Influence of the neutron-kinetic feedback parameter variation on an anticipated transient without SCRAM in a PWR

The complete failure of the reactor scram system upon request during an operational transient is called anticipated transient without scram (ATWS). According to the German regulatory guidelines, postulated ATWS events have to be analyzed with regard to their consequences on the safety of nuclear power plants.

Since the course of ATWS transients is determined by a strong interaction of the neutron kinetics with the thermal hydraulics of the system, coupled 3D neutron kinetic/thermal hydraulic code systems are adequate tools for the analysis of such transients. In the following, the coupled code system DYN3D/ATHLET is applied to the analysis of an ATWS transient. The objective of the present work is to perform a best-estimate analysis with consequent use of a 3D neutron kinetic code (DYN3D) in combination with an advanced thermal hydraulic system code (ATHLET) together with a quantification of differences in the course and the results of transients, which arise from the uncertainties of neutron-physical conditions.

Typically, the complete failure of the main feed water supply is assumed to be the bounding ATWS event with regard to the maximum primary coolant pressure, which can be reached during the transient. The situation is aggravated if the main coolant pumps remain in operation.

For this particular transient, the influence of different neutron-physical conditions on the course of the transient was analyzed. Variations of the reactivity coefficients of the moderator density, the moderator temperature (spectral coefficient) and the fuel temperature were assumed.

One of the most relevant safety parameters in this ATWS event is the primary circuit pressure. It has been found that the spreading of the first pressure maximum is influenced only by the variation of the moderator density coefficient. A variation of the Doppler coefficient contributes only to the second pressure peak. For that reason the spreading of the tolerance limits during the second pressure peak is higher.