This is the final article in the Truth About Nuclear Power series that discusses alternatives to the uranium fission process via the large Pressurized Water Reactor design, PWR. Earlier articles discussed the failings and disadvantages of small modular reactors, fusion reactors, and thorium reactors. This article discusses the hopes and disadvantages of the High Temperature Gas Reactor, which has been in development for decades. As with the other alternatives mentioned above, the very fact that such alternatives are researched gives the lie to the assertion that nuclear power is economic. If it were economic in any form, it would have already captured far, far more than the 11 percent (approximately) of the world’s electric power generation. Still, HTGR is yet another design that its advocates hope will be safe, economic, and the power plant of the future. Each of the HTGR components, and the system as a whole, will be described. The disadvantages are highlighted.
The HTGR can be designed in a few variations, the one that is discussed here has a nuclear reactor using small spheres to heat a circulating gas of high-pressure helium. The hot helium passes through a gas turbine that spins a generator to produce electricity. The exhaust helium from the gas turbine is still relatively hot, and passes through a series of heat exchangers in which the helium is cooled. The cool, low-pressure helium then is compressed in a compressor, typically in two stages with gas intercooling between stages. The compressor or compressors are also driven by the gas turbine described above. Hot, high-pressure helium from the second stage gas compressor is further heated against turbine outlet helium (one of the series of heat exchangers described above). Finally, completing the circulation loop, the hot high-pressure helium enters the nuclear reactor to absorb more heat.
The reactor is a complex chamber, almost certainly a vertical cylinder with thick walls and heads to contain the pressure and radiation. The reactor contains hundreds of thousands of radioactive spheres, known as pebbles, each approximately 2.5 inches diameter. The pebbles contain a core of fissile uranium with the uranium itself in thousands of microspheres. The pebble also has various layers of other materials, including graphite, to act as a neutron moderator and maintain the uranium’s integrity. The number of microspheres per reactor is on the order of 1 billion (10^9).
Pebble management is accomplished by adding fresh pebbles at the reactor top, and withdrawing pebbles from the bottom of the reactor. Heat output is regulated by control rods that extend vertically from the reactor top down into the bed of pebbles.
The claimed advantages are a high overall efficiency of approximately 50 percent, an inherently safe nuclear reaction that cannot under any circumstances melt down, and low cost.
A paper from the NRC from 2011 discusses the then-state-of-the-research; see link.
Finally, a report from 2004 by General Atomics describes the several highly radioactive, toxic, isotopes that are routinely produced and transported out of the HTGR reactor and into the circulating helium loop. A helium loop rupture would have dire consequences. See link. (note, this link downloads the article)
Whenever one sees a research proposal that involves inserting discrete material (in this case, pebbles) into a high-pressure system (the reactor at 1,000 psi), one must pause. The fact is, even with many decades of effort by talented and motivated engineers, the material feed hopper design has not been found that is sufficiently reliable to obtain high on-stream factors. In other words, moving the pebbles from atmospheric pressure into a lock hopper, pressurizing the lock hopper without blowing any seals, and dropping the pebbles by gravity into the high-pressure reactor simply has proven too difficult. One reference shows the pebble bed reactor operating in a research mode from 1967. That is almost 50 years ago (47 years at this writing).
With a mass of pebbles in the reactor, coaxing the spent pebbles out while keeping the live pebbles inside is a daunting task. With a hot, radioactive, 1000 psi reactor, it is indeed difficult to persuade any living being to step up and sort them out. Perhaps that can be a job for a radiation-tolerant robot.
The reactor will be very expensive, as in PWR reactors, due to the high pressure, high temperature, and radiation.
Keeping the pebbles from forming dead zones, where helium gas does not pass with sufficient velocity to remove the heat generated by fission, is also a monumental task. It is claimed by the HTGR advocates that the pebbles are completely safe and cannot meltdown even with zero cooling and all the control rods removed. One must pause at that statement, and ask exactly how did they arrive at that conclusion? Was the experiment performed? Did anyone take an actual, full-scale reactor full of radioactive pebbles, not necessarily new but at the worst possible condition, turn off the cooling and remove all the control rods? It is highly doubtful that the NRC (or any other responsible regulatory agency in any country) would have allowed such a full-scale test. Even if the pebbles cannot meltdown, there must be provided some means to remove the residual heat from the reactor. The materials used to form the reactor walls, heads, and internal structures will likely lose strength as the nuclear reactions proceed. It appears from the design that the three levels of containment are incorporated: the pebble itself is the first barrier to uranium, then the reactor vessel is the second, and the third is a containment building in which the reactor is placed. Yet more expense is required to build a suitable containment building.
Next, the closed-loop helium cycle consumes a substantial amount of power in the compression stages. In contrast to a natural gas-fired turbine, the HTGR must compress all of the helium. A gas-fired turbine compresses only the air that is burned with the natural gas. Therefore, the HTGR is automatically at a disadvantage from the requirement to compress all the gas that flows through the system. Also, since helium is a very low molecular weight gas, the compressor must “work harder” to achieve the same discharge pressure. This requires a larger compressor, and more capital to build and install it. Finally, operating high-pressure and high-temperature gas compressors on helium requires very sophisticated seals to prevent helium leaking into the atmosphere.
The very largest gas turbines to date, operating on natural gas and air, produce approximately 350 MWe. Blade temperatures and blade tip speeds inside the turbine are the limiting factors. Therefore, even if the reject heat is used to produce steam that produces electricity in a separate steam turbine and generator, nuclear HTGR will have approximately 500 MW output. Economy of scale begins to work against the plant. This effectively means HTGR can never, ever, be economic.
Next, the system relies on at least one gas-to-gas heat exchanger, which is inherently expensive because gas has a very low heat transfer coefficient. In essence, the exchanger must be huge to transfer the heat required. In addition, the gas-to-gas heat exchanger must be designed to withstand 1000 psi pressure, at relatively high temperature of approximately 1000 deg. F. The high pressure and high temperature also increase the cost. At least two additional helium gas heat exchangers transfer heat against other fluids, presumably cooling water in one instance, and unspecified fluid in the other. These also will have large areas because the gas, as before, has very low heat transfer coefficient. Leakage from tubes must be monitored closely to prevent any material from entering the helium loop.
Next, a reactor vessel rupture or helium piping rupture could send radioactive, hot, balls 2-1/2 inch diameter flying under 1000 psi pressure. Such an event must be considered, designed for, and mitigated as much as possible.
Next, long-term storage of spent fuel pebbles remains a problem.
Next, production of radionuclides from the reactor, transport of those radionuclides into the helium gas circulation loop, and release into the atmosphere in the event of a pipe or system rupture, is a very serious issue.
Finally, the NRC has identified an issue with graphite dust production and transport. This can be an extremely serious issue, as graphite is a form of carbon. Graphite dust is highly flammable and explosive in the proper concentrations.
A South Africa HTGR reactor was cancelled due to cost overruns, schedule pushed back, and insurmountable technical problems. A project in Germany was also cancelled. Two research reactor projects in the US were also cancelled, one in the early 1970s and the other in the 1980s. The latter plant, at 330 MWe, was likely not merely for research due to the rather impressive size. However, it was shut down due to recurring mechanical problems and what was described as “poor performance.”
The South African experience is described at this link.
The HTGR nuclear reactor system has, as described above, many serious technical challenges that must be overcome. Given the dismal experience in other industries with similar reactors operating at high pressure that attempt to inject a solid into the reactor, it is not surprising that the HTGR reactors also fail. It is true that low-pressure systems can be made to work, but the high-pressure ball injection and removal systems are problematic. The high cost of every component is also a factor. The inherently small electrical output will forever keep the plants from enjoying economy of scale – at least until another advance is made in the gas-turbine and compressor technology. The large size of the heat exchangers adds to the cost, primarily due to the low heat transfer coefficient of the helium gas. This is an immutable characteristic of gas heat exchange, and has been known for many decades. The dismal experience of researchers in several countries over several decades does not bode well for the future of HTGR. The most important issues, though, are the production of explosive graphite dust, and production of lethal radionuclides in the reactor that are transported into the helium circulation loop that includes the heat exchangers, turbine, and compressors.
As with the other new designs, small modular reactors, fusion, and thorium, HTGR is also a very distant pipe dream.
The Truth About Nuclear Power emphasizes the economic and safety aspects by showing that (one) modern nuclear power plants are uneconomic to operate compared to natural gas and wind energy, (two) they produce preposterous pricing if they are the sole power source for a grid, (three) they cost far too much to construct, (four) use far more water for cooling, 4 times as much, than better alternatives, (five) nuclear fuel makes them difficult to shut down and requires very costly safeguards, (six) they are built to huge scale of 1,000 to 1,600 MWe or greater to attempt to reduce costs via economy of scale, (seven) an all-nuclear grid will lose customers to self-generation, (eight) smaller and modular nuclear plants have no benefits due to reverse economy of scale, (nine) large-scale plants have very long construction schedules even without lawsuits that delay construction, (ten) nuclear plants do not reach 50 or 60 years life because they require costly upgrades after 20 to 30 years that do not always perform as designed, (eleven) France has 85 percent of its electricity produced via nuclear power but it is subsidized, is still almost twice as expensive as prices in the US, and is only viable due to exporting power at night rather than throttling back the plants during low demand, (twelve) nuclear plants cannot provide cheap power on small islands, (thirteen) US nuclear plants are heavily subsidized but still cannot compete, (fourteen), projects are cancelled due to unfavorable economics, reactor vendors are desperate for sales, nuclear advocates tout low operating costs and ignore capital costs, nuclear utilities never ask for a rate decrease when building a new nuclear plant, and high nuclear costs are buried in a large customer base, (fifteen) safety regulations are routinely relaxed to allow the plants to continue operating without spending the funds to bring them into compliance, (sixteen) many, many near-misses occur each year in nuclear power, approximately one every 3 weeks, (seventeen) safety issues with short term, and long-term, storage of spent fuel, (eighteen) safety hazards of spent fuel reprocessing, (nineteen) health effects on people and other living things, (twenty) nuclear disaster at Chernobyl, (twenty-one) nuclear meltdown at Three Mile Island, (twenty-two) nuclear meltdowns at Fukushima, (twenty-three) near-disaster at San Onofre, (twenty-four) the looming disaster at St. Lucie, (twenty-five) the inherently unsafe characteristics of nuclear power plants required government shielding from liability, or subsidy, for the costs of a nuclear accident via the Price-Anderson Act, and (twenty-six) the serious public impacts of large-scale population evacuation and relocation after a major incident, or "extraordinary nuclear occurrence" in the language used by the Price-Anderson Act. Additional articles will include (twenty-seven) the future of nuclear fusion, (twenty-eight) future of thorium reactors, (twenty-nine) future of high-temperature gas nuclear reactors, and (thirty), a concluding chapter with a world-wide economic analysis of nuclear reactors and why countries build them. Links to each article in TANP series are included at the end of this article.
Part Fourteen - A Few More Reasons Nuclear Cannot Compete
Part Fifteen - Nuclear Safety Compromised by Bending the Rules
Part Sixteen - Near Misses on Meltdowns Occur Every 3 Weeks
Part Seventeen - Storing Spent Fuel is Hazardous for Short or Long Term
Part Eighteen - Reprocessing Spent Fuel Is Not Safe
Part Thirty - Conclusion
Roger E. Sowell, Esq.
Marina del Rey, California