Most SMRs (including in particular iPWR SMRs) claim some level of enhanced

security compared to current large loop LWRs. This is justified by the following

features that are favorable or easier to implement for SMRs:

• Protection from external physical threats to the plant, such as an airplane crash, by full or partial underground placement of a plant combined with reinforced outer structure(s). This is easier done for a small than for a large plant, and typically includes placing at least the reactor and used fuel pool below grade.

• Access control and prevention of unauthorized access and intrusion by full or partial underground placement, limiting the number of access points and potential intrusion points (potentially to a single entry point). At the same time this reduces the necessary surveillance and protection force and thus has a positive economic impact.

• More difficult access to safety-relevant equipment, again due to underground placement.

• Inherent increased resilience of passive safety systems to sabotage or intentional mal — operation. Clearly, systems with fewer components, functioning on forces of nature, are more difficult to perturb and disable.

• Inherent safety features — accidents that cannot occur in the first place cannot be malevolently initiated either.

Potential synergistic use of the safety and security characteristics is illustrated in

Table 8.6.

Table 8.6 Synergy of safety and security features Feature Safety impact Security impact Integral vessel No external primary pipes, elimination of large break LOCA Compact design, feasible partial or full underground siting with enhanced physical protection Compact containment High design pressure; coupled vessel-containment performance in some designs limits coolant inventory loss Compact design, feasible partial or full underground siting with enhanced physical protection Inherent safety features, passive safety, safety-by­design Eliminates/reduces possibility of non­intentional initiation of certain accidents Eliminates/reduces possibility of malevolent initiation of certain accidents Compact rector building Seismic isolation feasible Compact design, feasible partial or full underground siting with enhanced physical protection

8.2 Future trends

Future trends for iPWRs are expected to focus on safety and economics, i. e., those features that simultaneously enhance safety as well as economics. In this author’s view, the most important design trends and features include:

• addressing safety from the very start, through safety-driven design (Safety-by-Design);

• using specific iPWR SMR characteristics to turn them into safety advantages;

• implementing inherent safety features;

• reliance on (exclusively) passive safety systems;

• introduction of additional features, levels of safety, and barriers to promote DID;

• partial or full below grade placement for security reasons;

• licensing with eliminated or reduced off-site EPZ;

• extending the post-accident grace period, with a larger, easier replenishable, ultimate heat sink;

• seismic isolation;

• advanced instrument and controls (I&C) to support safety in operation and status monitoring in off-normal conditions;

• advanced I&C for diagnostics/prognostics;

• fuel with enhanced accident tolerance.

Additionally, low-power-level systems will likely consider in their design:

• natural circulation for heat removal in normal operation;

• soluble boron-free operation (economically preferred for lower power levels);

• very long core life, or so-called ‘battery’ approach;

• near zero self-regulating excess reactivity, eliminating the possibility of prompt criticality, but usually limited to (very) low-power systems (such as ELENA, 3.3 MWth system envisioned for district heating (IAEA, 2012a)).

Related needs and challenges to enable or support desired safety features in the area of improved analytic capabilities and licensing include the following:

• Improving our understanding of passive systems and their vulnerabilities and failure probabilities needed for PRA. Currently, while the PRA approach is well established, specific probabilities and their uncertainties are not yet well quantified for iPWR SMRs.

• Better understanding and addressing the potential for common mode failure and potential for negative mutual impact of multiple units at the same site. As demonstrated by the Fukushima Daiichi accident, this is not a mere theoretical possibility. It requires developing reliable approaches to avoid such occurrences; otherwise, SMRs may not be able to fully claim (e. g. in licensing) the benefits of the smaller source term per unit.

• Risk-informed licensing. This will provide a framework for reducing the emergency planning zone based on adequate risk estimate rather than prescriptive values (Carelli et al., 2008).

Additionally, testing and validation will be necessary to address specific unique features:

• Testing and experimental validation of natural circulation phenomena and integral primary configuration. This will provide confidence in current models and support licensing.

• Validation of advanced analytical methods. Some novel or unique features (novel fuel, different flow regime from that in current reactors, novel components, etc.) will require new simulation codes or methodologies, that will need to be developed and validated based on carefully devised testing.

Some of the research needs include the following:

• Developing and improving reliable passive decay heat removal approaches, with indefinite grace period. This addresses the main challenge — ensured decay heat removal in all accident situations.

• Developing effective ‘firewalls’ between the nuclear and non-nuclear portion in co­generation applications. SMRs are more suitable for co-generation applications, which rely on effective separation.

• Developing novel components with improved performance. Some examples include novel fuel forms and designs, internal CRDMs and steam generators, integral pressurizer and fully immersed main coolant pumps.

Furthermore, considering trends in traditional PWRs and in non-LWR SMRs, the

following research trends, even if currently not pursued for iPWR SMRs, could be

expected to impact the next generation of iPWR SMRs:

• Fuel with enhanced accident tolerance, such as coated particle type (e. g., tristructural isotropic type, TRISO), originally developed for a very high-temperature reactor (VHTR), now as fully ceramic microencapsulated (FCM) considered for LWR. This fuel would potentially add margin in certain safety aspects, but it has significant challenges of its own, related to fabrication and cost. The author does not expect to see it applied to the first wave of iPWR SMRs.

• Use of other-than-oxide fuel, such as nitride or silicide. One possible driver is the desire for achieving a longer cycle, which could be justified for specific purposes, but it has its own development cost and challenges.

• Amplified negative feedback. Proposed for several high-temperature reactors and in some very low-power iPWRs (e. g., ELENA; IAEA, 2012a), primarily to be used for self­regulation. However, one needs to consider cooling reactivity insertion, to make sure it is not introducing a more negative than positive safety impact for general applications.

• Low-pressure systems. Inspired by lead or molten-salt-cooled reactors that offer attractive safety characteristics, this is clearly not applicable for mainstream iPWRs. However, specialized iPWR applications may be tempted to consider significantly lowering the operating pressure.