Status of Safety Analyses for the ESS Target

Status of safety analyses for the ESS target Rainer Moormann, PES-FZJ

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SAFERIB I - CERN/Geneva30.10. - 01.11.2002

Status of Safety Analyses for the ESS Target

R.Moormann, R.Bongartz, W.Kühnlein, J.Marx, H.Schaal, K.Verfondern (FZJ)

P.Berkvens (ESRF), P.Wright (RAL)


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Artist‘s view of the ESS Facility


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Main parameters


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Scheme of ESS target and of mercury circuit (15 t Hg, max. operation temperature: 250°C, max. temperature increase by decay heat: 35 °C)yellow =target block, red = mercury

Proton beam


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-The target is the only component in Spallation Sources, which requires detailed accident analyses: accidents, harming the public, are imaginable only for the target due to its substantial radiotoxic and chemical toxic inventory. Other components (accelerators etc.) need safety considerations for normal operation/- abnormal events only.

The preliminary safety analysis report (PSAR including updates) for the SNS target is under review with respect to its applicability for ESS


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Short description of PSAR Procedure of PSAR

(preliminary safety analysis report) :- screening of possible events- rough estimation of consequences and

frequencies (limited credit on safety measures = ‘unmitigated’ estimations)

- identification of relevant accidents by ‘risk

binning’; (risk = dose * frequency)

- cut off criteria: low frequencies (< 1.e-6/a) and low consequences (< 1 mSv)


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- more detailed estimation of source term and consequences for relevant accidents (‘unmitigated’ estimations)

- in case of transgression of dose limits:repetition of source term/consequence/- frequency estimations assuming additional ‘mitigating’ safety measures

- stepwise increase of accuracy, until dose limits are met (advantage: reduction of effort, identification of really important safety measures)


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Relevant accidents identified:• Fire within the target building• Hydrogen/moderator explosion with and without

subsequent fire• Loss of confinement (Leak in the target circuit)

– [Loss of mercury flow]– [Loss of heat sink]

• [Target hull heat-up by misadjusted proton beam]• Earthquake followed by explosion with and

without following fire• Crane drop accident


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Safety work at ESS

Main results of PSAR/SNS review are:

- PSAR/SNS is a valuable basis for ESS safety work

- Additional accident to be considered in case of a Be-reflector: Be-reflector fire

- European conditions require a more detailed safety analysis than performed within PSAR/SNS for the following reasons:


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• larger ESS radioactive inventory• smaller distance ESS – receptor• conservative consequence model for ESS

required, not a realistic one as for SNS• consideration of ingestion pathway for ESS

accidents required (contrast to SNS)• lower dba dose limits in EU:

effective dose/dba: 50 mSv in D 250 mSv for SNS


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Safety relevant nuclides in mercury target:

nuclide boiling point [K]

ESS target inventory

[GBq]

half life [d]

radiation type

dose/emission 1) ground cloud inhalation ingestion shine (γ) [ / ]Sv GBq

H-3 ( )HTO 373 2.3e-9 3) 4.6e-8 2) H-3 ( )HT 14

5.6 5 e( )total

4500 weak β 0 0 1.5e-11 3,4) 3.3e-9 4)

I-124 387 3100 4.2 β,γ 2.8e-6 1.0e-8 3.0e-5 3.4e-3 I-125 „ 14000 60 γ 1.8e-6 1.0e-10 1.8e-5 2.3e-3 I-126 „ 630 13 β,γ 3.9e-6 4.2e-9 6.3e-5 7.6e-3

Hg-193 629 1.9 6e 0.16 γ 3.6e-8 1.7e-9 6.5e-9 2.5e-10 Hg-194 „ 2.1 5e 1.9 5e γ 6.1e-3 1.3e-13 1.3e-6 5.0e-7 Hg-195 „ 3.2 6e 0.42 γ 7.5e-8 1.7e-9 8.1e-9 6.3e-10 Hg-197 „ 2.2 7e 2.67 γ 1.1e-7 5.2e-10 2.0e-8 3.0e-9 Hg-203 „ 1.5 7e 47 β, γ 7.5e-6 2.2e-9 1.8e-7 8.1e-9 Gd-148 3546 3.5 4e 2.72 4e α 0 0 2.2e-3 4.0e-7 Hf-172 4875 7.3 5e 683 γ 2.8e-4 7.7e-10 1.3e-5 1.9e-9 Au-195 3081 4.2 6e 186 γ 3.1e-6 6.2e-10 2.2e-8 5.9e-10

1) , (70 ), German directives for design basis accidents infant a continuous ingestion and ground shine effective doses (except for iodine ), : 250 , : 25 , thyroid minimum distance m emission height m2) preliminary ingestion model of , German rules3) ( ), including HTO resorption by skin preliminary model4)HT- oxidation to HTO in soil considered

: = , = Volatility classes green high volatile blue me ( ), = rcury intermediate volatility yellow low volatile

Overview on radiologically most relevant nuclides in the ESS-target (5 , 30 )MW y continuous operation


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Note: Long lived actinides are not found in spallation sources

Comparison of activities in LWR and ESS


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Conclusion on comparison of inventories:• Determination of spallation yield for long-lived

nuclides (Hg-194, Gd-148) is of particular relevance: ESS will perform evaluation of Hg irradiation experiments of ORNL, performed at SINQ

• Even for individual short lived nuclides the spallation yield is fairly uncertain (which is compensated by the large number of nuclides). Accordingly, short term Hg irradiation experiments are advisable


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Comparison of radiotoxicity and chemical toxicity of mercury

• Comparison is problematic, because only incorporation effects are comparable

• Comparison for toxicity by inhalation between the total inactive Hg-inventory and the inventory of one individual nuclide (Hg-197) reveals, that the chemical toxicity is about a factor of 10 - 30 higher than the radiotoxicity of Hg-197


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• Nevertheless, due to strong ground shine (Hg-194) and due to the large number of other radiotoxic nuclides, we expect, that radiotoxicity dominates the overall toxicity.Chemical toxicity may however not be neglected.


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Loss of confinement accidents1. Probabilistic and system studies

No Released media

Released activity

Probability / dose limit (mSv)

Unmitigated dose (mSv)

Counter measures

Remarks

Large failure of target confinement, release of Hg and cooling water into core container at operating temperature; release into environment via vacuum system, off gas system and stack

3.1-4

Hg, cooling water

H-3, Hg-,I-isotopes

SNS: U/50 ESS: A/0.3 per path

SNS: 3.2 ESS: > Limit

HEPA-Filter H-3-filter ?

Large failure in target circuit, release of Hg into target cell at operating te mperature; release into environment via Primary Containment Exhaust System (PCE) and stack

3.1-7

Hg “ SNS: A/5 ESS: A/0.3 per path

SNS: 2.2 ESS: >Limit

HEPA-Filter Hg-Adsorber

“

Failure of Hg-flow, local boiling, Hg- and cooling water release into core vessel via leak generated by proton beam, partial evaporation of Hg, release into environment via overpressure valve and stack

3.1-12

Hg, cooling water

“ SNS: A/5 ESS: A/0.3 per path

SNS: 200 ESS: >> Limit

Beam shutdown

Time for beam shutdown?

Failure of target heat sink, Hg- and cooling water release into core vessel via leak generated by proton beam, partial evaporation of Hg, release into environment via overpressure valve and stack

3.1-13

“ “ SNS: A/5 ESS: A/0.3 per path

SNS: > 250 ESS: >> Limit

Beam shutdown

Time for beam shutdown?

Leak in cooling water system (target shroud), release via target cell or core container and stack into environment

3.3-1

Cooling water

H-3 SNS: A/5 ESS: A/0.3 per path

SNS: 0.05 ESS: > Limit

H-3 filter in –PCE?

HT, HTO ?

Leak in reflector cooling system, release of cooling water into auxiliary system building, release into environment via stack

3.3-12

D2O H-3 SNS: A/5 ESS: A/0.3 per path

SNS: 0.22 ESS: > Limit|

H-3 filter in SCE?

“

Relevant ESS dba

A = anticipated within ESS lifetimeU = unlikely


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2. Deterministic examinations• Hg/H-3 thermochemistry has to be examined • JSNS work indicated, that mercury iodides (lower volatility than I2) may be formed:

- PSAR/SNS assumes a pronounced iodine release in loss of confinement accidents, because the chemical status of iodine is considered as elemental (high volatile). Resulting emissions are too high for ESS conditions.- Thermochemical calculations (using ORNL-code SOLGASMIX-PV) at ESS quantified the chemical composition. Validation will be performed.


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Equilibrium composition of an Hg/I and of an Hg/I/Fe system

Results of SOLGASMIX-PV


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Additional target and moderator safety work

Other accident classes in examination for ESS (probabilistic, system, source term and dose studies) are:

• External events (earthquake, fire, airplane crash..)

• Internal explosions


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• CFD (CFX-Code): calculations on distribution of moderator within target cell/- building in accidents (burnable gas mixtures etc.):

Leak in H2-pipe with spontaneous evaporation

1.5 kg of H2 released in 30 s

Air ingress with 0.15 kg/s at 288 K

• CFD-calculations on particle (Hg-droplets) transport in case of accidents

Examinations on moderator explosions


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General ESS safety work• Decommissioning, final waste disposal

• Activity transport with ground water flow

• Consequence estimations for accidents using COSYMA model (planned)

• Selected design work for safety systems

• Shielding philosophy and design

Documents

Status of Safety Analyses for the ESS Target