Topics #4 Safety Margins against Natural Hazards

Transcription

1 Technical Meeting on Safety Challenges for New NPPs Topics #4 Safety Margins against Natural Hazards Tsuyoshi Nakajima, NRA-Japan 1

2 Discussion Aspects Focused on natural hazard for design margin, especially in extreme external events Not for site evaluation but for design Could be tough to harmonize here address MS practices and key issues.. Goal: Common understanding how to secure safety margin for natural hazard 2

3 Definitions A cliff edge effect, in a nuclear power plant, is an instance of severely abnormal plant behaviour caused by an abrupt transition from one plant status to another following a small deviation in a plant parameter, and thus a sudden large variation in plant conditions in response to a small variation in an input. The safety margin is understood as the result of the conservative assumptions and conservative rules applied for the design that provides the structures, systems and components the capability to safely perform even in situations more severe than those postulated in the design basis without the incurrence of cliff-edge effects. 3

4 Information we have now are 1. TECDOC 1332 Safety margins of operating reactors Analysis of uncertainties and implications for decision making, OECD/NEA Safety Margin Evaluation-SMAP, Nov., SSR-2/1 (Rev.1), The TECDOC. Cliff edge effect and safety margin 5. CNRA, Green book Defence in Depth for NPPs Post the 1F accidents Not so clearly just only focused on Natural Hazards 4

5 SSR-2/1 Rev.1 PREFACE: Preventing severe accident through strengthening the plant design basis, including strengthening the independence of level four of defence-in-depth, consideration of external hazards and sufficient margins Footnote:. External hazards The design shall include due consideration of those natural and human induced external events (i.e. events of origin external to the plant) that have been identified in the site evaluation process. Causation and likelihood shall be considered in postulating potential hazards. In the short term, the safety of the plant shall not be permitted to be dependent on the availability of off-site services such as electricity supply and fire fighting services. The design shall take due account of site specific conditions to determine the maximum delay time by which off-site services need to be available Features shall be provided to minimize any interactions between buildings containing items important to safety (including power cabling and control cabling) and any other plant structure as a result of external events considered in the design. 5

6 SSR-2/1 Rev The design of the plant shall provide for an adequate margin to protect items important to safety against levels of external hazards to be considered for design taking into account the site hazard evaluation, and to avoid cliff edge effects. 5.21a. The design of the plant shall provide for an adequate margin to protect items ultimately necessary to prevent large or early radioactive releases in the event of levels of natural hazards exceeding those to be considered for design taking into account the site hazard evaluation The safety analysis shall provide assurance that uncertainties have been given adequate consideration in the design of the plant and especially that adequate margins are available to avoid cliff edge effects and large or early radioactive releases. 6

7 The TECDOC for SSR-2/1 Rev.1 Cliff-edge effects and safety margins: the requirements addressing cliff-edge effects and safety margins are discussed to provide a more detailed understanding of the concepts and their implications on the design. Functional independency, diversity, for instance on instrumentation, power supply or heat sink, as well as stronger safety margins and protection against external hazards, are among the measures to prevent common cause failures from stretching through different levels of defence in depth. 7

8 CNRA, Green book External Hazards and Events The importance of adequate consideration for extreme external hazards/events cannot be overstated given the impact that they can have on independent effectiveness of the SCCs provided at the various levels of defence, the creation of a common cause/fault, and on the capability of response on and off the site. This is well illustrated by the accident. When considering extreme external events particular attention should be given to cliff edge effects. Additionally, where there are significant uncertainties in the derivation of design basis external events then suitable margins should be included. 8

9 Practice #1 as EU stress test and others Practice in low-to-medium seismicity countries considers an increase of about 50% above design basis levels for an adequate evaluation of the BDB earthquake. may be interpreted as required 50% increase of the DB That a stress test did demonstrate the robustness of the SSCs important to safety against 50% increase seismic motion. +50%: France for earthquake. Just demonstration? +67%: USA for earthquake?. 9

10 Practice #2 in Japan for earthquake 1. More stringent criteria for active faults 2. More precise methods to define design basis seismic ground motion 3. Clarification of requirements for displacement and deformation in addition to the seismic ground motion Design basis for the ground motions are the same between DBA and DEC. Appropriate margins should be demonstrated in license applications. 10

11 Discussion Points 1. Identify natural hazard for MSs.. earthquake, tsunami, flooding, tornado, hurricane, volcano, fire, ice storm, or combination? 2. Identify factors to be effect on design margin Cliff edge effects, very low frequency but severe, 3. Share and categorize MSs practices for Each natural hazard Plant conditions such as DBA/DEC or DBEE/BDBEE Design basis of methodology or demonstration of robustness Extreme External Events. Consequence/frequency 4. How to apply codes & standards to fulfill some stress limits Conservative/Realistic/Best Estimate Envelope/Linear/Statistics/Detail model/. Enough for avoiding Cliff edge effects? 11

12 Conclusions 1. Defined cliff edge effects and safety margin more clear definitions are necessary 2. MSs practices for natural hazard in design earthquake, flooding include. tsunami, tornado, hurricane, winds missiles, volcano, fire, ice storm, aircraft crash, high tide, lighting attack 3. Approach for margins against flooding is clearer 4. Approach for margins against earthquake are different in MSs even if codes and standards are systematically applicable in the most MSs. depending on availability of historical data, updating time, uncertainties, built in margin, total level of gal and consequences 5. Should be discussed with seismic experts 12

13 Summary of MS Practices MS Events DBA DEC Remarks br ca tornado cn cz fi fr hu ir jp Eq, Tsu, Vo, po ua ru USA de 13