Neutron Scattering

ofthe reactor being a point source . In fact the core of the FRJ-2 consists of 25 tubular, 60 cm
high fuel elements arranged within a lateral grid of about one meter in diameter . The core is
submersed in and cooled by heavy waten streaming through the tubes . Figure 1 .1 shows a plan
cross sectional view ofthe reactor block .
The FRJ-2 is operated with highly enriched uranium 235U . With the existing relaxed fuel
element arrangement an essential neutron flux enhancement, e .g . by an order of magnitude,
were only possible with a corresponding but unwanted power increase . A different possibility
exists in compacting the core, a solution chosen for the high flux reactor at the Institut Laue-
Langevin in Grenoble, France . In fact, its core consists of a single annular fuel element of
40 cm outer and 20 cm inner diameter, respectively . Operated at 57 MW, a disturbed flux at
the beam tube noses of fia, =1.2x10 15 n / (cm2 s) is obtained .
Technical limitations
We have just established a relation between neutron yield and reactor power released as heat .
Disregarding for the moment investment and operation costs, the limiting factor for achievable
neutron yields is the power or, to be more precise, the power density in the reactor core .
This technically decisive factor, the power density (1VIW/liter), was not included in the
number given in the previous section, because it depends on the details of the reactor, in
particular the core size, the uranium enrichment and the fuel density in the fuel elements . The
size of the primary neutron source (reactor core, target volume, etc.) is important for a high
flux of thermal neutrons within the moderator . In Table 1 .A .1 of the Appendix a selection of
reactions is given and related to its neutron yields and power densities .
It is now well established that power densities in reactor cores cannot substantially be increased
without unwanted and impracticable consequences, such as liquid sodium cooling . In
particular, the service time of reactor vessel components like beam tube noses or cold sources
would become intolerably short due to radiation damage . Experience with the Grenoble High
Flux Reactor shows that these service times are of the order of seven years . Ten tunes higher
fluxes would result in impracticable service times under one year .
1 .4 Pulsed contra continuons sources
Regarding these arguments, we may ask ourselves, whether high flux reactors have already
reached a fondamental lirait . This were ce-tainly the case, if we expected a flux increase by
another order of magnitude like the one observed in reactor development since the fifties (sec
Table 1 .1).