Editing Subcritical reactor (section)

== Principle ==
Most current ADS designs propose a high-intensity [[proton]]  [[Particle accelerator|accelerator]] with an energy of about 1 [[GeV]], directed towards a [[spallation]] target or spallation neutron source. The source located in the heart of the reactor core contains liquid metal which is impacted by the beam, thus releasing neutrons and is cooled by circulating the liquid metal such as [[lead]]-[[bismuth]] towards a heat exchanger. The [[nuclear reactor core]] surrounding the spallation [[neutron source]] contains the fuel rods, the fuel being any fissile or fertile actinide mix, but preferable already with a certain amount of fissile material to not have to run at zero criticality during startup. Thereby, for each proton intersecting the spallation target, an average of 20 [[neutron]]s is released which [[Nuclear fission|fission]] the surrounding fissile part of the fuel and transmute atoms in the fertile part, "breeding" new fissile material. If the value of 20 neutrons per GeV expended is assumed, one neutron "costs" 50 MeV while fission (which requires one neutron) releases on the order of 200 MeV per actinide atom that is split. Efficiency can be increased by reducing the energy needed to produce a neutron, increasing the share of usable energy extracted from the fission (if a thermal process is used [[Carnot efficiency]] dictates that higher temperatures are needed to increase efficiency) and finally by getting criticality ever closer to 1 while still staying below it. An important factor in both efficiency and safety is ''how'' subcritical the reactor is. To simplify, the value of k(effective) that is used to give the criticality of a reactor (including delayed neutrons) can be interpreted as how many neutrons of each "generation" fission further nuclei. If k(effective) is 1, for every 1000 neutrons introduced, 1000 neutrons are produced that also fission further nuclei. Obviously the reaction rate would steadily increase in that case due to more and more neutrons being delivered from the neutron source. If k(effective) is ''just below'' 1, few neutrons have to be delivered from outside the reactor to keep the reaction at a steady state, increasing efficiency. On the other hand, in the extreme case of "zero criticality", that is k(effective)=0 (e.g. If the reactor is run for transmutation alone) ''all'' neutrons are "consumed" and none are produced inside the fuel. However, as [[neutronics]] can only ever be known to a certain degree of precision, the reactor must in practice allow a safety margin below criticality that depends on how well the neutronics are known and on the effect of the ingrowth of nuclides that decay via neutron emitting [[spontaneous fission]] such as [[Californium-252]] or of nuclides that decay via [[neutron emission]].

The neutron balance can be regulated or indeed shut off by adjusting the accelerator power so that the reactor would be below [[Nuclear reactor physics#Criticality|criticality]]. The additional neutrons provided by the spallation [[neutron source]] provide the degree of control as do the [[delayed neutron]]s in a conventional [[nuclear reactor]], the difference being that spallation neutron source-driven neutrons are easily controlled by the accelerator. The main advantage is [[inherent safety]]. A conventional [[nuclear reactor]]'s [[nuclear fuel]] possesses self-regulating properties such as the Doppler effect or void effect, which make these [[nuclear reactor]]s safe. In addition to these physical properties of conventional reactors, in the subcritical reactor, whenever the neutron source is turned off, the fission reaction ceases and only the decay heat remains.

[[File:ADR-schema.svg|thumb|The principle of operation of an accelerator-driven reactor]]