Nuclear reactions moderator

FIGURE 17.25 A schematic representation of one type of nuclear reactor in which water acts as a moderator for the nuclear reaction. In this pressurized water reactor (PWR), the coolant is water under pressure. The fission reactions produce heat, which hoi Is water in the steam generator the resulting steam turns the turbines that generate electricity. 

Graphite is used in nuclear power plants. A nuclear power plant converts nuclear energy to electrical power. Graphite acts as a moderator by slowing down the neutrons used in the nuclear reaction. 

The physical properties of isotopes differ slightly because of differences in atomic mass. For example, water that contains deuterium is called heavy water because the neutrons in deuterium add mass to the water molecule. Some nuclear reactors use heavy water to help keep the chain reaction going. The heavy water slows down (or moderates) the neutrons produced during nuclear fission so that they can be absorbed by the uranium fuel. You will learn more about nuclear reactions in Chapter 25. 

Table 21.1 compares some of the common properties of H2O with those of D2O. Deuterium oxide, or heavy water as it is commonly called, is used in some nuclear reactors as a coolant and a moderator of nuclear reactions (see Chapter 23). D2O can be... 

The production of energy by nuclear fission in a nuclear reactor must be a controlled process. Neutrons released from the fission of lose most of their kinetic energy by passage through a moderator (graphite or D2O). They then undergo one of two nuclear reactions. The first is capture by leading to further fission the second... 

Liquid sodium has attractive properties for its application as a working fluid in a fast neutron reactor with the ability to breed plutonium fuel by the reaction of ura-nium with the fast neutrons. Sodium does not act as a neutron moderator, its liquid state at atmospheric pressure reaches from 97.8 °C to 892 °C, its heat transfer properties are excellent and its nuclear reactions do not cause a long lasting activation. Sodium is the medium which is able to transfer the energy generated in the high density reactor core better than any other heat transfer fluid... 

Nuclear reactors are the most common source of neutrons. Inside the reactor, a sustained nuclear reaction of fissile material produces fast neutrons. When is used as the reactor fuel, 2-3 MeV neutrons are produced, along with other neutrons at other energy ranges. A neutron moderator slows the fast neutrons to reduce their energies to the thermal level. This is required to continue the chain reaction by further absorption of neutrons by surrounding atoms of Other neutron energy ranges are the epithermal between 0.1 and 1 eV and resonances between 1 eV and 1 keV. 

The following month, on April 22, Joliot, von Halban and Kowarski published a second paper in Nature concerning secondary neutrons. This one, Number of neutrons liberated in the nuclear fission of uranium, rang bells. Calculating on the basis of the experiment previously reported, the French team found 3.5 secondary neutrons per fissioa The interest of the phenomenon discussed here as a means of producing a chain of nuclear reactions, the three men wrote, was already mentioned in our previous letter. Now they concluded that if a sufficient amount of uranium were immersed in a suitable moderator, the fission chain will perpetuate itself and break up only after reaching the walls limiting the medium. Our experimental results show that this condition will most probably be satisfied. That is, uranium would most probably chain-react. 

In a system of the class described, a reaction chamber, a pair of pressure chambers having independent connections thereto, means in said system for sustaui-ing a thermal nuclear chain reaction comprising reactive composition in said reaction chamber including thermally fissionable material and liquid neutron moderator, and means for withdrawing from said composition the gaseous products of nuclear reactions therein, said Ulst-men-... 

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