Supplying Electricity & Hydrogen from the nuclear Pebble Bed Modular Reactor Andrew Kenny Energy Research Centre University of Cape Town South Africa Electricity & Hydrogen Production from the nuclear Pebble Bed Modular Reactor Andrew Kenny Energy Research Centre University of Cape Town South Africa

Advantages of Nuclear Power Safety: –By far the best safety record of any large scale source of electricity (full energy cycle) Waste: –Waste is small, solid, stable & easy to store so that it presents no danger to man and the environment Economic: –Among the cheapest, if not the cheapest, source of electricity in the USA, Europe and Asia

Advantages of Nuclear Power (cont) Sustainability: –Enough nuclear fuel to last until the Sun turns into a Red Giant (vast abundance of uranium & thorium in the Earth’s crust and oceans) Siting: –Nuclear stations can be sited wherever they are wanted (fuel is very cheap to transport) Emissions: –No gaseous pollutants or greenhouse emissions during operation –Among the lowest, if not the lowest, emissions of greenhouse gases / kWhe over full energy cycle

Number of Energy Accidents from 1969 to 1996 with at least 5 Fatalities ( Paul Scherrer Institut, "Severe Accidents in the Energy Sector“)

Greenhouse emissions for Full Energy Chain of different Generation Technologies

Disadvantages of Nuclear Power Public Perceptions –There is public fear of nuclear power –Associations with atomic bombs –The fear is deliberately exploited by anti- nuclear groups High capital costs –Existing nuclear power reactors have high costs per kilowatt –Long times for construction, licensing and commission - which adds to costs

Existing Nuclear Power Reactors 87% of the world’s nuclear electricity is from light water reactors These have evolved from nuclear submarine propulsion units, and are not ideal for land based power generation Features: –High power density –Low temperature, poor quality steam –Complicated active safety mechanisms –High capital costs ($1500 to $2000/ kW) –Large unit size (900 MWe +) –Long construction times

The Pebble Bed Modular Reactor The PBMR was conceived to meet three requirements: –1. Capital costs of $1000 / kWh –2. 24 month construction time / unit – metre Emergency Planning Zone (EPZ) No light water reactors can meet these requirements

Fundamental Design Philosophy The fundamental design philosophy of the PBMR is inherent safety. It is totally impossible for any safety system in the PBMR to fail because it does not have any safety systems. The control rods and small absorber spheres can shut the reactor down quickly but they are not necessary for safety. They are operating systems. Safety is inherent, passive, built into the reactor design. No human error or equipment failure can cause an accident that endangers the public. This includes total loss of cooling at 100% power

Features of PBMR (power unit) Coolant: helium (inert chemically and radiologically) Moderator: graphite Fuel: enriched uranium (about 9.5%) Configuration: Fuel pellets embedded in graphite spheres (“pebbles”) Power cycle: Brayton (heated helium drives gas turbine) Power density: about 6 kW/l (PWR: 50 kW/l +) Unit Size: about 165 MWe (400 MWt) –size limited so that surface area/mass always sufficient to ensure enough loss of radiant heat to prevent dangerous temperatures Highest coolant temperature: about 900°C Highest coolant pressure: about 90 bar Efficiency: 42% +

A Possible Plant Layout for the PBMR: Three Shaft

Reactor Compressors Turbine Gearbox Generator Recuperator Pre- cooler Inter- cooler Another possible layout for PBMR: Single Shaft

Fuel Element Design for PBMR

Features of Fuel Design Extremely stable fuel, even up to high temperatures Heat is generated throughout each “Pebble” –flat temperature profile; no hot spots Low  T from fuel to coolant –150°C (over 400°C for other reactors) –excellent for producing high temperature gas Multiple barriers around each fuel pellet and encasement in graphite –makes it extremely difficult to use waste fuel for weapons –makes waste disposal very easy (each Pebble is its own containment)

Fuel Performance

Post Failure Cooling

Control of PBMR Constant Temperature in reactor (about 1100°C), regardless of load. –Achieved by Doppler effect Control Rods are just for trimming or shut down. –Not used for day to day control. Small Absorber Spheres for shut down (boron carbide, B 4 C)). Helium Mass (Pressure) used for continuous control. –Series of helium tanks at different pressures inject into and receive from the primary loop. High Pressure Booster Tanks for quick power increases. By-Pass Valves on Compressors for quick power decreases. The PBMR is load-following

Fuel Handling & Waste Once a fuel element (“pebble”) enters the power unit, it does not leave. Pebbles are automatically sampled. If they still have useful energy, they are returned to the reactor. If not, they are dropped into a waste fuel vessel, and remain there for the life of the unit. When the unit is eventually decommissioned, the waste would be disposed of in the same way as conventional nuclear waste: kept in casks on site or sent to a special storage site

8 Unit Module (8 x 180 = 1440 MWe)

8 Unit Module

Additional Advantages of PBMR Small unit size and quick construction time allows electricity companies flexible planning –Not necessary to tie up large sums of capital for long periods Modular design offers standardisation and sharing of facilities

Hydrogen Production 1. Reforming –Steam reforming of methane –Releases CO 2 2. Electrolysis –Inefficient 3. Thermochemical water-splitting –Efficiencies of 50% + –No greenhouse gas emissions –Sulphur-iodine cycle most likely –At pilot stage but looks very promising –Requires high temperature heat: 700°C +

PBMR for Hydrogen Production Nuclear energy is a good source of heat without greenhouse emissions Water cooled reactors cannot give the high temperatures required Gas cooled reactors can The best is the Pebble Bed reactor because: –low  T between fuel and coolant –can deliver the highest temperature heat

PBMR in Thermochemical Cycle There would be three loops. 1. The primary loop, with helium cooling the reactor 2. A secondary, buffer loop, also using helium, transferring heat to the chemical process –(to protect the reactor against possible ingress of chemicals) 3. A final thermochemical loop producing the hydrogen from water, sulphur and iodine

Temperature Limitations The higher the temperature, the better –Greater efficiencies for electricity & hydrogen Limiting factors: –1. Heat exchangers –2. Above 1130 °C, diffusion of silver & caesium from the fuel (coats surfaces, causes maintenance problems) Prospects: –With likely improvements in fuel, can expect operating temperatures in reactor of 1350 °C by 2015 –This means gas temperatures of 1200 °C

Conclusion The PBMR offers clean, safe, cheap, sustainable electricity with low GHG emissions –electricity costs: about 3 c/kWh (USA) The PBMR is a good source of high temperature heat for hydrogen production

The End Thank You

Configurations for Electricity & Hydrogen Separate units for making electricity & hydrogen A combined unit that can make electricity and hydrogen A unit that makes hydrogen, which can then either be exported or sent to a fuel cell to make electricity

PBMR Circuit Outline

Deaths/GWy in Energy Accidents 1969 to 1996 ( Paul Scherrer Institut, "Severe Accidents in the Energy Sector“)

HTGR Development “Families”

Costs of Nuclear Electricity

Costs of Hydrogen Production $/kg Electrolysis: 0.06/kWh Methane reforming: $0.80/kg Expected for nuclear/thermochemical: $1.30/kg (source: Pioneering Science & Technology)

History of PBMR Project 1967 to Germany runs 15 MWe AVR reactor. Highly successful. Father of PBMR to Germany builds 330 MWe THTR reactor. Too big. Loses inherent advantages of AVR. Teething problems. Political problems. Shut down Eskom looks for future economic generating technologies, including nuclear. IST shows it the Pebble Bed concept. Eskom adopts it for study to Feasibility study, concept design, costing Cabinet support. Commercial partners. Licence & EIA application Business case completed. (McKinsey) Favourable Record of Decision on EIA by DEAT