Description of the ADS

Figure 5.8: Scheme of an Accelerator Driven System (ADS).

5.4.4.2 Description of the ADS The ADS consists of two main components:

1. The reactor core, whose fuel consists mainly of thorium. At the start-up of the ADS there will also be some U-235 or Pu or even transuranic waste in the fuel to increase the fission rate. The core is located close to the bottom of a double walled tank that is 25 meters tall and has a diameter of 6 meters (see Figure 5.9). The inner tank is filled with molten lead with a

7 The effective neutron multiplication factor (k ).

temperature of 600 - 700ºC, or lead-bismuth eutectic with a temperature of 400 – 500ºC. At the upper part of the tank there are heat exchangers that transfer the heat from the molten metal to a secondary circuit (molten metal or water), eventually forming steam that drives the turbine. The lead-bismuth or lead is circulated by natural convection, that is, the heat supplied by the fissions in the core heats the molten metal such that it rises to the top of the tank where it is cooled by passing through the heat exchangers and returns towards the bottom. The outer tank is cooled by passive convection of air, which will remove the decay heat in case the cooling is interrupted or the inner tank ruptures.

2. The proton accelerator, which is located outside the containment building, supplies a high-powered (≈ 10 MW) beam of high-energy protons (500 - 1000 MeV) through a shielded beam guide to the spallation source inside the reactor core. The accelerator may be either a LINAC or a cyclotron.

The proton beam hits the target that is located close to the centre of the reactor core, and causes spallations that produce about 30 neutrons per incident proton. The neutrons enter the fuel in the core and cause two important processes:

1. Transmutation of the thorium into protactinium, which decays with a half-life of 27 days to U-233, which is fissile. Thus, fuel is produced from the non-fissile material thorium.

2. Fission in uranium/plutonium or even transuranic waste, present in the fuel, and in the U-233 that has been produced from thorium.

The reactor core produces 1500 MW of heat which is transformed into 675 MW of electricity. Of this electricity, 30 MW is used to drive the accelerator, and the remaining 645 MW is delivered to the electricity grid (See Figure 5.10). For a reactor with k = 0.98, the power of the proton beam must be around 10 MW to produce 1500 MW of heat. The power from the beam is thus amplified by a factor of 150, which is the reason for the name “Energy Amplifier”. The more subcritical the core is, the higher the power of the beam must be. For instance, if a core with k = 0.95 is used, the beam power must be around 25 MW. A beam power of 10 MW is more than the largest accelerators are capable of today, but it is considered to be realistic to develop accelerators of this size. The world's most powerful cyclotron is located at the Paul Scherrer Institute (PSI) in Switzerland, and was as an example operating in 1999 for 6000 hours with a beam power of 1 MW and an availability of 91 % of the scheduled time in 1999 [106].

Figure 5.9: Carlo Rubbia's Energy Amplifier.

Figure 5.10: Schematic View of the Energy Amplifier System for Electricity Production.

The reason for using a lead-bismuth eutectic as coolant instead of pure lead is to lower the melting point from that of lead (327°C) to 123°C, which reduces the probability of frozen slugs clogging the flow channels. It also reduces the heating requirements during reactor shutdowns.

To reduce the induced radioactivity in the coolant, it would be desirable to use pure lead instead of the eutectic, but this is still under development.

In addition to its subcriticality, the EA is distinguished from most other reactor designs since it does not use control rods for the power control. The power of the reactor is proportional to the supplied beam power, which means that power control can be performed by adjusting the accelerator power. The same method is also used to compensate for reactivity changes caused by fuel burnup.

When compared with critical reactors, Accelerator Driven Systems have two specific characteristics:

1. If well designed, they prevent criticality accidents. It has been proposed [107] to take advantage of this sub-criticality in order to use certain types of fuel with poor neutronic properties: in particular, because of their small delayed neutron fractions, incineration of minor actinides appears to be feasible efficiently with ADS.

2. Spallation provides additional neutrons which can be used for increased breeding of U-233 or Pu-239. Another possible use of the additional neutrons is to transmute long-lived fission products and transuranics (TRUs).

In Norwegian media it has repeatedly been claimed that an ADS based on thorium cannot melt down like an ordinary reactor if the cooling is lost, because it is not critical. Unfortunately, this is not the case. A reactor without cooling will melt no matter how subcritical it is. When the reactor is shut down, the heat does not come from ongoing fissions in the fuel, it is the decay of the fission

products that produces the heat required to melt the reactor. The Energy Amplifier as shown in figure 5.9 will probably not melt if the heat removal from the lead-bismuth coolant stops, because it contains around 8000 metric tonnes of passively cooled metal that will absorb the heat. But, if the coolant leaks from the tank the core will certainly melt.