Broadening the role of reactors

Small-scale nuclear technology, like that of the the pebble bed high temperature reactor, fits the managerial, financial and operational rules and operating procedures that are used by the non-utility markets.

These are the markets of the non-professional energy producers. The members have core businesses for which energy is required, for example stand-alone heat generation; combined heat and power production units for industries like breweries or paper mills; stand-alone electricity generation on islands and in remote areas; and as prime mover on board ships.

The Nereus project aims to examine a “naturally safe, efficient reactor, easy to operate, ultimately simple and small”. The project considers the possibilities of using nuclear technologies for small-scale energy production.

THE HISTORY OF POWER

The important aspects of the history of energy production development have been:

•The amount of energy per kg of fuel increases.

•The combustion unit decreases in size for a given power level.

•Emissions change from “diluted and dispersed” to “confined and controlled”.

•The medium for transportation increases in purity, causing less corrosion and erosion.

•The energy transport system starts as an open (dirty) cycle, but becomes a closed (clean) cycle.

•The rotating energy conversion unit becomes standard.

•The complexity of the installation increases.

•The number of people required for operation and maintenance decreases.

•The weight and volume per produced kW decreases.

•The total process efficiency increases.

Historically, new systems have not appeared as a result of a shortage of a type of fuel, but because better systems became available. The historical trends of the use of power are given in the table opposite.

The challenges

The generation currently at college or university will be the leaders of society in 30-50 years. In view of the challenges posed for the future, these future generations will have to employ all possible means to produce this required amount of energy in the most cost-effective and environmentally friendly way.

The figure below left shows a concept for small-scale energy production. The energy production unit consists of a recuperative gas turbine directly coupled to an inherently safe nuclear heat source through a closed-cycle helium system.

We believe that this Nereus installation is smaller than comparable existing prime movers, is suitable as prime mover in an all-electric ship propulsion configuration, and is very much comparable in price per produced kWh to existing energy production installations, when calculations are based on through-life costing.

The waste heat and surplus electricity can be used for cargo heating or to produce fresh water through desalination and reverse osmosis, as is the existing practice.

The heart of the installation is an inherently safe helium-cooled nuclear heat source of 20MWt, directly coupled to a recuperative gas turbine which has an output of 8MWe.

The non-nuclear part

The non-nuclear part of the installation consists of a helium gas turbine, a helium-cooled generator, a recuperator, a heat dump, a mass flow power control system and an industrial heat exchanger.

The gas turbine is a “cool” one. Temperatures at the inlet of the turbine are low, so no expensive coatings or cooled blades are needed.

Lubrication oil in any closed cycle gas turbine will lead to pollution of the heat exchangers, and, in this case, of the nuclear heat source. Therefore, all bearings are helium-cooled magnetic bearings. The generator is also helium cooled.

Control of reactor criticality

On-line refuelling is a key element of the design. This ensures that there is no excess of reactivity in the core of an HTR, preventing the possibility of an accident like Chernobyl. However, on-line refuelling requires the constant adding of fresh fuel and the removal of spent fuel, requiring a complex installation. This refuelling system was applied in the Arbeitsgemeinschaft Versuchsreactor (AVR) in Jülich, Germany. In addition, the pebble bed modular reactor under study in South Africa uses such a refuelling system.

The Nereus installation is based on the “keep it simple, stupid” (KISS) principle. The intention is to use a burnable poison. Some materials have such high neutron absorbing characteristics that when placed in a reactor core, their concentration decreases because of transmutation as a function of time. Such materials are called burnable neutron poisons. They facilitate higher fuel concentrations in the reactor core and consequently a slower decrease in reactivity. The burnable poison also simplifies the control rod requirements, and only stop/start rods are needed.

Currently, Nereus is based on a three-year refuelling period. Therefore, the combination of the fuel enrichment and the burnable poison will be designed to produce 20MWt in the reactor, resulting in 8MWe at the generator, over a period of three years with a usage pattern of 90% load and 80% usage, resulting in 50,000MWh per year.

Control of energy production

The reactivity is strongly dependent on the temperature of the fuel.

HTR fuel has a negative temperature reactivity coefficient. When the temperature of the reactor temporarily decreases, its reactivity increases, its power generation increases, and the original temperature level is restored. It was extensively tested at the AVR in Jülich. This phenomenon is being used for power control in the Nereus reactor concept. As a result, the reactor can operate with “unmanned” control rooms (ie remotely). The power control output of the installation is delivered by the generator and is achieved by controlling the mass flow in the closed cycle system. This is not the optimal solution, but the one with the lesser number of parts.

After three years of operation, about 7m3 of fuel elements are removed from the core. This waste can be transported in shielded containers. One possible design has a diameter of less than 3m and a height of 5m. After 10 years, the waste will have decayed to intermediate level.

Safety test of the HTR

The 13MWe AVR test reactor has been used for testing purposes for 20 years.

A steam turbine was used as energy conversion unit. The core was cooled by helium in a closed cycle system and was pumped around by two electrical ventillators. The energy was used to raise steam in a heat exchanger and the steam used to drive the steam plant in a closed-cycle Rankine cycle.

Extensive tests on loss of coolant were carried out, and the conclusion from these tests was: “As a consequence of the inherent and passive safety characteristics of the design, the individual and social risks in relation to incidents and accidents are negligable.”

Special attention has been given to flooding. This is because of the potential application in countries like the Netherlands and on beaches of remote areas or islands producing fresh water.

The reactor can be designed such that on flooding of the core, the reaction would immediately stop. It should be noted that the present ship propulsion reactors do not comply with this requirement and must be stopped by active means in the case of being lost at sea.

The HTR propulsion unit is designed in such a way that inherent shutdown upon flooding is achieved as a consequence of the nuclear design. For this purpose, the moderator-to-fuel ratio in the core is an important design parameter, as is the inclusion of cooling channels in the inner parts of the graphite reflector, which are flooded in the case of ingress of water, thereby decreasing the neutronic coupling between the core and the reflector.

Suitable for small-scale?

The Nereus installation is suitable for small-scale energy production because of its inherent safety characteristics, such as:

•Removal of decay heat by natural draught.

•Negative temperature coefficient.

•Coated particle fuel (TRISO).

•Spherical fuel elements.

•Low power density.

•Fuel integrity maintained under all conditions.

•The gas turbine cycle has a higher efficiency, and so produces less thermal pollution than existing nuclear plants.

“Never before in history, was society confronted with a power so full of possible danger and at the same time so full of promises for the future of humankind and for peace in our world.” This was not a statement about nuclear technology, but one from the US Congress (1875) about the internal combustion engine.

These politicians had the courage and vision to take on the challenge that faced them.
Tables

History of energy trends