The Beguiling Promise of the HTGR

It’s easy to see why technologists fall in love with high-temperature gas-cooled reactors (HTGRs). These nuclear machines are remarkable inventions, at least on paper. But few have actually seen the real world for any length of time, and their real-world experience has been mixed.

The claims of high-temperature gas-cooled reactor (HTGR) advocates are impressive and persuasive. They say the reactors are versatile, scalable, and largely goof-proof. Because of their high temperatures, HTGRs are ideally suited for double duty in industrial cogeneration applications, which is why chemical companies and other users of process steam pine for their development. They don’t use water as coolant, nuclear moderator, or in managing spent fuel. They are even better than conventional light-water reactors (LWRs) in displacing carbon dioxide, as they can not only generate electricity but also back out industrial use of oil and natural gas burned to raise either low- or high-pressure steam. They can easily follow load.

The nuclear buzz of late has been all about small modular reactors, which are aiming for development in the next 10 years or so—the mPower, NuScale, and Westinghouse machines. These are aimed at supplementing the big Gen III machines such as the 1,000-MW Westinghouse AP1000s now under construction in Georgia and South Carolina. But the next, next generation of nuclear plant could be the HTGR, aimed at the 2030s timeframe.

That’s the heart of the case that two representatives from the Next Generation Nuclear Plant Ltd. (NGNP) Industry Alliance—Entergy’s John Mahoney and Fred Moore, a consultant who has retired from chemical giant Dow—made in the ELECTRIC POWER 2013 nuclear track in Chicago in May. The partnership (http://www.ngnpalliance.org) consists of a host of companies interested in the promise of the HTGR, including, among others in addition to Entergy and Dow, Areva, Westinghouse, ConocoPhillips, and SGL Group.

1. Another nuclear option. The HTGR nuclear heat supply system (NHSS) comprises
three major components: a helium-cooled nuclear reactor, a heat transport system, and
a cross vessel that routes the helium between the reactor and the heat transport system.
The NHSS supplies energy in the form of steam and/or high-temperature fluid that can
be used for the generation of high-efficiency electricity and to support a wide range of
industrial processes. Source: NGNP Industry Alliance

The current conceptual design is for a reactor that produces 625 MWth (300 MWe) of energy, which Moore noted is about the same size as today’s gas-fired combined cycle generators. The core outlet temperature of the helium coolant to a steam generator is 750C, compared with 300C for a conventional LWR. The reactor core is Areva’s Antares prismatic block concept, which consists of TRISO (tristructural-isotropic) fuel spheres packed into tubes that are then assembled into hexagonal fuel blocks (Figure 1).

The design, Mahoney and Moore emphasized, minimizes the need for developing advanced materials and is based on current fuel programs. Each tiny fuel particle is strong, fully encapsulated, and constitutes its own containment, able to withstand pressures of 1,000 atmospheres.

The safety features begin with the fact that the reactor has a strong negative void coefficient of reactivity, according to its advocates. If the reactor loses coolant, the reaction shuts down. The core has low power density, a safety plus. The helium is gaseous under all reactor conditions and is both chemically and radioactively inert. In a loss-of-coolant accident, there is no need for operators to shut down the machine; it shuts itself off. There is no requirement for backup power. Spent fuel doesn’t require cooling pools before it can go into above-ground, dry storage. HTGRs, according to its promoters, are beyond passive; they are essentially inert under accident conditions.

What’s needed to deploy these elegant machines? Some materials research remains, which is under way at the Department of Energy’s Idaho National Laboratory (INL). INL notes that while graphite has been used in past research and commercial HTGRs, “historical ‘nuclear’ grades no long exist. New grades must be fabricated, characterized, and irradiated to demonstrate acceptable non-irradiated and irradiated properties.”

This leads to a regulatory challenge. HTGRs are so different from conventional nuclear reactors that they will have to undergo a ground-up review at the U.S. Nuclear Regulatory Commission. Mahoney and Moore note that one commercial HTGR operated in the U.S., the Fort St. Vrain plant in Colorado, from 1976 to 1989. The Atomic Energy Commission (and later the NRC) allowed it to operate under an exception to its licensing rules, and the plant experienced serious operational problems. Convincing the regulators to licensing a new HTGR will be smack-dab on the critical path to deployment.

Kennedy Maize is executive editor of MANAGING POWER and a POWER contributing editor.

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