Control of occupational exposure when working within a reactor containment building at power

Matthew Lunn British Energy Generation Ltd. Sizewell B Power Station United Kingdom

Sizewell B Power Station

• 4 Loop, 2nd Generation Westinghouse PWR • 1200 MW, single unit • First criticality 31 Jan 1995 • 18 month refuelling cycles • Currently in Cycle 7: – November 2003 to Mar 2005

UK electricity prices

15 10 5 30 25 20

2001 2002 2003

0 Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec

Month

External radiological hazards

• Fission neutrons – Thermal & Intermediate energies – Fast neutrons (>25 keV); not measured directly by Albedo dosimeter • “k-factor” –

Prior knowledge

of neutron spectra & Albedo k-factor essential – Higher k-factor = harder neutron spectra • Activation & fission products – – 16 O(n,p) 16 N ;  emissions at 6.1 & 7.1 MeV 58 Ni(n,p) 58 Co ; 59 Co(n,  ) 60 Co ; 134 Cs/ 137 Cs ; – Crud still dominates radiation fields around certain components

Internal radiological hazards

• Surface contamination – Activation & fission products – Fixed & loose contamination <40 Bq/cm 2 • Airborne radioactivity – Particulate

alpha

< 0.001 Bq/m 3 – Particulate

beta/gamma & radioiodine

< 0.1 Bq/m 3 –

HTO

vapour ~10 to 60 kBq/m 3 • Internal monitoring & bioassay?

– Estimated doses <1mSv; no formal internal dose assessment required – (from static air sampling results & area occupancy)

Doserates @ 100% power

+21m (Main Operating Floor) ~0.01 to 0.10 mSv/h (  ) ~0.02 to 0.50 mSv/h (n) Average n:  ratio ~ 5 k-factor: 4.2 to 4.8

+28m (SG Steam Space) 0.02 to 0.15 mSv/h (  ) 0.08 to 3.00 mSv/h (n) Average n:  ratio ~10 k-factor: 5.1 to 5.5

+21m (Head Cable Bridge) ~ 0.50 mSv/h (  ) ~ 50.00 mSv/h (n) +6.5m (Ground Floor) ~0.01 to 0.03 mSv/h (  ) < 0.005 to 0.20 mSv/h (n) Average n:  ratio <0.5

k-factor: 2.0 to 2.8

+14m (LHSI Accumulators) <0.005 mSv/h (  ) <0.005 mSv/h (n) Average n:  ratio = 1 k-factor: 1.8 to 3.0

0.03 mSv/h

Radiation beams

3 mSv/h • Pipe & cable penetrations • • Bioshield gates  /n radiation beams – Very steep doserate gradients – Up to 20mSv/h per metre – Beams may not interact with personal dosimeters • No “multi-badging” of workers • Access to areas prevented using barriers 1 mSv/h

Justification of entries

• Triviality of dose – <0.05 man.mSv; no further justification or optimisation required • Lower doserates at 100% power – Especially near RHR system • Improved industrial safety • Resource minimisation – Flatten resource peaks (esp. scaffolding) where demand > supply • Improved outage mobilisation – Install temporary shielding to enable faster release of plant areas • Prevent a reactor trip (scram) – Avoid unit loss & dose from a forced outage recovery

Personal dosimetry

• EPD – Doserate alarm; 500  Sv/h – Dose alarm; 100  Sv – Dose alarm ~ 50% lower than usual • Albedo – Max. k-factor of 5.5 used for all assessments • Direct-reading electronic neutron dosimeter unavailable • So, staff told to assume…..

– Total dose ~ 10x EPD on 21m & above – Total dose ~ 2x EPD on 14m & below

Pre-job briefing tools

• ALARA Brief for all entrants • Plant Information Sheets – Photograph of equipment – Radiological survey data – Practical precautions (e.g. what side to stand) – Map showing location • Item Location Plans – A2 drawings showing location of plant items – Overlaid with general radiological conditions – Used in briefing room

180 160 60 40 20 0 140 120 100 80 2001

Scope of work at power

2002

Year

2003 Barrier Tape Corrective Maintenance Plant Operations RP/Chemistry Duties Scaffolding LLRT/MOVAT Plant Tours Minor Maintenance Transmitter Calibrations Condition Monitoring RPV Seal Leakage Sampling Other

Maintenance etc. doses

Estimated doses for containment work activities, excluding radiological protection, by year.

Calendar Year Neutron Collective Dose (man.mSv) Gamma Collective Dose (man.mSv) Collective Dose (man.mSv) Number of people Average Individual Dose (mSv) Maximum Individual Dose (mSv) 2001 15.120

2.775

17.895

43 0.416

1.595

2002 7.590

4.232

11.822

129 0.091

0.803

2003 16.800

4.991

21.791

119 0.183

1.052

Barrier Tape

RP Doses

Estimated doses for radiological protection activities inside containment, by year. Calendar Year 2001 2002 Neutron Collective Dose (man.mSv) 5.610 2.600 Gamma Collective Dose (man.mSv) ~ 2.000 ~ 2.000 Collective Dose (man.mSv) Number of people ~ 7.610 8 ~ 4.600 22 Average Individual Dose (mSv) Maximum Individual Dose (mSv) ~ 0.951 ~ 1.590 ~ 0.209 0.430 2003 3.800 1.608 5.408 22 0.246 0.781 Barrier Tape

Conclusions

• Wide variation in practices around world • Little OE or published data available during planning – Setting dose constraints was difficult….

– Difficulty in preventing increase in work scope • Individual & collective doses remained low….

– c.f. national limits & company dose constraint of 10mSv • Some jobs justified & optimised when worked at power – e.g. RHR scaffolding & lagging • Some minor jobs not optimised when worked at power – e.g. Transmitter calibrations, corrosion surveys?

• Proposed dose constraints for future containment entries – 1.5 mSv per annum – 15 man.mSv per annum