Question

rods containing uranium
neutron-absorbing material
nuclear reactor
a power plant needs to slow down the fission reactions in its nuclear reactor. in which direction should the uranium-containing rods in the diagram be moved?
○ left
○ right
○ up
○ down

Explanation:

Step1: Recall nuclear reactor control

In a nuclear reactor, uranium rods (fuel) undergo fission, releasing neutrons. Neutron - absorbing materials (like control rods) regulate the reaction. To slow down fission, we need to increase neutron absorption or decrease the interaction between uranium and neutrons.

Step2: Analyze rod movement effect

The uranium - containing rods are among the neutron - absorbing materials (the red/yellow parts are neutron - absorbing? Wait, no: the green rods are uranium - containing (fuel), and the red/yellow are neutron - absorbing. Wait, correction: The green rods are uranium (fuel), and the red - yellow are neutron - absorbing material. To slow down fission (reduce the rate of neutron - induced fission in uranium), we need to move the uranium rods so that more of them are in contact with the neutron - absorbing material, or move them to a position where there is more neutron absorption. Wait, actually, in a nuclear reactor, the control rods (neutron - absorbing) are moved, but here the rods containing uranium are the fuel rods. Wait, no, maybe I got it reversed. Wait, the diagram: "Rods containing uranium" (green) and "Neutron - absorbing material" (red - yellow). To slow down the fission reactions, we need to reduce the number of neutrons available to cause fission in uranium. So we need to move the uranium rods (fuel) so that they are more exposed to the neutron - absorbing material? Wait, no. Wait, the fission occurs in the uranium. The neutron - absorbing material absorbs neutrons, so if we move the uranium rods (fuel) up, then more of the uranium is out of the reactor core (or more in contact with the neutron - absorbing material? Wait, no, the neutron - absorbing material is in the core. Wait, the core is the blue area. The uranium rods are green, going into the core. The neutron - absorbing material is the red - yellow vertical bars. So the uranium rods are between the neutron - absorbing bars. To slow down fission, we need to have less uranium available for fission, or more neutron absorption. Wait, actually, when we move the uranium rods (fuel) up, we are removing them from the core (the blue area with the neutron - absorbing material). Wait, no, that would reduce the amount of uranium in the core, so the number of fission reactions would decrease (since there's less fuel). Wait, but the other way: if we move the uranium rods down, they are more in the core. Wait, no, maybe I mixed up fuel and control rods. Wait, control rods are the neutron - absorbing ones. But in this question, the rods containing uranium are the fuel rods. So to slow down fission (reduce the reaction rate), we need to move the fuel rods (uranium - containing) so that there is less fuel in the core, or more interaction with neutron - absorbing material. Wait, the neutron - absorbing material is in the core (the red - yellow bars). The uranium rods (green) are inserted into the core, between the neutron - absorbing bars. So if we move the uranium rods up, we are pulling them out of the core (out of the blue area with the neutron - absorbing material). Wait, no, the core is the blue region. The uranium rods are going into the core. So the length of the uranium rod inside the core: if we move it up, less of the uranium rod is in the core (so less fuel available for fission, so the reaction slows down). If we move it down, more of the uranium rod is in the core (more fuel, more fission). Wait, but the neutron - absorbing material is in the core. So when the uranium rod is in the core, it's surrounded by the neutron - absorbing material? No, the neutron - absorbing material is the red - yellow bars, and the uranium rods are the green ones, between them. So the neutrons are produced in the uranium (fission), and some are absorbed by the neutron - absorbing material. To slow down the fission, we need to have more neutrons absorbed, or less uranium undergoing fission. So if we move the uranium rods up, we are taking them out of the core (the area with the neutron - absorbing material), so there is less uranium in the core, so less fission. Wait, but the other option: moving up. Let's think again. In a nuclear reactor, to slow down the fission reaction (decrease the power), we can either insert control rods (neutron - absorbing) deeper into the core (to absorb more neutrons) or remove fuel rods (uranium) from the core. In this diagram, the uranium - containing rods are the fuel rods. So to slow down the reaction, we need to move the fuel rods (uranium - containing) up, so that less of them is in the core (the region with the neutron - absorbing material), thus reducing the number of fission events. If we move them down, more of the fuel rod is in the core, increasing fission. Moving left or right doesn't make sense as the diagram is vertical. So the correct direction is up.

Answer:

up (the option "up")