Small-sized reactors of different types: Regulatory framework to be re-thought?

Transcription

1 Small-sized reactors of different types: Regulatory framework to be re-thought? TM on Challenges in the Application of Design Safety Requirements for NPPs to SMRs, IAEA, Vienna, 4-8 Sept

2 Starting position/background Severe accidents in nuclear reactors are rare in absolute (3...5 events/ reactor-years) and also in relative terms (compared to other industrial activities); statistics do not contradict PSA results The basic physical process and current technology make today s large reactors susceptible to disturbances and operational deficits Potential large radioactive releases with frightening consequences impair public acceptance; another severe accident somewhere may put the further use of nuclear energy at risk almost everywhere Long-lived nuclear waste (actinides) call for inconceivably long husbandry times and ensured stability of the host geological formation and future societies

3 Tightened-up requirements Overarching goal:make civilian use of nuclear energy safer and more acceptable by reactor and fuel cycle designs that are less dependent on reliable human performance and more robust against malicious interventions/instability of our societies. More specific requirements and ways to achieve them: Elimination of potential reactivity induced accidents by core design or at least controllability by passive means, e.g. by a) sub-critical systems b) weak, negative reactivity coefficients c) small reactivity surplus at start-up with fresh fuel. Forgiveness against loss of active core cooling e.g. by a) low power density and power size (to avoid exceeding critical temperature limits), b) strategies to avoid high fission product inventory, e.g., by dispersed fuel, c) temperature resistant fuel cladding and structural materials, d) sufficient heat storage capability and inherent/passive heat transfer mechanisms No use of reactive, toxic materials or fluids, or avoiding direct contact of reacting substances

4 Tightened-up requirements (continued) Securing structural integrity to avoid geometric disorders or loss of confinement of radioactive inventory, e.g., by a) low primary circuit pressure or rupture proof vital components, b) radiation resistant and robust core structures, c) underground siting for protection against extreme external impact. Avoidance/incineration of long-lived radioisotopes by fuel cycle designs allowing for reduced long-term stewardship e.g., by a) a switch to fuel cycles (thorium) with drastically smaller generation of long-lived minor actinides or waste burner core designs, b) striving for enhanced closed fuel cycles or for long-term stable, high burn-up spent fuel. Ensuring intrinsic proliferation resistance characteristics, e.g., by a) avoiding use of highly enriched uranium (HEU) and high-grade plutonium generation, b) reprocessing spent fuel only if there is a clear plan to minimize the amounted time during which weapons-grade material, notably plutonium, is in separated form, c) online reprocessing or facilities/processes including fuel fabrication at reactor location

5 Basic characteristics of selected SMR concepts Design Approach Evolutionary Innovative - revolutionary Highly innovative - exotic Design Features Water mpower NuScale Sodium PRISM Gas (Helium) HTR-PM Molten Salt SaWB Lead BREST Neutron spectrum thermal fast thermal semi-thermal, fast fast - power density [MW/m 3 ] < (?) pressure [MPa] 14.1? unpressurized 80 unpressurized unpressurized - type pool pool; IHX loop pool, overflow; IHX loop - purpose burner waste burner burner waste burner converter - fuel (enrichment) UO 2 (5%) (U,TRU)O 2 (15%) (U,Th)O 2 (8.5%) U,Th,TRU dissolved (UPu)N - power size MWe 311 MWe 100 MWe 50 MWt 300 MWe - basic safety approach integral design, passive passive inherent/passive inherent/passive inherent/passive - structural material metallic metallic ceramic salt - metallic metallic - construction factory factory on-site/factory factory on-site - siting (reactor) underground underground underground underground above ground

6 General assessment of selected SMR concepts,varying by coolant, neutron spectrum, power density, operating pressure Reduced power size (proportionally reduced fission product inventory) and operating pressure (most concepts unpressurized) lower significantly the potential of catastrophic releases and related driving forces Increased safety margins and rugged design characteristics promise to eliminate the risk of severe core damage, triggered by loss of active heat removal (including SBO events) Active systems and need for early operator actions are largely avoided; need for enforced containment structures, emergency planning and even remote siting is often negated Some fast reactor concepts deserve special attention regarding RIA; specific accident scenarios (fierce chemical reactions, flawed fuel addition, overcooling/freezing or water/air ingress) call for thorough analysis Note:Most designs are at a conceptual stage, safety assessments are mostly preliminary, etc., all giving rise to huge uncertainties

7 Preliminary assessment of selected SMR concepts against tightened-up requirements ranking from excellent () to very poor (--) Candidate reactor concepts varying coolant, neutron spectrum Key requirements Water thermal (Large EPR) Sodium fast (PRISM) Molten Salt fast (SaWB) Helium thermal (HTR-PM) Lead fast (BREST-300) Elimination of RIA /~ Forgiveness against Loss of Active Core Cooling - avoid exceeding critical temperatures - avoid high fission product inventory - provide sufficient heat storage & transfer capacity n.a. 1 ~ n.a /~ 1 Structural Integrity - avoid high operating pressure [suitability of underground siting] - -- [-] Use Non-chemically Reactive/ Toxic Materials (non-stable) 3 [?] [] 4 [] 4 3 [] Avoid Long-lived Radioisotopes -- Enhance Proliferation Resistance - avoid high enriched uranium - - ~ due to small power size 2 in case of dispersed fuel & due to small power size 3 not pressurized but high static load 4 foreseen 5 intermediate cycle (IHX) foreseen 6 close to HEUlower limit

8 Need for modernized regulatory approach? Most developers pretend to basically apply well-established safety objectives and fundamental principles, notably defense in depth, but claim that classical approaches are inappropriate, too burdensome, need to be adapted to the characteristics of the particular innovative SMR concept Most designers do not question the current safety approach, based on prevention of severe accidents and measures taken to limit their consequences, but argue that classical accident scenarios can be deterministically excluded We basically share the view, that due to intrinsic features of most considered SMR concepts the vulnerability of barriers and the necessity of lines of defense need to be re-assessed, i.e. whether failure of the primary circuit must be assumed in case of unpressurization, of the first barriers in case of temperature resistant (ceramic) fuel elements or whether a secondary (leak-tight) containment is adequate

9 Adaptation of regulatory requirements and associated challenges Pure application of current rules and best practices is not meaningful; it poses unnecessary barriers to the deployment to most SMR concepts Adaption to innovative/exotic design features is challenging and results in a shift of safety proofs to material properties, demonstration of sufficient quality/validity of small/large scale experiments and computer codes, etc. Elimination of classical accident scenarios and design base accidents raises the question of sufficient completeness of newly considered accident scenarios and how to cope with lack of sound knowledge and huge uncertainties Note:For most fast reactors, RIA deserve special attention/measures; some concepts link the reactor closely to elements of the fuel cycle, use highly enriched fuel, foresee below ground siting and off-site fabrication

10 Key safety characteristics of the HTR-M as an example

11 Take home messages Selected, highly innovative SMR concepts show a high potential to achieve extremely ambitious safety requirements due to safety characteristics, significantly different current large NPP designs Regulatory framework for very promising SMR concepts must be re-thought to avoid inadequacy, unnecessary (economic) burden and show stoppers Adaptation of (IAEA) safety principles and regulatory requirements as well as education and training of the respective staff pose a tremendous technical and organizational challenge, to be taken up in a timely fashion

12 Thank You for your attention!

13 Tightened-up requirements Overarching goal:make civilian use of nuclear energy safer and more acceptable by reactor and fuel cycle designs that are less dependent on reliable human performance and more robust against malicious interventions/instability of our societies More specific requirements 1) Elimination of potential reactivity induced accidents (RIA) by reactor core design or at least controllability by passive means Forgiveness against loss of active core cooling, including total loss of power, by design and passive/inherent means Securing integrity of vital components to avoid geometric disorders or loss of confinement of radioactive inventory; protection against extreme external impacts Use of chemically non-reactive, non-toxic materials/fluids or avoid direct contact of reactive substances Avoidance/incineration of long-lived radioisotopes (actinides) by fuel cycle designs, allowing for reduced husbandry times Ensuring intrinsic proliferation resistance characteristics of the fuel (enrichment), fuel cycle and related processes 1) See paper IAEA-CN-251 for ways to achieve them