1. Practical limitations of detecting personal contamination at a PWR
Matthew Lunn & Guy Renn
British Energy Generation Ltd.
Sizewell B Power Station
Leiston
Suffolk
United Kingdom
Introduction

2. Development of limits for surface contamination
Dunster (1955)
“Maximum Permissible Levels, MPL”
10-3 to 10-4Ci/cm2 (, )
Dunster (1963)
“Derived Working Levels, DWL”
10-4Ci/cm2 (, )
Today, 2002;
1 DWL = 4Bq/cm2 (, )
Hp (0.07) ~ Sv/h.
Limits for surface contamination
In UK - derived from historical practice
Earliest published work from UKAEA Harwell (1950’s & 1960’s)
Limitation of dose from various exposure pathways
Prevention of contamination of monitoring equipment
Limits set in 1960’s found to be adequate & used to modern day.

3. Monitoring Arrangements at SZB
CCA; C2
> 4Bq/cm2 (,)
>0.4Bq/cm2 ()
Personal & Equipment Monitoring
“Defence in Depth” - in common with all users of radioisotopes
CCA - (Definition)
Located within RCA - (definition)
Personal Monitoring between areas
NO MONITORS AT SITE BOUNDARY.
Non-controlled areas
RCA; R2
< 4Bq/cm2 (,)
< 0.4Bq/cm2 ()

4. Installed Monitoring Equipment
NE Technology HFM7
CHOICE OF MONITORS:-
HFM (Definition) + CM Paddle probe - Low risk/low occupancy SCR’s
IPM 8 - Monitor of choice at high risk/high occupancy SCR’s (+ RCA boundary).
Both gas filled proportional counter detectors
NE Technology IPM8

5. Original Calibration Protocol
Applicable to all detectors:
WANO Review (1999):
Relevance of calibration nuclides?
Determine contact efficiency to 36Cl
(P-Factor = 2)
Set alarm to 0.5 DWL
Check detector response to 55Fe or 99Tc (In contact)

6. Which Nuclide? Co-60 target nuclide
Originally used Cl-36 - beta energy much higher than Co-60 beta
(reason for setting alarm at 0.5 DWL)
Co-60 itself - short T1/2 v.expensive to replace sources
Tc-99 close to Co-60 beta, long t1/2 used on US LWR’s
Eventually chose C-14, long t1/2 & close to clothing attenuated Co-60 beta energy (from personal communication from NRPB)

7. What efficiency? What efficiency?
variability in clothing - detector distances between individuals & within individuals.
Scale Bars = 20mm
Determination of C-14 efficiency at various distances.

8. 14C Efficiencies Contact efficiency fairly low ~ 22%
Drops off rapidly:-
14% at 20mm 8% at 40mm 1% at 80mm
tried settings alarms for different efficncies
lowest alarm without unit tripping out on fault alarm equated to 20mm.

9. Revised Calibration Protocol
Body Detectors:
Determine 20mm efficiency to 14C
(P Factor = 2.6)
Set alarm to 1 DWL
Check detector response to 55Fe
(In contact)
Hand/Foot Detectors:
Determine contact efficiency to 14C
(P Factor = 2.6)
Set alarm to 1 DWL
Check detector response to 55Fe
(In contact)

10. Summary of alarm settings
Describe slide
Efficacy of Old & New alarm settings tested with contaminated clothing
100cm2 cotton patches spiked with various known activities of Co-60 or Mn-54
Worn through IPM 3 times. Number of alarms recorded.
“MDA” defined as lowest specific activity to generate 2/3 (66%) alarms.
Results…..

11. “Minimum Detectable Activities”
40 (>40)
8 (20)
4 (8)
2 (20)
20 (40)
40 (40)
4 (20)
8 (40)
20 (>40)
60Co
54Mn
MDA against Old alarms in parentheses
MDA against new alarms in bold
Both in Bq/cm2
36Cl - typical MDA in range of Bq/cm2 (5 - 10DWL)

12. Operational Experience
Old Alarm Settings:-
2000/2001
85313 RCA Entries; 139 Alarms = ‰
New Alarm Settings:-
2001/2002
55334 RCA Entries; 91 Alarms = ‰
Increased Sensitivity:-
Small increase in alarms due to NORM (Rn daughters)

13. Summary 50% reduction in IPM body alarm levels.
5% reduction in hand & foot alarm levels.
60Co “MDA” improved by factor of x
But; some body parts still have MDA > 1DWL
“Defence in Depth”
Strict contamination control at workplace
Have contaminated persons been leaving RCA undetected?
Unlikely