Preliminary design studies should allow selection of the most promising concept, which could be adapted to the different nuclear policies of the European Union members. In a first step, the XADS is viewed as an actinide incinerator. In this context, the fast spectrum is clearly the most appropriate. Two cooling system concepts are considered: the lead-bismuth eutectic (LBE) and helium. Both concepts have advantages in terms of safety and potentialities.

The Pb-Bi can be used both as the spallation target and the core coolant. When used in the core reflector, it allows optimization of long-lived fission product transmutation (TARC effect [57]). Furthermore, the liquid metal can provide a passive way to help residual heat extraction by natural con­vection. As compared with liquid lead, the eutectic decreases the operating temperature, which allows operation of the system at lower temperature, where the problems of structural material corrosion are simplified. Such a demonstrator would not be completely representative of an industrial reactor, concerning the power density in the fuel. Furthermore, bismuth is a very expensive metal and the presence of 209Bi leads to a significant produc­tion of 210Po during irradiation, a highly radiotoxic nucleus. The LBE concept appears to be a good way to test and develop the concept of a lead-cooled core, even if it would be difficult to consider industrial LBE reactors on a large scale.

The gas concept minimizes neutron slowing-down and facilitates reach­ing very fast neutron spectra, which optimize minor actinide incineration. The inspection of the fuel elements during operation is made much easier with gas compared with liquid metal, which requires coolant draining to detect any structural deterioration. The main problem of a gas cooled concept concerns residual heat extraction in an accident situation. Further­more, most of the fuel considered for use in a gas cooled reactor contains carbon (SiC, graphite, etc.), which forbids the presence of oxygen, and thus the presence of air, in the gas. The gas cooled XADS would operate at low temperature, with classical oxide fuels. This configuration would not be representative of a full-scale gas cooled reactor, which would operate at high temperature, with innovative fuels. However, the gas cooled XADS would be a first step in the development of such reactors.

In this context, different cores are studied: a small LBE core (about 20- 40MWth), a larger (80MWth) LBE cooled concept, and a gas cooled core (100MWth). In the gas cooled system, two types of fuel are considered: a standard cladded pin fuel element and a pebble-bed core concept. The spallation target may be a liquid heavy metal (LBE) which could be separated or not from the accelerator by a window. For the gas cooled concept only, a solid spallation target (tungsten) is being studied.

As can be seen, a large range of options is examined, more or less innovative, in order not to be stopped by a technological impossibility, and at the same time encouraging innovative options which could find an application beyond the ADSR concept.

The fuel considered would be, in a first phase, a MOx fuel with high plutonium concentration (around 20-25%). This type of fuel is relatively standard and would allow a precise study of the subcriticality characteristics. In a second step, different innovative fuels can be considered, such as highly enriched minor actinide fuels or the pebble-bed concept. The XADS appears to be an ideal tool to test new types of fuel in favourable safety conditions, as long as it operates far enough from criticality.

One of the goals of this project is to develop a common integral safety approach on both lead and gas cooled cores. A specific analysis of the subcriticality characteristics has to be performed, in order to validate the well-known advantages of the ADSR. In this respect, present experiments, for example MUSE [134] (Cadarache, France), should provide essential

GAS COOLED ADS DEMO

REACTOR VESSEL

PROTON BEAM

Pb-B. INLET

j_-r —► Pb-B. OUTLET

REACTOR VESSEL

REMOVABLE HEAD

CHECK VALVE

COOLANT

INLET

SHUTDOWN

COOLING SYSTEM (2.)

REACTOR THIMBLE

REACTOR VESSEL

CORE FEEDING PLENUM

CORE SUPPORT PLATE

Figure 13.3. Sketch of the gas-cooled ADSR demonstrator proposed by Framatome ANP.

results, in particular concerning the control of the different multiplication factors.

Once the subcriticality specificity is fully understood, it is clear that such a demonstrator could be an ideal tool to test different aspects of the safety of lead or gas cooled fast reactors, whether subcritical or not.

Figure 13.3 shows a first design of a gas cooled (He) core proposed by Framatome ANP. The power of such a reactor is around 100 thermal MW.

Concerning the accelerator, different aspects will play a role in the determination of the chosen type, such as reliability, availability, stability and reproducibility of the power control. Linac and cyclotron types of accelerator will be investigated. The purpose is to provide a proton beam in the range 600 MeV to 1 GeV. For a thermal power of the whole system of 100 thermal MW a beam intensity of a few mA is needed, depending on the subcriticality level which will be chosen. On one hand, the cyclotron concept requires a limited space but is limited in energy or intensity. On the other hand, the Linac concept is not limited in power specification, but requires a huge and expensive installation (several hundred metres for the proton beam). The investment budget will have a determining influence on the choices made.

The beam transport line could reach the spallation target horizontally or vertically. In the first design proposed by Framatome ANP, the beam arrives vertically above the spallation target. This kind of design implies a con­gestion of structural and handling elements above the core: Pb-Bi circulation (spallation target), fuel element handling structures, beam line (and par­ticularly the magnet). In this sense, a beam hitting the spallation target horizontally, associated with vertical element management and horizontal Pb-Bi circulation (on the other side), could simplify the congestion problems. Obviously, the safety aspects of this kind of design must be precisely studied.