Power plant design
High hopes for hydride1 January 2009
Uranium hydride fuel is the key to Hyperion’s small, transportable, nuclear power reactor. By Otis Peterson
The Hyperion Power Module, a self-contained, self-regulating reactor, is breaking new ground in the nuclear industry. This small, but powerful unit, often referred to as a “battery,” is complementing large-scale existing nuclear reactors by filling a heretofore unmet need for moderately-sized power applications either distributed or dedicated.
Employing proven science in a new way, Hyperion provides a safe, clean power solution for remote locations or locations that must currently employ less than satisfactory alternatives. For the first time, the advantages of nuclear power – efficient, cheaper, non-polluting with no greenhouse gas emissions – are available for remote locations without designing and building individual, massive, and costly conventional power plants that are built to serve large areas. Meeting all the criteria of the Global Nuclear Energy Partnership (GNEP), the Hyperion Power Module (HPM) was specifically designed for applications in remote areas where cost, safety, and security is of concern.
Generating nearly 70MWt and 25-30MWe, the HPM is a unique, small transportable reactor that takes advantage of the natural laws of chemistry and physics, and leverages all of the engineering and technology advancements made over the last fifty years.
The Hyperion reactor was invented at the Los Alamos National Laboratory in the USA. Through the US government’s technology transfer initiative, the exclusive licence to develop and commercialise the invention was granted to New Mexico-based Hyperion Power Generation, Inc. (HPG). The company has now retained the nation’s top nuclear power design and engineering teams, including staff from US federal laboratories, to further develop the reactor. It will continue to partner with industrial leaders for the reactor’s production, operation, and maintenance.
There are four main applications for the Hyperion reactor:
• Military bases (independent, baseload power).
• Oil & gas recovery and refining, including in oil sands and shale recovery.
• Remote communities lacking accessibility to a source of electrical generation.
• Quickly installed back-up and emergency power for disaster areas.
The development of Hyperion globally is important because it will help:
• The Environment: Hyperion’s clean nuclear power can replace the pollution and greenhouse gas-emitting fossil fuels in many applications.
• Humanity: Hyperion provides a safe, non-polluting manner in which to provide electric power to remote regions for pumping and cleaning water. The lack of clean water is at the root of much poverty and societal instability.
The HPM has only a one and one-half meter diameter core. With no mechanical intrusions in that core, the reactor will be able to be sealed at the factory, sited underground, and eventually returned to the factory for fuel recycling and refuelling after a useful life of seven to ten years. Another benefit of the underground siting is the additional anti-tampering and anti-terrorist protection provided.
The key to the success of Hyperion will be its fuel – uranium hydride powder, which allows the hydrogen moderator to easily move in and out of the core. The physical characteristics of uranium hydride, a combined fuel and neutron energy moderator, are ideal for the generation of safe nuclear power. The reactor operates at an optimum temperature of 550°C, selected as the goal for the so-called Generation IV reactors by the US Department of Energy (DoE). At 550°C, the dissociation pressure for the hydrogen above the hydride is approximately eight atmospheres, which permits easy transportation of the gas without presenting significant high-pressure risk. The temperature-driven mobility of the hydrogen contained in the hydride can change the moderation, and therefore the reactor criticality, making the reactor self-regulating.
The hydrogen forced out of the core during any over-temperature excursion reduces the neutron energy moderation necessary for nuclear criticality. The Hyperion Power Module is inherently fail-safe, since any temperature increase from excess activity immediately reduces the criticality parameters and thus the power production. The consequent power reduction causes the temperature to decrease and that temperature decrease eventually reverses the process, resulting in relaxation oscillations that quickly damp out to steady-state operation.
Hydride materials have long been recognised as possible controls for self-regulating nuclear reactors. R E Magladry was granted four US patents related to use of the material. G A Linenberger, J D Orndoff, and H C Paxton wrote about “enriched-uranium hydride critical assemblies” in Nuclear Engineering & Science journal as early as 1960. In addition, uranium hydride was demonstrated to be a successful reactor fuel very early in the nuclear era although the hydride was cast into blocks using a polymeric binder to prevent the hydrogen from escaping. This binding of the fuel precluded any observation of the self-regulation characteristics inherent to the material.
While the science of the Hyperion reactor has been around for this long period, it has not been implemented because the conditions for self-regulation have not been explored and the limits on those conditions delineated. But now the critical modelling has been performed and the critical features and design criteria for exploiting the safety and self-regulation advantages of hydride materials have been established, which make the reactor practical for construction and deployment.
Uranium hydride (UH3) stores vast quantities of hydrogen, equivalent to the density of hydrogen in water or in liquid hydrogen. This hydrogen, however, is volatile and is easily driven out of the hydride by any increase in temperature over the operating point of 550°C. The resulting decrease in moderator density drives the core reactivity negative. This drop in moderation occurs with very little change in core temperature since the excess power goes into the dissociation of the hydride. Potential over-excursions in temperature are limited by this chemical conversion in a manner similar to other phase changes, such as the boiling of water. A decrease in core temperature reverses the process by causing hydrogen absorption that increases the moderator density and thereby increases the core reactivity.
The customary control of nuclear power devices by the mechanical insertion and removal of control rods has been replaced in the Hyperion reactor by the self-regulating, temperature-driven desorption/absorption of the moderating hydrogen. The complex arrays of detectors, analysers, and control systems responsible for the safety and stability of conventional nuclear reactors have been superseded by the fundamental science and properties of the active materials. The system safety requires high-volume gas transport between the core and a hydrogen storage medium. This dictates that both media be contained in a common chamber with substantial open space for this gas flow. The gas storage medium must have substantial storage capacity, suggesting another metal hydride, such as depleted uranium, with excessive surface area to absorb the escaping gas. As a final safety measure, an overpressure relief valve is included in the gas container to release the hydrogen if it ever exceeds a preset value, thereby eliminating any possibility of catastrophic temperature increase.
Finally, if the temperature ever does rapidly escalate for any reason whatsoever, the phenomena that makes the popular TRIGA reactors safe will takeover and shutoff the nuclear activity. This is another inherent property of hydride moderator/ fuels. The safety record for TRIGA is without peer and so dependable that the US Nuclear Regulatory Commission (NRC) has granted the reactor type a licence for unattended operation.
The core can include a fuel container with a hemispherical bottom, topped with a cylindrical volume of equal diameter. An alternative design might be cylindrical, of height equal to the diameter. In both cases, the top surface of the powder is flat and open to the hydrogen atmosphere, which allows the hydrogen to be exchanged. Outside the core is a collection of trays to hold another hydride to absorb the hydrogen expelled from the core. Depleted uranium, whose chemistry is identical to the fissile material, is the obvious storage material. These volumes are separated by a thermal insulator to dampen thermal feedback. A sealed chamber chamber confines the hydrogen gas as it is exchanged between the core and the storage media.
The temperature of the storage media is controlled to a fixed value, which determines the hydrogen pressure within the sealed chamber by either absorbing or desorbing the gas. The ambient hydrogen pressure, in turn, fixes the effective temperature of the core by forcing hydrogen into the core if its temperature is below the dissociation temperature for that pressure, and permits the hydrogen to escape if the core temperature is above the dissociation temperature.
In this manner the temperature of the core is slaved to that of the storage medium. The gas-tight container is sited in a secure vault, fabricated from reinforced concrete. To protect the external environment, this inner vessel confines the hydrogen gas and provides containment for all radioactive materials. The containment vessel is designed for double confinement.
On top of the sealed vessel is the heat exchanger that transfers the thermal power generated by the reactor to secondary heat pipes that are larger and longer so they can reach ground level and subsequently transfer the thermal power to a working fluid used directly as a heat supply or used to drive rotating machinery to produce electricity.
The requirement for multiple confinement adds extra stages of heat transfer to the heat extraction system to ensure that any failed heat pipe does not become a conduit for contamination escape.
The gas-tight construction will be breached by only a small number of gas ports. The gas ports are important because they permit the addition and removal of the hydrogen gas for startup and shutdown. Sufficient hydrogen gas to initiate nuclear activity will be admitted into the chamber only when it is located in its intended operating site. Additionally, these ports permit maintenance of the contained gas. Neutron-absorbing gases such as Xe-135, which poison the fission reactions, are removed, hydrogen is added to replace that which is lost to diffusion out of the container, and inert gases, which inhibit hydrogen absorption, are removed. The power module is made completely safe from inadvertent nuclear startup by evacuating a substantial volume of the hydrogen out of the chamber, thereby removing the moderation required for criticality.
The incorporation of gas ports into the inner chamber affords an additional advantage. Mixtures of deuterium and protium can be used for fine control of the criticality of the module when it initially possesses excess reactivity. The module must be able to achieve criticality over a large range of enrichment, for example from 5% down to 3%, to perform multiyear operations. This criticality is easier to control in the initial high-enrichment phase of operation using a significant fraction of deuterium as the neutron energy moderator. Deuterium, in part because its mass is twice that of the neutrons, is almost an order of magnitude less effective as an energy moderator than protium, which is almost identical in mass to the neutron. Adjusting the ratios of these two isotopes of hydrogen contained in the chamber throughout the life of the power module afford advantages to the fine control of the system criticality.
The hydride reactor can maintain control of the fission activity for a large excess of reactivity and fuel, which permits extended operational life and significant burnup of the fuel. A reactor designed to burn a significant fraction of the fuel must be large enough to still reach nuclear criticality at the terminal enrichment design point. For example, a reactor that can produce power at an enrichment of 3.5% would have a critical dimension (equal height and diameter cylinder) of 1.44m while the size required for nuclear criticality at an initial enrichment of 5% (reactor grade) would be only 1.1m in height and diameter. These calculations used MCNP (the Monte Carlo neutron transport code) assuming fully stoichiometric uranium hydride, and assuming that half of the volume is invested in cooling tubes.
Taking five years to burn the fuel from the starting 5% down to 3.5% will produce about 70MW of thermal power on average. Assuming a nominal conversion factor for electrical production of 40% would make this device a 27MW electrical generator. Designing the reactor to operate at higher initial enrichments would permit even higher percentages of fuel burnup while still maintaining the compact nature of the device.
One of the remarkable advantages of this reactor concept is the novelty of the fuel form. The hydride chemistry greatly simplifies the problems normally associated with nuclear fuel reprocessing. At the end of the useful life of the original charge of fuel, the module will be returned to the factory. Adding heat to the fuel drives any remaining hydrogen off, leaving uranium metal. This metal can be stripped of its fission product contaminants by simple zone refining. The small fraction of the processed fuel that contains the concentrated waste may require further processing to extract residual actinides to be blended back into the fuel fraction. Such extraction and recycling of the actinides would remove the long-lived radioactive components from the waste and reduce concerns over its long-term management. Reuse of the fuel would require blending in an admixture of enriched or otherwise fissile material to bring the fissile component up to the original 5%, reactor-grade design level. This reprocessing requires only the addition of power to process the fuel, thereby adding no new material to the waste stream. The fission fragments can be further concentrated if it is economically useful or can be further processed to extract economically valuable radiation sources.
The simplicity of the process and the zone refining equipment makes reprocessing this fuel economically viable. This permits the contaminated but unburned fuel to be recycled, greatly reducing the waste stream and dramatically improving the economics of future nuclear power production. Only the fission fragments mixed with some residual uranium require permanent disposal.
Actinide hydrides with high melting temperatures make near-ideal combined fuel and moderator materials for producing an entirely new paradigm for exploiting nuclear energy. The mobility of the hydrogen contained within the hydride controls the fission activity without the need for any mechanical components or human oversight.
The Hyperion reactor, which takes advantage of these features, is therefore self-regulating and stable in power generation and inherently safe from any over-excursions in temperature. Modelling of the kinetics of the fission activity and the gas transport has shown that the reactor responds to transient effects with damped oscillations in power. The inherent formation of the hydride into powder form makes the hydrogen diffusion rate rapid enough not to interfere with the system kinetics and permits the rapid expulsion of the gas by the fluidisation of the powder. The high melting temperatures of uranium and thorium hydrides prevent the fusion of the powder at temperatures compatible with steam cycle conversion of the thermal power to electrical power. The reduced chemical form of the fuel greatly simplifies the reprocessing of the fuel by permitting the zone refining extraction of fission fragments. This should make recycling the actinides economically attractive and minimise both the waste requiring disposal and the long-term radioactivity risks of that waste.
The Hyperion hydride nuclear reactor could dramatically change the way we exploit nuclear energy. The individual power modules will be modest in size, capable of generating 27MW or more of electricity each. The small size permits the units to be placed at dispersed locations underground, making it possible for the site and the control to be local to the end use. The inherent safety reduces the manpower requirements for any installation. The small size and simplicity reduce investment risk and make the invention attractive for mass production of turnkey units. Simplified fuel preparation and reprocessing add to the economic attractiveness.
Finally, the dispersed and underground location of the modules makes them safe from most outside threats. These dramatic changes in the manner in which nuclear power could be generated will again make it an attractive competitor to fossil fuel power generation.
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