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High-Temperature Reactors
Published in William J. Nuttall, Nuclear Renaissance, 2022
The pebble bed spherical fuel elements are fabricated from Triso fuel. Each HTR-10 fuel pebble contains about 8,300 Triso particles totalling in all 5 g of 17% enriched uranium. The reactor core contains approximately 27,000 fuel elements, or pebbles, which circulate in multiple passes through the core. The pebbles emerging from the reactor are moved using a pulse pneumatic fuel-handling system. Each fuel element is assessed individually for damage and nuclear burn-up. Those that have not yet reached the desired burn-up level are sent back pneumatically to the top of the reactor core [108].
Radiation protection in the nuclear industry
Published in Alan Martin, Sam Harbison, Karen Beach, Peter Cole, An Introduction to Radiation Protection, 2018
Alan Martin, Sam Harbison, Karen Beach, Peter Cole
There has been some interest over the years in the high-temperature gas-cooled reactor (HTGR), which uses helium gas to cool ceramic uranium fuel. In one design, known as the pebble bed reactor, the fuel consists of many thousands of ceramic spheres, through which the helium passes to remove the heat. The temperature of the helium gas as it exits the core is much higher than in existing gas-cooled reactors and so a direct turbine cycle should theoretically be possible. However, the HTGR presents a number of novel engineering and materials challenges which have so far prevented its commercial exploitation.
Nuclear and Hydropower
Published in Roy L. Nersesian, Energy Economics, 2016
The primary safety feature of a pebble bed reactor is its low fuel density with a power density only one-thirtieth that of a PWR. The reactor is inherently safe for a total loss of coolant—no core meltdown occurs in a PWR as there is no core. A loss of coolant heats the pebbles up to a maximum temperature of 1,600°C, well below the 2,000°C needed to melt the ceramic coating surrounding each bit of fissionable fuel. As pebbles heat up, the frequency of fissions drops, which lowers the power output of a pebble reactor to a level where more heat escapes through the reactor wall than is produced by nuclear reactions. The reactor cannot crack, explode, melt, or spew hazardous materials: it simply remains at an idle temperature with pebbles intact and undamaged. Known as passive nuclear safety, the reactor’s low fuel density allows more heat to escape than is generated in the absence of coolant rather than having to depend on an active nuclear safety feature such as inserting control rods and maintaining coolant. A pebble bed reactor is inherently safer than traditional reactors. It is impossible to have a runaway reaction as occurred at Chernobyl by a sudden withdrawal of control rods that caused the reactor to go supercritical or to have a partial core meltdown as at Three Mile Island or a total core meltdown as at Fukushima by a loss of coolant.
Review of the Fluid Dynamics and Heat Transport Phenomena in Packed Pebble Bed Nuclear Reactors
Published in Nuclear Science and Engineering, 2023
Rahman S. Almusafir, Ahmed A. Jasim, Muthanna H. Al-Dahhan
One of the most essential characteristics of pebble bed nuclear reactors is that the reactor core is designed such that a maximum fuel element temperature of 1600°C is not surpassed during any accident. Active core cooling is not required for decay heat removal during accidents. It is sufficient to discharge the decay heat to the cavity coolers located outside the reactor pressure vessel using passive heat transport processes such as heat conduction, radiation, and natural convection.21 The bed structure, coolant flow dynamics, pressure drop, and heat transport, which determine the thermal-hydraulic characteristics of the PBR, are among the essential phenomena that need to be well understood for the proper design and safe performance of these reactors. The advantages and disadvantages of nuclear PBRs are summarized in Table II.
FHR, HTGR, and MSR Pebble-Bed Reactors with Multiple Pebble Sizes for Fuel Management and Coolant Cleanup
Published in Nuclear Technology, 2019
Charles W. Forsberg, Per F. Peterson
A pebble-bed reactor with two or more sizes of pebbles creates options that have not been previously explored. In pebble-bed reactors, the greater heat transfer per unit volume of the smaller pebbles enables higher power densities relative to the large pebbles that enables higher power densities in the reactor core (smaller reactor) and creates a new pathway to actinide burning. The small pebbles could be used for coolant cleanup, from tritium to noble metals. In MSRs, multiple pebble sizes may address the challenge of replacing graphite damaged by neutron irradiation, a design option that translates into smaller reactor cores that implies less salt and a lower reactor fissile fuel inventory.
Modal Decomposition of the Flow in a Randomly Packed Pebble Bed with Direct Numerical Simulation
Published in Nuclear Technology, 2022
Mustafa Alper Yildiz, Elia Merzari, Thien Nguyen, Yassin A. Hassan
The world`s nuclear power plant fleet consists predominantly of second- and third-generation (Gen II and Gen III) light water reactor designs. Even though these reactors have operated with limited significant accidents for the total operating time, the design of Gen II and III reactors have inherent risks in unforeseen accidents, as witnessed in 2011 when the Fukushima-Daiichi plant suffered major damage after a tsunami. The pebble bed high-temperature reactor concept is one of several designs put forward by the Generation IV International Forum based on its improved passive safety characteristics.1,2