Small Modular Nuclear Reactors: problems of wastes and weapons prolifration

Small-modular-reactor-dudOne size doesn’t fit all: Social priorities and technical conflicts for small modular reactors  Science Direct,  M.V. RamanaZia Mian 2014.04.15 .

“…………..Waste reduction and proliferation risk

One way that designers have tried to reduce the quantity of radioactive waste generated has been to move to reactors that use fast neutrons.23 Because a significant fraction of their energy is released through the in situ breeding of fuel, fast reactors do require far less uranium to be loaded and produce less nuclear waste. Even in those cases where the spent fuel unloaded from such an SMR is not reprocessed and ultimately prepared for final disposal, the quantity of waste generated would be significant reduced [107].

The total amount of plutonium generated with these designs, however, is much larger than for light water reactors. More importantly, the concentration of plutonium in the spent fuel is about 6–7 times higher than in LWR fuel. This characteristic translates directly into a higher risk of proliferation because would-be proliferators have to divert only a much smaller quantity of spent fuel to a clandestine reprocessing plant to produce adequate material to make one or more nuclear weapons.

Proponents of such SMRs usually claim that their designs are proliferation resistant because the plutonium in the spent fuel is not separated out routinely. However, this assertion assumes that the reactor and spent fuel are used as intended, and does not account for misuse or diversion. Indeed, the very definition of proliferation resistance from the IAEA—characteristics of a nuclear energy system that impede the diversion of undeclared production of nuclear material or misuse of technology by states in order to acquire nuclear weapons or other nuclear explosive devices [114]—involves the possibilities of misuse and diversion.

Further, despite the higher costs associated with reprocessing, there are many countries that persist with reprocessing spent fuel, including Russia and India. Many other countries have ambitious plans to embark on reprocessing. Further, outside of the United States, most fast spectrum SMRs are being designed to operate in a closed fuel cycle [43]. The high concentration of plutonium will make the reprocessing of spent fuel from SMRs that much more economically attractive. Reprocessing has traditionally been associated with greater proliferation risks and thus the widespread of adoption of the technology would enhance the risk of nuclear weapon proliferation

 Waste reduction and safety

The use of fast neutron reactors to reduce the amount of waste generated also has implications for safety. In thermal reactors, the core is typically in its most reactive configuration when it is operating normally at full power. Any change to this configuration in an accident would therefore decrease the power being produced. In fast reactors by contrast, collapsing the fuel into a reduced volume increases the rate at which the chain reaction occurs. If this were to happen quickly enough, the pressure in the fuel would rise fast enough to lead to an explosion. This could fracture the protective barriers around the core, including the containment building, and release large fractions of the radioactive material in the reactor into the surroundings. Such a “core disassembly accident” (CDA) has therefore been an important concern among the fast reactor design community ever since the first fast neutron reactors were constructed [115]. CDA studies have been conducted for nearly all of the fast reactors constructed or proposed in the United States and Western Europe [116]. Core meltdown accidents can also occur without disassembly: two U.S. fast reactors have had partial core meltdowns.

Another safety problem that affects some fast reactors, including SMRs, is due to the use of molten sodium as coolant. Though sodium has some safety advantages, it reacts violently with water and burns if exposed to air.24 Further, when sodium absorbs a neutron, it is converted to an intensely radioactive isotope called sodium-24, a major problem when any component in the reactor has to be repaired. One of the persistent problems in sodium cooled fast reactors built so far has been the propensity for leaks to develop, especially in the steam generators. These have occurred in almost all countries and at various stages of the operational life, suggesting that there are fundamental reasons for such leaks [117]. These safety problems are likely to afflict SMRs that are based on the same principles as well.http://www.sciencedirect.com/science/article/pii/S2214629614000486

4.4. Proliferation resistance and economics

As discussed earlier, one avenue that has been pursued by SMR designers to improve proliferation resistance is through increasing the period between refueling. A good example of a reactor design that seeks to achieve greater proliferation resistance is the Atoms for Peace Reactor (AFPR-100). AFPR-100 is a 100 MW(e) reactor that was originally designed to have no need of on-site refueling [87]. Over the years, however, its design has been evolving (in part because of problems with the fuel that had not been envisaged) and therefore cannot be treated as finalized.25

The longer lifetime of the fuel—in this case, designed to be the lifetime of the reactor itself—comes at a price: increased fueling cost. The problem is that the operator of the reactor would have to pay upfront for two or three decades’ worth of fuel rather than being able to pay for annual or biennial loading of fuel at a time. A second factor driving up the cost is the higher uranium enrichment level needed to keep the reactor working through its lifetime. Together, these render the cost of fueling the reactor much more expensive. For example, the average enrichment level of fuel for the AFPR was about 11.3 percent [118]. This could result in a cost for uranium enrichment and fuel fabrication to be about $8000/kg as opposed to about $2500/kg for 4% enriched uranium. The total premium for the use of a larger upfront loading of uranium with higher enrichment depends on the details of the design, but we estimate that it could be of the order of $30/MWh more than a conventional LWR. This is a substantial economic disincentive.26 The case of “nuclear battery” type SMRs is likely to be similar………..

Table 1.

Generic SMR designs and the relationship of technical characteristics to various desirable goals; the implicit comparison is with standard-sized light water reactors.

SMR family Technical characteristic Desirable criteria
Cost Safety Volume of waste Proliferation risk
iPWR Smaller size, lower fuel burnup Higher Increased Larger Increased
HTGR Lower power density and higher enrichment level Higher Increased Mixed impact Mixed impact
Fast Reactors Higher power density and higher fissile content, molten metal coolants Higher Decreased Smaller Increased

http://www.sciencedirect.com/science/article/pii/S2214629614000486

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