Joseph M Harrer and James G. Beckerley

1- 1 NUCLEAR AND FOSSIL FUELS

Many instrumentation systems are required to monitor, control, and regulate the nuclear and chemical processes of an operating nuclear power plant Although some of the systems are identical to those used in fossil-fueled power plants, many are entirely different

Perhaps the most important reason for differences between the instrumentation systems of conventional (fossil-fueled) and nuclear power plants is that conventional plants operate with continuous fuel feed and nuclear plants operate with stored feed In a conventional plant fuel is fed continuously to the combustion chamber, in present-day nuclear plants all the nuclear fuel necessary for many months of operation is in the reactor all that time Because of the large fuel inventory, an increasing nuclear reaction rate caused by equipment failure or malfunction will not be stopped by exhaustion of fuel Perhaps in future nuclear plants (e. g, those using fluid fuels), the inventory of fuel in the reacting region can be reduced and the associated hazard thereby diminished. In the meantime nuclear power plant instrumentation must be dependable to prevent damage.

Another reason for instrumentation differences is the concentration of heat energy in the two types of fuel About 40 billion Btu of heat is released in the fissioning of 1 lb of pure nuclear fuel On the other hand, about 14 thousand Btu is released in the burning of 1 lb of coal Since nuclear fuels are such concentrated heat sources, the heat flux from a nuclear fuel can exceed the capabilities of the coolant to remove heat Consequently, the details of nuclear-fuel performance must be measured The develop­ment of instrumentation systems for monitoring fuel performance has been a major effort in nuclear power technology.

The heat energy of a nuclear fuel comes almost entirely from the fission fragments At the time of their formation, the fragments have kinetic energies that correspond to particle tcmpcratuics of about 101 2°С In present-day solid fuels, the fission-fragment energies are immediately shared

CHAPTER CONTENTS

1-1 Nuclear and Fossil Fuels…. ……………………………………. 1

1-2 Definitions…………………………………………………………………………………………………… 2

1-2 1 Nuclear Terms…………………………………………………………………………. . 2

1-2.2 Fission-Process Terms……………………………………………………………………………… 3

1-2.3 Nuclear-Reactor Terms……………………………………………………………………………. 3

1-3 Nuclear-Reactor Kinetics…. ……………………………………. 4

1-3.1 Point Kinetics Without Delayed

Neutrons………………………………………………………………………………………………… 4

1-3 2 Point Kinetics with Delayed

Neutrons…………………………………………………………………………………………………. 5

1-3.3 Reactivity…………………………………………………………………………………………………. 6

1-3.4 The Inhour Equation. . . 8

1-3.5 Effects of Reactivity Insertions ……………………………………………………………… 8

1-3.6 Reactivity Changes………………………………………………………………………………….. 8

(a) Xenon 135……………………………………………………………………………………. 10

(b) Samarium-149 …………………………………………………………………………….. 15

(c) Fuel Burnup………………………………………………………………………………… 17

1-3.7 Three-Dimensional Kinetics. 18

1 4 Nuclear Power Plants………………………………………………………………….. . . 19

1-4 1 Types of Plants……………………………………………………………………………………….. 19

1-4.2 Sensed Vanables. .19

References………………………………….. … . . . 21 with the surrounding approximately 1022 atoms/cm3 of the fuel (usually uranium metal or oxide or carbide), and the average temperature of the nuclear fuel is many orders of magnitude less than the initial fission-fragment tempera­ture. In fact, the rate of coolant flow is regulated to keep the interior temperature of the nuclear fuel in the same general range (a few thousand degrees centigrade) as typical fossil-fuel temperatures Should coolant flow be impeded, however, the nuclear fuel temperature could become much higher. One of the primary purposes of nuclear power reactor instrumentation is to prevent this

Another difference between nuclear and conventional power plants that affects instrumentation is the presence of strong nuclear radiation fields in certain regions of a nuclear reactor The interaction of these fields with sensors and with electrical components can cause a deterioration in signal and system performance Because of this, the materials and techniques used in nuclear power plant instrumentation often differ from those used in fossil- fueled power plants

1- 2 DEFINITIONS

Nuelear power plants are based on eoneepts that require eareful definition In subsequent ehapteis man) definitions of speeiali/ed terms are presented whenever relevant to the diseussion In this section definitions of a number of fundamental terms basie to an understanding of nuelear power plant operation are given The definitions are listed by eoneept rather than alphabetically Most of the defini­tions are taken verbatim from the American National Standard Glossary of Terms in Nuclear Science and Technology 1