As a result reactor designers have paid great attention to the inherent safety of reactors which can be achieved by negative temperature and power
coefficients and fail-safe control systems. It can be said with some confidence that present-day thermal reactors are safe in the sense that under no
conceivable circumstance can they explode like a bomb, and control systems have been designed which can, in the event of any malfunction on the part of the reactor or its associated plant, automatically and rapidly shut down the reactor, i.e. make it subcritical by a substantial amount, in a very few seconds.
1. According to the paragraph, inherent safety of reactors can be achieved by . （C） A. the operators;
B. positive temperature and positive power coefficients; C. negative temperature and negative power coefficients; D. passive safety system and positive power coefficients.
2. The best title of the passage may probably be . （D） A. fail-safe control system；
B. thermal reactor safety and operation; C. automatic protective system; D. inherent safety design of reactors.
2.2. Passage II
The biological shield should contain some hydrogen compound to slow down fast neutrons, and be dense enough to attenuate gamma radiation effectively. Concrete satisfies both these requirements fairly well and is suitable for landbase reactors. Barytes (重晶石) concrete, containing the heavy element barium, and steel-shot concrete have been used for biological shields. They are more dense than ordinary concrete, with improved shielding properties, however their higher cost offsets this advantage. The biological shield for a marine reactor, which is usually a fairly compact pressurized water reactor, must satisfy a minimum space and weight requirement. This leads to a shield design which consists typically of alternate layers of water (for fast neutron slowing) and steel (for gamma ray attenuation).
3. According to the above passage, is not possible for constructing biological shield? （C）
A. steel; B. concrete; C. graphite; D. paraffin wax.
4. Which of the following sentences is not true? （D）
A. The biological shield is designed mainly to slow down fast neutrons and attenuate gamma radiation.
B. The marine reactor uses alternative steel and water layers as its biological shield.
C. The combination of heavy element concrete and steel bars could improve the shielding properties.
D. The biological shield should use hydrogen element to slow down fast neutron and attenuate gamma radiations.
2.3. Passage III
In order to mitigate the effects of large release of steam (an potentially of
radioactivity) in the containment, two full capacity independent safety systems are provided; the reactor building spray system and the reactor building emergency coolers. The systems are designed to provide cool water to
condense discharge steam and to prevent containment pressure from reaching its design limit. Individual systems differ considerably but a typical system may be described as follows; The initial capacity of the systems in removing heat from the containment atmosphere is typically 253GJ/hr.
When a containment pressure of 4psig is reached, the emergency coolers of the reactor building are actuated. In their post accident mode, the system consists of three units each with a fan and an emergency cooler. As the reactor building air is circulated across a tubular heat exchanger, a portion of steam is
condensed. These coolers alone would be capable of returning the containment pressure to near atmospheric within 24 hr after an accident. When the pressure reaches a level of 10 psig, the second safety system, the reactor building spray system, is automatically actuated. It consists of a pump, piping, headers, and spray nozzles arranged uniformly under the containment dome. It can spray borated water into the reactor building at a rate of 11.35m3/min. A sodium hydroxide additive is also provided in the spray water to increase the retention of iodine, and hence, to reduce its concentration in the containment atmosphere in the event of a sizable breach of fuel cladding.
5. Two full capacity independent safety systems are provided for the design purpose of . （A）
A. condensing the steam released into the containment when pressure exceeds design limit.
B. maintaining the high pressure in the containment C. discharging large amount of steam
D. mitigating the effect of radiation hazard to the containment.
6. The reactor building spray system will be actuated automatically . （C） A. after 24 hours after the accident; B. immediately after the accident;
C. when the containment pressure reaches a level of design limit;
D. when the air in the containment is circulated through the heat exchanger.
7. Which of the following statements is INCORRECT? （B） A. The emergency cooler consists of fans and heat exchangers.
B. The emergency cooler system can spray borated water into containment. C. All the two system are applied for returning the containment pressure to be blow atmospheric after the accident.
D. Sodium hydroxide additive is provided in the spray water to reduce the amount of radioactive fission produces.
2.4. Passage IV
Many reactor-years of operating experience have shown that it is not the fission chain reaction in the reactor core that is the most likely source of malfunction and accidents, but the “conventional” components of the power plant such as pumps, valves, switches, relays and parts under stress such as pressure vessel or pipework. Human error on the part of operating and
maintenance staff has also proved to be a rather frequent source of trouble in nuclear power plant.
These factors are not peculiar to nuclear power plant, but they assume great importance because of the hazardous nature of nuclear reactors. Designers have to ensure that all systems should as far as possible be fail-safe and
redundant, i.e. if one system fails to function correctly, another is available to fulfill the same function.
As stated above, nuclear reactors cannot explode like nuclear bombs. This
primarily because of the fast acting negative thermal feedback due to Doppler broadening of the 238U absorption resonances. In addition in thermal reactors where neutrons are moderated, the prompt neutron lifetime Lp is the order of 10-4 second; in a bomb, since the neutrons are unmoderated, the prompt neuron lifetime is of the order of 10-8 seconds. Finally, reactor fuel consists typically of 2 to 3 percent 235U, where as nuclear weapons contain almost pure 239Pu. The net effect of these difference is that, even in a reactor which is totally out of control and gone prompt critical, the reactor period will not be much less than a second or so. In a nuclear bomb, the period is of the order of nanoseconds.
(1) According to the text, the most likely source of troubles in a nuclear comes from 1. Core of the reactor 2. Conventional components 3. Parts under stress 4. Human error
(2) The first sentence of second paragraph “These factors are not peculiar to …”, what does “These factors” mean? 1. Human error
2. Malfunction of conventional components 3. Fission chain reaction in the core 4. All of the above
(3) What make the pressurized water reactors can not explode like a nuclear bomb? 1. Doppler broadening of the 238U absorption resonances 2. The neutrons are moderated
3. PWR fuel contains much less fissile isotopes than nuclear bomb 4. All of the above
(4) The last two sentences “The net effect of … is of the order of nanoseconds.” Give a comparison about the reactor period, what does the author most likely to express? 1. The reactor period is too short comparing with a nuclear bomb 2. The 239Pu in a nuclear bomb is more easily to go prompt criticality
3. The nuclear reactor can not generate large amount of heat as a nuclear bomb does within
very short time, therefore nuclear reactor can not explode.
4. The author wants to give us a reference about the data of the reactor period time.
2.5. Passage V
The primary functions of the control rod drive mechanisms (CRDM) are to insert or withdraw rod cluster control assemblies and gray rod control assembles into or from the core to control average core temperature at a designed speed. A schematic diagram of CRDM is given in Fig 1. Control rod withdrawal one step involves six actions.
1) Moveable Gripper Coil B-on. The
Fig 1. Control rod drive mechanism
latch-locking plunger rises and
swings the movable gripper latches
into the drive rod assembly groove. A small axial clearance exists between the latch teeth and the drive rod.
2) Stationary Gripper Coil A-off. The force of gravity, acting upon the drive rod assembly and attached control rod, causes the stationary gripper and plunger to move downward 1/16 inch, transferring the load of the drive rod assembly and attached control rod to the movable gripper latches. The plunger continues to move downward and swings the stationary gripper latches out of the drive rod assembly groove.
3) Lift Coil C-on. The 5/8-inch gap between the movable gripper pole and the lift pole closes, and the drive rod assembly rises one step length.
4) Stationary Gripper Coil A-on. The plunger rises and rises the gap below the stationary gripper pole. The three links, pinned to the plunger, swing the
stationary gripper latches into a drive rod assembly groove. The latches contact the drive rod assembly and lift it a small fraction of an inch. The small vertical drive rod assembly movement transfers the drive rod assembly load from the movable gripper latches to the stationary gripper latches.
5) Movable Gripper Coil –off. The latch-locking plunger separates from the movable gripper pole under the force of a spring and gravity. Three links,
pinned to the plunger, swing the three movable gripper latches out of the drive rod assembly groove.
6) Lift Coil C –off. The gap between the movable gripper pole and the life pole opens. The movable gripper latches drop 5/8 inch to a position adjacent to a drive rod assemble groove.
Repetition of the above six actions will make another step of withdrawal movement of control rod.
(1) According to the text, the most likely source of troubles in a nuclear comes from 1. Core of the reactor 2. Conventional components