Aspects of the Interface between Severe Accident Management and Emergency Preparedness and Response

Transcription

1 Aspects of the Interface between Severe Accident Management and Emergency Preparedness and Response George Vayssier, NSC Netherlands, Bob Lutz, Lutz Consultancy, USA

2 Why this paper? Nuclear Power Plants (NPPs) have extensive sets of Severe Accident Management Guidelines (SAMG) All NPPs also have an Emergency Plan /Emergency Preparedness (EP) Yet, the authors experience says: there is limited interface between these two Better interaction may lead to better knowledge of the real threat and prevent, for example, unnecessary evacuation with all its vast and often catastrophic consequences Upgraded SAMG currently being implemented may address some of the issues that follow but there is no experience yet NSC Netherlands 2

3 Observations -1v8 The plants SAMG offer extensive guidance to mitigate the consequences of a severe accident Tries to bring the NPP to a safe and stable end state Mitigate any releases during the evolution of the accident But there is mostly no guidance to estimate the upcoming source term as a function of time There is extensive on- and off-site radiation protection, but usually based on measurements, not on predicted source terms Prediction may also help on-site support (repairs, use of portable equipment) by field operators One should not only estimate the potential source term /release, but also the probability of the source term/release, which may vary along the SAMG execution Inform the authorities both of increasing as well as decreasing risk during SAMG execution NSC Netherlands 3

4 Observations -2v8 Most SAMG have been set up so that NPP staff does not need to be severe accident experts Focussed on following a pre-defined set of parameters, with limited coupling to the phenomena which the associated guidelines should counteract. E.g. for the Westinghouse Owners Group SAMG seven parameters need to be followed (generically), they are thresholds for individual severe accident guidelines (SAGs) Inside the SAMG there are Computational Aids but they serve the SAGs, not information needs, such as the amount of core damage E.g. for the BWR Owners Group SAMG various levels in the Reactor Pressure Vessel (RPV), and variables for the pressure suppression pool and the containment NSC Netherlands 4

5 Observations -3v8 In addition, all SAMG has been developed such that there is no need to recognize the underlying scenario But the underlying scenario will determine what the source term will be See e.g. the PSA level 2 end states As a consequence, NPP staff is mostly not trained to handle severe accident phenomena, but to handle parameters But this will only work as long as the accident evolves inside or close to the scenarios used for the design of the SAMG NSC Netherlands 5

6 Observations 4v8 Consequently, most operators/tsc are not trained to recognize the scenario, hence cannot predict the source term If the accident evolves outside the SAMG design basis, the SAMG may not work` any longer, and other actions may be needed. Operators /TSC are usually not trained for such needed deviations from the default guidance Routine A/M` versus Emergency A/M` Recognition of the scenario may become even more difficult Limited guidance for recognition of the plant damage state was available in the SAMG by the Combustion Engineering OG and Babcock & Wilcox OG (and some Areva) Has been discontinued in the new PWROG SAMG NSC Netherlands 6

7 Observations 5v8 SAMG may include deliberate releases to protect system or component functions Various places in BWROG SAMG actions irrespective of plant releases` - but no reference to EP /off-site consequences Example: can we invoke releases when just a nearby school or hospital is evacuated? Teams developing SAMG and EP guidance have often limited (no?) interaction See e.g. NS-G-2.15 (SAMG) and EPR NPP Public Protection 2013 have no cross references; statements from the latter (next slide): NSC Netherlands 7

8 Observations 6v8 To act promptly means to act before the beginning of a severe release3. The timing of a release is unpredictable, therefore actions to protect the public, in order to be most effective, need to be initiated immediately near the nuclear power plant when predetermined criteria are exceeded`` Radioactive releases warranting protective actions off the site are unpredictable``. The containment could fail at any time and it is impossible to predict when this will happen`` Examples of causes of containment leakage/ failure that are not predicable that can result in a severe release are: (a) hydrogen explosions, (b) overpressure, (c) by-pass, and (d) failure to isolate. The only release pathway that is predictable is intentional venting, and in most cases releases by this route would be smaller``?? NSC Netherlands 8

9 Observations 7v8 SAMG is limited in time after exit of SAMG longterm accident management (LTAM) is taken up Includes site decontamination, repair / replacement of failed equipment But how to maintain highly active components?? Treatment of possibly large quantities of radioactive waste water How do we treat large quantities of run-off water? Is usually neither considered in the plant EP, nor in potential off-site consequences But run-off water potentially damages drinking water sources, fishing in rivers and see, etc. NSC Netherlands 9

10 Observations 8v8 EP is in charge of local measures E.g., shelter, iodine pills, evacuation But are modern communications considered? Messages via cell phones, Facebook, Twitter spread very rapidly, may cause havock, chaos Exceeds speed of authorities at a high rate May lead to misinformation, spreading rapidly over the internet May lead to spontaneous evacuation, disturbing largely the actions by authorities Possible blockage of intended communication channels NSC Netherlands 10

11 Conclusions from these observations - 1v2 Training of NPP staff should include the physics of severe accidents so that they are able to estimate the underlying scenario as far is needed to estimate the upcoming source term to meaningfully deviate from default guidance, should the need arise Additional tools should be developed /available tools should be used to help them estimate the upcoming source term and its probability May include tools to deliver information needs (e.g. amount of core damage), recognition of the EPRI plant damage states and, possibly, the underlying scenario NSC Netherlands 11

12 Conclusions from these observations - 2v2 Teams developing SAMG and guidance for EP should communicate better: No SAGs with irrespective of releases`- consequences of actions should be estimated EP teams should try to obtain information/ estimates from the NPP about the upcoming source term long before any release Radiation protection section of the plant ERO is responsible, should be supported by the TSC Text now with unpredictable` in EP documents should reflect the severe accident knowledge today and the SAMG actually applied Long-term A/M needs to be considered Effect of modern communication methods (Twitter, Facebook) to be considered in EP NSC Netherlands 12

13 Tools to recognize scenarios and predict source terms Existing: Core Damage Assessment method -> Emergency Action Levels (EALs) Containment integrity to be added -> may decrease EAL Use of library of a limited number of MAAP runs select candidates Possible (used?): rapid, interactive sev. acc. simulation codes, to be checked and updated on a limited number of measured parameters Example: OECD Halden CAMS method (EU SAMOS project) Advanced: Bayesian Belief Networks (BBNs) Where no tools exist, following parameter trends rather then point values may also help but this does not seem to be available in existing SAMG Programs which incorporate such techniques: WOG Core Damage Assessment Guideline (CDAG, slide 14) SPRINT program by NRG (next slides 15-18) Similar exists in Germany FASTNET project run by IRSN (see slide 21) NSC Netherlands 13

14 Core Damage Assessment Required by the USNRC after TMI-2 Example in WCAP (public, ~ 2000) Based on containment radiation monitoring and a combination of indicators (core temperature, containment H2, RVLIS, other) Guideline available for plant use Basis for determining EALs from the core damage level Includes the EOPs in use at the moment Does not contain behaviour of containment and the potential release at venting or failure Would be good to have such as some higher EAL Older methods: Sampling of the containment atmosphere (many uncertainties plus time lapse) NSC Netherlands 14

15 Objective for development of BBN (NRG, M. Slootmaker) NSC Netherlands 15

16 Example of BBN (from NRG Petten)

17 Example from Exercise-1 (NRG -Petten) NSC Netherlands 17

18 Example from Exercise-2 (NRG -Petten) NSC Netherlands 18

19 OECD Halden CAMS model architecture (built in in EU SAMOS project next slide is iteration loop) Plant Data Data Adquisition MAAP 4 Predictive Simulator + Tracking Simulator Signal Validation Diagnosis Module Fitting Module Man-Machine Interface PSA Module Strategy Generator Critical Function Monitor

20 Re-start MAAP4 calculation upon User s demand changing the corresponding MAAP4 system status from t=t B Is the INITIATOR EVENT in MAAP4 the same as in Diagnosis module? NO Re-initialize MAAP4 calculation from t=0 upon User s demand YES NO Re-start MAAP4 calculation upon User s demand changing the corresponding MAAP4 system status from t=t A Is there any SAG Entry Criteria reached at the plant? NO Are the SYSTEM STATUS in MAAP4 the same as in Diagnosis module? YES YES YES Is this SAG entry criteria reached in MAAP4 simulation? NO Are the PLANT STATES (FP BARRIERS) in MAAP4 the same as in Diagnosis module? NO Re-start MAAP4 calculation from t=t C changing the corresponding MAAP4 plant states upon User s demand NO Is the trend of this SAG parameter in the plant the same as in MAAP4? YES YES Not re-start MAAP4 CAMS - SAMOS

21 FASTNET project by IRSN (D. Vola) Based on on line expert opinion and so necessitate significant levels of expertise and practice for the emergency management teams. NSC Netherlands 21

22 Conclusions Interaction between EP and SAMG development teams should be improved Text in EP guide should be commensurate with severe accidents insights and use of SAMG SAMG should mention and estimate the impact on EP at statements which involve releases Proper tools and the associated guidance to be developed /if available to be used in the plant ERO to estimate the upcoming source term and its probability include also changes in risk Consequences from LTAM should be considered NSC Netherlands 22