RRFM 2009 Transactions - European Nuclear Society

3.3 Reference concept and alternative options for the GFR
GFR potential and deployment scenario
The helium cooled fast reactor is an innovative nuclear system with such attractive features
as a chemically inert and optically transparent coolant, as well as a quasi-decoupling of the
reactor physics from the state of the coolant. Other advantages of the GFR relate to its
potential to operate at high temperature (at least 850°C), which enables in principle the
production of hydrogen or synthetic hydrocarbon fuels in a sustainable manner. On the
downside, since gas is a poorer coolant than liquid metals, key aspects demonstrating the
viability of the GFR include development of a refractory and dense fuel, and robust
management of accidental transients, especially cooling accidents.
A status on the GFR pre-viability has been made at the end of 2007, ending the preconceptual
design phase. A reference set of design options has been proposed for a
2400 MWt GFR [3,4].
The feasibility of the GFR is essentially linked to two demonstrations: the mastery
(fabrication, thermo-mechanical behaviour) of a high fissile content refractory fuel, and the
implementation of appropriate safety systems for the prevention and a robust mitigation of
accidental scenarios (especially depressurization). Because there is no experience available
on the GFR, a first step for demonstrating its feasibility is the operation of a 50-100 MWt
experimental reactor, ALLEGRO, to qualify its specific fuel, materials and operating
principles. Ideally, R&D results expected by 2012-15 could support a decision to construct
ALLEGRO, possibly as a European Joint Undertaking. The next step would be a prototype
GFR that could come 10-15 years after.
A refractory fuel concept for the GFR: reference option and alternatives
The GFR fuel should comply with:
• an operating temperature of 1200°C in normal condi tions and 1600°C in accidental
conditions (to offset the gas poor efficiency as coolant);
• a high fissile atom density and high thermal conductivity, thus triggering a renewed
interest for carbide or nitride fuels;
• a power density in the range of 100 MW/m 3 as a trade-off between minimizing the
plutonium content (lower boundary) and safety (slow-down of adiabatic heat-up).
Attempts to transpose attractive features of HTR fuel particles to fast neutron cores (fission
product confinement, very high temperature resistance, thermal conductivity…) remained
unsuccessful. Two concepts are presently under study: (1) a macro-structured plate-type
fuel and (2) a cylindrical pin-type fuel, similar to LWR and SFR fuel elements, with changes
to enable it to meet GFR requirements Preliminary studies finally led to select a macrostructured
plate-type fuel as the reference. However, the alternative design based on
ceramics clad fuel pins is thoroughly investigated, too.
The reference fuel element consisting of fuel pellets arranged in cells within a ceramics clad
plate is shown on Figure 2. Each cell contains a fuel pellet composed of mixed uranium,
plutonium and minor actinides. The clad is made of composite silicon carbide reinforced with
SiC fibres (SiC-SiCf) for an increased mechanical resistance. The sub-assembly is
composed of a stack of such plates axially piled up in a triangular array and enclosed in a
hexagonal wrapper.
Mixed carbide (U,Pu)C is considered the optimum choice for the actinide compound,
combining excellent neutronic and physical properties (high melting temperature, satisfactory
thermal conductivity).
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