Wilton, 9 years ago (2012) published a thesis while at Lund University, Sweden on the subject topics. It is instructive to review the early impressions of SMRs in the early stages of their development and their potential in 2012 against present conditions as of November, 2021 (see SMRs). Wilton intended to construct a framework for comparing various SMR designs. Of the total population of SMRs, the majority is still in the early stage of development. Therefore, only one design was selected per reactor type, with the exception of light water-based SMR of which two designs are currently under licensing review. The assessment was based on the level of progression which has been made on the design; the minimum progression having been determined to be the preliminary design stage.
The selection was subsequently analyzed with regard to its passive safety, for which the thermal inertia in combination with the SA:V was considered to be a proxy; its proliferation resistance, for which several utility function were taken from literature; and its economic potential, for which the levelized cost of electricity was determined.
For a thorough evaluation, several additional criteria, such as the public acceptance of certain nuclear fuel cycles, should also be taken into consideration. In addition, for some of the criteria that were evaluated, it needs to be wondered whether they were developed to the fullest possible extent. The passive safety evaluation in particular was identified as not taking into consideration sufficient informative parameters to render any definitive conclusion premature. Therefore, it was not deemed possible to come to a definitive conclusion about what SMR could be considered the most desirable by means of a multi-criteria analysis.
The proliferation resistance was split up in the front-end and the back-end of the fuel cycle. In the front end of the fuel cycle the best overall performers were found to be the reactors utilizing low enriched uranium, like the NuScale and mPower reactors, or natural uranium fuel, like the PHWR-220. This is due to the larger number of SWUs required to produce weapons-grade uranium out of low enriched uranium fuel. In addition, a natural uranium fuel cycle could imply the unavailability of enrichment facilities in general, which removes the possibility of their use in a clandestine operation. For the back-end of the spent fuel cycle the FUJI reactor was ranked the most proliferation resistant reactor, with a nearly perfect score following the almost complete absence of plutonium in its spent fuel.
Additionally, the HTR-PM, which benefits from a large mass of spent fuel, produces the second lowest amount of plutonium SQs per GWe∙y. A large spent fuel mass is also the reason that the PHWR-220 rated relatively resistant to proliferation, which can be considered surprising due to the relatively large amounts of spent plutonium SQs in the spent fuel. The large mass of spent fuel the SQs are divided over however resulted in a relatively low plutonium percentage. Considering that plutonium has the potential for direct-use and reactor-grade uranium has to be enriched first, the proliferation resistance of the back-end of the fuel cycle was considered more important than front-end of the fuel cycle for all reactors save the MYRRHA.
The latter utilizes plutonium as its fuel. The passive safety was approximated by assessing how quickly the temperature of the materials in the reactor environment increases when the removal of heat from the primary circuit is interrupted. An analysis resulted in the identification of multiple structures which could aid in dissipation of heat.
In this thesis:
1) the primary coolant,
2) a passive heat removal system using radiative heat transfer, and
3) the presence of large in-core deposits with a high heat capacity, such as graphite, were included in the analysis.
It was concluded that all but one reactor (the 4S) possessed a higher thermal inertia than a state-of-the-art large-sized reactor, the EPR. However, this is a contestable item. The best overall performer is the HTR-PM, this is due to its low average core power density and high degree of thermal inertia as a result of the large volumes of graphite present inside the reactor core. Other notable performers are the SVBR-100, which employs an intricate passive heat removal system, and the FUJI reactor, which contains a large volume of graphite moderator/reflector that doubles as heat sink, similar to the HTR-PM. Additionally, all of the abovementioned reactors have a relatively large SA:V in addition to a high degree of thermal inertia.
The economic potential was measured through the levelized cost of electricity, which includes all costs that need to be incurred during the reactor’s lifetime, ranging from construction to decommissioning and decontamination. Considering that SMRs are partially designed to be affordable for privately held
companies, competitive interest rates were approximated by means of the CAPM in favor of the government-backed lending schemes, which typically come at lower interest rates. Furthermore, variables such as the specific overnight costs, cumulative load factor and operational lifetime were not considered to be constant, but input as probability density functions, which is more in line with the uncertainty inherent to the cost estimation process of reactor designs. This is of particular importance due to the majority of the designs investigated in this thesis still requiring a first-of-a-kind construction experience.
The resulting cost distributions are generally higher than conventional cost estimates for nuclear power. This can be explained by the overall higher specific capital costs of SMRs, which have been widely documented to be the major cost component in the cost of electricity. Furthermore, SMRs are typically designed to operate co-generation mode, meaning that additional useful output in the form of heat credits or desalinated water is generated, which reduces the generating costs. These and similar possibilities for cost reductions were not taken into consideration in this thesis.
Of the SMR designs, when looking at the cost of electricity alone, the HTR-PM was found to have the lowest levelized cost of electricity. This can be solely attributed to the relatively low specific overnight costs estimate, in comparison to conventional light water reactor designs, published by various authors (e.g. [178]).
It should be noted these estimates could prove to be optimistic as a result of the authors, or their sources, being directly or indirectly involved with the HTR-PMs development. However on a separate note, based on the three models discussed in this thesis, the HTR-PM can be considered to be a strong contender. In the end only actual operational experience can determine whether the designs investigated in this thesis are feasible for participating in any meaningful way towards reducing our dependency on fossil fuels.
History is littered with examples of overly optimistic prognoses of the role of nuclear power in our energy order. Although this does merit some skepticism towards ‘miraculous new designs’ offering unparalleled safety, negligible waste issues and electricity too cheap to meter, skepticism should ultimately not restrain us from developing and building any reactors of which it can be verified that they can perform at or above the governing standard.
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