Press Release Back Number(2006)

The target ground motion for improving the seismic safety is about 1,000 gal (seismic tremors at bedrock) for horizontal ground motion and about 700 gal (seismic tremors at bedrock) for vertical ground motion. The table below shows the basis for setting the target ground motion.

The evaluation targeted facilities critical to seismic resistance design. *1

The number of facilities subject to evaluation is shown in the table on the right.

*1: Facilities critical to seismic resistance design include the reactor itself and its peripheral piping, as well as equipment used to shut down or cool the reactor, equipment used to contain radioactivity, fuel equipment and power source and electric power equipment related to these.

The reactor building and its piping, electrical circuits and machinery underwent seismic response analysis at the target ground motion using an analysis model created to combine the local ground, reactor building and the reactor building's large equipment. Based on the acceleration and seismic force found in this analysis, engineers then evaluated the seismic safety margin through a process of evaluation with response magnification*2 and spectrum modal analysis*3. The seismic safety margin of exhaust pipes and other pipes and ducts outside the building was evaluated with seismic response analysis at the target ground motion.

During the seismic safety margin evaluation, for each component making up the facility (i.e. each component being evaluated), the calculated value (generated value) for the stress, etc. resulting from the target ground motion was confirmed to make sure it was within the standard value.

*2: Response magnification is a technique to calculate generated stress, etc. by taking the ratio of response acceleration from the original design and the response acceleration resulting from the target ground motion, and multiplying it by the stress from the original design.

*3: Spectrum modal analysis is a technique used on piping and equipment that have multiple support points and shake in complicated ways. The technique calculates generated stress, etc. by analyzing maximum response acceleration based on the pipe's or equipment's unique cycle, the response acceleration at this unique cycle, and so on.

Engineers evaluated the seismic safety margin of the Reactor No. 4 facilities following completion of the project to improve this margin. The evaluation confirmed that these facilities critical to seismic resistance design, including those in which no modification was necessary, achieved the target seismic safety margin.

The following table provides examples of evaluation results for particularly critical facilities that were evaluated: those that stop, cool or contain the reactor.

These evaluation results have been summarized in the Hamaoka Nuclear Power Station Reactor No. 4 Seismic Safety Margin Improvement Project Evaluation Report. This report contains results of seismic response analysis at the target ground motion and seismic safety margin evaluation procedures and results (for representative facilities and about 130 points used as components evaluated).

*4: Standard values are criterion values used to confirm margins for target ground motion. The standard values used were ones stipulated by specifications or otherwise found to be suitable in tests, etc.

*5: The shearing strain value is evaluated for each floor of a building; the value used is the deformation of the top of the shear wall of each floor divided by the shear wall height.

*6: Insertability is the ability of control rods to insert within a standard time during emergency shutdown of a reactor. Specifically, it means that they can be inserted within 1.62 seconds so that 75% of the neutrons they are capable of absorbing will be absorbed.

*7: The value in the control rods' generated value space is the maximum value for fuel assembly relative displacement (i.e. the displacement between the upper and lower ends and the center).

*8: The value in the control rods' standard value space is the standard value found when confirming, during vibration testing, whether the control rods can be inserted within the standard time, even if there is relative displacement of 40mm in the fuel assembly.