A boiling water reactor is a light-water reactor (LWR) using H2O as both moderator and coolant. The fuel rods are simply placed in the water, which is allowed to boil under pressure, producing steam directly to the turbines. The water, however, is exposed to radioactive material. A pressurized water reactor (PWR) or the European version called EWR contains the water under 153 atm of pressure so that it cannot boil at its temperature of 322°C. This water goes to a heat exchanger to transfer the energy to outside water which does not touch any radioactive material. All the reactors in France are PWRs. Standardizing to a single type reduces the risk of accidents.

A CANDU (Canadian Deuterium Uranium) reactor was invented because Canada had no enrichment facilities. It burns natural uranium containing only 0.7% U235. With so few fissionable nuclei, the moderator has to be heavy water, D2O. Hydrogen would absorb too many neutrons. The fuel rods are double tubes, the inner tube contains the fuel pellets and cooling water. Gas in the outer tube insulates the heat from the moderator, which is at room temperature. No thick domed vessel is necessary to contain the reactor. With so little U235, the power output is only 20% of other LWRs, so the fuel has to be replenished often. It is done continuously, going from one end of the rods to the other. There is no proliferation risk due to enriched fuel, but plutonium is produced and comes out with the expended fuel. It can be stolen more easily since it comes out continuously instead of at a fixed time under heavy guard [41].

AGRs are early (advanced gas-cooled reactors) developed in England using a graphite moderator and 600°C carbon dioxide as a coolant.78 Natural uranium could be used at lower temperatures where a low-absorbency “Magnox” fuel cladding could be used, but enrichment is needed for the advanced types. Yet another acro­nym is the European pressurized reactor (EPR), a safer type being constructed in Finland and France. These two projects have been delayed by cost overruns and safety protests.

Liquid-metal fast breeder (LMFBR) reactors are an entirely different breed. Fast refers to the fast, or prompt, 2-MeV neutrons emitted in fission. In LWRs, these neutrons have to be slowed down by the moderator before they can cause U235 to fission. In breeders, the fuel is U238 with 10% Pu239, and U235 is not used. Twelve percent of the fast neutrons cause fission in the U238, and the rest are captured. But as Fig. 3.59 shows, the capture of a neutron by U238 produces an atom of Pu239, which is a good fuel. Those neutrons that do not get captured immediately eventually slow down and cause U238 and Pu239 to fission. By covering the chamber with a uranium blanket, which can be made of depleted uranium from an LWR, more plutonium can be produced than is used. Breeder reactors can breed fuel from natural uranium.

No moderator is needed; in fact, it is essential not to have any material that will slow down the 2-MeV neutrons. However, there has to be a coolant, and the coolant must not slow down or capture the neutrons either. There are only two elements in the peri­odic table that can be used: sodium (Na) and lead (Pb). These can be used in liquid form and do not capture many fast neutrons. Sodium, which melts at 98°C, is chosen for convenience in spite of its nasty nature. Although it is harmless when combined with another nasty element (chlorine) in table salt, pure sodium will explode if it touches water. It is the liquid metal in LMFBR. These reactors cannot go critical with normally enriched uranium. A chain reaction requires 10-12% enrichment.

This technology has been well tested in the Superphenix reactor on the Rhone river in France. The 3,000 tons of sodium coolant was in its own closed loop, and heat was exchanged to a secondary sodium loop not exposed to radioactivity. Steam was created in a second heat exchanger. The reactor ran between 1995 and 1997, producing 1.2 GW of electricity between repairs. The sodium ran at 545°C and never boiled, so there was no high pressure. The fuel elements had thicker walls than in LWRs and produced twice the energy per ton. Sodium leaks have been the main problem. A smaller LMFBR, the Monju in Japan, developed a leak in the intermediate coolant loop in 1996. No radioactivity was released, but the sodium fumes made people sick. The reactor was restarted in 2010.81 LMFBRs are ready for the next generation of reactors. Gas cooling in the intermediate heat loop is the only improvement needed.

Reactor Control

A chain reaction requires active control. The reproduction ratio of neutrons has to be exactly one. Too few neutrons, the reaction dies. Too many, the reaction runs away. The reaction rate depends on the temperature of the moderator (how much it absorbs) and the freshness of the fuel. Fission occurs so fast that it would be impos­sible to stop a chain reaction except for a lucky circumstance. A small fraction of the neutrons are delayed. In uranium, 0.65% and in plutonium, 0.21% of the neu­trons from a fission event are emitted only after 10 seconds. Since every neutron is needed, the chain reaction does not proceed instantaneously; there is a time lag. The moderator and coolant in the reactor have high heat capacity, so the temperature inside the reactor changes even more slowly. There can be as much as 20 minutes to react to a temperature change. Control rods made of boron carbide (BC), a powerful absorber, are moved in and out of the moderator to control the neutron population. This is normally done automatically, and reactors have run for years without trouble. The few accidents that have occurred are due to human error in response to an abnor­mal condition. The danger is not only when the chain reaction is going too fast and the temperature rises. If the temperature goes too low, voracious neutron absorbers like Xe135 can accumulate and poison the reactor. It cannot be restarted until all the Xe135 has built up and then decayed with a half-life of 8 hours [41].