rod

internal pressure high, thus the

gap between fuel and cladding could

be enlarged. If fuel swelling is the

dominating process, as in case of

MOX fuel, even gap closure might

occur which leads to pellet- cladding

interaction which finally enhances

significantly the stress level in the

cladding. A priori, which effect

dominates cannot be estimated with

simple engineering judgment. Therefore,

a code prediction is inevitable

in order to get reliable estimates about

dominating processes.

Fuel swelling

Fuel in a fuel rod accumulates fission

gases in the fuel matrix during normal

operation. E.g., small gas bubbles of

micrometer size appear within the

fuel grain at higher burn-up levels.

Because the accumulation of fission

gas in the fuel matrix is limited, some

quantity of fission gas will get released

from fuel.

There is a well-known interlinkage

between the accumulation of fission

gas and swelling of the pellet. The

more the fuel accumulates fission gas,

the more the fuel swells.

The same mechanism is true for

the long-term storage, but here helium

is accumulated instead of fission

gases. This helium stems from the

decay of α-emitting actinides.

Patrick Raynaud [RAY 15] from

US.NRC has compiled fuel swelling

correlations for UO 2 fuel and PuO 2

fuel which refer to the α-decay in the

fuel (Figure 1). Correlating parameter

is dpa (displacement per atom). This

compilation reveals a swelling mechanism

which saturates at a certain

maximal swelling level. Consequently,

the swelling can be expressed as

exponential function:

upper bounding values (1)

and

mean values (2)

where ∆a is the change of lattice

parameter, a 0 is the undeformed

lattice parameter.

The parameter dpa correlates with

time. Raynaud /RAY 15/ provides for

60 GWd/t UO 2 fuel the relation

dpa(t) =0.01172 t 0.72246 , where t is

measured in years. In case of MOX

fuel, this relation can be multiplied by

3, because MOX fuel has 3-times more

α-decays, see figure 5.3 on page 54 in

[SON 17].

The swelling mechanism, as correlated

above, refers mainly to the production

of Frenkel pairs and helium

atoms at interstitial positions in the

crystal structure of UO 2 . The effect

AMNT 2018

TESPA-ROD Code Prediction of the Fuel Rod Behaviour During Long-term Storage ı Heinz G. Sonnenburg

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