NuclearReactor_FRM2.exe - a simplified online simulation of the new research reactor FRM2 in Munich
The FRM-II in Munich is a nuclear reactor that produces neutrons for research, medicin and technology. The reacor core is kept very small - it consists only of one fuel element - for the following reason: in this way the probability that a neutron generated by fission can leave the reactor is much higher than in a large power reactor. With this trick the share of neutrons per unit fissioned uranium which is available for experiments and for the irradiation of material is especially high. The amount of energy that cannot be used and of radwaste which is produced during the operation is kept low in this way. To keep the chain reaction running in such a small reactor core highly enriched uranium has to be used to avoid that a lot of those neutrons that remain in the core are absorbed by the non-fissile uranium isotope U-238.
In the simulation you can see the fuel element in the centre of the reactor surrounded by the moderator tank, which is filled with heavy water (D2O). This is the second measure to keep the neutron gain high, since heavy water absorbes less neurons than normal water. With other words, heavy water is the better moderator than ordinary water. Just the outer tank that serves for radiation protection is filled with light water. Also the coolant, which is pumped through the fuel element, is normal H2O, of course very pure one.
To start up the reactor, use the neutron source and the control buttons for the safety rod and the control rod. Take care not to exceed the measuring range of the neutron detector (3 % - 90 %), otherwise you'll produce a a safety shout down called SCRAM and you can restart from the beginning. Avoid this by changing the measuring range of the neutron detector in time. Another reason for a SCRAM is a too high reactor power (120 %). For more details on how to control the reactor - click here!
When the power gets close to 100 %, you will see how the produced neutrons stream out of the irradiation channels. At the real reactor, experimental facilities are connected to these ducts. Our small virtual reactor has two neutron guides: one for thermal (slow) neutrons (right) and a second one for fast neutrons (left). The thermal neutrons are simply taken from the cloud of neutrons that swirr around close to the fuel element in the moderator tank. The corresponding neutron guide simply ends close to the location where the highest neutron density is found. A strong flux of fast neutrons is generated by putting a convertion target made from fissile uranium in front of the second duct. The thermal neutrons induce fissions in this plate, which eliberate a stream of fast fission neutrons. Thermal neutrons contained in this stream are filtered out by a special absorber foil.
To tell the truth, we used some tricks to increase the flow of neutrons coming out of the irradiation channels: The converter target is programmed in a way that it releases fast neutrons preferably in the direction of the neutron duct. This will not work at the real reactor, but there we have much much more neutrons in the reactor than we can simulate on the PC. That's why the collegues in Garching manage to produce the high fluxes without such not-existing tricks. But we used a similar trick for the thermal neutron guide, too.
The reactor was officially put in operation on 9. June 2004 in a public ceremony. With the program NuclearReactor_FRM2.exe you can do the same in a virtual way on your PC at home. With the addition of a glass of sparkling wine - which must be excluded from a real reactor control room - you can surely create a little festive atmosphere, too.