September-October 2017 NPJ

24 NuclearPlantJournal.com Nuclear Plant Journal, September-October 2017 TRISO particle acts like a small pressure vessel to retain all fission products during operation of the plant.After plant operation they act as small used fuel containers, all embedded in a graphite matrix to prevent any of the used fuel or its fission products to migrate into the biosphere. The beauty of the graphite matrix is that it will not dissolve in water or air for millions of years – as proven by the coal fields. The pebbles will pass six times, on average, through the core, as it flows under gravity from the top to the bottom of the core in about six months, one pebble at the time. That is denoted as the average residence time . After six months, if it has achieved its full burnup of about 160,000 MW days per tonne (of heavy metal), it gets replaced by a fresh pebble. Otherwise, the pebble is returned back into the core. As noted it will, on average, return six times to the core. The beauty of such a long fuel cycle, is that it allows for in-situ utilization of the in-core bred plutonium. The uranium 238, when it gets bombarded by fast neutrons, turns into plutonium, and thereafter, the plutonium produced during reactor operation, at the end of the fuel cycle is being used in-situ by about 80%. That means that whatever remains of the plutonium vector at the end of life is completely unusable for weapons proliferation purposes. The pebbles as it comes out, finally, can thus not be mischievously used by whoever would get hold of it. This was part of the Xe-100 design philosophy. 4. How does a steam cycle operate in Xe-100? In the Xe-100 a concentric pipe connects the reactor with the PCU. The heated helium (750° C) passes through the inner part of the concentric pipe, while the cold (250° C) helium passes on the outside. The helium passes through a helical coil steam generator with two circulators. The circulator circulates the helium through the reactor, entering at about 250° C. From there it is guided into the reactor from the top where it is heated to 750° C, where it passes through the helical coil steam generator. Feedwater is heated up to form steam for the typical Rankine cycle. The higher temperatures allow for a higher net efficiency of about 38%. 5. Is Xe-100 a generation four reactor? Shippingport reactor, considered to be a generation one reactor, when commercialized, became known as generation two. An example of generation three was the AP600. These plants have not been built but have been licensed. The designers continued to increase size tomake economic sense, leading to the AP1000 and the EPR. These reactors become known as generation three plus, since they have been designed to be more inherently safe. The Xe-100 is a true, generation four reactor, designed so that a core meltdown would be physically impossible. Other characteristics include a high level of proliferation resistance featured by the fuel cycle, more cost efficient with particular reference to affordability. 6. Is material corrosion an issue with Xe-100? In the case of the Xe-100 the working medium is helium. Helium is both chemically inert and radiologically transparent. Corrosion is prevented in our system by observing the helium flow velocities through the graphite blocks. Apart from that, the layout is comprised of a pebble bed reactor core, surrounded by a meter-thick graphite reflector. The reflector blocks are seismically supported by a metallic core barrel. Between the reflector blocks and core barrel a small inner gap exists, while an outer gap exists between the core barrel and RPV. Inside the gaps the flow is mostly stagnant, therefore not prone to any corrosion activity. 7. Is the steam generator also corrosion proof? In the helical coil steamgenerator helium is present on the shell side. On the helical coil tube side, there is steam. The tube material and design is aimed at avoiding vibrations. After having said all that we are still having to decide exactly what that material is, but right now it looks like incoloy 800H, which is a higher temperature-able material. Incoloy 800H, have been well-proven where it was used on the AVR to operate for 16 years with an outlet temperature of 950° C. Despite the high outlet temperature, the pressure drop over the pipes were kept low. 8. What’s the target date Xe-100 commercialization. A number of permits have to be obtained. Of course, the limit will be the NRC license. Proven Technology... ( Continued from page 23) Rendering of Xe-100.