Določanje difuzijske dolžine nevtronov v vodi

Čižmek, Sebastjan (author), Snoj, Luka (mentor), Ambrožič, Klemen (co-mentor)

Motivacija za magistrsko nalogo je izdelava eksperimentalne naprave za merjenje difuzijske dolžine nevtronov v vodi ter primerjava izračunanih in izmerjenih difuzijskih dolžin nevtronov v vodi pri različnih temperaturah. V prvem delu obnovimo teoretične osnove, definiramo Boltzmannovo enačbo za transport nevtronov in iz nje izpeljemo enogrupni difuzijski približek. V nadaljevanju uporabljamo odprtokodni program OpenMC za simuliranje transporta delcev z metodo Monte Carlo, izvedemo vrsto simulacij in iz rezultatov izračunamo difuzijske dolžine nevtronov v vodi, grafitu, betonu, polietilenu in parafinu. Pri temperaturi 20 °C znaša izračunana difuzijska dolžina nevtronov v vodi 7,27 cm, v grafitu 26,11 cm, v betonu 11,96 cm, v polietilenu 5,02 cm in v parafinu 6,38 cm. Zraven preverimo tudi vpliv temperature vode na difuzijsko dolžino ter, kako se vrednosti difuzijske dolžine razlikujejo, če se meritev izvede v zloženi kopici materiala in v ploščah, ki se nahajajo v vodi ali na zraku. Izkaže se, da so vsi trije načini določanja difuzijskih dolžin dobri. Na podlagi dobljenih rezultatov načrtujemo dimenzije bazena za eksperimentalni del in izračunamo pričakovane hitrosti doz na robovih, ki znašajo 85,8 nSv/h. V drugem delu sestavimo eksperimentalno napravo za merjenje difuzijske dolžine. Za bazen uporabimo rezervoar za vodo volumna 1m3, ki ga napolnimo z demineralizirano vodo. Ker preverjamo temperaturno odvisnost difuzijske dolžine, rezervoar toplotno izoliramo, vanj pa namestimo grelca skupne moči 11kW. Temperaturo vode nadziramo s petimi Pt100 temperaturnimi senzorji, vzdržujemo pa jo s vklaplanjem in izklapljanjem grelcev preko kontaktorja. Za mešanje vode in enakomerno temperaturo po bazenu poskrbi potopna črpalka. Kot izvor se uporabi Am-Be izvor nevtronov, kot detektor pa He-3 plinski detektor. Meritve fluksa nevtronov izvedemo na devetih oddaljenostih stran od izvora, pri šestih temperaturah vode. Izmerjena difuzijska dolžina nevtronov v vodi pri temperaturi 25 °C znaša 7,36 cm, kar se zelo dobro ujema z izračunano, ki znaša 7,32 cm. Negotovosti izmerjene difuzijske dolžine ocenimo na podlagi negotovosti v razdalji detektorja od izvora in temperature vode. Relativne vrednosti negotovosti se gibajo med 1% - 2%. Sledi primerjava izmerjenih in izračunanih rezultatov ter zaključek, kjer predstavimo možnosti nadgradnje eksperimentalne naprave in izboljšave rezultatov.

• Language: Slovenian
• Keywords: Difuzijska dolžina nevtronov, Am-Be izvor, He-3 detektor, radiacijske doze, ščitenje pred sevanjem, Monte Carlo transport nevtronov, Boltzmannova transportna enačba, difuzijski približek, OpenMC.
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-150448
• COBISS.SI-ID: 165921283
• Publication date in RUL: 17.09.2023

Secondary language

• Language: English
• Title: Determination of neutron diffusion length in water

Abstract

The motivation for this master’s thesis is building an experimental set-up for measuring the diffusion length of neutrons in water. In the first part, we cover the theoretical basics, define the Boltzmann equation for neutron transport and derive the one-speed diffusion approximation. In the following, we use the OpenMC open source program to simulate particle transport using Monte Carlo methods, preform a series of simulations and use the results to calculate the diffusion lengths of neutrons in water, graphite, concrete, polyethylene and paraffin. At a temperature of 20°C, the calculated diffusion length of neutrons in water is 7,27 cm, in graphite 26,11 cm, in concrete 11,96 cm, in polyethylene 5,02 cm and in paraffin 6,38 cm. In addition, we also check the effect of water temperature on the diffusion length and how the diffusion length values differ when the measurement is made in a stacked pile of material and in plates in water or air. All three methods of determining diffusion lengths are shown to be good. Based on the results obtained, we plan the dimensions of the pool for the experimental part and calculate the expected dose rates at the edges, which amount to 85,8 nSv/h. In the second part, we set up the experimental apparatus. For the pool, we use a water tank with a volume of 1m3 filled with demineralised water. Since we are checking the temperature dependence of the diffusion length, we thermally insulate the tank and install two heaters with combined heating power of 11kW. The water temperature is monitored by five Pt100 temperature sensors and maintained by switching the heaters on and off via a contactor. A submersible pump ensures that the water is mixed and that the temperature is uniform throughout the tank. For the source, we use a Am-Be neutron source and for the detector we use a He-3 gas filled detector. Neutron flux measurements are preformed at nine different distances from the source and at six different temperatures. The measure diffusion length of neutrons in water at 25°C is 7,36 cm, which agrees very well with the calculated one of 7,32 cm. Uncertainties in the measured diffusion length are estimated from uncertainties in the distance of the detector from the source and the water temperature. The relative uncertainties values range from 1% - 2%. A comparison of the measured and calculated results is followed by a conclusion where we present the possibilities of experiment apparatus upgrades and result improvements.

• Keywords: Neutron diffusion length, Am-Be neutron source, He-3 gas detector, radiation doses, radiation shielding, Monte Carlo neutron transport, Boltzmann transport equation, one-speed diffusion approximation, OpenMC.