Nuclear energy presents a sustainable energy supply and cheap electricity. It has been indicated to minimize the emission of greenhouse gases. Unfortunately, nuclear energy also has some shortcomings in sustainability. One important issue on this emanates from the disposal of spent fuels discharged from the reactors. Therefore, transmutation and partitioning of the minor actinides in the spent fuels will be necessary for the self-sustainment of the nuclear industry.
The radiotoxic inventory of the geological repositories will be reduced up to a factor of 100 if minor actinides and plutonium will be completely transmuted and recycled. Researchers have indeed investigated extensively with varying fuel cycle strategies, reactor systems and neutron spectrums. Fast reactors have been identified as the most promising candidates.
Unfortunately, transmuting minor actinides in fast reactors have been found to have significat safety concerns on the performance of the reactor cores. It has been demonstrated that the high amount of minor actinides such as neptunium and americium, initiates severe deterioration of coolant density reactivity feedback in the heavy-liquid-metal-cooled fast reactors. Researchers have found that it is important to reduce the core power by about 4% for every 1% addition of americium loaded in a lead-cooled fast reactor. If minor actinides are added in a reactor core, its neutronic safety performance becomes worse.
Since the economy of a nuclear reactor is dictated by the operational power level, it is uneconomical to undertake minor actinides transmutation in a critical fast reactor, which must be operated in a reduced power level in a bid to maintain sufficient safety margin. Fortunately, the accelerator driven subcritical system has a larger safety margin in minor actinides transmutation as opposed to the critical fast reactors.
In view of the fact that transmutation in the current operational reactors is insufficient and poses some safety concerns, Professor Youqi Zheng and colleagues from Xian Jiaotong University, China, proposed an accelerator-driven subcritical transmuter, which was named highly efficient industrial transmuter (HEIT) in a move to address these concerns. Their research work is published in International Journal of Energy Research.
The proposed system utilizes uranium-free metallic dispersion fuel and has high power density. The authors focused on the transient analysis of the highly efficient industrial transmuter in order to stablish its feasibility in the future nuclear applications. They analyzed cladding stresses, cumulative creep damage fractions and temperatures. In addition, the researchers investigated the burnup dependence and evaluated three transients: the beam overpower, the unprotected transient overpower, and the unprotected loss of flow.
The authors observed that the highly efficient industrial transmuter core remained safe without scram in a majority of transient cases. From the results, there was indicated that there will be enough safety margins from fuel pin failure. In the unprotected loss of flow transient, the cladding cloud exceeded the rapture limit in approximately half an hour when no shutdown responded. This was reference to the positive coolant density coefficient caused by the minor actinides loading.
The creep damage fraction as well as the maximum temperature was observed to change with the depletion owing to the delay heat fraction and power distribution variation. For the unprotected loss of flow transient, the end of lifetime was bounding, while for the case of beam overpower and unprotected transient overpower transients, both the end of lifetime and the beginning of lifetime ought to have been accommodated.
Youqi Zheng, Mingtao He, Liangzhi Cao, Hongchun Wu, Xunzhao Li and Shengcheng Zhou. Reactor core transient analysis of an innovative high-level nuclear waste transmuter with metal fuel. International Journal of Energy Research, volume 41 (2017), pages 1322–1334.
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