Experience gained during the first tritium experiment with relevance to the full D-T phase of JET

Fusion Engineering and Design 22 (1993) 77-84 Nurth-t lolland

Experience gained during the first tritium experiment with relevance to the full D - T phase of JET The JET Team JET Joint Undertaking. ,qhingdon, ().ton. OXI4 3I:',4, Utffted Kingdom

Handling Editor: G. Casini Submitted 15 September 1992, accepted I1) November 1992

In November 1991 a series of plasma discharges fuelled with a mixture of deuterium and tritium producing fusion power in excess of 1 MW were carried out in JET for the first time. The first tritium experiment (FTE) required careful prcpar~ttitm vnd the implementation of the experiment, the subsequent clean up phase and maintenance of the tokamak resulted in accumulation of relevant and valuable experience for the full D-T phase of JET. The FTE is briefly described together with the resulting implications for tokamak maintenance work and waste management during the present period of installation of new components inside the wtcuum vessel. The experience gained is summarised in the light of the technical requirements for the full D-T phase.

1. Introduction

Control of the influx of impurities into the plasma is one of the main problems that need to be solved for steady state plasma opcration. A potential method of achieving this is the operation with a pumped divertor [1] which forms the basis for the cxtcnsion of the J E T programmc from the end of 1992 to the end of 1996. The active phasc of J E T which was originally planned for 1992 will now be delayed to 1996. During this extension preparations will also be completed for processing of the tritium containing exhaust gases from the tokamak and subsystems and for making the tokamak and its auxiliary systems fully tritium compatible. In order to gain timely information on the operation of tritium fuelled plasmas including retention in wall material, opcration of diagnostics, radiation monitoring, and waste handling, it was decided in 1991 to prepare for the FTE which would involve the use of a small quantity of tritium. The lessons learned could then be taken into account in the final configuration of the systems for the full D - T phase of JET. Preparations for the FTE involved the obtaining of statutory approvals, the submission of a safety assessment, the design, installation, and commissioning of special equipment for the storage and introduction of tritium

and for the collection of tritiated exhaust gases from the vacuum vessel and the neutral injection systems. Further preparations involved the training of staff. The FTE consisted of two series of experiments, in the first series, 2 out of 16 neutral beam sources were fed with a mixture consisting of 1% tritium in deuterium. The remaining 14 sources were fed with pure deuterium. During the second phase the 1% mixture was replaced with nominally pure tritium. During and after the D - T plasma discharges of tritiated exhaust streams of the tokamak and neutral beam systems were collected by cryopumping and subsequent absorbing onto uranium beds (U-beds) [2]. Only small fractions of the collected exhaust streams could not be absorbed and were released through a stack if the residual activity was found to be sufficiently low or alternatively processed at an elevated temperature with subsequent absorption [3}. The bulk of the residual tritium which remained after the injection experiments in the vacuum vessel and neutral injection system was removed during the subsequent clean up phase [4]. This was followed by resuming normal experimental operation with deuterium plasmas. A few weeks after the FTE the tokamak exhaust could be reconnected to its normal mechanical backing pumps, whereas the neutral injection beam line used for tritium injection into the plasma required

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The JET Team / Experience gained during the first tritium experiment

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a much longer period of collection of the regenerated gases. This required that in addition to the two installed U-beds with a total capacity for hydrogen of approximately 1000 standard litres, a further pair of U-beds had to be integrated in the gas collection system. Some two months after resuming routine experimental operation, the beam line system could finally be reconnected to its conventional backing pumps. During the collection of the exhaust streams, samples were taken for tritium analysis and the quantities of recovered gas as well as the tritium content was measured by means of instrumentation built in to the gas collection system. Together with measurements and calculation of the injected amounts and the release through the stack, a detailed accountancy of the tritium was undertaken [4]. The FTE aimed at only a few short plasma discharges restricting the usage of tritium such that the resulting activation of the vacuum vessel would not increase significantly above the level resulting from routine deuterium operation, hence minimis-

ing the impact on the major shutdown which started at the end of February, 1992. Maintenance activities inside the vacuum vessel (and Neutral Beam system) had therefore for the first time in JET to be carried out in a tritium contaminated environment. This also had a direct impact on the management of waste arising from these areas. During the final D - T phase of JET, neutrons liberated by D - T fusion reactions will activate the structure of the materials to such an extent that any subsequent man entry into the vessel will be precluded. Maintenance of the tokamak and auxiliary systems inside the torus hall will then only be possible with remote handling means [5]. The amount of exhaust gases to be processed during the D - T phase is such that a dedicated processing plant, the Active Gas Handling System (AGHS) [6], will be required instead of the gas collection system devised for the FTE. However, key components of the AGHS were used in the design of the special equipment for the gas introduction and the gas collection

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Fig. I. Layout of gas collection system.

The JET Team / Experience gained during the first tritium experiment system used during the FTE including pressure regulators, valves, pressure gauges, ionisation chambers, Ubeds, etc.

2. Preparations for the VI'E The installation of the AGHS had not yet been completed when the FTE was conducted and thereforc special equipment had to be designed and commissioned for gas introduction as well as exhaust gas collection and for the storage of tritium.

2.1. Tritium introduction system Two ion sources (PINIs) of one of the two J E T neutral beam line systems were modified for tritium service in the experiment. The six remaining PINIs of the beam line system and the eight PINIs of the second neutral injector were left unmodified for deuterium operation. For the modified PINIs the gas was supplied from two U-beds (Mark 4 Amersham type), one loaded with deuterium for commissioning and one loaded with tritium for tritium injection. The U-beds and manifolds were cncloscd in a ventilated glove box. A detailed description is given in [7]. By heating the U-beds absorbed gas is desorbed and can be expanded into a volume. During the pulse, tritium is fed between extraction grid number 4 and the first stage neutraliser at ground potential allowing an all metal supply system, without electrical breaks. Either one or two PINIs could be used. The amount of gas used during the injection pulse is derived from the pressure decrease in the expansion volume (in communication with the Ubeds) during the pulse. Thc accuracy of this measurement is dependent upon the desorption characteristics during the pulse when the pressure in the U-bed decreases. This configuration requires that during the experiment the bed temperature remains elevated in order to provide gas pressure. This leads to permeation of tritium through the hot U-bed walls.

2.2. Gas collection system The gas collection system designed for the FTE which is schematically shown in Fig. 1 is connected via valves with the vacuum pumping ducts of the torus and the neutral injection systems. Details of the equipment and its operation during the FTE is given in [3] and the performance of the ionisation chambers is described in [8]. The system comprises an inlet manifold which contains various diagnostic elements such as ionisation

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chamber, mass spectromcter, sampling manifold and pressure gauge. Connected via a valve with the manifold is a tubular liquid helium cooled cryopump to condense exhausted gases. This cryopump contains a small amount of activated charcoal for cryosorption of helium. By regeneration of the cryopump gases can be expanded into a reservoir where again the tritium content (ionisation chamber) as well as the pressure can be measured. This is followed by absorption of the hydrogen species on a U-bed. Two U-beds were originally installed with a total absorption capacity of approx. 1000 standard litres of gas. Residual gases which could not be absorbed could then be pumped via a rotary pump out of the reservoir and discharged via a stack which was monitored for tritium, provided that the residual tritium activity did not exceed the daily discharge limit.

2.3. Storage Four Amersham Mark 4 type U-beds were activated at JET and their discharge characteristics were established. Tritium was loaded onto two U-beds by a commercial tritium supplier. The first one was pre-loaded at JET with deuterium to which 1.85 TBq (50 Ci) of tritium was added giving an overall 1% tritium mixture. The second bed was loaded with 88.8 TBq (2400 Ci) of tritium. The other two U-beds were loaded with deuterium for tests. A secure storage facility for tritium which includes an intruder alarm and tritium monitor, both hardwired to a permanently manned site incident desk was prepared at JET for storage of the tritium before and after the FTE.

2.4. Procedures Operation of all tritium handling plants was done according to formal operation procedures which were approved through the JET management control system. Formal meetings were held at daily intervals to discuss progress and in particular problems. Any minor deviation from approved procedures required formal approval. Significant changes, eg the addition of a pair of U-beds after the PTE, were separately safety assessed and endorsed by the Fusion Safety Committee.

2.5. Safety aspects Prior to the FTE, JET had been in operation with hydrogen and deuterium plasmas since 1983 and the necessary approvals for such operation, including that

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The JET Team / Experience gained during the first tritium erperiment

for the discharge of tritium produced by fusion reactions, had been in place for some time. Tritium was processed and handled in bulk quantities for the first time during the experiment which meant that safety arrangements and approvals required considcrable revision. Details of safety aspects and approvals of the FTE are given in [9]. These included: (i) Radioactive Discharge Authorisations, (ii) Safety justification for the tritium handling systems and torus, (iii) Emergency arrangements, (iv) Training. It was only possible to make all these arrangements and to obtain the relevant approvals within the required short timescale because JET had kept the relevant authorities fully informed of plans for D - T operation and had made significant progress on obtaining approval for the final phase of the project. The approvals required are summarised below. Nuclear sites in the UK, such as reactors and reprocessing plants, are regulated by the UK Nuclear Installations Inspectorate (Nil) which is part of the Health and Safety Executive (HSE). The Nil have decided that JET is not required to be licensed by them but would be subject to the normal regulations governing the use of radioactive materials. This is consistent with the situation in the tritium light source industry which handles comparable amounts of tritium (several tens of grams on one site). JET is therefore legally required to be registered by Her Majesty's lnspectorate of Pollution (HMIP) to keep or use tritium and other radioactive substances. The storage and disposal of tritiated and other radioactive wastes is also required to be authorised by HMIP. In addition, JET has a duty under the JET Statutes to satisfy the UKAEA (on whose site JET has been constructed) in advance of any radioactive operation taking place, that the arrangements conform to the UKAEA standards. As the radioactive operations of the UKAEA are licensed by the NII, this means that in effect JET must conform to the standards required under a license. A consequence of this was the establishment of a Fusion Safety Committee to examine the safety of proposed operations. This committee was analogous to Nuclear Safety committees on sites licensed under the Nuclear Installations Act and included several members from outside the JET project. Formal approval to start operation with tritium was given by the Safety and Reliability Directorate (SRD) of the UKAEA.

In addition to the above formal approvals, the Ionising Radiations Regulations required JET to meet certain specific requirements which included limits for occupational and public exposure, dosimetry service approval, agreement on measurement of losses and contingency planning.

2.5.1. Radioactil'e d&charge authorisations Prior to the experiment, JET had been negotiating with HMIP for a number of years to obtain the level of authorisation appropriate for the full D - T phase and this was grantcd in mid 1991. The likely maximum level of routine discharges of tritium, activated air and dust to atmosphere and tritium and activation products to the river Thames wcrc calculated on the basis of 5 x 1022 neutrons per year using up to 90 g of tritium with 30 g in daily circulation. These levels wcrc justified by showing that the dose to the most exposed individual off-site ( < 20 ~zSv/year) was well within the absolute limit of 0.5 mSv/year imposed by by HMIP and the agreed limit for the project of 50 ~Sv/year. However the authorisations impose an overriding requirement that Best Practical Means (BPM) must be used to limit the environmental impaci of the operations. This had several implications, the most important being that even though the Authorisation would permit discharge of the full amount of tritium used for the experiment, BPM required that options other than direct discharge had to be considered and led to the requirement for collection of as much of the tritium as possible using the technology of cryogenic pumping and absorption of hydrogen isotopes on U-beds. A review of the likely performance of the gas collection system and the evolution of tritium from the torus indicated that a daily discharge of less than 10 GBq should be achievable. This value was set as a management target above which a special BPM case would need to be made. Although the evolution of tritium from the machine was somewhat different to that estimated before the experiment in that clean-up of the neutral injection system took much longer than expected, it was found to be a reasonable operational limit.It was exceeded only by a small margin on a few occasions when there was significant unprocessable gas. 2.5.2. Safety jt~tification for the tritium handling systems and torus The requirements for safety justification had been established a number of years previously in consultation with SRD. As the methodology and assessment

The JET Team / Experience gained during the first tritium ~periment standards had been agreed for the level of risk posed by operation in full D - T and many design features of tritium processing systems had already been accepted by SRD, it was possible to produce the safety justification for the experiment on a relatively short timescale of six months. The radiation dose to persons on and off the site which would result from the accidental release of radioactive materials had also been calculated for the full D - T case. For a release of tritium as HTO from the top of the torus building under Class F weather conditions, the maximum dose on-site was 12 p.Sv/TBq and off-site 0.6 p.Sv/TBq. The off-site dose calculations have been recently confirmed independently by Ontario Hydro, Canada,using the E T M O D code and been extended to include dose from HT (0.003 p.Sv/TBq). Similar calculations were carried out for gaseous and solid activation products although the source term and consequences were much lower than for tritium. Staged safety submissions were made to ensure that potential delays were identified early. In particular a Preliminary Safety Report was prepared which specified the design features and operational limits so that agreement in principle could be obtained from the Fusion Safety Committee and SRD on key issues such as the form of containment, operating pressures and inventory limits. This was then justified through a Safety Analysis Report (SAR) which was accepted in August 1991. A number of actions on J E T arose from this report, one of which was to revise the hazard assessment to include more explicit consideration of certain hazards including external events. On completion of these, and a thorough review of operational safety aspects (operational procedures, waste handling facilities etc), SRD endorsed the experiment in October 1991. The SAR had three main functions: to describe the final design configuration for the machine and ancillaries; to specify the accident sequences which could lead to a release of tritium (this having been identified in the preliminary analysis as the major hazard) and perform a probabilistic analysis; and to compare the public and worker risks against standards. The SAR also specified which machine sub-systems used in the D - D phase were not suitable for use with tritium and should be isolated. For example, diagnostics which discharge into the torus hall; radiofrequency heating systems containing SF6 which could poison catalysts in the ED system; and systems such as the pellet injector in which tritium contamination could cause maintenance difficulties. Faults on the J E T machine such as vacuum leaks

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were reviewed and the steps taken to mitigate against their effect were discussed. From this and the other accident sequences identified from the hazard assessment, a number of accident sequences, which required fault tree assessment to demonstrate their acceptability, were selected. These events included failure of PINI insulator or vacuum windows, overheating of U-beds and leakage from tritium containment systems. The criterion for the acceptability of the design for JET tritium systems agreed with SRD is that for any single accident sequence, the product of the frequency and the amount released is less than 0.37 TBq/year. This has been applied through the design safety review of the AGHS as a means of ensuring that the overall risk targets are met. The A EA standard for public risk is that the total risk of premature death to the member of the public most at risk from all fault sequences on the whole site should not exceed 10 -~' per year and the risk from any particular fault sequence should not exceed 10 -v per year. If the products of release and frequency are summed and multiplied by the dose per unit release (0.59 p~Sv/TBq) and by the risk coefficient (3 x 10 -2 per Sv), a value of 1.4 x 10 s per year is obtained. Even with the pessimism arising from the use of nonaverage conditions and hypothetical members of the public, this is well within the above target. The above criterion of 0.37 T B q / y e a r corresponds to a risk from any accident sequence of 7 x 10 '~ per year thus ensuring that the individual accident sequence risk target above is complied with. 2.5.3. Emergency arrangements The Ionising Radiations Regulations require contingency plans to be established to minimise the radiation exposure of employees and members of the public in the event of an accident. J E T has set up an Emergency Plan which forms a framework for the various procedures, facilities and communications which are necessary. For the full D - T phase, the plan is capable of being extended to cope with the assessment and communications necessary to determination the need for and implementation of off-site restrictions in the event of a large accidental release. The quantity of tritium used for the experiment was 0.25 g (about 90 TBq) which, if it were all released as HTO under adverse weather conditions would lead to maximum short term doses of about 1 mSv on-site and about 50 ~Sv off-site. The emergency arrangements were therefore concerned with the protection of on-site personnel. These included a trained Incident Response

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The JET Team / Experience gained during the .first tritium experiment

Team whose action was initiated through a new Incident Desk in the control room: a room specifically designated for assessment of the incident: and a series of Site Incident Procedures. These specified, amongst other things, the action to be taken in the event of failures of tritium containments and fire in radiation controlled areas. Two emergency exercises were held prior to the experiment. JE T was obliged to demonstrate that its staff had received adequate training in all aspects which were necessary for safe operation of the experiment. An auditable training programme had been in operation for a number of years and this was extended to deal with the new requirements which were in three forms: (i) General training on tritium, its properties, methods of detection and emergency response was given to all staff affected by the FTE. (ii) Specific training for operators of tritium plant. Although detailed written procedures were provided for this plant and tested out inactively prior to the experiment, it was essential that with full shift working, there were sufficient personnel to be able to deal with abnormal events. (iii) Training for those with specific emergency duties.

ered after phase I. Of the calculated amount of tritium injected during phase 2 approximately 27% was collected in the torus exhaust stream within approximately 1.5 hours of the experiment and the bulk of the tritium deposited in the neutral injection system was recovered when the injector cryopump was regenerated a few hours after the second tritium plasma discharge.

4. Clean up of tokamak and neutral beam system A few days after the injection experiment operation was resumed aimed at releasing as much as possible of the remaining torus and neutral beam system tritium inventory for collection by the gas collection system. 4. I. Clean up of tokamak The release of tritium from the torus dropped rapidly from initially 2.2 × 1() ~ Bq to approximately 10 '~ Bq per pulse after 15 pulses. Several other techniques were tried, however the release kept falling and the Torus was reconnected to its mechanical backing pumps some 3 weeks after the start of the clean up phase. Details are give by G. Saibene et al. [6].

3. D - T discharges

4.2. Clean up o f neutral beam system

D - T plasma discharges were carried out in two distinct phases. Phase 1 covered a two day period during which a total of nine plasma charges with two neutral injection sources injecting beams consisting of 1% tritium in deuterium were carried out. This allowed testing of all systems including the recovery of tritiated gases and their measurements. During the second phase covering one day, the two converted PINIs were fed with 100°~ tritium. Two plasma discharges with two PINIs injecting tritium were carried out. Both discharges resulted in a peak D - T fusion power in the range of 1.6 MW. During a few preliminary conditioning pulses and the two injection pulses the U-bed supplying tritium remained hot. This gave rise to laermeation of 16 GBq tritium through the hot wall of the U-bed thereby requiring a BPM justification for exceeding the J E T imposed daily discharge limit of 10 GBq. During the 1% and 100% experiments 0.925 and 36.18 TBq of tritium were discharged from the U-beds. Regeneration of the cryopumps in the neutral beam line system together with the gas recovered after the tritium injection pulses showed that within the error of measurement nearly all of the injected tritium had been recov-

Most of the tritium remaining in the neutral beam system was thought to be implanted in the beam stopping elements. The release of tritium from the elements was attempted by impinging these with deuterium beams. The efficiency of this technique could be monitored by measuring the neutron detector signal from the resulting fusion reactions. This showed that this was highly successful in reducing the implanted tritium to a small fraction of the initial amount. Some implanted tritium would only be removed by beams hitting the element during a simultaneous plasma discharge as the torus magnetic stray field influences the trajectory of the ion beams. The amount of tritium released during successive regenerations remained however at such a level that the tritium had to be absorbed onto the U-beds of the gas collection system for a much longer time than had originally been anticipated. At present it cannot be concluded with certainty whether this long decontamination period is due to a small release from the blackened (anodised) liquid nitrogen cooled cryopanels which are known to have a water inventory of several hundred grammes or to slow release of implanted tritium from beam stopping elements. There are indications however, that the latter

The JET Team / Experience gained during the first tritium experiment may be the case [9,11]. The continued release of tritium required installation of two additional U-beds when the first two U-beds became fully charged with approx. 1000 bar l of hydrogen species. A special procedure was written for this and implemented after formal approval had been obtained. Connecting into tritium contaminated pipe work was achieved without any measurable release of tritium into the secondary containment. Transfer of the absorbed gas loads from the first two U-beds onto the new ones in relatively large batches allowed a more accurate measurement of tritium content than had been possible for the large number of small amounts of gas of varying batch size and gas composition, thereby obtaining an improvement in the overall accountancy accuracy. It took approximately two months before the neutral injection system used for tritium injection could be reconnected to its normal backing pumps.

4. 3. Impurity processing A few days after the start of the clean up phase the total activity contained in residual gas which could not be adsorbed by the U-beds exceeded JETs own imposed daily discharge limit. As it was expected that the majority of the activity would be due to tritiated hydrocarbons produced by the interaction of plasma with carbon wall materials, attempts were made to crack these on a hot U-bed. One U-bed was slowly heated up to 500°C whilst continuously circulating residual gas from the expansion reservoir through the hot U-bed followed by purging the cold U-bed. When the temperature exceeded 450°C the residual activity measured by the ionisation chamber in the reservoir started to decrease and a very low level remained when 500°C was reached. It is assumed that tritiated hydrocarbons reacted with the hot uranium powder to form uranium carbide and that the released hydrogen species absorbed on the second cold U-bed. It should be noted that similar U-beds will be used in the AGHS for impurity processing.

5. Implications for the divertor shutdown Personnel access was gained into the vacuum vessel at the beginning of March 1992 at the start of the shutdown operations. These involved complete removal of all internal vessel components followed by cleaning of the inner shell by a wet grit blasting technique [12] before commencing installation of the divertor coils and finally the other internal components.

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Initially measured tritium and air concentration in the (ventilated) vacuum vessel ~vas 86 B q / m 3 which decreased to approximately 10 B q / m 3 after 20 weeks of shutdown work when the in-vessel cleaning started. A few weeks after the vessel cleaning the level dropped to approximately 1 B q / m 3 and the surface contamination generally to below level of detection. Up to and including the in-vessel cleaning campaign, in-vessel operators had to wear full air line suits due to the presence of beryllium. The tritium levels did not require this, although the suits did of course limit the exposure of personnel by a large amount and consequently urine sampling did not result in tritium dose rate credited to any in-vessel operator. A detailed account of the Health Physics aspects is given in [13]. The tritium level of components removed from the vacuum vessel and surface outgassing was generally such that a contaminated component had to be stored in areas connected to a ventilation system discharging through a monitored stack. For interim storage machine components are stored in insulated ISO containers which are connected to the stack of the AGHS which is fitted with on-line tritium monitoring as well as samplers. Possibly up to 50% of material stored in the ISO containers will be eventually declared waste. Secondary wastes are kept in intermediate storage pending sorting, sampling, compacting and disposal via a waste sorting facility [14]. Several batches of 200 l drums of compacted low level waste (LLW) have been despatched from JET for disposal in the UK LLW repository. Arrangements and procedures for waste disposal are covered by a series of QA documents which fulfil the guideqines issued by the operator of the UK LLW repository.

5.1. Future plans for tritium operations The present JET programme includes an active phase during 1996. Prior to the full D - T phase, a short shutdown is planned to make the last preparations for tritium operation. It should be noted that the envisaged D - T neutron production rate of 5 × 10 22 per year precludes man entry into the torus hall for maintenance work not long after the start of this phase. All maintenance operations will then have to be undertaken by remote handling means. The total envisaged tritium site inventory is 90 g, ie, a factor of 360 higher than for the FTE. Emergency procedures may therefore have to be extended to cover off-site actions. In order to gain further experience with D - T plasmas prior to the full D - T phase, the present planning foresees a second restricted tritium experiment possi-

84

The JET Team / Experience gained during t/w first tritium e.rperiment

bly involving the introduction of up to 10 timcs more tritium than during the F F E . This c x p e r i m e n t would require the use of the A G H S and therefore provide the o p p o r t u n i t y for tritium commissioning of the A G H S in conjunction with the t o k a m a k and auxiliary systcms. T h e experience gained during the F T E suggests that this may be highly desirable.

6. Conclusions T h e F T E has provided a test bed for several key A G H S c o m p o n e n t s and instruments. It required thc o b t a i n i n g of statutory and o t h e r approvals similar to those required for the full D - T phase albeit at a smaller scale. T h e p r e p a r a t i o n of a S A R and its end o r s e m e n t using the same m e t h o d o l o g y and risk standard as for latter tritium e x p e r i m e n t s was a very useful exercise. E m e r g e n c y p r o c e d u r e s were written and may be e x t e n d e d for increased site inventory. T r a i n i n g of staff for these o p e r a t i o n s showed that the effort required was considerably larger than had originally been anticipated. All tritium h a n d l i n g was d o n e in accordance with approved p r o c e d u r e s u n d e r a special mana g e m e n t structure. T h e use of tritium on the J E T site required extension of the radiological protection ins t r u m e n t a t i o n and the setting up of access controlled at surveyed areas including a secure store. A waste sorting facility was b r o u g h t into o p e r a t i o n for the shutdown following the F T E and an extensive Q A prog r a m m e covering all aspects of waste m a n a g e m e n t at J E T and disposal from the site is in operation. Radiological surveys carried out have confirmed the need for additional shielding in certain areas as well as for the r e p l a c e m e n t of air with a low activation gas as

heat t r a n s f e r m e d i u m in thc torus baking plant. Important results were o b t a i n e d on the clean up of the t o k a m a k and neutral b e a m systems.

References [1] M. Huguet et al., JET Status and Prospects, in: Proc. 14th Syrup. on Fusion Engineering, San Diego, USA, 30 Sept-3 Oct 1991. [2] M. Huguet el al., Fusion Engrg. Des., to appear. [3] J.L. Hemmerich el al., Fusion Engrg. Des.. to appear. [4] G. Saibene el al., Fusion Engrg. Des., to appear. [5] T. Raimondi, The JET experience with remote handling equipment and future prospects, in: Proc. 15th SOFT. Utrecht, 1988 (Elsevier, Amsterdam, 1989). [6] R. Haange et al., Status and prospects of JET tritium operation, Fusion Technol. 21 (1991) 253-256. [7] L. Svensson et al., The gas introduction system used for tritium neutral beam injection into JET, in: Proc. SOFT17, Rome, 1992 (Elsevier, Amsterdam, 1993)pp. 1231). [8] C. CaldwelI-Nichols el al., The design and performance of the JET ionisation chambers for use with tritium, in: Proc. SOFT-17, Rome, 1992 (Elsevier, Amsterdam, 1993) pp. 1142. [9] A.C. Bell el al., Fusion Engrg. Des., to appear. [10] H.D. Falter et al., Hydrogen isotope exchange in the JET neutral beam injection system, in: Proc. SOFT-17, Rome, 1992 (Elsevier, Amsterdam, 1993) pp. 481. [11] W. Obert et al., Regeneration and tritium recovery from the large JET neutral injection cryopump system after the FTE, ibid. pp. 1191. [12] S.M. Scott et al., Decontamination of the JET w~cuum vessel from beryllium and tritium, ibid. pp. 1216. [13] R.M. Russ et al., Health physics and environmental implications of JET's FI'E, ibid. pp. 1769. [14] S.J. Booth et al., Radwaste management at JET, ibid. pp. 1690.