Studies on excess reactivity in the large size innovative fast reactors in a closed NFC
Closure of the nuclear fuel cycle is now seen as an approach to solving the pressing problems of the existing nuclear power industry and as a way to develop 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 an acceptable level of radiological equivalence [1, 2]. Fast reactors using plutonium recovered from spent nuclear fuel with its repeated reprocessing are a key, systemically important component of CNFCs, in which long-lived radioactive waste, the so-called minor actinides (MA), can be successfully incinerated. The suite of FR and CNFC technologies is being intensively developed in Russia within the framework of the «Proryv» project stream [3 – 5].
Apart from solving problems related to SNF and radwaste, FRs should demonstrate the new level of safety that has not been reached before. In this safety-grade reactor plants, the possibility of a Chernobyl-type severe accident would be deterministically eliminated. In particular, V.V. Orlov suggested that excess reactivity value in FRs should not exceed βeff., thus eliminating prompt neutron reactivity accidents . This is achieved by ensuring a 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 in demand within the framework of the «Proryv» project. Provided that the core breeding ratio is close to 1, the new technology makes it possible to implement equilibrium mode characterized by the stability of both reactivity and isotopic composition of the fuel. However, the reactors should be operated in a transient mode for a long period (more than 10 years), and this operation requires special measures to control the reactivity scram during the core lifetime.
The article presents the main directions of automated modeling of the entire life cycle of the FR core operating in a 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.
The authors present the results of modeling the entire life cycle of fast reactors with lead and sodium coolants in a closed fuel cycle, as well as the results of solving some practical problems to implement the concept of low excess reactivity in order to eliminate of prompt neutron reactivity accidents. It was demonstrated that the introduction of MAs had beneficial effect on the maximum value of excess reactivity, and the mass fraction of MAs might be the parameter that makes it possible to transform transient mode into CNFC equilibrium mode.
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- Egorov A.V., Khomyakov Yu.S., Rachkov V.I., Rodina Ye.A., Suslov I.R. Minor Actinides Transmutation in Equilibrium Cores of Next Generation FRs. Nuclear Energy and Technology. 2019, v. 5 (4), pp. 353-359; DOI: https://doi.org/10.3897/nucet.5.46517.