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Chapter 15: Problem 22

Explain in some detail what happens if control rods are too effective at absorbing neutrons so that each fission event produces too few unabsorbed neutrons.

Short Answer

Expert verified
Answer: The consequences of control rods being too effective at absorbing neutrons in a nuclear reactor include a reduced rate of the nuclear fission chain reaction, decreased heat and steam production, lowering the output of the turbine-generator system, and reduced electricity generation. While this negatively impacts energy production, it also prevents the risk of uncontrolled reactions that could lead to accidents or meltdowns.

Step by step solution

01

Understand the role of control rods in a nuclear reactor

Control rods are an essential component of nuclear reactors that control the rate of the nuclear fission reaction. They do this by absorbing neutrons released during fission events, which prevents the chain reaction from accelerating uncontrollably. Control rods are made of materials with a high neutron absorption cross-section, commonly boron, hafnium, or cadmium.
02

Define neutron absorption

Neutron absorption is the process by which a neutron is captured by an atomic nucleus. Absorbing neutrons during fission events serves to control the number of neutrons available for other fissions, thus controlling the rate of the chain reaction. This process is crucial to maintaining a stable nuclear reactor.
03

Describe the consequences of too few unabsorbed neutrons

If control rods are too effective at absorbing neutrons, this means that there will be too few unabsorbed neutrons for each fission event. Since neutrons are needed to cause further fission events, a decreased number of unabsorbed neutrons will lead to a reduced rate of the nuclear fission chain reaction. If the reaction rate drops too much, this could result in the reactor going subcritical – at this point, the reactor will not be able to achieve a self-sustaining chain reaction and will effectively shut down.
04

Effects on energy production and safety

Due to the reduced rate of fission events, the nuclear reactor will produce less heat. This, in turn, will lead to decreased steam production, reducing the output of the turbine-generator system and lowering electricity generation. Although this may negatively impact electricity production, having control rods that are too effective also prevents the risk of an uncontrolled reaction, which could lead to accidents or meltdowns.

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Most popular questions from this chapter

In a year, an average American uses about \(3 \times 10^{11} \mathrm{~J}\) of energy. How much mass does this translate to via \(E=m c^{2} ?\) Rock has a density approximately 3 times that of water, translating to about \(3 \mathrm{mg}\) per cubic millimeter. So roughly how big would a chunk of rock material be to provide a year's worth of energy if converted to pure energy? Is it more like dust, a grain of sand, a pebble, a rock, a boulder, a hill, a mountain?

A particular fission of \({ }^{235} \mathrm{U}+\mathrm{n}(\) total \(A=236)\) breaks up. One fragment has \(Z=54\) and \(N=86\), making it \({ }^{140} \mathrm{Xe} .\) If no extra neutrons are produced in this event, what must the other fragment be, so all numbers add up? Refer to a periodic table (e.g., Fig. B.1; p. 375 ) to learn which element has the corresponding \(Z\) value, and express the result in the notation \({ }^{\mathrm{A}} \mathrm{X}\).

Since each nuclear plant delivers \(\sim 1 \mathrm{GW}\) of electrical power, at \(\sim 40 \%\) thermodynamic efficiency this means a thermal generation rate of \(2.5\) GW. How many nuclear plants would we need to supply all 18 TW of our current energy demand? Since a typical lifetime is 50 years before decommissioning, how many days, on average would it be between new plants coming online (while old ones are retired) in a steady state?

The world uses energy at a rate of \(18 \mathrm{TW}\), amounting to almost \(6 \times 10^{20}\) J per year. What is the mass-equivalent \(^{83}\) of this amount of annual energy? What context can you provide for this amount of mass?

Operating approximately 450 nuclear plants over about 60 years at a total thermal level of \(1 \mathrm{TW}\), we have had two major radioactive releases into the environment. If we went completely down the nuclear road and get all \(18 \mathrm{TW}^{89}\) this way, what rate of accidents might we expect, if the rate just scales with usage levels?
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