The nuclear part
Control of Reactor Reactivity
In the reactor core, during its fuel cycle, a chain of fission reactions has to be maintained. Because of the fissions, the amount of fuel decreases gradually and so does the reactivity (ability to sustain the chain reaction). Refueling has to take place. Most studies on the pebble bed HTR assume on-line refueling. Consequently there is no excess reactivity in the core of an HTR and an accident like the one in Chernobyl cannot occur. But on-line refueling means a constant adding of fresh fuel and removal of spent fuel.
Figure 1: Reactivity control by burnable poisons
(EFDP = effective full powers days)
Especially the fuel removal implies a complicated installation, which may be prone to breakdowns. However, the NEREUS installation is based upon the principle to Keep It as Simple as possible (KISS), our intention is to use burnable poison. Some materials have such a high neutron absorbing property, that when placed in the reactor core their concentration diminishes because of transmutation as a function of time. Such materials are called burnable neutron poisons. They facilitate higher fuel contents in the core, while keeping the reactivity constant during burn up.
The burnable poison also simplifies the “control rod” requirements, only stop/start rods are needed. On indications from the small-scale energy production branch, intensive studies were performed recently to improve the working of the burnable poison even further. A recent development is to introduce the burnable poison in the pebbles in the form of ‘burnable particles’. In this way, as you see in Figure 6, it is possible to deliver a nearly flat line of reactivity over the whole usage period.
The figures at the curves denote the diameter (in mm) of the very small cylinders (‘needles’) of burnable poison, embedded in the fuel pebbles. With a diameter of 0.8 mm the reactivity (in cold state, without xenon poisoning) of the reactor as a function of time (expressed in effective full power days, EFDP) is almost constant which eliminates long-term reactivity control by control rods (Van Dam, H., 2000).
At the moment we are working on a three-year refueling period (this operation is foreseen during the docking period of a ship). So the combination of the fuel enrichment and the burnable poison will be designed to produce 20 MW power in the reactor, resulting in about 8 MWe at the generator, over a period of three years with a usage pattern of 90% load and 80% usage, which is about 50,000 MWh electric per year.
Control of Energy Production
The reactivity is strongly dependent upon the temperature of the fuel. HTR’s fuel possesses a negative temperature reactivity coefficient, which can even be improved by the use of burnable particles as mentioned in the last section. This implies that when the temperature of the reactor temporarily decreases to some extent, its reactivity increases, its power generation increases and the original temperature level is restored. So the reactor functions like a thermostatic device.
Figure 2: Self-regulating power control at a step change of 100 – 40 – 100%.
It was extensively tested at the AVR in Jülich (see figure 5 of this chapter). This phenomenon is being used for power control in the NEREUS reactor concept.
This “self-regulating power control” makes this fuel safe and very suitable for unmanned power plants, such as on board ships. This applies especially to two markets the project is aiming for: the stand-alone electricity production market and the market for prime movers on board ships. Both markets prefer to work with “unmanned power plants” and “unmanned engine rooms”. The power control output of the installation is delivered by the generator and is achieved by controlling the mass flow in the closed cycle system. This is not the optimal solution, but the one with the lesser number of parts. It is simple and well understood. After all if the fuel is cheap, the necessity of a maximum of efficiency at partial load is less important than when fossil fuels are used.
Removal of the Decay Heat
After shutdown of a nuclear reactor, the radioactivity of the fission products gives rise to production of some decay heat, which gradually decreases. The completely passive removal of this decay heat is an essential part of the inherently safe nuclear installation. For this purpose there is a space between the outside of the reactor drum and the inside of the biological shielding, through which air flows, driven by natural draft. This cooling will be there all the time and is established, without any ventilators etc. in a natural way. For this purpose a normal ship’s funnel construction (100 kW) can be used during normal operation.
The funnel can also be used as transport route for refueling, maintenance and repair by replacement by the pool-management system. The cooling air must be supplied through air filter units at the open decks. This passive heat removal system is always in operation and removes about 0.5% of the heat.