The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

PRESENT STATUS OF NEXT TERM PROJECT IN KURRI
S. Shiroya, A. Yamamoto 1, H. Moriyama, H. Unesak:i, M. Inoue, T. Misawa andY. Kawase,KURRI, Kumatori-cho, Sennnan-gun, Osaka 590-0494, Japan

Abstract
A final goal of the next tenn project in Kyoto
University Research Reactor Institute (KURRI) is toestablish a new neutron source of accelerator driven
subcritical reactor (ADSR) as a substitute for the current
research reactor KUR. A ring cyclotron has become amain candidate of an accelerator system. A conceptual
design study has been carried out to examine the nuclear
features of a research reactor type ADSR. In addition, abasic experimental study on the ADSR was initiated byusing the Kyoto University Critical Assembly (KUCA).
From these studies, it became very clear that the nuclear
characteristics of ADSR are essentially detennined by theneutron multiplication process in the subcritical reactor.

1 INTRODUCTION
A final goal of the future plan in KURRI proposed in
1996 is to establish a new neutron source of ADSR as a
substitute for the current research reactor KUR of 5 MW
to promote the joint use program among Japanese universities [1]. Now, a ring cyclotron system has becomes amain candidate for this plan in consideration of the construction cost and the space in the KURRI site. The
future plan can be divided into two phases. The firstphase is the introduction of a 70 MeV ring cyclotron forthe basic study on the ADSR, which wiii be carried out incombination with the KUCA. Based on the fruits of thefirst phase, the second phase will be the introduction of a500 MeV ring cyclotron for the construction of theADSR as a neutron source for the joint use program.
A conceptual design study has intensively been carriedout to examine the nuclear features of research reactor
type ADSR [2-6] by using the MCNPX code [7]. Moreover, a basic experimental study on the ADSR was also
initiated fonnally in 2000 by using the KUCA, where asubcritical nuclear fuel assembly can be driven by 14
MeV neutrons generated from a Cockcroft-Walton type
accelerator installed in the KUCA building.

be". This report mainly dealt with the KUR, since it isthe largest among university reactors in Japan [6].
For the future of KURRl, this report concluded asfollows: (1) It is reasonable to continue the operation ofKUR for a while by reducing uranium enrichment of fuel.
(2) It is advisable to promote the present study on thenew neutron source by introducing a proper accelerator
for the KUCA. (3) Since a considerable time is generally
needed for the complete shut down procedure of nuclear
reactor, it is desirable to initiate the preparation of thiskind for the KUR with looking at the status in researchfrom a medium- and long-tenn point of view. In conclusion, this report fonnally approved the first phase of thefuture plan to be adequate as a next tenn project inKURRI. However, the second phase is stiii open, sincethe High Intensity Proton Accelerator Project proposed
jointly by the Japan Atomic Energy Research Institute(JAERI) and the High Energy Accelerator Research Organization (KEK) has already been initiated in Japan.

Table 1: Sspect'fitcatton of70 MeVR'mg C;ycIotron
Ion Source
TypeMulticusp Ion SourcelOrnA
Maxtmum Current
Extraction Energy50keVRFQInjection Energy
Output EnergyOperation Mode
Average Beam Current
Frequency
RF Power
Total Length
Rine Cyclotron
K Value
Injection Radius
Extraction RadiusInjection Energy
Output Energy
Maximum Beam Current
Number of Sectors
Magnet Weight
Frequency
RFPower

2 PRESENT SITUATIONAt the end of the last year, a scientific advisorycommittee of the Ministry of Education, Science, Sports
and Culture {Monbusho) in Japan completed a report
entitled "what the research reactor of university should

50keV
400keV

cw

5mA
50MHz
lOOkW
2m
70
0.31 m
3.42m
400keV
70MeV
2mA
4720ton
50MHz
200kW

In response to this report, KURRl settled the following
policies: (1) The operation of KUR will be continued fora while; the highly enriched uranium (HEU) fuel will be
used until March 2006, and the low enriched uranium

1 Guest Assoc. Prof. during Oct. 1999- Sept. 2000. Present affiliation and address: Kumatori Works, Nuclear Fuel Industries, Ltd.;950 Ohaza-Noda, Kumatori-cho, Sennan-gun, Osaka 590-0481, Japan.

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(LEU) fuel after April 2006. (2) The preparation will be
initiated from now for the coming discontinuation of theKUR in the future. (3) Through the full equipment of theKUCA and the introduction of the 70 MeV ring cyclotron shown in Table 1, KURRI will promote the research
including new topics. The basic study on the ADSR will
be promoted by using neutrons with higher energy, andthe new research will be initiated in the fields of medicaltreatment, material science and so on.
Then, KURRI provided a draft of budget for the fiscal
year 2002 including the full equipment of KUCA and theintroduction of the 70 MeV ring cyclotron. This draftwas submitted to the Ministry of Education, Culture,Sports, Science and Technology (MEXT) in Japan. At
the same time, KURRI began the negotiation in theframework of an agreement with the local public; it has
successfully been completed in August 2001.

3

depending on the properties of both the neutron production and absorption in material. The power distribution
in the core does not strongly depend on either the subcriticality or the proton energy, and also on either the eigenvalue or the fixed source calculation. These results indicate that the eigenvalue calculation is still useful for theconceptual design of the ADSR.
The time-dependent behavior of neutrons after theproton pulse injection was examined to obtain an imageof the reactor dynamics in the ADSR in relation with apulse operation of the ADSR type neutron source [5,6].
This kind of behavior after the pulse injection of 14 MeV
neutrons was also examined in relation with the KUCAexperiments. Note that the effect of delayed neutrons can
not be taken into account in the MCNPX calculation.
Fuel position [1,0], keff = 0.980, SOOMeV Proton, I rnA, W target, Time unit= lj..l s

CONCEPTUAL DESIGN STUDY

l.OE+OOurel.OE-01

By introducing the MCNPX code, a conceptual design
study on the ADSR was carried out through the neutronics calculation under an assumption that the proton
beam were incident upon a bulk target with a size of thecurrent fuel element settled at the center of the KUR [2].

l.OE+15-I.OE-02§ia l.OE-03.€-e~ I.OE-04~"'§ I.OE-05~ ~~~~!!!i~~il~~i!~!l!llili!

- - Target [0,0] Eigenvalue Cal.
--Fuel [1,0] Eigenvalue Cal.

z l.OE-06
I.OE-07

I.OE-08
l.OE-10l.OE-08

I.OE-06

l.OE-04!.OE-02

l.OE+OO

l.OE+02

Neutron energy (MeV)

Figure 2: Evolution of neutron spectrum.

Figure 1: Comparison between spectra obtained by
the eigenvalue and fixed source calculations.
The nuclear features of the research reactor typeADSR were examined by changing the design parameters systematically. For an example, the neutron spectra
obtained by the eigenvalue and fixed source calculations
are shown in Fig. 1. One can find that the main shape ofneutron spectrum is very similar with each other becauseof the neutron multiplication process due to the fission
chain reaction in the subcritical core, although thespallation neutrons can not be treated in the eigenvalue
calculation. It was also found that the neutron flux levelin the core becomes larger with increasing kerr and incident proton energy, and the thermal power of the ADSR
is expected to become higher than that in the KUR when
the proton energy is selected to be 500 MeV. The target
material has a certain impact on the neutron flux level

A typical result for the evolution of neutron spectrumafter the proton pulse injection is shown in Fig. 2. This
figure shows the following phenomena: (1) Neutrons
with energy of more than 20 MeV decay out in a moment
after the pulse injection. (2) Fast neutrons decay rapidly
in the period of less than several Jls. (3) The neutron
spectrum approximately reaches an asymptotic one identical to that of the subcritical core at around 10 Jls because of the neutron multiplication process in the ADSR.
It was also found that no essential difference between the500 MeV proton and the 14 MeV neutron injection in thetime-dependent behavior of neutrons,

4 BASIC EXPERIMENTS
A series of basic experiments on the ADSR wasformally initiated in 2000 by using the combination of asolid moderator core of the KUCA and a CockcroftWalton type accelerator [5,6,8]. A tritium target settled
outside a polyethylene moderated/reflected core loadedwith the HEU fuel was bombarded by a deuteron beam ofaround 200 keY to generate 14 MeV pulse neutrons asshown in Fig. 3.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

6 ACKNOWREDGEMENTThe KUCA experiments were performed as a JOmt
study among Prof. Toshikazu Takeda's group of Osaka
Univ., Prof. Yoshihiro Yamane's group of Nagoya Univ.
and KURRI. The present study is partially supported by
the Grant-in-Aid for Scientific Research (A) from theMEXT and the JAERI's Nuclear Research Promotion
Program (JANP).

REFERENCES

Figure 3: Image of the KUCA experiment.
Neutrons were detected by the optical fiber detector[9]; the Th02 detector was used to monitor 14 MeV neutrons, and the 6Li enriched LiF detector to measure theneutron flux level in the core. To examine the dependence of the neutron multiplication and the neutron decay
on k.rr, the subcriticality was systematically varied by adjusting the stroke of the control and safety rods inserted
into the core. To measure the neutron flux distribution,
the LiF detector was axially traversed along the fuel rod
with a constant speed to facilitate the conversion fromthe time-dependent neutron counts to the reaction rate
distribution. For the neutron decay measurement, the arc
pulse of the accelerator was employed as a trigger signal
to accumulate the data of time-dependent neutron counts
after the pulse injection of 14 MeV neutrons.
The analysis of the KUCA experiment was executed bya continuous Monte Carlo code MVP [10] in combination with JENDL-3.2 [11]. The 235 U number density offuel was reduced by about 5% in consideration of a bias
observed in the criticality analysis of the HEU fuelledcore [12]. It was found that the neutron multiplication isapproximately expressed as a function of 1/(1-k.rr) for theKUCA experiment. A large difference was observed
between the measured and calculated behaviours in theneutron decay. However, this discrepancy can beexplained in consideration that the MVP calculation can
not take into account the effect of delayed neutrons:

5 CONCLUSIONS
The following conclusions can be derived:(1) The nuclear features of the ADSR are practicallydetermined by those of a subcritical core because theneutron multiplication process becomes dominant.
(2) The accuracy both for the measured and calculated
subcriticality has an essential importance for thedesign study on the ADSR. The requirement for theprecision of neutronics calculation would become
more severe than that for the conventional nuclear
reactor, since the neutron multiplication does notdepend on k.rr but approximately on 1/(1-k.rr).

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