6. The term nuclide is commonly used describe an atomic species whose nuclei have a specif ed composition, that is to say, a nuclide in nature is a species having given atomic and mass num bers <o 03 7. Such nuclides, having the same atomic number but different mass num ber, are called isotop e,e.g,three forms of uranium isotopes in nature with the atomic number 92 but mass num be r 234, 235 and 238, respectively. soe/0 8. The unstable substances undergo spontaneous change, i.e., radioactive decay, at definite rates 9. The radioactive decay is associated with the emission from the atomic nucleus of an electri cally charged particle, either a alpha particles, ie, helium nucleus, or a beta particles, ie,an electron. <o 10. In many instances of gamma rays, which are penetrating electromagnetic radiation of high energy, accompany the particle emission. <OV></OV 11. The most widely used method for representing the rate of radioactive decay is by means o f the half -life, which is defined as the time required for the number of radioactive nuclei to decay to half its initial value <</0S 12. Since the num ber of nuclei (or their activ ity) decays to half its initial value in a half-life period, the number (or activity) will fall to one-fourth by the end of two half-life periods, and to less than I percent of its initial value after seven half-life period UNIT 6 REACTOR CONTROL<O 1. In the normal operation of a reactor, the functions of the control system may be divided in to three phases, ie. startup, power operation and shutdown. oOx<0S 2. If the potentially unsafe conditions should arise, a protection system would automatically sh ut down the reactor <OS</osh 3. An essential requirement of the control system is that it must be capable of introducing eno ugh negative reactivity to compensate for the build-in(excess) reactivity at initial startup of th e reactor <OV></O 4. Four general methods are possible for changing the neutron flux in a reactor, they involve t emporary addition or removal of (1) fuel,(2)moderator, (3)reflector,(4)a neutron absorber. 0v><O 5. The control material commonly used in pressurized water reactor is alloy of 80(weight)perc ent silver. 15 percent indium and 5percent cadmium soexxos6. The term nuclide is commonly used describe an atomic species whose nuclei have a specifi ed composition, that is to say, a nuclide in nature is a species having given atomic and mass numbers.<O ></O > 7. Such nuclides, having the same atomic number but different mass number, are called isotop e, e.g., three forms of uranium isotopes in nature with the atomic number 92 but mass numbe r 234, 235 and 238, respectively.<O ></O > 8. The unstable substances undergo spontaneous change, i.e., radioactive decay, at definite rates. <O ></O > 9. The radioactive decay is associated with the emission from the atomic nucleus of an electri cally charged particle, either a alpha particles, i.e., helium nucleus, or a beta particles, i.e., an electron.<O ></O > 10. In many instances of gamma rays, which are penetrating electromagnetic radiation of high energy, accompany the particle emission.<O ></O > 11. The most widely used method for representing the rate of radioactive decay is by means o f the half –life, which is defined as the time required for the number of radioactive nuclei to decay to half its initial value.<O ></O > 12. Since the number of nuclei (or their activity) decays to half its initial value in a half-life period, the number (or activity) will fall to one-fourth by the end of two half-life periods, and to less than 1 percent of its initial value after seven half-life periods. UNIT 6 REACTOR CONTROL<O 1. In the normal operation of a reactor, the functions of the control system may be divided in to three phases, i.e. startup, power operation and shutdown.<O ></O > 2. If the potentially unsafe conditions should arise, a protection system would automatically sh ut down the reactor.<O ></O > 3. An essential requirement of the control system is that it must be capable of introducing eno ugh negative reactivity to compensate for the build-in (excess) reactivity at initial startup of th e reactor.<O ></O > 4. Four general methods are possible for changing the neutron flux in a reactor, they involve t emporary addition or removal of (1) fuel, (2) moderator, (3) reflector, (4) a neutron absorber.< O ></O > 5. The control material commonly used in pressurized water reactor is alloy of 80(weight) perc ent silver, 15 percent indium and 5percent cadmium.<O ></O >