Assessment model for annual worker radiation dose calculation for the 400MWth PBMR plant
Abstract
Increasing demands on energy supplies have renewed interest in high temperature gas-cooled reactors such as the Pebble Bed Modular Reactor. However, public pressure due to nuclear power plant accidents, such as at Chernobyl, has highlighted radiation safety as a primary concern. In the radiation safety assessment and the safety planning of a nuclear facility, worker dose assessments play an important role. For water-cooled nuclear power plants, these assessments are based on operational experience gained and data collected during the design and operation of existing reactor designs.
Most of the research on high temperature gas-cooled reactors was stopped as far back as the 1980s. Information on safety assessments for these reactors is therefore outdated. This forces a new approach, as the 400 MWth Pebble Bed Modular Reactor is a first-of-a-kind development with design parameters set higher than previous designs. Most worker dose assessments are retrospective studies, based on historical or existing information captured in dose and maintenance records.
With the innovative Pebble Bed Modular Reactor, a new integrated dose assessment and integrated dose assessment model were developed, as operational experience and dose measurements are absent. These were developed within the processes and procedures defined for the nuclear industry. It is recognised that when operating experience or radiation monitoring data is not available, dose estimation requires extensive research, compilation of theoretical scenarios and innovative models. A new simplified dosimetric formula was developed based on the exposure determinants that will contribute to the representative worker’s dose. This formula makes it possible for the conceptual exposure scenarios to be quantified in order to arrive at an annual worker’s dose. Sensitivity analyses of the key input parameters were conducted to assess their impact on the results. This formula aims to calculate a conservative estimate of the upper limit of annual worker doses received on the plant.
Implementing the integrated assessment model provided the design team with new quantitative and qualitative information. Quantifying the annual worker dose brought an improved understanding of the level of radiation hazard present on the plant. It provided a method to carry out comparative assessments for various combinations of alternative design and maintenance concepts. This is especially important during system optimisation evaluations. It also established a quantitative benchmark for comparison of design improvements.
The results of this study were biased towards upper limit values, due to a lack of safety analysis results for normal operating conditions. Most of the available safety design analyses focused on abnormal plant conditions and accident scenarios during early design phases. It is recognised that as a design matures, more information regarding normal operating plant conditions becomes available. This requires regular updating of the assessment model to ensure that the selected missions are representative of actual exposure scenarios expected.
This integrated assessment and integrated assessment model proved to be a useful engineering tool. It provided the engineers with feedback on the adequacy of the integration of safety considerations early in the design process. The results of the worker dose assessment and the insights gained from the assessment also allow for easier compilation and changes of safety programmes and procedures for the operating plant.
Die toenemende vraag na energie het tot hernude internasionale belangstelling in die ontwerp van hoë temperatuur gasverkoelde reaktors gelei. Die Korrelbed Modulȇre Reaktor Kragsentrale is ’n voorbeeld van hierdie tipe reaktors. Stralingsveiligheid in die ontwerp van kernkragaanlegte is van primȇre belang. Dit is kernkragongelukke soos die van Chernobyl in 1986 wat tot openbare kritiek van kernkrag gelei het. In die analise van stralingsveiligheid en die veiligheidsbeplanning van ‘n kernaanleg, speel werkerdosisontledings ’n belangrike rol. Vir waterverkoelde kernkragaanlegte is werkerdosisontledings gebaseer op operasionele ervaring en empiriese data wat versamel word tydens die ontwerp en bedryf van bestaande kernaanlegte.
Navorsing op hoë temperatuur gasverkoelde reaktors is reeds so lank terug as die 1980’s gestop. Die bestaande inligting oor stralingsveiligheidaspekte van hierdie tipe reaktors is om hierdie rede verouderd. ’n Nuwe benadering word vereis in die ontwerp van die 400 MWth Korrelbed Modulȇre Reaktor Kragsentrale, wat ’n ‘eerste-van-’n-soort-ontwikkeling’ is. Die ontwerpparameters van hierdie aanleg is hoër as die van vorige ontwerpe.
Die meerderheid van beskikbare werkerdosisontledings is terugblikkende studies, gebaseer op historiese inligting van dosis- en onderhoudsrekords. Die ontwerp van die innoverende 400 MWth Korrelbed Modulȇre Reaktor verg dat ’n dosisontledingsmetode gevind word, wat in die afwesigheid van operasionele ervaring en dosismetings uitgevoer kan word.
Die doel van hierdie studie is om ’n nuwe geïntegreerde dosisontledingsmetode te ontwikkel om werkerdosisontledings uit te voer tydens die ontwerp van ‘eerste-van-’n-soort’ ontwikkelings, veral waar operasionele ervaring en empiriese data afwesig is. Hierdie is binne die prosesse en prosedures van die kernindustrie ontwikkel. Dit word erken dat dosisramings uitgebreide navorsing, opstel van teoretiese scenario’s en nuwe modelle vereis wanneer operasionele ondervinding en data oor stralingsregulering afwesig is.
‘n Nuwe prosedure, gebaseer op die blootstellingsdeterminante van die dosis van ’n verteenwoordigende werker, is ontwikkel. ’n Vereenvoudigde formule maak dit moontlik om die konseptuele blootstellingscenario’s te kwantifiseer. Hierdie scenario’s is verteenwoordigend van onderhoud en inspeksies op die aanleg wat ’n virtuele werker jaarliks uitvoer.
Sensitiwiteitsanalises is gedoen om die invloed van die belangrikste invoerparameters op die resultate te evalueer. Hierdie vereenvoudigde metodologie poog om ’n konserwatiewe beraming van die boonste limiete van die jaarlikse werkerdosis te bereken.
Implimentering van die metodologie verskaf nuwe kwantitatiewe en kwalitatiewe inligting oor die stralingsvlakke op die aanleg. Die geïntegreerde model maak dit moontlik om vergelykende studies van verskillende kombinasies van alternatiewe ontwerpe en onderhoudskonsepte uit te voer. Dit is veral belangrik tydens sisteemoptimiseringsevalusies. Dit verskaf ook ‘n kwantitatiewe basislynkriterium vir vergelyking van ontwerpverbeterings.
Die resultate van hierdie studie neig na die boonste verwagte limiet waardes van jaarlikse werkerdosisse, weens ’n gebrek aan beskikbare veiligheidsanaliseresultate vir normale operasionele toestande. Die veiligheidsanalise fokus op abnormale toestande en ongelukscenarios in die vroeë ontwerpfases van ’n aanleg. Soos meer detail tydens die ontwerpproses verkrygbaar word, moet hierdie evalueringsmodel hersien raak. Dit is om te verseker dat die geselekteerde missies verteenwoordigend van realistiese normale blootstellingscenario’s is.
Die geïntegreerde model kan as ’n nuttige ingenieurshulpmiddel gebruik word. Vroeg in die ontwerpproses, verskaf dit ’n terugvoermeganisme aan ingenieurs oor die geskiktheid van die integrasie van veiligheidsvereistes. Die resultate van die werkerdosisontleding, en die insigte verkry uit die ontleding, vergemaklik identifisering van voorstelle en veranderinge vir die radiologiese veiligheidsprogram en -prosedures vir die operasionele aanleg.
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