Feasibility of Accelerator-Driven System with Current Technology T.-Y. Lee and H. S. Lee Pohang Accelerator Laboratory

Threats to Nuclear Power Fukushima accident and its on-going effect was a major blow to nuclear power. To restore status of nuclear power as the future energy source, We need safe nuclear power plant

We Still Need Nuclear Power Environment issues (global warming): Nuclear power is the least CO2 emission energy source and the most economic

Accelerator-Driven System Sub-critical reactor + Proton Accelerator (compensates for sub-criticality (insufficient neutrons) by generating neutrons at hard target) Accelerator is a safety switch which provides safety to reactor. But, is ADS free of Fukushima-type accident ?

Thorium fuel is in liquid form as molten salt. If Thorium is used…. Thorium can be used as the fuel in ADS instead of Uranium. Thorium is 4-times more abundant than Uranium. Thorium is fertile, but Thorium Fuel Cycle is fissile. Thorium fuel is in liquid form as molten salt. Thorium reactor was first studied in Oak Ridge Lab. in 1960s.

How It Is Safe Fukushima Reactor Accident Molten Salt Thorium Reactor 1. Reactor Shutdown by earthquake 2. Power Out by tsunami. Cooling Facility down, 3. Overheat by decay heat 4. Fuel cladding melted, 5. Hydrogen gas from fuel cladding 6. Hydrogen gas leakage from reactor Explosion! Reactor Shutdown by accelerator-off Air Cooling. No need to be located near shore If it begins to overheat, a little plug melts and salt drain No hydrogen No explosion !

Thorium Reactor

Transmutation, Another Aspect of ADS Using it fast neutrons, ADS can transmute long-lived nuclear waste (Minor Actinide) to short-lived waste (several hundred years) and reduces the amount substantially. ADS has so many good properties. Why is ADS unrealized yet?

Accelerator requirements for ADS are difficult to satisfy Two major problems High proton beam energy and power: Proton energy around 1 GeV is effective in neutron production. A typical target frequently referred to is 1 GeV proton beam and 10 MW power (lower proton energy may be used with higher beam power) .

Current Research: MYRRHA (Belgium)

Stability Issue Commercial requirement is stricter 2. Operational reliability: Accelerator operation should be very stable. (Accelerators often stop). Commercial requirement is stricter

Energy & Power Issue in Accelerators Linear Accelerator Achieved 1 GeV. Power has not reached it yet. Large size, huge cost 1 GeV, 1.4 MW, 350 m length

Energy & Power Issue in Accelerators Cyclotron 1. PSI cyclotron: 0.59 GeV, 1.2 MW, 4.5 m radius. To be upgraded to 2.4 MW. 0.59 GeV is usable. 2. 1 GeV, 10 MW cyclotron design exists (NIMB 113, 1, 1996). 3. Small size, relatively economic Accelerator requirements such as power and stability are still difficult to satisfy

ADS with Current Technology? It is difficult to build a giant accelerator of ADS capacity. Hence, Distribute power and stability to multiple accelerators Cyclotrons Reactor Power can be added to give 10 MW or even higher.

Stability of Multiple Accelerators P: Stability of an accelerator, 0<p<1, (ex: p=0.9). Suppose that 5 accelerator can give 10 MW or even higher. Gain: the probability that all accelerator are down is extremely low as Hence, there will always be a few accelerators operating. Loss: the probability that all accelerators are operating is lower than p, as Hence, all accelerators will not always be operating. ADS with multiple accelerators can be safely operated by adjusting the k-parameter.

k-parameter k>1 : Super-critical. Nuclear bomb. k=1: Critical. Nuclear Reactor. k<1: Sub-critical. Reactor down. But, in ADS, the 1-k portion is furnished by accelerators. For safe ADS operation, k should not be too close to 1. A typical number is 0.98

Operation of Multiple Accelerators ADS with multiple accelerators can be operated by adjusting k with control rod. 5 cyclotron active: 4 cyclotron active+1 cyclotron down: 3 cyclotron active+2 cyclotron down: 2 cyclotron active+3 cyclotron down: 1 cyclotron active+4 cyclotron down: 5 cyclotron down: reactor down Increased danger is only temporary. Danger increases Probability decreases Conclusion: ADS with multiple accelerators can be safely and stably operated.

Summary Thank You for Listening!! Accelerator-driven sub-critical reactor is a promising candidate for the future energy source that is both safe and economic. Thorium fuel makes the sub-critical reactor safer and more economic. Multi-beam scheme may realize ADS with current technology. Thank You for Listening!!