Solid targets are generally made in the form of rods, spheres, or plates. Spallation is a violent reaction in which the target is bombarded with high energy particles like neutrons, which disintegrates the nucleus of the target through nuclear reaction. The result is the emission of protons, neutrons, alpha particles and other fission and evaporation products releasing considerable amount of heat. During the spallation reaction process, the conversion of several MW proton energy into neutrons by the spallation target takes place. This is a big thermal load, and so the spallation target has to be cooled. There are two options of cooling in ADS spallation target, namely by using the coolant of subcritical reactor primary system, or by using an independent loop, which is preferably used where the operating temperature is less than 5500C, which is a tolerable temperature of target window.
There is a new concept for a high power spallation target, namely gravity driven Dense Granular Target (DGT), where Granular Tungsten (W) particles are used. DGT can also use other heavy metal particles. W particles behave like both solid and fluid similar to sand flowing in a hourglass. W particles would flow from the hopper to the spallation zone under gravity and would be bombarded with a proton beam from a high energy linac, when neutron would be generated.
The grains will pass through the spallation region quickly and discharge from the orifice of the hopper to avoid being melted down. If the geometry is well designed, the flow rate of the hopper can be constant like an hourglass. If there is any damage to the W grains the same can be replaced online. The system is provided with a sensing device that would stop the proton beam in case of overheating. The arrangement is shown in FIG – 10a. Fig – 10b shows the circuit of grain supply and associated heat exchangers that ensure safe operation.
Core design
So far no ADSR system is operating on commercial basis. However, the project MYRRHA (multipurpose hybrid research reactor for high-tech applications) at present is in operation under the Belgian Centre for nuclear research. This is a research reactor operating since 1995, aiming to demonstrate the feasibility of the ADS. It can be operated in both ADS mode and subcritical mode.
Estimated cost of the project is about Euro 287 million. Reactor building size is about 90m (length) x 49m (wide). From its constructional and operational features, one can get an idea about a commercial ADSR nuclear plant, which may be installed in the near future. It consists of a superconducting linear proton accelerator of length about 100 m delivering its beam to a windowless spallation target coupled to a pool type subcritical core, with Lead- Bismuth Eutectic (LBE) coolant. A diaphragm inside the reactor vessel separates the hot and cold LBE.
The primary, secondary, and tertiary cooling systems have been designed to evacuate a maximum core power of 110MWt . The primary pump delivers the LBE to the core with a mass flow of rate of 453 l/s at a pressure of 300kPa as shown in FIG – 11. Primary cooling system consists of two pumps and four primary heat exchangers. The secondary cooling system is a water cooling system, while the tertiary system is an air cooling system. The thermal connection between the primary and secondary cooling syatems is provided by the primary heat exchangers.
Saturated pressurized water at 2000C is used as secondary coolant. In case of primary pump failure, the proton beam is automatically shut off when the reactor is used in ADS mode and in critical case the shutdown rods are inserted. The decay heat removal is done by natural convection. Main charecteristics are as folows: Total core reactor power – 110MWth, total primary mass flow rate – 9500kg/s, LBE mass inventory – 4500Ton, Core inlet temperature – 2700C, LBE outlet temperature from core – 3500C, temperature of saturated steam at secondary cooling loop – 2000C. FIG – 12 shows the geometry of the core and FIG – 13 shows cutway view of the MYRRHA reactor.
The MYRRHA system is provided with a proton accelerator delivering a proton beam of 600MeV at a beam current of 3-4mA in continuous wave to the core. To extract or insert the fuel assemblies, the robot arm can move up or down and grip the fuel assemblies. The geometry of the core is of hexagonal shape containing about 171 hexagonal channels, with 10.45 cm between opposite faces as shown in FIG – 12. 78 hexagonal channels are stuffed with the fuel assembly rods, that surround the spallation target, followed by next hexagonal ring, which contains 30 channels through which LBE coolant flows, and then comes 42 number of cells that house MgO reflectors. Rest number of channels house control rods and IPS experimental devices. All are housed in a stainless steel reactor housing of approximate size 10m (diameter) x 20m (height).
The following 3 fuel mixtures can be used:
- Mix – 1. Standard MOX fuel containg natural uranium – 62%, Pu -26% and O-12% (fuels are in oxide form).
- Mix – 2. Initial fuel mixture containing natural Th-232 -58%, Pu -30% and O -12%.
- Mix – 3. Th/U mixture containing Th- 232 – 70%, U-233 -18% and O- 12% (6).
Indian context
A zero-power, subcritical assembly BRAHMMA driven by a neutron accelerator to generate neutrons has been designed and commissioned by BARC (India) for investigation of neutronic proprties of ADSR. This research reactor is capable of changing keffvalues to suit the type of fuel. It has beryllium oxide reflector resulting in a compact mass. The subcritical assembly can be driven by either 2.45 MeV neutrons or by 14.1 MeV neutrons. An important component of this equipment is 1 GeV proton accelerator, which has been planned to be built using superconducting technology.
Concluded
Rathindra Nath Biswas is a 1964 batch Chemical Engineering graduate from Jadavpur University, Calcutta. He further specialized in design & engineering of Benzol Rectification Plant in Giprokoks, USSR. He retired as Head, MECON, Durgapur Site Office.
