RadiationProtectionDosimetry(2012),Vol.151,No.1,pp.36–42 doi:10.1093/rpd/ncr450 AdvanceAccesspublication6January2012 VERIFICATION OF SCREENING LEVEL FOR DECONTAMINATION IMPLEMENTED AFTER FUKUSHIMA NUCLEAR ACCIDENT HaruyukiOgino*,TakeshiIchijiandTakatoshiHattori RadiationSafetyResearchCenter,NuclearTechnologyResearchLaboratory,CentralResearchInstitute ofElectricPowerIndustry(CRIEPI),2-11-1Iwado-kita,Komae,Tokyo201-8511,Japan *Correspondingauthor:[email protected] ReceivedAugust232011,revisedNovember32011,acceptedNovember42011 Thescreeninglevelfordecontaminationthathasbeenappliedforthesurfaceofthehumanbodyandcontaminatedhandled objectsaftertheFukushimanuclearaccidentwasverifiedbyassessingthedosesthatarisefromexternalirradiation,ingestion, inhalationandskincontamination.Theresultshowsthattheannualeffectivedosethatarisesfromhandledobjectscontami- natedwiththescreeninglevelfordecontamination(i.e.100000countsperminute)is <1mSvy21,whichcanbeconsidered astheinterventionexemptionlevelinaccordancewiththeInternationalCommissiononRadiologicalProtectionrecommenda- tions.Furthermore,thescreeninglevelis alsofoundtoprotecttheskinfromtheincidenceof adeterministiceffectbecause theabsorbeddoseoftheskin thatarisesfromdirect depositiononthesurfaceof thehumanbodyis calculated tobelower thanthethresholdofthedeterministiceffectassumingapracticalexposureduration. INTRODUCTION National Institute of Radiological Sciences (NIRS) andguidelinesofFukushimaMedicalUniversity(3). Off the Pacific Ocean, near the Tohoku district, an Meanwhile, on 19th March, the Nuclear Safety Earthquakeoccurredat14:46on11March2011JST Commission of Japan (NSC) determined the screen- (Japan Standard Time) and generated atsunami that ing level for decontamination to be 100000 cpm. inundatedtheFukushimaDaiichinuclearpowerplant The revised screening level corresponds to a dose oftheTokyoElectricPowerCo.(TEPCO),resultingin rate of 1 mSv h21 at a distance of 10 cm, stipulated anuclearaccidentofanunprecedentedscaleandover as a standard for decontamination in the case of alengthyperiod.Asaresultofthisaccident,thepres- contamination on the surface of the body for sure venting of primary containment vessels, explo- generalresidentsintheManualforFirstResponders sions at reactor buildings and other incidents caused to a Radiological Emergency(4) given by the radioactive materials to be released into the environ- International Atomic Energy Agency (IAEA). The ment.AccordingtotheestimationoftheNuclearand IndustrialSafetyAgencyofJapan(NISA)(1),160PBq measured values are those measured using a Type of131I,18PBqof134Csand15PBqof137Csweredis- TGS-136GMsurveymeterwitha5-cmbore. charged into the air from reactor units 1–3 of the Withregardtothecontaminationofresidents,the FukushimaDaiichinuclearpowerplantduring11–16 Fukushima prefecture has been implementing March2011.Countermeasureshavebeen taken, such screening surveys for residents in the prefecture in- astherestrictionofthedistributionandconsumption cluding people evacuated from within 20 km of the ofcontaminated foodstuffs(2)and the implementation Fukushima Daiichi nuclear power station in cooper- ofascreeninglevelfordecontamination. ation with the Nuclear Emergency Response Local On 11th March, the Fukushima prefecture deter- Headquarters. Most of the 219743 people checked mined the screening level required for whole body asof20thAugustwereunderthe100000cpmlimit. decontamination at 100000 counts per minute Decontamination was performed for 102 people (cpm) and that partial decontamination by wiping exceeding 100000 cpm, but their contamination would be performed in the case of detection of levels fell to below the criterion after such radioactivity .13000 cpm but ,100000 cpm, decontamination(5). based on the opinion of experts in radiation medi- Thescreeninglevelfordecontaminationof100000 cine dispatched from the Ministry of Education, cpmdeterminedbytheNSChasbeenappliedtonot Culture, Sports, Science and Technology (MEXT), onlythesurfaceofthebodyforgeneralresidentsbut and doctors and other professionals from the also contaminated objects handled by emergency #TheAuthor2012.PublishedbyOxfordUniversityPress.Allrightsreserved.ForPermissions,pleaseemail:[email protected] ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionNon-CommercialLicense(http://creativecommons.org/ licenses/by-nc/3.0/),whichpermitsunrestrictednon-commercialuse,distribution,andreproductioninanymedium,providedtheoriginalworkis properlycited. SCREENINGLEVELFORDECONTAMINATION workers (e.g. vehicles, equipment, machinery, tools). Thesurfacedensityofradioactivecontaminationis Theauthorshavedevelopedadoseassessmentmodel given by equation (1), where A (Bq cm22) is the i for surface-contaminated objects such as manually, surface density of radioactive contamination for closely and remotely handled objects to derive radionuclide i, N (count s21) is the measured count isotope-specificclearancelevelsforsurfacecontamin- rate,N (counts21)isthebackgroundcountrate,1 b e,i ation(6–8).Inthisstudy,thescreeninglevelfordecon- (%2p21)istheinstrumentefficiencyforradionuclide tamination implemented after the Fukushima i, W (cm2) is the area of the detector window, 1 is s,i nuclear accident is verified from the viewpoint of the source efficiency for radionuclide i and f (%) is i dose assessment using the dose assessment model theemissionrateofbetarayforradionuclidei. developedinpreviousstudies(6–8). A ¼ðN(cid:2)N Þ=ð1 (cid:3)W (cid:3)1 (cid:3)fÞ: ð1Þ i b e;i s;i i MATERIALSANDMETHODS A count rate of 100000 cpm corresponds to 1670 Evaluationofradioactivecontaminationdensity countspersecond.Thebackgroundcountrateshould behigherafterFukushimanuclearaccident,buthere Immediatelyaftertheatmosphericreleaseofradioio- it was conservatively set to zero. The beta-ray emis- dines(e.g.131I)andradiocaesiums(e.g.134Cs,137Cs) sion rates of 131I were set as 89.4 % foran energyof in the Fukushima nuclear accident(1), radioiodines 606keV,7.4%foranenergyof334keV,2.1%foran were the main target of measurement for the deter- energyof248keV,0.6%foranenergyof304keVand minationofmedicaltreatmentforthethyroidbyspe- 0.4 % for an energy of 807 keV, and those of 134Cs cialisedmedicaldoctors.Nowthatsomemonthshave weresetas70.2%foranenergyof658keVand2.5% elapsedandtheradioiodineswithshorthalf-lifetimes for415keV, thoseof 137Cswere setas 94.4% foran (e.g. 8 d for 131I) have almost completely decayed, energyof 514keVand 5.6 % for1.176MeV. Thein- radiocaesiums have become the dominant radionu- strumental efficiency for beta rays was uniformly set clidesconsideredintheevaluationofradioactivecon- as0.4,conservativelyconsideringthemaximumbeta- tamination. Figure 1 shows the trend of the rayenergiesof 131I, 134Cs and 137Cs. The source effi- concentrationsofradioactive131I,134Csand137Csin ciency was set as 0.5 in accordance with the ISO airmeasuredbyTEPCOatthesiteoftheFukushima 7503-1(10),consideringthatthemaximumbeta-rayen- Daiichinuclearpowerplant(9).Onthebasisofobser- ergies of 131I, 134Cs and 137Cs are higher than 0.4 vations such as those shown in Figure 1, in this MeV, although an experimental study has shown a paper,thesurfacesofobjectsandbodiesareassumed higher source efficiencyof radiocaesiums for various to be contaminated with 131I, 134Cs and 137Cs, the materials(11).Theareaofthedetectorwindowwasset ratio of contamination densityof 134Csto 137Cswas as20cm2,whichisthatforanAlokaTypeTGS-136 set to 1 and the ratio of 131I to these radiocaesiums GMsurveymeterwitha5-cmborethathastypically wassetto100,10,1,0.1and0.01. beenappliedforthemeasurement forscreeningafter theFukushimanuclearaccident. Methodologyofdosimetricassessment The dose rate depends on the surface area of radio- active contamination regardless of the contamin- ation density, although the applied screening level for decontamination of 100000 cpm has been set upon considering a dose rate of 1 mSv h21 at a dis- tance of 10 cm from a radioactive surface(4). Here the dose rates at a distance of 10 cm from a surface were calculated for various contaminated areas (i.e. 100, 300 cm2, 0.1, 0.3, 1, 3, 10 m2) using the QAD- CGGP2 code(12), assuming that the whole areawas uniformly contaminated by 131I, 134Cs and 137Cs with surface density levels that correspond to the screening level for decontamination of 100000 cpm calculatedbyequation(1). With regard to the methodology of dose assess- Figure 1. Radioactive concentration of 131I, 134Cs and mentfromsurface-contaminatedobjects,theoriginal 137CsintheairmeasuredatthesiteofFukushimaDaiichi dose assessmentmodelforthederivationof isotope- nuclearpowerplant. specific surface clearance levels (Bq cm22) was 37 H.OGINOETAL. developed by integrating the existing stylised ap- Table 1 shows the dose conversion factors (mSv proach of IAEA for protecting people in the waste y21 Bq21 cm2) of 131I, 134Cs and 137Cs developed and transport safety fields given by the Safety for each type of surface-contaminated object. The Reports Series No. 44(13) and TECDOC-1449(14), so effective dose from whole body exposure and the that an appropriate conservative model can be com- skin-absorbed dose that arise from these objects prehensively applied to the radiation, transport and were calculated using the dose conversion factors waste safety fields(6–8). In the dose assessment, the giveninTable1. exposure scenario was classified into three categor- In the assessment of skin contamination, there ies: manually handled objects, closely handled should be a direct pathway after the Fukushima objects and remotely handled objects. The assessed nuclear accident, such as radiocaesiums existing in exposure pathways were external irradiation and the the atmospherebeingdeposited on thehumanbody, inhalation of resuspended radionuclides from closely in additionto theaboveassessmentviathehandling andremotelyhandledobjects,andingestionviacon- of a surface-contaminated object. Here the assess- taminated hands and skin contamination from ment of the skin-absorbed dose and the effective manuallyhandledobjects. dosethroughdermalabsorptionbecomesimportant. In the developed model, the external irradiation The skin-absorbed dose that arises from such direct was assessed using the QAD-CGGP2 code(12) as- deposition is given by equation (2), where E q,i suming independent surface-contaminated areas for (mGy) is the skin-absorbed dose for radionuclide i, closelyandremotelyhandledobjects.Theinhalation A (Bqcm22)isthesurfacecontaminationdensityof i was assessed using the resuspension rate (h21), the the body for radionuclide i, CF (mGy h21 Bq21 b,i volume of the room (m3), the exchange rate (h21), cm2)istheskin-absorbeddoseratetothebasallayer the breathing rate (m3 h21) and the dose conversion of the skin epidermis for beta irradiation (4 mg coefficient of inhalation for each radionuclide (Sv cm22) for radionuclide i, CF (mGy h21 Bq21 g,i Bq21) as parameters. The ingestion was assessed cm2)istheskin-absorbeddoseratetothebasallayer using the area for ingestion from the contaminated of the skin epidermis for gamma irradiation (7 mg area (cm2), the transfer factor from contaminated cm22) for radionuclide i and T (h) is the exposure objects to the hands, the transfer factor from the duration. These skin-absorbed dose rates are given handstothemouth,thefrequencyofingestion(h21) in Radiation Protection 65 of Commission of the and the dose conversion coefficient for each radio- EuropeanCommunities(17). nuclide (Sv Bq21) as parameters. The dose conver- sion coefficients for inhalation and ingestion were E ¼A (cid:3)ðCF þCF Þ(cid:3)T: ð2Þ q;i i b;i g;i given in International Commission on Radiological Protection(ICRP)Publication68(15)forworkersand On the other hand, it can be considered that the inPublication72(16)forthepublic.Thedeterministic methodology for the dose assessment of dermal ab- approach was applied in the derivation of isotope- sorption is still under development, in contrast to specific surface clearance levels(6) following the the completed methodologies for the skin-absorbed stylised approach developed by IAEA(13, 14), and dose and internal exposure pathways (i.e. inhalation moreover the probabilistic approach was applied and ingestion), although the National Council on using a Monte Carlo calculation code to verify the Radiation Protection and Measurements (NCRP) validityof the deterministic approach(7). For further has recently developed abiokinetic model for radio- details of the assessment methods, see relevant nuclide-contaminated wounds and procedures for papersbyOginoandHattori(6–8). their assessment, dosimetryand treatment in Report Table1. Doseconversionfactorsforeachsurface-contaminatedobjects. Radionuclide Doseconversionfactors Manuallyhandledobjects Closelyhandledobjects Remotelyhandledobjects Ingestiona Skinb Externala Inhalationa Externala Inhalationa 131I 1.2(cid:3)1023 5.2(cid:3)1024 7.9(cid:3)1025 1.7(cid:3)1026 5.9(cid:3)1025 2.6(cid:3)1027 134Cs 1.9(cid:3)1024 1.7(cid:3)1021 1.5(cid:3)1023 5.7(cid:3)1026 7.3(cid:3)1024 8.6(cid:3)1027 137Cs 1.5(cid:3)1024 1.9(cid:3)1021 6.8(cid:3)1024 4.8(cid:3)1026 3.2(cid:3)1024 7.1(cid:3)1027 aEffectivedoseforexternalirradiation,ingestionandinhalation(mSvy21Bq21cm2). bSkin-absorbeddoserateforskincontamination(mGyy21Bq21cm2). 38 SCREENINGLEVELFORDECONTAMINATION No. 156(18). Considering the above, one is not con- results, it was found that the notification by the cernedherewiththeassessmentof theeffectivedose NSCthat100000cpmcorrespondstoadoserateof through dermal absorption in this paper, the aim of 1mSvh21atadistanceof10cmfromthesurfaceis which is to prove a reasonable starting point for the conditionally applicable, and that the dose rate verification of screening levels for decontamination becomeshigherthan1mSvh21whenthesurfacesof aftertheFukushimanuclearaccident. objects are contaminated overa large area as shown inFigure2. The annual dosesthat arisefromhandling objects contaminatedwith the screening level fordecontam- RESULTSANDDISCUSSION ination are shown in Table 3. The dominant Table 2 shows the set of surface contamination pathwayforcasesIandIIwasfoundtobeingestion densities that correspond to the screening level of from manually handled objects, and the annual ef- 100000 cpm. Figure 2 shows the calculated dose fective doses were calculated to be 0.50 and 0.44 rates at a distance of 10 cm from a radioactive mSv y21, respectively. Other pathways such asexter- surfacefordifferentareas.Itwasfoundthatthedose nal irradiation and inhalation from closely handled rate becomes higher than 1 mSv h21 when the area objectsandremotelyhandledobjectswerecalculated of the radioactive surface is .200 cm2 assuming to be lower than the ingestion from manually case Vand .800 cm2 assuming case I. From these handled objects. On the other hand, the dominant pathwayforcasesIII–Vwasfoundtobeexternalir- radiation from closely handled objects, and the Table 2. Surface densities for radioactive contamination annual effective doses were calculated to be 0.35, that correspond to the screening level for decontamination 0.51 and 0.53 mSv y21, respectively. Doses from (100000cpm)foreachcase. other pathways such as ingestion from manually handled objects and inhalation from closely and re- Caseno. Assumedratio Surfacedensityfor motely handled objects were calculated to be lower ofradionuclides radioactive thanthatforexternalirradiation. contamination (Bqcm22) Considering thepractical screeningunder high-ra- diation conditionsafterthe Fukushima nuclearacci- (131I:134Cs:137Cs) 131I 134Cs 137Cs dent, the effect of the counting loss due to the resolving time of the GM survey meter should be CaseI 100:1:1 410 4.1 4.1 takenintoaccount.Withregardtothetypesofradi- CaseII 10:1:1 360 36 36 ation considered in this paper, gross count rates are CaseIII 1:1:1 150 150 150 assumed to only consist of beta rays emitted from CaseIV 0.1:1:1 23 230 230 both radioiodines and radiocaesiums as shown in CaseV 0.01:1:1 2.4 240 240 equation (1), although gamma rays emitted from these radionuclides can also be measuredwith lower counting efficiency than that for beta rays. The countinglossisnolongernegligibleunderhigh-radi- ation conditions such aswhere the screening level of decontamination was raised to 100000 cpm owing to the relatively long resolving time of the order of 100 ms(19). By using a correction formula for the counting loss given by equation (3), where t (s) is the resolving time, m (count s21) is the measured count rate and n (count s21) is the true count rate (count s21), the factor for counting loss, defined as the ratio of the true count rate to the measured count rate, was calculated to be (cid:2)1.2, when the TGS-136GMsurveymeterindicatedacountrateof 100 000cpm and the resolving timewas assumed to be (cid:2)100 ms for the Aloka TGS-136 GM survey meter, which was typically applied in the practical screeningaftertheFukushimanuclearaccident(3). The above result implies that the surface contam- ination densities in the practical screening under high-radiation conditions where counting loss is no Figure 2. Dose rate at 10 cm distance from a radioactive longer negligible may be (cid:2)1.2 times higher than surfaceasafunctionofareaofsurfacecontamination. those shown in Table 2, and the factor for counting 39 H.OGINOETAL. Table3. Doseassessmentforhandlingsurface-contaminatedobjectswiththescreeninglevelfordecontamination(100000cpm). Caseno. Annualdose Manuallyhandledobjects Closelyhandledobjects Remotelyhandledobjects (0.1m2) (1m2) (10m2) Ingestiona Skinb Externala Inhalationa Externala Inhalationa CaseI 5.0(cid:3)1021 1.7(cid:3)100 4.1(cid:3)1022 7.6(cid:3)1024 2.8(cid:3)1022 1.1(cid:3)1024 CaseII 4.4(cid:3)1021 1.3(cid:3)101 1.1(cid:3)1021 9.9(cid:3)1024 5.8(cid:3)1022 1.5(cid:3)1024 CaseIII 2.4(cid:3)1021 5.6(cid:3)101 3.5(cid:3)1021 1.9(cid:3)1023 1.7(cid:3)1021 2.8(cid:3)1024 CaseIV 1.1(cid:3)1021 8.3(cid:3)101 5.1(cid:3)1021 2.4(cid:3)1023 2.4(cid:3)1021 3.6(cid:3)1024 CaseV 8.5(cid:3)1022 8.7(cid:3)101 5.3(cid:3)1021 2.5(cid:3)1023 2.5(cid:3)1021 3.8(cid:3)1024 aEffectivedoseforexternalirradiation,ingestionandinhalation(mSvy21). bSkin-absorbeddoseforskincontamination(mGyy21). loss can also be the same for the results of the dose calculated annual effective doses for all cases are assessmentshowninTable3.Evenwhenmultiplying lowerthantheinterventionexemptionlevel. the results shown in Table 3 bythe estimated count- The effective doses from ingestion via the domin- ing loss, it can be found that the annual doses that antpathwaysforcasesIandIIwerecalculatedtobe arise from handling objects contaminated with the 0.50 and 0.44 mSv y21, respectively. These effective screening level for decontamination are still lower doses correspond to equivalent doses for the thyroid than1mSvy21. of 10 and 8.8 mSv y21, respectively, considering the ingestioneffectivedosecoefficientof131Ifora1yold m child upon intake (1.8(cid:3)1027 Sv Bq21) given by n¼ : ð3Þ 1(cid:2)tm ICRPPublication72(16)andtheingestionequivalent dose coefficient of 131I for a 1 y old child upon The ICRP mentions in Publication 111(20) on the intake (3.6(cid:3)1026 Sv Bq21) given by ICRP protection of people living in long-term contami- Publication 67(24). These dose coefficients are given nated areas after a nuclear accident or a radiation for a 1 y old child because the dose conversion emergency that the reference level for the optimisa- factors for each of the surface contaminated objects tion of protection of people living in contaminated given in Table 2 were calculated for this critical age areas should be selected from the lower part of the inpreviousstudies(6–8). 1–20 mSv y21 band recommended in Publication The ICRP mentions in Publication 40(25) on 103(21) for the management of this category of generalguidanceondosevaluesfortheintroduction exposure situation, and that past experience has of countermeasures that the lower equivalent dose demonstratedthatatypicalvalueusedforconstrain- level for early- and intermediate-phase countermea- ing the optimisation process in long-term post- sures is 50 mSv y21 for individual organs preferen- accident situations is 1 mSv y21. The ICRP also tially irradiated and that the upper equivalent dose mentions in Publication 82(22) on the protection of levelis500mSvy21.Theequivalentdoselevelof50 the public in situations of prolonged radiation ex- mSv y21 for the thyroid recommended by the ICRP posure that an intervention in commodities is has been applied to the food safety regulations exempted if the additional annual dose is (cid:2)1 mSv implemented after the Fukushima nuclear acci- y21. Furthermore, the ICRP also mentions in dent(2).Consideringtheresultofdoseassessmentfor Publication 104(23) that exemption or exclusion for the thyroid from handled objects contaminatedwith naturallyoccurringradioactivematerial-basedindus- thesamelevelasthescreening levelfordecontamin- triescouldbehandledwithanindividualdosecriter- ation(i.e.10and8.8mSvy21),itwasfoundthatthe ion of (cid:2)1 mSv y21 excluding the dose from radon. calculated annual equivalent doses are lower than In this context, the additional annual dose of (cid:2)1 the lower equivalent dose levels forearly- and inter- mSv y21 can be considered as the intervention ex- mediate-phase countermeasures for individual emption level in existing exposure situations. organsrecommendedbyICRPPublication40(25). Considering the result of dose assessment for With regard to skin contamination, the annual handledobjectscontaminatedwith thesamelevelas absorbed dose that arises from handling surface- the screening level for decontamination after the contaminated objects was calculated to be 87 mGy Fukushima nuclear accident, it was found that the y21, as shown in Table 3, and the calculated 40 SCREENINGLEVELFORDECONTAMINATION Table4. Skin-absorbeddoseratefromdirectdepositiononthebodywiththescreeninglevelfordecontamination(100000cpm). Caseno. Skinabsorbeddoserate(mGyh21) 131I 134Cs 137Cs Total(131Iþ134Csþ137Cs) CaseI 9.9(cid:3)1021 7.9(cid:3)1023 1.1(cid:3)1022 1.0 CaseII 8.6(cid:3)1021 6.8(cid:3)1022 9.2(cid:3)1022 1.0 CaseIII 3.7(cid:3)1021 2.9(cid:3)1021 3.9(cid:3)1021 1.1 CaseIV 5.5(cid:3)1022 4.4(cid:3)1021 5.9(cid:3)1021 1.1 CaseV 5.8(cid:3)1023 4.6(cid:3)1021 6.2(cid:3)1021 1.1 can be considered to prevent the skin from the inci- dence of a deterministic effect such as erythema or epilation, because (cid:2)3000h ofirradiationisrequired to reach the minimum threshold (i.e. 3 Gy). Furthermore, the entire process of the turnover of the skin, from the birth of basal cells to the forma- tion of surface corneocytes and desquamation, is accomplished within 14–75 d in humans, during which continuous cell proliferation is required in the basallayertomaintainthetissue(27–30). Finally,althoughthecalculatedabsorbeddosesof the skin were found to be unlikely to reach the threshold of a deterministic effect on the skin in a practical exposure situation, countermeasures for preventing the contamination of the human body should be applied in advance for parts of the body that may be contaminated, and even parts of the body possibly contaminated below the screening level should be decontaminated to a level as low as reasonably achievable to ensure optimal radiation Figure 3. Skin-absorbed dose from direct deposition on protection. bodyasafunctionofexposureduration. CONCLUSION absorbeddoseratesofskinthatarisefromthedirect The screening level of 100000 cpm that has been deposition of radioiodine and radiocaesiums on the applied for decontamination of the surface of the body are shown in Table 4. Figure 3 shows the human bodyand handled objects implemented after absorbed dose of the skin from direct deposition on the Fukushima nuclear accident was verified by the body for different radioactive surface contamin- assessing the doses that arise from external irradi- ationdensities. ation, ingestion, inhalation and skin contamination. Thethresholdof adeterministiceffectontheskin The annual effective dose from handling contami- observed in a clinical studyon acute irradiation has nated objects was found to be lower than 1 mSv, been reported by the IAEA(26) to be 3–10 Gy for which can be considered as the intervention exemp- erythema, .3 Gy for epilation, 8–12 Gy for dry tion level. The screening level of 100000 cpm can desquamation, 15–20 Gy for moist desquamation, also prevent the skin from the incidence of a deter- 15–25 Gy for blister formation, .20 Gy for ulcer- ministic effect assuming a practical exposure ation within the skin and .25 Gy for necrosis duration. (deeperpenetration). Even assuming that a similar deterministic effect can be observed in the case where the time integra- FUNDING tion of the absorbed dose rate of the skin from chronic irradiation (e.g. 1.0 mGy h21) reaches the This work was supported by the Radiation Safety lowest threshold of acute irradiation (.3 Gy), the ResearchProjectoftheCentralResearchInstituteof implemented screening level for decontamination ElectricPowerIndustry(CRIEPI). 41 H.OGINOETAL. REFERENCES 15. International Commission on Radiological Protection. Dosecoefficientsforintakesofradionuclidesbyworkers. 1. Nuclear and Industrial Safety Agency of Japan. ICRP Publication 68. Ann. ICRP 24(4). Pergamon Evaluationofthestatusofreactorcoresinunits1–3of Press(1994). the Fukushima nuclear power plant, 6 June (2011) (in 16. International Commission on Radiological Protection. Japanese). 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