Nuclear fission

[3]Identification of SAM strategies

The main objective of severe accident management (SAM) is to mitigate the consequence of a severe accident and to achieve a long term safe stable state. For successful and efficient SAM first the endangering processes and their likelihood must be recognized. Then SAM strategies have to developed, taking into account their potential benefit and their requirement in terms of human and system resources. It is trivial to ask for such a balanced approach, but it is more than difficult to realize it.
From a theoretical point of view it is very desirable to identify and define SAM based on a well-structured approach, applying full scope PSA models. Such an approach is certainly feasible for the implementation of SAM which involve no too difficult human action, or which consider plant states which can largely be represented by the existing analysis models. Examples for such SAM with limited complexity are passive autocatalytic recombiners (PARs) to cope with hydrogen challenge, or containment venting procedures. PSA can very well quantify the risk reduction due to such SAM, or help designing the SAM (e.g. positioning the PARs inside the containment, or define the design requirements for venting systems).
However, it has to be recalled that L2 PSA deals per definition with plant conditions which are so severely disturbed that it has not been possible to avoid core melt – although preventing core melt is assured by probably the most sophisticated systems and procedures which exist in the history of industry. Therefore, dealing with SAM under core melt conditions has to acknowledge a difficult, probably chaotic and dangerous environment. Staff which has to take action carries the burden that a catastrophic technical or human failure has occurred, and that a disaster is imminent where their health or life is at risk. Evaluating system availability or human actions under such conditions obviously is extremely challenging. In addition, still considerable uncertainties exist in the accident simulation codes, so that the related results are not always a sound basis for judging SAM.
In particular after the Fukushima Dai-ichi disaster there was direct need for rational installment of additional safeguards against extreme and unforeseen circumstances. For almost all plants additional hardware and/or SAM procedures have been or are being implemented. Unfortunately, L2 PSA has only rarely been used as guidance in the decision process. This may be partly due to the difficulty of the issue as mentioned above, and partly to the pressing time constraints which called for urgent action without time available for extensive analyses. A third momentum may be the fact that in some cases the cost for performing detailed analyses may be comparable to the cost of a SAM procedure under consideration.
Having said that, it remains to be stressed that there is unanimous agreement provisions should be made for efficient SAM under severe accident conditions. Furthermore, the selection and design of SAM should be as reasonable as possible. Adequate PSA certainly is a very good basis for decision making. After the hasty activities in the wake of the Fukushima Dai-ichi events, it is advisable to apply PSA now for checking the benefit or possible improvements of the updates made.
Within the issue of applying PSA for the implementation of SAM there are – among others - the following remarkable challenges:

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  Safety grade equipment and also operational equipment should be taken into account.
  
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  Is the SAM analysis restricted to the plant operating staff, or is a crisis team (internal or external to the plant) part of the PSA modelling?
  
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  How to address adverse environmental conditions due to external hazards?
  
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  How to model multi-unit issues (mutual support and/or spread of negative impact from an affected plant to the next one(s))?
  
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  How to model the decision process when there is a conflict of interest (e.g. limited amount of water is available, but two SAM actions require water)?
  
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  How to deal with opposing requirements (a classical issue is venting the containment: it leads to immediate environmental releases, but prevents later catastrophic release)?
  

The tables below present the main risk issues and objectives in case of severe accident phenomenon for PWR and BWR respectively, and some corresponding SAM strategies able to avoid or to limit radiological releases. These tables are just a set of examples and do not represent a complete list. In each plant specific PSA pertinent screening is needed for potential SAMs, followed by an assessment of their impact on the accident evolution.

3.1Main risk issues and objectives in case of severe accident phenomenon - PWR

Risk/Objectives	SAM Strategies or design provisions
In-vessel phase
Confirm entry in SAM	Criteria depending on reactor status (e.g. full power, shutdown state, SBO). Change priority : containment function instead of core integrity.
Get efficient emergency teams	Emergency team activation (local, national, utility, public bodies, … ). Communication, radioprotection, data transmission… Strategy to keep control room, emergency control, crisis centers habitability (radiation protection, team rotation …).
Activate / repair any system which might be useful	Identify systems which are operable and systems which have failed ore are not operable, or could be brought back to operation. Identify systems which are strictly necessary to manage the severe accident.
Decrease RPV pressure	Reduce the RPV pressure to support use of low pressure systems. Reduce pressure to lower than 0.5 MPa (value depending on the NPP design) to avoid DCH during vessel rupture.
Prevent Induced Steam Generator tube rupture	RCS depressurization. Limit SG depressurization. Feed SG with water.
Prevent Containment isolation failure	Check the containment isolation. Close the containment if needed (specific procedures depending on initial reactor state – full power, shutdown states …).
Prevent gaseous release through ventilation	Control the ventilation device (filtration) and limit non filtered release.
Control contaminated liquid release in auxiliary building or in environment	Reinjection of contaminated water in the containment. Isolate leakage. Use circuit with intermediate heat exchangers to avoid direct contamination of the environment. Limit the circulation of contaminated water outside of the reactor containment.
Control of flammable gases (H2)	PARs, igniters, containment inertisation strategy … Control of in-vessel water injection. Control of containment spray system activation.
Control the containment pressurization	CHRS, FCVS, …
Prevent large releases	SG isolation, ventilation control, spray the containment, depressurize the containment, flood the containment.
Prevent vessel rupture	In-vessel water injection. External flooding of RPV (IVR). Containment flooding.
Confirm plant status	Instrumentation use to identify core melt, containment status, radioactive contamination and release.

Risk/Objectives	SAM Strategies or design provisions
Vessel rupture phase
Prevent containment failure due to DCH at RPV failure or vessel uplift	RCS depressurization. Control in-vessel water injection. Containment design (containment design pressure, geometry of internal structures to limit corium dispersion, geometry of the cavity to limit vessel uplift … (for new design).
Prevent containment failure due to ex-vessel steam explosion	Prevent vessel rupture (IVR). Limit water in reactor cavity. Geometry of cavity (large cavity and small flow paths limit risks) (for new design).
Confirm plant status	Instrumentation use (RCS pressure during core melt, vessel rupture).

Risk/Objectives	SAM Strategies or design provisions
Ex-vessel phase
Get efficient emergency teams	Activate additional support for plant-external SAM and related decisions (e.g. venting strategy).
Prevent basemat failure due to MCCI	Prevent vessel rupture (IVR). Optimize geometrical features: large area for corium spreading, large width of the basemat (core-catcher for new design, upgrade for existing NPPs). Suppress containment bypass in the basemat (e.g. close pipes). Control water injection: for corium cooling, to allow corium spreading, to quench the corium after the vessel failure ….
Control of flammable gases (H2, CO)	PARs, igniters, containment inertisation strategy …. Monitor containment atmosphere conditions. Control containment spray system activation.
Control the containment pressurization	CHRS, FCVS, …. Apply containment venting system. Apply containment heat removal circuits able to withstand severe accident conditions.
Prevent gaseous release through ventilation	Control the ventilation device (filtration) and limit non filtered release.
Control contaminated liquid release in auxiliary building or environment	Reinjection of contaminated water into the containment. Isolate leakage. Use circuit with intermediate heat exchangers to avoid direct contamination of the environment. Limit the circulation of contaminated water outside of the reactor containment.
Prevent large releases	Control the pH in the sump, SG isolation, ventilation control, spray the containment, depressurize the containment, flood the containment, protect containment venting filter.
Confirm plant status	Instrumentation use to identify containment status and radioactive contamination and release.

Risk/Objectives	SAM Strategies or design provisions
Other
Mitigate a SFP accidents	Strategy if the SFP is inside the reactor containment. Strategy if the SFP is outside the reactor containment.