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The Pebble Bed Modular Reactor

Cheaper, Safer and Almost Carbon Free
by Kevin McKern
March 13th, 2006

Accepting both the peak oil hypothesis and that climate change is real raises questions about the future of nuclear power; its cost, safety and full cycle greenhouse impact. Additionally, as an Australian reader objected to my last item "Nuclear Energy Back in the Mainstream" because I recommended Uranium miners as an investment I feel its important to place my understanding of the facts on the record.

Nuclear reactors generate energy from fission. An atom of uranium splits into two, releasing energy plus two neutrons; and if either of those neutrons hits another uranium atom it can cause that atom to split, which releases more energy and another pair of neutrons, a chain reaction. Most nuclear power today is produced by large PWRs.

According to a 2005 IAEA report, Chernobyl caused 56 direct deaths; 47 accident workers and 9 children who died of thyroid cancer. Additionally it was estimated that as many as 4,000 people may ultimately die from long term accident-related illnesses. Greenpeace, amongst others, dispute that study's conclusions and presume the toll was higher.

Whatever the true toll of the Chernobyl accident, even conceding a worst case scenario, what most characterises the contribution of civilian nuclear power to world energy production is its relative safety compared to all other means of energy production.

In terms of direct deaths per terawatt produced since 1972, Coal killed 342, Hydro 883 and natural gas 85, but only 8 fatalities were recorded per terawatt of nuclear power.(1) In fact, this statistic vastly underestimates the relative hazards of fossil fuels as the indirect deaths from pollution caused by Coal powered stations worldwide is estimated at over 5 million per year.

A 1000 MW(e) coal plant, depending on sulphur content, sends annually millions of tons of Carbon dioxide, 44 000 tonnes of sulphur oxides and 22 000 tonnes of nitrous oxides into the atmosphere causing acid rain and poor human health. Additionally, there are 320 000 tonnes of ash containing 400 tonnes of heavy metals for which abatement procedures themselves produce as much as 500 000 additional tonnes of solid waste that must be disposed of.

If the potential future climate change impact of the billions of tons of carbon emitted yearly from conventional power plants is taken into consideration, the death toll of say, heat waves in Europe or drought in Africa may, sooner or later, need to be added to the already massive indirect costs of conventional power.

Reactor Types

In a Pressurised Water Reactor (PWR), the fuel (ceramic pellets) is packed into fuel rods. Fission heats water to a temperature of about 320 C and via a heat exchanger this heat generates steam that drives turbines in another loop.

The coolant water also serves to slow the neutrons down, allowing them to be absorbed by other uranium atoms, that is, the water acts as the moderator.

PWRs were built based on experience gained building reactors for submarines, where a high power density was required and in theory, if the coolant is lost the chain reaction stops. In practice heat from short lived decay products keep the core hot. A large PWR can produce so much power that without coolant flow the reactor can be damaged and it is this high power density that demands a massive containment structure and safety systems and personnel.

A Pebble Bed Modular Reactor (PBMR) has thousands of pebbles rather than fuel rods. About the size of billiard balls, each micro sphere has a core of enriched uranium, about half a millimetre across, surrounded by three layers, pyrolytic carbon, silicon carbide and graphite. Pebbles are added to the top of the reactor and taken from the bottom. The fuel pebbles removed are inspected and replaced and otherwise returned to the reactor. You do not need to shut the reactor down to refuel, unlike a PWR.

Helium is used as the coolant, entering the core at 482 C and leaving at 900 C. The high temperature of the helium and the fact that it is directly coupled to the gas turbine make a PBMR much more efficient than a PWR. A single reactor produces only about 110 MW. But, if more power is required at a site, up to 10 PBMRs can be located together and run from a common control suite; hence the name modular.

A PBMR has a number of features that should make it much safer than a PWR. The use of pebbles means it has a considerably lower power density in the core and with a much greater surface area pebbles are better at dissipating heat. A loss of coolant therefore cannot result in a meltdown that damages the reactor. The biggest advantage of a PBMR is that as the pebbles heat, fission slows. In the event of a catastrophic cooling-system failure, the core temperature climbs to 1,600 degrees Celsius - comfortably below the balls' 2,000-plus-degree melting point - and then falls, making the reactors walk-away safe.

A few tons of high level waste a year has to be disposed of carefully underground.

Costs

Vattenfall, the Swedish energy company produces electricity from Nuclear, Hydro, Coal, Gas, Solar Cell, Peat and Wind energy and has produced accredited Environment Product Declarations for all these processes.

Vattenfall finds that averaged over the entire lifecycle of their Nuclear Plant including Uranium mining, milling, enrichment, plant construction, operating, decommissioning and waste disposal, the total amount CO2 emitted per KW-Hr of electricity produced is 3.3 grams per KW-Hr of produced power.

Vattenfall measures its CO2 output from Natural Gas to be 400 grams per KW-Hr and from coal to be 700 grams per KW-Hr.

Thus nuclear power generated by Vattenfall emits less than one hundredth the CO2 of Fossil-Fuel based generation. In fact Vattenfall finds its Nuclear Plants to emit less CO2 over the lifecycle than even green energy production mechanisms such as Hydro, Wind, Solar and Biomass.

Of course, all these methods emit much less carbon than fossil fuel electricity and they all have a respected place in our energy future. Until cheap and ultra efficient large energy storage systems become available only nuclear power can replace large coal burning plants.

Once PBMR's are in full production they may be able to generate energy at about 1.7 US cents per kWh, well below the costs of new coal, gas or wind plants, and far below the cost of other nuclear power.

In conclusion, I'll quote from James Lovelock, who's research ultimately saved the planet when he discovered CFCs in the atmosphere in 1973.

"Opposition to nuclear energy is based on irrational fear fed by Hollywood-style fiction, the Green lobbies and the media. These fears are unjustified, and nuclear energy from its start in 1952 has proved to be the safest of all energy sources. We must stop fretting over the minute statistical risks of cancer from chemicals or radiation... If we fail to concentrate our minds on the real danger, which is global warming, we may die even sooner, as did more than 20,000 unfortunates from overheating in Europe last summer."

The sooner construction starts on these reactors the better.

by Kevin McKern
March 13th, 2006
website: www.kontentkonsult.com
email: kevinmckern@iinet.net.au

(1) Severe Accidents in the Energy Sector, Paul Scherrer Institut, 2001
(2) Risk analysis at http://www.phyast.pitt.edu/~blc/book/