Studies on excess reactivity in the large size innovative fast reactors in CNFC

Closure of nuclear fuel cycle is now addressed as an approach to solving vital problems of existing nuclear power and as pathway to development of the large-scale nuclear power based on fast reactors (FR). Closed nuclear fuel cycle (CNFC) makes it possible to ultimately solve the problem of spent nuclear fuel (SNF) and reduce radwaste related radioecological hazard down to acceptable level of radiological equivalence [1, 2]. Fast neutron reactors using plutonium extracted from SNF with its repeated recycling are key, systemically important component of CNFC, in which long-lived radwaste, so-called minor actinides (MA) can be successfully incinerated. Suite of FR and CNFC technology is being intensively developed in Russia within the framework of «Proryv» project stream [3-5]. Apart from solving problems related to SNF and radwaste, FR should demonstrate the new level of safety, which has not been reached before. In this safety-grade reactor plant, the potential of Chernobyl type severe accident would be deterministically eliminated. In particular, V.V. Orlov proposed that excess reactivity value in FR would not exceed β_eff, thus eliminating prompt neutron reactivity accidents [6]. This is achieved by assuring high breeding ratio of the core (equal to at least 1). Certainly, such a possibility theoretically exists if high-density fuel is used in the reactor core. In this view mixed nitride uranium-plutonium, (MNUP) fuel is called for within the framework of «Proryv» project.On condition of the core-breeding ratio close to 1 the new technology allows realization of equilibrium mode characterized by stability of both reactivity and fuel isotopic composition. However reactor should be operated in transient mode for a long time period (more than 10 years), and this operation requires special measures for controlling reactivity windup during core lifetime.In the article presented are the main guidelines of automated modeling of the entire life cycle of the core of FR operating in CNFC with repeated fuel recycling. A designated algorithm of plutonium mass fraction selection assuring reactor criticality in the beginning of the core run was worked out for making critical loadings. Results of simulation of the whole life cycle of fast reactors with lead and sodium coolants in the closed fuel cycle are presented, as well as the results of solution of some practical problems concerning implementation of low excess reactivity concept in order to eliminate prompt neutron reactivity accidents. It was demonstrated that MA introduction had beneficial effect on max value of excess reactivity, and MA mass fraction might be the parameter allowing transient mode transformation into equilibrium mode of CNFC.

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