Core disruptive accident

Transition phase studies (beyond the initiation phase of a Hypothetical Core Disruptive Accident)... 

The degraded core research at PNC addresses the fuel failure propagation during local-fault accidents, and physical phenomena during core disruptive accidents (CDAs). 

The analytical results for unprotected loss of flow and unprotected transient overpower as design basis accidents are presented below beyond design basis accidents were not considered at this stage. However, risk of a core disruptive accident due to local blockage of the core is expected to be low since there are no wrapper tubes in the RAPID core. 

Main and auxiliary cooling systems are driven by natural convection. The inherent safety features of the core are enhanced to avoid a core-disruption accident even in anticipated... 

Main and auxiliary cooling systems of the PBWFR are driven by natural convection. The inherent safety features of the core are enhanced to avoid a core disruption accident even in anticipated transients without scram (ATWSs). Specifically, void reactivity for the case when the core, the axial blanket, and the plenum are totally voided is limited by 3 (design modifications are foreseen to make this effect negative). The bum-up reactivity swing during 15 years of operation without refuelling is minimized down to 1.5% AK/K. 

Another problem was posed by the requirement for redundancy, diversity and overall reliability of the decay heat removal scheme for an unusual spectrum of operating and accident situations including the already mentioned hypothetical core disruptive accident, and various external events, especially large earthquakes and airplane crashes. 

Creation of design criteria and methods for furnishing proof of the mastering of the Hypothetical Core Disruptive Accident (HCDA) in the following main areas ... 

The AGT 4 conducts safety studies, especially in connection with phenomena in the core. Assessments of sub-assembly faults, sodium fires, containment loadings, and, in particular, the core disruptive accident are of great importance for supporting the work of the designers team. 

For collection and retention of fuel debris inside the reactor vessel, to prevent formation of a critical mass and contact between fuel and the reactor bottom in beyond design core-disruptive accidents, a special localizing device is provided (Fig. 9.34). This is a tray shaped as truncated cone, with an outer conical shell and a tube with a cap under which there are orifices to allow coolant to flow from the plenum beneath the device. The inner surface of the device is lined with molybdenum alloy sheets. Decay heat from the device is removed by natural convection of primary sodium to the intermediate heat exchangers. This natural convection capability in all loops is increased by virtue of a more favourable layout of the components. 

All requirements of the regulations concerning nuclear, radiological and technical safety are met. The annual radiation dose to an individual of the local population at a distance of 3 km during normal operation of the plant away will not exceed 0.004 mSv, which is much lower than the limit set in Russia and is quite n igible compared with the natural radiation background (less than 0.2%). The assessed value of the frequency of severe core-disruptive accidents for BN-800 is less than lxE-6 per reactor-year. The probability of a radioactivity release corresponding to the limit for severe accidents is not more than l.E-8 per reactor-year. 

There are two features of contemporary fast reactors which have attracted particular attention on the grounds of safety core disruptive accidents (CDAs) caused by positive reactivity transients which may be exacerbated by the fact that coolant void coefficient of reactivity is under some circumstances positive (if the coolant is sodium) and sodium fires. There is no doubt that there will be continuing work during the coming decade to improve safety in both of these areas. 

H. K. Fauske, The role of core disruptive accidents in design and licencing of LMFBRs, Nucl. Saf 17(5), 550-567 (1976). 

The designers of KALIMER suggest that no intervention in the public domain beyond the plant boundary as a consequence of any hypothetical core disruption accident would be required. 

The only BDBEs considered for the KALIMER are hypothetical core disruptive accidents (HCDAs). The probability of such accidents is less than 10 plant-year, which is so low that such events fall into the residual risk classification. However, these events may have the potential consequences that would justify their consideration in the design. 

An important design objective for the KALIMER is to enhance the level of safety to eliminate the need for any intervention in the public domain beyond the plant boundary as a consequence of any hypothetical core disruption accident within the plant. To reduce the core damage probability, a sufficient margin in the fuel and core design is provided. The negative power reactivity coefficient is also crucial in preventing the core damage. 

CheUapandi, P., Srinivasan, G.S., Chetal, S.C., 2013. Primary containment capacity of prototype fast breeder reactor against core disruptive accident loadings. Nuclear Engineering and Design 256, 178-187. 

Nakamura, T., Kaguchi, H., Dcarimoto, I., Kamishima, Y., Koyama, K., Kubo, S., Kotake, S., 2004. Evaluation method for strucmral integrity assessment in core disruptive accident of fast reactor. Nuclear Engineering and Design 227, 97—123. 

JSFR utilizes the advantage of economy of scale by setting the electricity output of 1500 MWe and it has an economic competitiveness that benefits from advanced design, such as simplified and compact structure of the reactor, integration of the intermediate heat exchanger (IHX) and the primary circulation pump, shortened piping layout, and reduction of loop number. Furthermore, a special effort has been made to meet the safety requirements, which include enhancement of passive safety capabilities and the in-vessel retention of degraded core under a core disruptive accident. 

Extensive studies of the LIPOSO accident, including experimental programmes, were performed on the understanding of the various phenomena involved in a pessimistic scenario, on the prevention and protection means and on the calculation models including all the uncertainties. The final result was that the Superphenix safety provisions were able, thanks to the reactor and the core design, to eope with this type of fast loss of core flow accident, despite pessimistic assumptions, which was initially considered as a main initiator of a core disruptive accident. 

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