5. Characteristics of abwr Accident Analysis

Characteristics of ABWR accident analysis with newly-adopted internal pumps and new-type control rod drives are summarized next

(1) Adoption of internal pumps

Internal pumps are adopted in the ABWR design, instead of the previously used recirculation loops thus eliminating the possibility of a large break at a liquid phase region. Therefore, the facility design has been focused on medium and small breaks, enhancing the high pressure ECCS (adoption of two HPCF and RCIC systems in the ECCS).

These changes eliminated the possibility of core exposure during "loss of reactor coolant", which has been assumed to occur in existing reactors, and core flooding can be maintained throughout the event Moreover, the fuel cladding temperature does not rise due to core exposure, but the fuel cladding temperature reaches its highest value due to the boiling transition occurring immediately after the accident (due to a recirculation pump trip by assumption of a loss of power).

Examples of HPCF pipe rupture analysis, which show the change in the reactor water level and the maximum fuel cladding temperature, are provided in Figures 7.3.3 (1) and (2), respectively.

In the "loss of reactor coolant flow" accident, ten internal pumps are assumed to trip simultaneously, and boiling transition occurs due to a temporary mismatch between heat generation and heat removal caused by a large reduction rate of core flow. ABWRs are provided with logic circuits for scram on rapid core flow reduction so as to mitigate the fuel cladding temperature rise after this boiling transition.

The "locked-rotor of a reactor coolant pump" accident would give a small effect on the nuclear reactor even with one pump seizing among ten pumps, and as the result is sufficiently enveloped in that of the "loss of reactor coolant flow", its accident analysis is omitted for ABWRs.

Fig. 7.3.3 (1) Change in the reactor water level during a HPCF double-ended pipe break accident (ABWR)

Fig. 7.3.3 (2) Temperature change at the position with the maximum fuel cladding temperature during a HPCF double- ended pipe break accident (ABWR)

(2) Change in control rod drive design

The design of control rod drives (CRDs) for ABWRs has been drastically changed from the previous one to reduce the possibility of a CR drop accident. For a CR drop accident related to the previous drive, it is assumed that a CR drops when it is separated from its CRD at the coupling section. For ABWRs, the bio-net mechanism is adopted (a coupling would not become separated unless rotated by 45 degrees). Therefore, for ABWRs, it is assumed that a CR drops together with the CRD driving shaft, which makes the drop speed slightly slower than that for the previous design; 0.95 vs. 0.7 m/s. However, the fundamental accident sequence is not different from the previous one.

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NSRA, Japan