Small Modular Reactors And the Nuclear Non-Proliferation: A View From the Safeguards Standpoint

Author: Ярушевська Альона

Nuclear power generation continues to hold its ground in providing humanity with energy that does not require fossil fuels and is not greenhouse-emitting compared to coal-fired power plants. The global energy industry is currently getting ready for deployment of the IV Generation advanced nuclear reactors in the not-too-distant future. Small modular reactors (SMRs) with a capacity of up to 300 MW are expected to have great prospects. This concept offers such advantages as the possibility of using manoeuvring modes, lower construction costs for such reactors and a wider choice of sites for their location due to modularisation. In this way, SMRs can gradually replace high-capacity large nuclear reactors and fossil fuel power plants, becoming a safer and more affordable alternative together with renewable energy sources.

To ensure that nuclear energy continues to serve exclusively peaceful purposes, the new technology should leave as few opportunities to divert it for the creation of nuclear weapons as possible. First and foremost, we are talking about the possible loss of control over nuclear material or weaponisation of nuclear fuel. This is no less important than nuclear and radiation safety under current circumstances.

The very SMR design has a number of innovations that reduce the likelihood of their use for military purposes, in particular:

• SMRs are smaller than traditional reactor designs, making them potentially less demanding to monitor. In other words, the area of monitoring, safety and security is smaller, and therefore easier to control.
• SMRs require less fuel, which means that radioactive emissions in case of emergency will be lower. Thus, the critical mass of fissile material is smaller than in conventional reactors. Quantity matters in the production of nuclear weapons.
• High fuel burn-up making it unsuitable for nuclear weapon applications due to its longer operation.
• Certain SMR designs have a core sealed at the manufacturing plant or require less frequent (once every 300 months) reloading. This considerably complicates the use of fresh or spent nuclear fuel for military applications.
• Number of modules at a single site: One of the potential advantages of SMRs is that one or more additional modules can be added successively to an existing power plant. This would allow using one common spent fuel pool for all modules.

Along with this, given the differences between some SMR technologies and those of previous generations, it poses new challenges to nuclear non-proliferation. This requires new approaches to monitoring compliance with the non-proliferation safeguards enshrined in the 1968 Nuclear Non-Proliferation Treaty. Detailed considerations on this issue were presented in the report “Impact of Small Modular Reactors on Nuclear Non-Proliferation and IAEA Safeguards” by the Vienna Centre for Disarmament and Non-Proliferation (VCDNP).

What Are the Ways to Verify Nuclear Material?

The IAEA uses two approaches for operating light water reactors (LWRs): item accountancy and nuclear material containment&surveillance measures to prevent undeclared movement of nuclear material.

The inventory of nuclear material in the reactor and spent fuel storage facilities can be verified by visual inspection, non-destructive analysis (NDA) and through containment&surveillance (C&S) measures.

Remote monitoring is based on an all-digital approach with a communication system that is independent of the monitoring system. Since LWRs are not refueled more than once a year, this makes it possible to seal the reactor pressure vessel head and verify the safety of nuclear material during refueling periods by IAEA inspectors in person, and by remote monitoring during the rest of the time . 

The activities to be performed during the IAEA inspection are the following:

• Audit of accounting records and comparison with reports submitted to the IAEA.
• Examination of operating records and reconciliation with accounting records.
• Verification of fresh fuel before reactor core loading. In order to detect possible diversion of fresh fuel, the verification is carried out by item counting, serial number inspections, non-destructive analysis and other methods. For reactors using a mixture of uranium and plutonium oxides in their cores, an additional check of the seal verification is done, assuming that the fuel is obtained from an IAEA safeguarded enterprise. If not, additional NDA measures are applied, and the fuel is kept sealed if in a dry storage facility or under surveillance if in a wet store.
• The fuel in the core is verified by item and serial number identification following refueling and before the reactor vessel is closed. In facilities that use a mixture of uranium and plutonium oxides as their fuel, loading is naitained either by on site or underwater surveillance. After verification, C&S measures are applied to ensure that the reactor core remains unchanged.
• Spent fuel ponds are verified after the spent transfer canal gate has been sealed or after the reactor core has been closed. In addition to evaluating the C&S measures, inspectors verify spent fuel by evaluting the Cherenkov glow using non-destructive techniques.

It is light-water SMRs that will be potentially the first to be exported to other countries because of their similarity to the most common operating pressurized water reactor type.

What Challenges to the Potential Diversion of Nuclear Materials Do SMRs Pose?

These are the their design features that complicate the IAEA’s monitoring of compliance with safeguards on the peaceful use of nuclear materials.

On-load refuelled reactors, that, so to speak, can be refuelled on the run, require safeguards to be considered that are related to more frequent spent fuel management compared to off-load refuelled reactors. The repeated movements of relatively small irradiated direct use items provide an opportunity to install non-destructive assay instrumentation inside the primary containment to facilitate IAEA monitoring measures. This, however, may require the design engineer to consider the use of unattended systems that are remotely monitored or require periodic on-site servicing by inspectors.

At the same time, the issue of safeguards for such reactors has already been resolved in the CANDU heavy water reactors (LWRs) that are widely operated in Canada and a few other countries and in which the design also allows replacing used fuel with fresh fuel without shutting down the reactor. Therefore, the experience in incorporating safeguards into such SMRs can be borrowed from high-capacity reactors.

However,  spent fuel verification in the spent fuel pool by inspectors can be challenging for designers, who need to consider ways to minimise spent fuel movement, especially if the irradiated fuel to be inspected is stacked in layers.

Safety assurance considerations include provisions for the continuity of knowledge (i.e., assurance that no changes have occurred between inspections) of the core using radiation sensor-based core discharge monitors and other devices. The purpose is to facilitate IAEA verification and maintain continuity of knowledge of irradiated fuel placed in layers for storage, as well as to remotely monitor IAEA equipment to verify its proper operation.

Some SMR designs use fuel with a higher (above 20%) degree of uranium enrichment. This means that more safeguards will be required for such reactors, increasing costs.

SMRs that use non-transparent substances such as molten sodium or lead-bismuth as coolant will not allow for for optical viewing of the fuel in the core or spent fuel storage facility, unlike water-cooled reactors. Therefore, the IAEA will need access to operator viewing systems that should be incorporated into the reactor design at the development stage.

What is important, however, is that most SMRs still remain at the design stage, with only a few already undergoing pre-commissioning tests. Consequently, nuclear non-proliferation safeguards can be foreseen in them in advance.

Considering Safety Safeguards in SMRs

Nowadays, one of the LWR types, the integrated pressurized water reactor (iPWR), or supercritical water reactor, is being licenced by the U.S. Nuclear Regulatory Commission. The problem with this type of reactor is that their preliminary designs do not mention the idea of IAEA safeguards-by-design.

The fuel in iPWRs is reloaded during outage periods, when the nuclear inventory in the reactor and storage areas can be verified through visual inspection, NDA, or C&S measures. Since all nuclear material is stored in the form of fuel assemblies and remains unchanged during its lifetime in the facility, it can be counted and identified. The fuel assemblies that are delivered from the manufacturing plant to the SMR site are also subject to inspection. 

Molten salt reactors (MSRs). The wide range of fuel cycles (uranium, thorium) and reactor technologies (can operate in the thermal or fast neutrons) strongly affects safeguards and non-proliferation, with significant differences between the two subcategories of these reactors: liquid fuel and solid fuel ones.

The VCDNP lists these challenges to safeguards as caused by the unique core of liquid salt reactors:

• homogeneous highly radioactive mixture of fuel, coolant, fission products and actinides;
• high operating temperature of fuel salt, which is always kept above the melting point of the salt and highly corrosive environment of the fuel salt;
• presence of frozen fuel, which may potentially require other safeguards compared to those for liquid salt fuel;
• fuel with a potentially low concentration of fissile material in the salt mixture;
• fuel reprocessing and refuelling.

In order to inspect a molten salt reactor, bulk material accountancy may be needed for front and back end of the nuclear fuel cycle. The problem here is that the techniques and instrumentation for bulk material accounting cannot be directly applied to SMRs of this type. The homogeneous fuel mixture, not contained in fuel assemblies, makes traditional item counting and visual accountability of salt fuel impossible. There are no current safeguards approaches for power reactors that have to take into consideration the nominal MSR fuel form as a homogeneous mixture of fuel, coolant, fission products, and actinides.

Traditional safeguards can be applied only to some molten salt SMR designs that use solid fuel.

The safeguards implications for salt reactor designs would be different if a thorium fuel cycle is used. Additional complications will arise here, as the resulting radiation signatures will not be the same as those produced by the uranium fuel cycle. And the existing IAEA inspection regimes are based on the uranium-plutonium cycle.

Another concern is the possibility of the reactor misuse for producing more U-233 by modifying its fuel salts composition. Experts believe that it is important to examine how the fuel cycles of MSRs involve fissile and fertile materials in terms of  location and distribution of the nuclear material inventory, production rate and consumption rate of these materials, as well as their chemical, physical and isotopic changes. As a result of these changes, each reactor will have distinct radiation signatures depending on the different aslt chemistries and the nuclear fuel processing techniques, which will depend on how much fissile material is generated. This poses a new challenge for non-proliferation safeguards.

It is expected that by means of nuclear material accounting it will be possible to confirm that all unaccounted for material is included in the list authorized by the IAEA. Thus, high-precision measurement systems may be required to apply safeguards to this type of reactor. Therefore, existing safeguards need to be expanded to accommodate different fuel cycles of salt reactors and other reactor technologies.

Challenges for non-proliferation in high-temperature reactors are related to the presence of a large number of billiard-ball-sized pebbles inside the core that are manufactured without individual serial numbers. They reside in the safeguards  grey area because their properties allow for both item and bulk accounting options. There is a lack of clear safeguard approaches for high-temperature reactors/pebble-bed modular reactors, so this area represents a gap in safeguards for the peaceful use of nuclear energy.

Difficulties associated with traditional item counting are aggravated by the online refuel capability. Hundreds of thousands of small fuel spheres contain grams-quantities of nuclear material that cannot be uniquely identified when moving into or out of the reactor during online reloading.

The VCDNP proposes a hybrid approach to IAEA safeguards that includes monitoring of fuel flow, redundant advanced C&S measures, and bulk nuclear material accountancy and verification techniques.

Reactors with fast neutron spectrum are called breeders because U-238 easily absorbs fast neutrons to create Pu-239, and in some cases producing more fuel than they consume. Fast reactors are refuelled during periods of outage, during which the nuclear material inventory in the reactor and storage areas can be verified by visual inspection, NDA, and C&S measures.

In terms of safeguards, the IAEA considers fast reactors to be similar to LWRs. However, the possible presence of separated plutonium in unirradiated fresh fuel represents a risk of violating safeguards. The amount of separated plutonium is higher than in reactors containing LEU, and this signifies that IAEA safeguards implementation may require more effort than for LWRs.

The main safeguards challenge in a closed-cycle fast breeder reactor is the reproduction of high-purity Pu-239 isotope. Accordingly, controlling plutonium isotope composition is the main barrier against nuclear proliferation.

What is the Bottom Line?

Due to their unique features, SMRs pose certain challenges to the current international safety safeguards regime. Specifically, the types of fuel, coolants, and reactor configurations raise new challenges for IAEA safeguards. However, experts are highly confident that each of the SMR designs can be safeguarded to prevent nuclear proliferation, as most technologies are still under development. This requires new, more effective tools and close interaction between the IAEA and reactor developers at the design stage to identify and explain the technology’s challenging elements at the onset.

Furthermore, SMRs include technologies whose safeguards resemble those of LEU-fuelled LWR designs. These are, among others, light water SMRs already mentioned in this article. From the safeguards standpoint, such reactors are most similar to VVERs operated in Ukraine.

Therefore, when choosing between technologies, those should be picked up that are safer, more environmentally friendly, require less investment and payback faster, and meet safeguards and NRS rules and regulations. Or those better than the actual requirements, which, of course, has to be proven by the designer.