RRFM 2009 Transactions - European Nuclear Society

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Challenges
When operating BR2, fresh fuel is initially loaded in a position at the periphery of the core and
progressively moved towards the core center as the burnup accumulates. Furthermore, burnable
poisons incorporated in the fuel help to mitigate the reactivity drop induced by the burnup.
Fig.4: H1 core cross section
and the Aluminium-water
central plug
The RJH fuel does not
incorporated burnable
poisons and there is only
one possible irradiation
position. Hence, in this
irradiation, the challenge
is to operate BR2 with a
fuel element in its center,
operating at a required
power level, while still
being able to operate the
reactor at a convenient
power level for the other
users at the periphery.
This is achieved by several means. First, there is the choice of materials surrounding the central
fuel, a mixture aluminium-water which presents optimum absorption – moderation –
transmission characteristics when the fuel is fresh and most reactive. Then, by playing on the
number of Iridium production targets surrounding the central fuel, it is possible to compensate
the decrease of reactivity of the central fuel caused by burnup accumulation and still operate
BR2 at constant power. At the end of life, the aluminium-water mixture has to be replaced by
beryllium, which was found better suited for depleted fuel.
The accuracy of the BR2 load determination before each cycle will be a critical element for this
irradiation.
Power generated in the RJH Fuel Element
The summary Table 1 contains the calculated power deposited into the cooling water between
the fuel plates, and in both plugs. The total power in the fresh RJH fuel element was calculated
for the model with Al plug in the central trap of the fuel element and for the power of BR2
equal to Q BR2 =62 MW.
Irradiation
1 Fission and decay reactions 4.77 MW
2 γ prompt in cooling water 61 kW
3 γ prompt in Al cladding 77 kW
4
28 Al decay in the cladding 4 kW
5 Deposit of energy by γ delayed emitted by fission products to 47 kW
the cooling water and to Al cladding
6 Total in fuel element, MW 5.0 MW
Table 1. Thermal power in the RJH fuel element.
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