Joseph F Mech

1- 1 INTRODUCTION

1- 1.1 Reactor-Power Measurement

Since a nuclear power plant generates power from the heat produced by nuclear fissions, the power level is commonly measured b> observing the “radiations” directly associated with the fission process Fnergetic fission fragments, neutrons, photons, and other particles are produced at the time each fission occurs 1 ’ The number of these radiations, or components of these radiations, is proportional to the number of fissions The rate of appearance of these radiations is proportional to the fission rate and, thus, to the reactor power level

Most fission fragments are radioactive and continue to emit betas and gammas long after the fission events m which they were created * In addition, the fission neutrons and some of the more energetic photons can induce radioactivity This induced radioactivity also persists long after the creating process The radiations from the total residual radioactivity, variously called afterglow, decay heat, or fission-product activity, contribute up to 5% of the reactor heat However, the decay heat is not an indicator of reactor power but is related to the history of operation of the reactor

The most desirable way to make any measurement is to use the most direct method Reactor power therefore should be measured by detecting the prompt fission radiations The fission fragments and beta radiations are short range and are stopped in the reactor fuel However, the neutrons and gamma rays accompanying fission are sufficiently penetrating to be detected at some distance The technology for reactor power measurement is based on the detection of neutrons or gammas or both.

‘Less than 1% of the fission fragments emit neutrons Most neutrons are emitted in the first few seconds after the fission event that created the fragments See Chap 1, Sec 132

CHAPTER CONTENTS

2 1 Introduction…………………………………………………….. …………. . .22

2-1 1 Reactor Power Measurement………………………………………………….. ……. 22

2 1.2 Interactions with Matter…. … 22

2 13 Accepted Detection Principles……………………………………………………….. 23

2 1.4 “Out-of (ore” Defined…………………………………………………………… ……… 23

2 15 Use of Out-of-Core Sensors in Reactors…. 24

2 2 Principles of Gas Ionization Sensors…. . -25

2-2 1 Ionization Chambers…. 25

2-2 2 Compensated Ionization Chambers………………………………………………. 28

2 2 3 Counters…………………………………….. … . … 3q

2-3 Mechanical Features of Gas Ionization Sensors… 32

2-3 1 Structural Design………………………………………………………………………………. 32

2 3.2 Materials of Construction…. . . -32

2-4 Other Radiation Sensors. … . … -33

2-4 1 Proportional Counters……………………………………………………………………… 33

2-4 2 Self-Powered Detectors…………………………………………………………………… 34

2-4.3 Activation Detectors………………………………………………………………………. 34

2-4.4 Solid-State and Scintillation Detectors…. 34

2-5 Installation……………………………………………………………………………………………………. 34

2-5.1 Cables……………………………………………………………………………………………….. 35

2-5 2 Hardware………………………………………………………………. ……….. 35

2 5.3 Circuits…. 35

2-5 4 Immersion in Coolant…………………………………………… ………. 35

2-6 Environment………………………………………………………………………………………………… 3 s

2 7 Life and Reliability……………………………………………………. …. -36

2-8 Typical Specifications of Commercial

Gas Ionization Sensors……………………………………………………… . * 36

References……………………………………………………………………………………………………………. 41