Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The objective of this research was to develop a neutron detection system that
incorporates a mass spectrometer to measure high neutron flux in a nuclear
reactor environment. This system consists of slow and fast neutron detector
elements for measuring fluxes in those energy regions respectively. The detector
should further be capable of withstanding the harsh conditions associated
with a high temperature reactor. This novel detector which was initially intended
for use in the PBMR reactor has possible applications as an in-core
neutron and indirect temperature-monitoring device in any of the HTGR.
Simulations of a generic HTGR core model were performed in order to
obtain the neutron energy spectrum with emphasis on the behavior of three
energy regions, slow, intermediate and fast neutrons within the core at different
temperatures. The slow neutron flux which has the characteristic of a Maxwell-
Boltzmann distribution were found to shift to larger values of neutron flux
at higher energies as the fuel temperature increased, while fast neutron flux
spectra remained relatively constant. In addition, the results of the fit of
the slow neutron flux with a modified Maxwell-Boltzmann equation confirmed
that in the presence of the neutron source, leakage and absorption, the effective
neutron temperatures is above the medium temperatures. From these results,
it was clear that the detection system will need to monitor both slow and
fast neutron flux. Placing neutron detectors inside the reactor core, that are
sensitive to a particular energy range of slow and fast neutrons, would thus
provide information about the change of temperature in the fuel and hence act
as an in-core temperature monitor.
A detection mechanism was developed that employs the neutron-induced
break-up reaction of 6Li and 12C into α-particles. These materials make excellent
neutron converters without interference due to γ-rays, as the contributions
from 6Li(γ,np)4He and 12C(γ,3α) reactions are negligible. The mass spectrometer
measures the 4He partial pressure as a function of time under high vacuum
with the help of pressure gradient provided by a high-vacuum turbomolecular
pump and a positive-displacement fore-vacuum pump connected in series. A
cryogenic trap, which contains a molecular sieve made of pellets 1.6 mm in diameter,
was also designed and manufactured to remove impurities which cause
a background in the lighter mass region of the spectrum.
The development and testing of the high flux neutron detection system
were performed at the iThemba Laboratory for Accelerator Based Sciences
(LABS), South Africa. These tests were carried out with a high energy proton
beam at the D-line neutron facility, and with a fast neutron beam at the
neutron radiation therapy facility. To test the principle and capability of the
detection system in measuring high fluxes, a high intensity 66 MeV proton
beam was used to produce a large yield of α-particles. This was done because
the proton inelastic scattering cross-section with 12C nuclei is similar to that of
neutrons, with a threshold energy of about 8 MeV for both reactions. Secondly,
the secondary fast neutrons produced from the 9Be(p,n)9B reaction were also
measured with the fast neutron detector.
The response of this detection system during irradiation was found to be
relatively fast, with a rise time of a few seconds. This is seen as a sharp increase
in the partial pressure of 4He gas as the proton or neutron beam bombards
the 12C material. It was found that the production of 4He with the proton
beam was directly proportional to the beam intensity. The number of 4He
atoms produced per second was deduced from the partial pressure observed
during the irradiation period. With a neutron beam of 1010 s−1 irradiating the
detector, the deduced number of 4He atoms was 109 s−1. When irradiation
stops, the partial pressure drops exponentially. This response is attributed to
a small quantity of 4He trapped in the present design.
Overall, the measurements of 4He partial pressure produced during the
tests with proton and fast neutron beams were successful and demonstrated
proof of principle of the new detection technique. It was also found that
this system has no upper neutron flux detection limit; it can be even higher
than 1014 n·cm−2·s−1. The lifetime of this detection system in nuclear reactor
environment is practically unlimited, as determined by the known ability of
stainless steel to keeps its integrity under the high radiation levels. Hence, it is
concluded that this high flux neutron detection system is excellent for neutron
detection in the presence of high γ-radiation level and provides real-time flux
measurements. / AFRIKAANSE OPSOMMING: Die doel van hierdie navorsing was om ’n neutrondetektorstelsel te ontwikkel
wat hoë neutronvloed binne in ’n kernreaktor kan meet. Die stelsel bevat
twee aparte detektorelemente sodat die termiese sowel as snelneutronvloed
gemeet kan word. Die detektor moet verder in staat wees om die strawwe
toestande, kenmerkend aan ’n hoë temperatuur reaktor, te kan weerstaan. Die
innoverende detektorstelsel, oorspronklik geoormerk vir gebruik in die PBMR
reaktor, het toepassingsmoontlikhede as in-kern neutron- sowel as indirekte
temperatuurmonitor.
Simulasies van ’n generiese model van ’n HTGR reaktorkern is uitgevoer
ten einde die neutronenergiespektrum in die kern by verskillende temperature
te bekom met klem op die gedrag van neutrone in drie energiegroepe: stadig
(termies), intermediêr en snel (vinnig). Daar is bevind dat die stadige
neutrone, wat ’n Maxwell-Boltzman verdeling toon, in intensiteit toeneem en
dat die piek na hoër energie verskuif met toename in temperatuur, terwyl die
vinnige neutronspektrum relatief onveranderd bly. ’n Passing van die stadige
spektrum op ’n gemodifiseerde Maxwell-Boltzmann verdeling het bevestig dat
die effektiewe neutrontemperatuur weens die teenwoordigheid van bronterme,
verliese en absorpsie, hoër as die temperatuur van die medium is. Hierdie resultate
maak dit duidelik dat die detektorstelsel beide die stadige sowel as die vinnige neutronvloed moet kan waarneem. Deur detektorelemente wat sensitief
is vir die onderskeie spekrale gebiede in die reaktorhart te plaas, kan
informasie bekom word wat tot in-kern temperatuur herleibaar is sodat die
stelsel inderdaad as indirekte temperatuurmonitor kan dien.
Die feit dat alfa-deeltjies geproduseer word in neutron-geïnduseerde opbreekreaksies
van 6Li en 12C is as die basis van die nuwe opsporingsmeganisme
aangewend. Hierdie materiale funksioneer uitstekend as neutron-selektiewe
omsetters in die teenwoordigheid van gamma-strale aangesien laasgenoemde se
bydraes tot helium produksie via die 6Li(γ,np)4He en 12C(γ,3α) reaksies, weglaatbaar
is. Die massaspektrometer meet die tydgedrag van die 4He parsiële
druk binne ’n hoogvakuum wat met behulp van ’n seriegeskakelde kombinasie
van ’n turbomolekulêre en positiewe-verplasingsvoorpomp verkry word. ’n
Koueval met ’n molekulêre sif, bestaande uit 1.6 mm diameter korrels, is ontwerp
en vervaardig om onsuiwerhede te verwyder wat andersins as agtergrond
by die ligter gedeelte van die massaspektrum sou wys.
Die ontwikkeling en toetsing van die hoëvloed detektorstelsel is te iThembaLABS
(iThemba Laboratories for Accelerator Based Sciences) gedoen. Dit
is uitgevoer deur gebruik te maak van die hoë energie protonbundel van die
D-lyn neutronfasiliteit asook van die bundel vinnige neutrone by die neutronterapiefasiliteit.
Om die beginsel en vermoë te toets om by ’n hoë neutronvloed
te kan meet, is van die intense 66 MeV protonbudel gebruik gemaak om ’n hoë
opbrengs alfa-deeltjies te verkry. Dit is gedoen omdat die reaksiedeursnit vir
onelastiese verstrooiing van protone vanaf 12C kerne soortgelyk is aan die van
neutrone, met ’n drumpelenergie van 8 MeV vir beide reaksies. Tweedens is
die sekondêre vinnige neutrone afkomstig van die 9Be(p,n)9B reaksie ook met
die neutrondetektor gemeet.
Daar is bevind dat die reaksietyd van die deteksiestelsel tydens bestraling
relatief vinnig is, soos gekenmerk deur ’n stygtyd van etlike sekondes. Laasgenoemde
manifesteer as ’n toename in die parsiële druk van die 4He sodra die
proton- of neutronbundel op die 12C teiken inval. Daar is verder bevind dat
die 4He produksie direk eweredig aan die bundelintensiteit is. Vir ’n neutronbundel
van nagenoeg 1010 s−1, invallend op die neutrondetektor, is vanaf die
gemete parsiële druk afgelei dat die produksie van 4He atome sowat 109 s−1
beloop.
In die geheel beoordeel, was die meting van die 4He parsiële druk tydens
die toetse met vinnige protone en neutrone suksesvol en het dit die nuwe meetbeginsel
bevestig. Dit is verder bevind dat die meetstelsel nie ’n beperking op
die boonste neutronvloed plaas nie, maar dat dit vloede van selfs hoër as 1014
s−1 kan hanteer. Die leeftyd van die detektorstelsel in die reaktor is prakties
onbeperk en onderhewig aan die bevestigde integriteit van vlekvrystaal onder
hoë bestraling. Die gevolgtrekking is dus dat die nuwe detektorstelsel uitstekend
geskik is vir die in-tyd meting van ’n baie hoë vloed van neutrone ook in
die teenwoordigheid van intense gammabestraling.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/19999 |
| Date | 03 1900 |
| Creators | Singo, Thifhelimbilu Daphney |
| Contributors | Papka, Paul, Wyngaardt, Shaun M., Dobson, Robert T., Smit, Frederick D., Stellenbosch University. Faculty of Sciences. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 164 p. : ill. |
| Rights | Stellenbosch University |
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