January 13, 2005

Old Atoms, New Tricks 

A big problem with current nuclear reactor designs is that they are not very intrinsically safe: if the cooling systems fail, the fuel rods will overheat with very unpleasant consequences - in the worst case, a core meltdown. Also, if too many control rods are withdrawn all together, the power production of the reactor can increase sharply beyond the design limits, and (together with cooling problems) this is Chernobyl.

Actually, the number of nuclear accidents is very low, but they can be very serious - although only chernobyl had a wide impact on people and environment. But that was enough for the general public to see nuclear energy as a barely tolerable risk.
Yes, reactors can be made safer, and more containment structures can greatly mitigate the consequences of core damage. But adding layers of safety makes things more complicated, and enormously more expensive. The big and thick steel pressure vessels that encase the reactor cores cost many million dollars alone. Thus, nuclear power stations are often not economically viable, because they will not produce cheap energy.

A different basic design is needed to address those issues altogether.
So we come to the pellet bed reactor. The idea is not really new - the first reactor of this kind was proposed in the USA in the '40s by Farrington Daniels, but then rod-fueled and water-cooled reactors were preferred - not really for technical reasons.
In this kind of nuclear reactor, the fuel is in form of small balls arranged in a bed (much like a fixed-bed chemical reactor) with gaseous helium passing through the bed to carry heat away. The use of helium alone resolves many problems, because helium is chemically inert and a quasi-ideal gas. On the other hand, high-pressure steam and superheated water are corrosive and at high temperature can react chemically with the fuel or rod materials (generally, zirconium).

The pebble-bed reactor was studied by different groups, but no-one came up with a commercially viable design. Until a few years ago, when a Chinese research group (with considerable help from German groups and companies) activated a 10 MW experimental unit, named HTR-10 (high temperature reactor, 10 MW). Not casually this reactor was developed in China: the rapid Chinese development requires huge amounts of electricity, and China is devoting great resources to research and development in many fields. Wired has a good divulgative article, but a much more in-depth description of the reactor is in the monographic Volume 218 of Nuclear Engineering and Design, October 2002. And now don't tell me that your barber shop does not have Nuclear Engineering and Design!

In this reactor, the fuel is uranium oxide-loaded graphite balls, 6 cm in diameter and containing 5 g of enriched uranium each, that move slowly down a properly designed chamber, lined with graphite bricks to reflect back neutrons, and then boronated carbon bricks for thermal insulation and radiation shielding. The coolant is gaseous helium at about 3.0 MPa, which enters the core at 250 C and exits at about 700 C, while the fuel balls will reach the maximum temperature of 800 - 870 C during normal operation. The hot helium then is routed to a heat exchanger in onrder to produce steam to drive the turbine, and circulated back to the core by a blower. There are 10 control rods to control the fission and eventually shut down the reactor, plus containers filled with neutron-absorbing balls ready to be discharged in the core if emergency shutdown is required.
The fuel balls exit one by one at the bottom of the reactor, and they are automatically examined to discard the damaged and burnt-up ones while those still suitable are reloaded at the top.

In case of accident and cooling failure, the fission and decay heat will be dissipated through the reactor walls in the housing cavity, and then to the outside by oversized natural-circulation water coolers. But even in the case of failure of these coolers, the reactor is designed in such a way that the fuel elements will not exceed the safe temperature of 1600 C (a maximum of 1033 C is estimated in case of accident). That's the main feature of the HTR, indeed: even with no core cooling, the temperature of fuel elements will not excedd safe limits and that depends from geometric and physical factors, not intervention of automatic or manual safety systems. Another, subtler, safety mechanism is the so-called "negative reactivity temperature coefficient": for reasons quite difficult to explain, in this kind of reactors the fission rate decreases when the temperature of the fuel increases, thus generating less heat.

If a conventional water-cooled reactor loses its cooling, damage to fuel rods is very likely (it depends wether the reactor is shut down quickly enough or not). There may be no radiactivity dispersion, but it's not a comfortable situation anyway.

The fuel elements itself are very safe: they contain low enriched uranium as spherical particles of uranium oxide coated with a multi-layer coating of pyrolitic carbon (similar to graphite) and silicon carbide: this coating has the function of containing the fission products (which are mainly solids and gases) up to 1600 C. These coated particles are subsequently mixed with graphite and pressed in a ball with a 10 mm uranium-free layer at its periphery (The process is actually very complex, and likely to be rather expensive). The fraction of free uranium (uranium out of the coating) is very low, no more than 0.01% of the total. This means that only a minimal amount of fission products will be released in the cooling helium. Given that graphite, pyrolitic carbon and silicon carbide all have high corrosion resistance, the spent fuel balls pose a less serious disposal problem than spent fuel from conventional reactors. In fact, spent rods are first left to cool down in a water basin, and after some steps of processing, the radiaoctive waste is disposed of in special containers, but even the best containers will be corroded after a few hundred years - that's why radioactive waste dumps need to be placed in stable and dry geological formations, because water accelerates corrosion and carries away the radioactive elements. I don't know exactly the corrosion rate of graphite and SiC, but my guess is that these materials will happily last many thousand years.

At a first glance, the HTR seems also a scarcely efficient system to produce plutonium for weapons use, and that's a factor to consider.

China is expecting to build many Modular HTR units, beginning from 2006 with a demonstrative plant, in order to reduce the consumption of fossil fuels and produce cheap and abundant electricity. Different turbine options are being evaluated, the most interesting one being the direct helium turbine and steam turbine cycle: with this design, hot helium is directly passed into a gas turbine, and the residual heat in the effluent used to produce steam before helium is returned to the reactor's core. This solution is very efficient, but it would require re-design and re-testing of many parts of the reactor.
Research and development is underway, both for the HTR-Module and to improve the fuel production process.

Also Western research groups are studying HTRs, because electricity production is not only a Chinese issue.
The real problem - not in China but in the power-hungry West - is to make the public accept this kind of reactors. The Chinese group trusts the fact that the safety concept of the HTR is easier to understand even for people without engineering knowledge, but I am afraid that it will be rather difficult - especially in Europe where the enviro-folks are ready to rattle the Chernobyl spectre everytime anything nuclear is mentioned. Time will tell, but for my part I would be glad to see HTRs being built.


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