Research reactor facilities, recent developments at Imperial College

CH0100350

RESEARCH REACTOR FACILITIES & RECENT DEVELOPMENTS AT

IMPERIAL COLLEGE, LONDON.

S J. FRANKLIN, PROF. AJ.H. GODDARD and J. O'CONNELL

EAS,

ICCET,

T.H.Huxley School of the Environment, Earth Science and Engineering,

Imperial College of Science Technology & Medicine, Silwood Park, Ascot SL5 7TE-UK

ABSTRACT

The 100 kW CONSORT pool-type Reactor is now the only Research Reactor in the UK.

Because of its strategic importance, Imperial College is continuing and accelerating a

programme of refurbishment of the control system, and planning for a further fuel charge.

These plans are described and the progress to date discussed. To this end, a description of the

enhanced Safety Case being written is provided here, and refuelling plans discussed. The

current range of facilities available is described, and future plans highlighted.

Introduction

The Imperial College (IC) Research Reactor is based at the Silwood Park Field Station, about 25 miles

west of London. Designed jointly by GEC and the IC Mechanical Engineering Department, it first went

critical on 9th April 1965. It is part of the Centre for Environmental Technology (ICCET), which is an

interdisciplinary postgraduate research and teaching department split between the South Kensington

campus and Silwood Park. The Environmental Analysis Section (EAS), of which the Reactor Centre is a

part, provides a service to both academia and industry for reactor physics training, isotope production and

irradiations, using core tubes, beam tubes, horizontal & vertical thermal columns and irradiation facilities

incorporating fast pneumatic transfer. A specialist Analytical Services Group using the Reactor provides

Neutron Activation Analysis. Other groups include Environmental Pathways and Processes Research

Group, who utilise the wind tunnel and aerosol research facilities, as well as the IC-PMS facility. The

Reactor has strategic importance for the UK, as it is now the only remaining Research Reactor in the

country. It is therefore important to put in place refurbishment programmes and maintain and upgrade the

safety case. This paper will focus on the Reactor Centre.

Usage of the Imperial College Reactor

With the closure of other UK Research Reactors the number and variety of users of the Imperial College

Reactor is increasing. Some of these users can have their requirements met quite easily and can use

existing Reactor and associated facilities, the only requirement being that new safety cases have to be

developed for their particular work.

Some new users do, however, have extremely specialised needs and new Reactor facilities which have to

be safety justified before commissioning need to be provided. In this respect, the original design of the

CONSORT Reactor gave a lot of flexibility in that various beam tubes and thermal columns were

provided. Several of these facilities have been modified or brought into use recently to enable specialised

facilities to be developed.

The Reactor QA system developed for the Site Licence enables new experiments and/or modifications to

be developed and authorised either internally or by reference to the independent Nuclear Safety

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Committee and the Licensing Authority, depending on the nature and level of potential risks involved.

Such modifications and/or experiments need to be provided with appropriate justification for safety,

following well-established safety principles and guidelines. Expertise exists both within the Centre and

from external consultants in order to provides guidance and advice.

It can be seen that new types of work using new or existing facilities on the Reactor can be

accommodated and/or developed to a customer's specific requirements usually within a reasonably short

space of time, depending on the level of potential risks involved. Such work that has been developed and

carried out recently includes:

• Production of high activity tracers for use in industrial process monitoring.

• Production of low activity radiolabelled tracers used in the medical industry.

• Provision of testing facilities for neutron detectors over a wide range of Reactor powers from 1.5

watts to several kilowatts.

• Development of a foil-based flux-monitoring system giving rapid results to users of certain facilities.

• Provision of non-destructive testing facilities using neutron beams together with associated counting

systems.

• New Reactor experiments developed and installed in order to provide training for students from

external sites.

Fig.l shows a general breakdown of

the CONSORT Reactor operations by

the type of work last year. Currently,

isotope production leads the list of

tasks at the Reactor Centre.

However, this breakdown will change

this year, as even more use is made of

the Reactor Centre as a whole as a

training facility. New facilities under

safety case development will also be

installed by IC customers.

Analytical

Services

Support

Univeisity

Support

Detector

Calibration

Reactor

Physics

Training

Isotope

Production

Fig. 1 Breakdown of Customer Utilisation

These new facilities are all in addition to the well-established facilities already associated with the

Reactor. Such facilities provide for neutron activation analysis, radioisotope production, as well as

existing experimental facilities for student training.

Background to Licensing Issues

Under the Conditions of the 1994 Nuclear Site Licence granted by the UK regulator, the Nuclear

Installations Inspectorate of the Health & Safety Executive (HSE) requires the Reactor Centre to develop

and enhance QA documentation. This paper focuses on the Site Licence Conditions (SLCs) that are

devoted to the regular review of safety case documentation and the regular review of maintenance

requirements throughout the lifetime of the plant. Although it is recognised that the low power and low

overall source term of this Research Reactor make it a special case, CONSORT nevertheless operates

under the same 35 Site Licence Conditions as the other UK nuclear installations. In common with all UK

installations, further work is planned to develop the existing decommissioning safety case; to take

advantage of the experience learnt at the UK's Risley Reactor and JASON. As with other nuclear

installations, CONSORT has a Nuclear Safety Committee (NSC) of internal and external experts, who

review the work in place for the Reactor.

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Operations

monitor

the

Reactor systems

for

signs

ageing

are

ongoing,

and

have been discussed

previously

[2].

This paper reports

the

progress

that programme,

and the

plans

development

and

extend

the

existing safety case

The

Safety Report

SLC

requires that arrangements

are

made

review

the

safety case

appropriate intervals.

The

arrangements require review every

years;

the

latest review began

late

1995, and the

extended report

due to be

completed

in May 1999. It is

intended that

the

layout

of the

revised report will follow both

the IAEA Guidelines

[4], and the HSE

guidance provided

to its own

Nuclear Installations Inspectorate

[5,6].

This will update

the

safety case from

its

current deterministic basis

take advantage

the modem

techniques developed

Nuclear Power Plants

and

incorporate probabilistic safety assessment.

In order

assess

the

likely behaviour during steady state

and

transient events,

hydrodynamic

and

point

kinetics model

has

been constructed using

the

PARET code, which

has

been applied

to the

IAEA standard

Research Reactor

[3].

Firstly,

all

potential faults have been identified

in all

parts

of the

Reactor system.

Event trees have then been constructed

enable

the

probabilistic safety assessment

to be

made. This will

be combined with

radiological assessment

normal operational behaviour

and

behaviour during faults.

Particular emphasis place

sensitivity studies where parameters have uncertainties that

are

difficult

quantify.

5. Technical Specification

CONSORT

is a 100 kW

light water moderated, reflected/shielded

and

cooled pool-type Reactor, with

the

water cooling being essentially convective.

The

core consists

of 24

fuel elements, each with

shroud

square cross-section

and

containing

12-16

high-purity aluminium clad plates

of a

UA1 alloy with Highly

Enriched Uranium

(HEU) at

80%.

The

excess core reactivity

controlled

stainless steel clad cadmium

control rods

(one

safety, plus

two

control).

stainless steel

rod is

used

for the

fine adjustments that

are

required

feedback changes

due to

changing coolant temperature throughout

working

day, as the

Reactor

only operated during normal office hours.

The first fuel charge

had an

inventory

approximately

3 kg

235

U, providing

excess reactivity

of 1%.

The original core

was

supplied

to the

UKAEA Dounreay

BNFL Springfields Mark

design

in 1964,

with

Mark

assemblies (with straight plates) supplied

in 1976, and 5 in 1983 to

raise available

reactivity. Other papers

can

provide more detail about

the

Reactor systems

[1].

6. Refuelling

In common with many other Research Reactor operators,

IC is

exploring

the use of Low

Enriched

Uranium

(LEU) for the

next fuel charge. Because refuelling

planned

interfere with operations

only

a minimal degree,

the

intention

is to

maintain

the

same size core, with

the

same geometry fuel. Work

has

begun

analyse

the

fuel design based around

the

uranium silicide route

offered

UKAEA

Dounreay/BNFL Springfields, which uses enrichments

of ~

20%.

It is

most important

maintain

the

flux

levels that

are

currently available

in the

experimental facilities

CONSORT.

Control Rod Timing: the Story So Far

In common with Nuclear Power Plants world wide,

the

CONSORT

NSC has

encouraged investigation

any trend

control blade drop time. Following

trip,

the

electromagnetic clutch disengages,

and the

blades drop

their stainless steel supporting tapes into channels between extra-spaced fuel.

The

drop

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essentially under gravity, although there is friction from the drive unit drum. A braking system then slows

travel beyond around 60% insertion. The rationale is that distortion in components due to irradiation has

caused incomplete control rod insertion in some PWR designs. During routine monthly testing, variations

had been observed in control rod drop time, although the specification had at no time been violated.

The NSC brief was that no modifications to the actual control rod mechanism would be made. The full

experiment performed has been described previously [2]. A combination of the built-in drop time

measurement facilities, which are used during monthly testing, and are initiated and stopped by micro-

switches for the top and bottom of rod travel, and optical sensors scanning regularly spaced marks on the

control blade tape are used. These together give the rod drop profile can be obtained (for the coarse rods),

although this is subject to some manual data assessment due to data logging with the MCA. Nevertheless,

neither of these methods has indicated any trend in control blade drop times, which remain within

specification.

However, in an effort to gauge the behaviour during the first part of the control blade insertion, plans are

underway to develop a new measurement system that monitors the time between the scram initiation and

50-60%

insertion; the most important part of the shutdown. The ideal situation would be a measurement

of the trip signal initiation to 50% insertion. A method of ascertaining this is under development.

8. Replacement and Upgrades of Reactor Control System.

The Reactor has now been operating safely for almost 33 years. During this time various equipment

upgrades have taken place and routine maintenance programmes have been carried out based on the best

advice available, good engineering practice and previous operational experience. These programmes have

been adapted throughout the lifetime of the Reactor to ensure that up-to-date practices and modern

advances are followed. The Reactor Site Licence formally requires mat safety documentation is

developed to demonstrate the safety of all Reactor operations and that there is a periodic review of all

such safety documentation. These are not new requirements but have lead to an ongoing review of the

nucleonic instrumentation and safety circuits in order to demonstrate that they will continue to perform

reliably with safety maintained, or indeed enhanced, for further operation of the Reactor.

The Reactor control system utilises 108 PO 3000 type relays. These were designed originally for use in.

telephone exchanges and as such operate many times during the working day. In the Reactor control

system they operate on average twice a day. No failures have ever been reported in the operating life of

the Reactor. Data was obtained to give reassurance as to their expected reliability and lifetime. This was

further reinforced by physical testing and maintenance performed in-house. Data has also been obtained

on cable ageing in order to provide reassurance for future operation. Again, in-house testing and checking

of cable integrity was performed in order to reinforce these findings.

There are five independent power monitoring instrumentation channels. Each channel consists of a

module containing several of the following units: power supply, EHT supply, amplifiers, alarm units and

trip units. This instrumentation had been replaced over a period of time in the early 1980s, but is now

ageing and could give rise to potential maintenance problems, as some of the components they use are no

longer obtainable. Complete modules are also not available to buy "off the

shelf',

and it would be

extremely expensive and time consuming to have a suite of new instrumentation built to order by an

outside supplier. As the original circuit diagrams and technical documents for each unit were available, it

was decided to build new units to the original designs that would be updated to utilise modern

components and reflect modern practices and design advances. The new units are built in-house, although

an external consultant provides the updated designs and component schedules. Soak testing of each new

unit prior to installation is also carried out externally to the original test schedules. Units are built, tested

and installed individually so that, over a period of time, the whole instrumentation suite will be replaced.

Old replaced units are then refurbished and kept as spares. The replacement strategy has concentrated on

169

firstly replacing those units where

component degradation has been

observed or suspected. The

progress of the replacement

programme for instrumentation

channel modules is shown in

Table 1. Currently a new unit is

undergoing soak testing by the

consultant electronic engineer,

and will then be installed in

Shutdown Amplifier 2 when

appropriate.

Unit

Type

Pulse Channel:

Log d.c. Channel:

Shutdown Amplifier 1:

Shutdown Amplifier 2

Linear Channel

N°.

of Units

Replaced

N°.

of Units Awaiting

Replacement

Table 1. Progress With Power Monitoring Channel Instrumentation

Replacement

It is planned to continue with the replacement of all units in the instrumentation system and to speed up

the process in order to complete this within the next few years. Other items within the control system will

also be replaced including a power transformer/converter and several more of the contactors. Other, more

minor, items in the control and instrumentation systems will be addressed as and when appropriate.

Various other ancillary items connected to the control and instrumentation systems have already been

changed in recent years. These include: chart recorders measuring Reactor power, chart recorders for

temperature and activity monitoring systems, alarm bell timers, associated switches, and contactors in

motor driven systems. Great attention has also been paid to the Reactor cooling system. This has

undergone a great deal of refurbishment with parts being cleaned, replaced or modified as necessary.

It is believed that these actions, together with tiie appropriate documentation required, will enable the

CONSORT Reactor to continue to be operated safely, and indeed with safety enhanced, for the

foreseeable future.

9. Conclusions

It is a busy time at the Imperial College CONSORT Reactor centre, with a great deal of work being done

to maintain this strategic facility for the future. The production of the enhanced safety case will produce a

case for operating the Reactor for the next ten years, before a full review is due again. A rolling

programme of component replacement is continuing, and has been very successful to date. Reactor

operations have been maintained and no major problems have been identified. Refuelling plans are being

considered.

10.

References

[1] G.D.Burholt, T.D.MacMahon, "The University of London Reactor Centre, Ascot, England",

J.Radioanal. Chem. 53(1-2), 1979, p 365.

[2] J.O'Connell, R.Miller, Proc. Intl. Topical Sem. Management of Ageing of Research Reactors,

Hamburg, Germany, 1995, IAEA-SR-190/34, p 343.

[3] W-L-WoodrufF, Argonne National Laboratory, USA.

[4] "Safety Assessment of Research Reactors and Preparation of the Safety Analysis Report", IAEA

Safety Series No. 35-G1,1994.

[5] "Safety Assessment Principles for NPPs", HSE, 1992.

[6] "Tolerability of Risk from Nuclear Power Stations", HSE, 1992.

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