Approaches to achieve long refuelling interval

Design approaches for extending refuelling interval differ according to the choices for fuel and for the neutron spectmm. The reactor concepts described in this report employ four major fuel systems  

These different fuel systems are capable of very different discharge bum-ups, controlled by very different physical phenomena. 

The situation is quite different for fast neutron spectmm reactors where burnable poisons do not exist but excellent neutron economy allows for plutonium breeding to be conducted internal to the core lattice itself to produce new fissile materials that can be bred in-situ to completely compensate reactivity loss with bum-up. 

A second consideration, which is peculiar to the fast reactors, exists. Some licensing authorities may simply forbid fast reactor designs to exhibit a positive coolant void worth, even when the overall power coefficient of reactivity is negative. For deployments of fast reactors in countries where that would be the case, two options are available  

As an example, the first choice has been made for the 4S-LMR CRIEPI Design (15) while the second choice is made for the near-term configurations of the SVBR-75/100 (18). 

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