Neutron Sensors — In-Core

Howard H Stevens

3- 1 INTRODUCTION

In core neutron sensors accomplish one or more of the following (1) confirm calculated core performance (2) confirm core operating safety margins, (3) provide input data for fuel management and (4) detect the existence of xenon induced power asymmetries or oscillations (see Chap 1, Sec 1 — 3 6(a) for a discussion of xenon)

The first of a kind power reactor core is usually more thoroughly instrumented than cores of duplicate or similar reactors because of the need to confirm calculated nuclear and thermal performance This practice has become preva lent as core designers have become more dependent on computer codes for nuclear and thermal data and less dependent on critical experiments data The first of-a-kind plant therefore serves as a field laboratory to confirm design calculations It is not intended to generate new experimental data

When reactor complexity, size, power output, power density, or neutron flux level increases beyond certain limits, safe operation of the reactor over its lifetime cannot depend on data from out of core instruments The reactor operator must have available to him the outputs from in-core neutron sensors so he can determine if the core is operating within prescribed safety limits

In large power reactors in-core neutron sensors provide the data needed for carrying out an effective fuel manage ment program and monitor for the existence of xenon induced power asymmetries or instabilities It is normally possible to detect the presence of such asymmetries or instabilities with out of-core instruments, but in-core sen sors must be used to ascertain their exact nature and to provide the data from which an operator can carry out effective control actions (l e, equalizing asymmetries or damping out oscillations) In-core neutron sensors also provide the data for correlating core performance with the response of the out of-core sensors

To be effective, the in-core sensors should provide data continuously or, at a minimum, at periodic intervals while the reactor is operating in its normal mode Reactor

CHAPTER CONTENTS

З 1 Introduction 42

3 2 In Core Environment 42

3 3 In Core Neutron Flux Sensing 43

3 3 1 In Core Fission Chambers 44

(a) Uranium Form 44

(b) Uranium Enrichment 44

(c) Uranium Surface Area 45

(d) Type of Till Gas 45

(e) Fill Gas Pressure 45

(f) Emitter Collector Gap 45

(g) Dimensional Tolerances 45

(h) Operating Characteristics 45

(l) Operating Ranges 47

(j) Traveling In Core Fission Chambers 49

3 3 2 Boron Lined Chambers 49

3 3 3 Self Powered Neutron Detectors 50

(a) Operating Principles 50

(b) Construction and Materials 51

(c) Sensitivity 52

(d) Emitter Burnup 53

(e) Response Characteristics 54

(f) Connecting Cables 56

shutdown should not be required to collect the data, such as would be the case if activation foils were used for flux distribution measurements or if gamma scanning of fuel elements were used for determining irradiation history

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